More Building Science
In the last two blog posts, I shared highlights from a talk given by building scientist Michael Blasnik at the Department of Energy’s National Weatherization Training Conference, particularly the problems with inaccurate computer models in determining weatherization strategies and effective home energy improvement measures.
Blasnik also shared his list of common strategies for home weatherization that don’t save much energy. To keep from wasting time chasing windmills and wasting taxpayer money, he recommends strategies to avoid or closely evaluate. These include:
- Furnace tune-ups. While a safety check is a necessity, annual furnace tune-ups don’t produce a marked improvement in efficiency, according to a study by the Oak Ridge National Laboratory. Blasnik says that a maintenance cycle of four to five years is typically adequate unless there are problems with the equipment.
- Sizing the furnace properly. Appropriate sizing for cooling systems is critical, but Blasnik says that there is no evidence to support that an oversize furnace is detrimental to the efficiency of the unit. Further, oversizing can compensate for duct design and installation issues. Of course, that’s not a good reason to settle for sloppy workmanship and oversizing furnaces. The take away is that it is not worth the investment to replace an properly functioning oversize furnace in an existing home.
- Basement duct sealing. Sealing ductwork in an uninsulated crawl space or attic can save 8-15%, but one study shows only a 0-3% savings with duct sealing in basements. Blasnik feels that these improvements are negligible because, for all intents and purposes, basements are “indoors.” Waste heat and equipment leakage preheat infiltration. He recommends fixing gaping holes but not to obsess about mastic application.
- Floor insulation. Adding floor insulation in results in up to 39 therms per year of savings, which calculates to around 25 years for payback. The reasoning for basements is because they behave more like inside than outside. If there is heating equipment in the basement, then ambient air is heated by waste heat, which in turn allows for regain in living space above the uninsulated floor. Insulating the floor would eliminate this positive effect on energy bills. Crawl spaces may see slightly more savings than basements when adding floor insulation, especially in very well ventilated crawl spaces with sealed ducts. However that addition can make ductwork even more outside. Alternatively, perimeter wall insulation would be a much cheaper choice and can achieve similar savings.
- Caulking and weatherstripping. These efforts show minimal savings, typically reducing infiltration by less than 10% and a savings of 0-3%. Blasnik cited a Canadian study that concluded that intensively caulking and weatherstripping extremely leaky windows can save about 0.5 therms per year, equating to around 50 cents.
- Window replacement. I find this statistic very interesting because I have been a proponent of historic window preservation for years. Replacing windows can save 2 to 3 therms ($2 to $3) per year per window, resulting in a payback period of over 100 years. Like all of these caveats, the existing conditions of homes should be scrutinized. Jalousie windows or superleaky units without storms could see better savings, but adding storm windows is more cost effective with higher savings potential. In the case of cooling, it’s more cost effective to try shading windows with landscaping, solar screens, or window film.
- Tankless gas water heaters. Tankless water heaters can save up to 1/3 of a home’s water heating requirements, or 35 to 75 therms per year. When replacing a tanked water heater, keep in mind that tankless heaters are expensive, topping $2,000, a 30- to 40-year payback. When coupled with efficient layout and very low-flow fixtures, savings can increase and the payback period can decrease, especially in new construction. One factors that plays into energy costs here: In homes with tankless water heaters, occupant behavior can increase energy usage. Endless hot water for showers and longer wait times at the sink can become the dirty truth in these homes. Most tankless units have a minimum activation flow rate, meaning that a trickle of water will not result in hot water coming out of the tap. A very low-flow showerhead does not trigger the flame in some instances, and homeowners turn on the bathroom sink to compensate.
- Cooling your attic. According to Blasnik, you won’t save much by insulating your rafters and cooling the space in the attic, unless your AC ductwork is located up there. In that case, savings could be up to 15%.[Editor’s note: on this issue, see Michael Blasnik’s posted comments below.] Blasnik says that the absolute worst approach is to add a powered attic ventilation fan above a leaky ceiling. This will depressurize the attic and, since the ceiling is leaky, the living space of the home. The depressurized home will force warm, moist air into conditioned space through holes in the envelope, pulling air out of the home and resulting in a warmer indoor environment.
Other things that Blasnik mentioned that don’t save cash:
- Changing furnace filters monthly. Once every season is adequate.
- Opening the refrigerator quickly and cleaning coils.
- Using drapes to insulate windows. This can actually create convective loops in the air space between the window and the drapes.
- Energy-saving kits. Two CFLs will not solve your astronomical energy bills.
When improving the energy efficiency of the existing housing stock, care must be taken to gather information about each specific home as well as the empirical trends in the science of this industry. The best approach is to gather historical utility data, thoroughly inspect homes, use diagnostic equipment, and have knowledge of what works.
The important underpinning of all three of these blogs is really a necessary mindset best recorded by the ancient Egyptians, Greeks, Muslims, and the societies that followed: Empirical evidence is absolutely necessary to support universal truths. Without the knowledge of that evidence, untruths can be repeated so much by so many people that they become “fact.” Every American should understand the scientific method enough to know the importance of seeking truth that is sufficiently supported by evidence. That’s what Blasnik does for us. The trouble with dogma in energy efficiency is that it doesn’t save the planet or anyone’s money. You wouldn’t go to a brain surgeon who learned about cutting through his intuition, so let’s not do that with our homes. Heed Aristotle’s words:
Thus it is clear that we must get to know the primary premises by induction; for the method by which even sense-perception implants the universal is inductive. … It follows that there will be no scientific knowledge of the primary premises, and since except intuition nothing can be truer than scientific knowledge, it will be intuition that apprehends the primary premises. … If, therefore, it is the only other kind of true thinking except scientific knowing, intuition will be the originative source of scientific knowledge.
Check out these references for some good building science knowledge:
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31 Comments
Good stuff, but needs to be taken in context
Rob - I agree with most of what you and Michael are suggesting here, however the comment about rafter insulation needs to be addressed in more detail. If you actually do an excellent job of insulating and air sealing the ceiling plane, then I agree with the point. In my experience, however, hardly anyone is capable of actually doing this particular job effectively, especially if there are a lot of recessed lights, knee walls, access hatches, etc. In some cases it is actually cheaper to insulate the roofline than the ceiling adequately, even if the duct system isn't in the attic. I always like to find relatively foolproof solutions as it is hard to get all the different trades to work together effectively all the time.
Where's the Posting?
Rob,
I cannot find your blog on effective home energy improvements. The above link just goes to the Think Spot main page.
Very True
When I was building, we always opted to insulate the roof deck. It gave homeowners the opportunity to have insulated storage space, and in my house, makes a great home office. Someone at GreenPrints this year made the same point about insulation installers. Blasnik made the point that weatherization crews actually do a very good job of installing insulation. I do not have the experience to make that sort of claim, but Blasnik did state that his MO is to analyze lots and lots of data. The few times that we specified fiberglass batts, our own crew went ahead of the installers and meticulously air sealed. Then we inspected the job for quality assurance, and our green rater would inspect as well. Those were walls, but the same could apply to ceilings. I'll do some digging and see if I can find more info on this topic and report back.
The other blog in this series will be posted soon. Thanks.
Too many experts
No two houses are the same. A lot of money is going to be needlessly spent ineffectively retrofitting houses on the recommendations of self appointed experts before this is realised. Well done! I await your updates with interest.
Mostly good, but don't throw the baby out...
I agree with most of the article, I think there's too much being done that makes too little difference. I think most of us in the biz are trying to steer consumers towards intelligently focusing their efforts. Don't replace windows, add insulation, and such.
One thing in the list that I strongly disagree with is "Changing furnace filters monthly. Once annually is adequate." This is utter malarkey and can lead to some seriously dangerous situations with combustion furnaces. I strongly suggest you remove this item.
I have never seen a home where a once a year changing of furnace filters is adequate. Every two months is much more realistic, but it really all depends upon runtime. If you're in the dead of winter, and moving air through the filter 24/7, you probably do need the monthly change. But really, consumers have to be regularly checking the filter.
Otherwise, great site, great articles!
Good point
Filters need to be visually inspected often. It totally depends on the system: oil vs gas, sealed vs open flame. There is no blanket answer, no silver bullet. Thanks for making a good point.
energy savings
Is there anyone who can tell me how much the new government mandated light bulbs actually save an average houshold per year?
It depends
Rich,
If you replace ten 60-watt incandescent bulbs with ten 13-watt CFLs, and if your electricity costs about 10 cents a kWh, you might save about $51 a year -- depending on how many hours a day you operate your lights.
For a more precise calculation, use this spreadsheet:
http://www.energystar.gov/ia/business/bulk_purchasing/bpsavings_calc/CalculatorCFLs.xls
clarification...
I'm glad you enjoyed my talk at the conference. I would like to make a couple of clarifications.
About furnace filters, I actually said you can change them once a season, not once a year. The commenter that claims that air moves through them 24x7 in mid-winter would only be correct if the homeowner set the fan to ON, which would be a big mistake and huge waste of energy. For furnaces with the fan set to AUTO, they typically run about 1000 hours per winter in cold climates. For a standard cheap filter, that's a reasonable amount of use and actually makes the filter a better filter. If the homeowner uses a higher quality filter, then they should change it more frequently, because those filters actually filter stuff out of the air. I hate to be too picky Rob, but I fail to see why fuel type or sealed vs. atmospheric combustion would have anything to do with filter changing needs.
My main point was that frequent filter changing is touted often as an energy saver and it isn't.
Your description of what i said about cooling an attic is kind of confused. I did not address conditioning an attic and moving the insulation to the roof line, which can be a good strategy in some circumstances. My point was about power attic ventilators, radiant barriers, and 'cool" roofs. These are three strategies for making the attic cooler to try to reduce cooling loads even though the insulation is at the ceiling plane. My point was that the savings are trivial from these measures if the ceiling is well insulated and there are no ducts in the attic. I was not talking about roof rafter insulation as an alternative to ceiling insulation.
People may want to see the actual presentation handout -- I haven't checked if waptac has posted it yet....
Thanks for the coverage and interest.
link to presentation
I see that waptac has posted many of the presentations from the conference at http://www.waptac.org/sp.asp?id=9595 My talk is about two thirds of the way down the page but you may want to grab several other useful presentations there as well. The pdf files they created are a little hard to read because of the layout they chose, but it seems to be all there.
Thank you Michael Blasnik
Michael,
Thanks for taking time to comment.
I would love to hear your comments about this blog also
https://www.greenbuildingadvisor.com/blogs/dept/think-spot/how-save-energy-0
Thanks Michael
I really appreciate the comments Michael. I did enjoy your presentation.
The 'gas vs oil' comment that I made above was in response to T. Inoue's comment, "can lead to some seriously dangerous situations with combustion furnaces.' I should have clarified that in an open ombustion system, if the fire box is compromised and the filter if located adjacent to the combustion chamber, it could lead to issues. I have seen systems like this before and it seems like that would not only be a CO, but also a fire safety issue.
Thanks for clarifying the attic cooling section. I'm not sure if it was the bier garden in Indy the night before your presentation or my wandering mind but in retrospect, I was confused on that item. Great to have your comments.
If anyone every has the chance, I highly recommend seeing Michael Blasnik speak.
Oversized heating and cooling equipment
As a heating/cooling contractor I can tell you that over sizing your heating equipment can cost you plenty. With today's high efficient heating and cooling systems they are designed to operate with efficiency and if your duct system is wrong you should try and correct that issue. Over sized units lead to high heating/cooling bills and premature failure of the equipment. In some cases this failure can become dangerous to the home owner (CO2 in the Duct system).
I know many cannot replace their duct work in a cost effective way, but installing oversized equipment to compensate for shotty workmanship can be a waste of money and a risk to your health or the next home owners.
There are no short cuts to efficiency, either you do it right or get some one that can help you. Sorry if I sound sarcastic, but I see this every day and have to help people make proper choices. Sometimes these choices are not what they had in mind, but you must start with a heat loss/ heat load calculation and then use that to design a proper energy efficient comfort system. Many times it can be a simple repair to the duct system to correct the improper duct sizing (Adding dampers), but without a heat loss/heat load calculation it is just an un-educated guess and that probably was how the first system was design in the first place.
equipment sizing
My mention of duct systems and system sizing was actually one of the main arguments in favor of downsizing the equipment. Many duct systems are poorly designed and have excessive static losses. Reducing the size of the equipment means less air flow is needed which means better air flow through these poor duct systems. Another reason in favor of smaller equipment size is first cost.
But in terms of efficiency, modern sealed combustion gas furnaces are relatively insensitive to sizing. You can install an 80 kbtu furnace instead of a 60 kbtu furnace and it will likely have no measurable impact on heating energy usage. For air conditioning, proper sizing has always been emphasized due to the better humidity control provided by right-sized equipment. But recent research in Wisconsin (by ECW) and Florida (by FSEC) both found that changing air conditioner equipment size did not lead to any measurable energy savings and no significant differences in comfort.
Arguments in favor of over-sizing the equipment include faster recovery from temperature set-back (important in older homes where setback can provide good savings) and lower duct system conduction losses.
I don't think we should all way oversize heating and cooling equipment, but I do think that claims about efficiency impacts are way over-stated. Kevin states that over sized units lead to high bills and premature failure. Can you cite any studies to support this claim? Also, by CO2 in the duct system I assume you are referring to CO?
Also, people seem to think that Manual J was carved in stone from the mountain top -- it's actually just another crude model of estimating building loads.
equipment sizing
Thank for responding Michael, I look forward to being a part of your community. A friend of mine emailed me your site and he knows how passionate I am about energy efficiencies when it comes to heating and cooling. I hope I can add to your discussion and learn more as well.
Yes I meant CO and not CO2; I was typing to fast and got ahead of myself.
The reason I posted to your blog is that I have seen many customers disheartened by the lack of savings they achieved when going with an oversized high efficient heating system. They where expecting more return for the dollar and they got less while still being unhappy with the way their home feels during the seasons.
You ask do I have proof of the premature failure of heating equipment. Yes the manufactures of these units will tell you that and they spend millions researching their products before they sell them on the market. Most high efficient heating systems (above 90%) condense while they operate. This condensation traps an acidic vapor that if not removed properly will rot the heat exchanger. Just like if you build a chimney to high the gas will get trapped or if you oversize an A/C system it will cool the air, but will not remove the moisture. You can slightly oversize the unit, but that is not what most contractors do they drastically over size.
Next many contractors try to defy science, warm air rises and cool air falls. If you look at many (Over 75%) of warm air systems they are either installed with all the registers in the floor or all in the ceiling. This causes your system to have to work harder to overcome the law of science, not a good idea and doesn’t really work well. That is why warm-air systems are the cheapest to install, they cut way to many corners. If you installed it properly you would need a register in the floor and ceiling of every room, except baths and kitchens. In Europe where energy costs are a lot more, over 90% of heating systems are water based. Here warm air is over 90% and since most are oversized they are doomed for higher heating cost and very uncomfortable home owners. Don’t under estimate the power of feeling cold, it can make a person turn the heat up to above 75 degrees.
I am not a fan of set-back I believe you waste more than you save. You have to reheat what you already have heated and building materials can take a long to heat up. Many people that have warm air systems set the high temp. to above 70 degrees to compensate for the feeling of cold. In a radiant floor system you can achieve comfort at 65 all day long.
Think of it this way your car gets better fuel mileage on the highway then in the city. So a heating system that constantly runs and tries to maintain temp is more efficient than one that tries to recover quickly. Quick recovery means you waste more in the exhaust than it puts in the home. That is way many in the heating and cooling industry think that the ASME ratings are not true. It is not 80 degrees every day in the summer and -5 in the winter, so your system has to know how to operate differently in October than is does in January, most system do not.
The fact of the matter is that with all heating and cooling systems the manufactures have done the hard work for you, you just need to follow their lead.
reply to Kevin
Kevin-
First, this is not my blog. I was just responding to the conversation.
Second, almost everything you state is simply incorrect ranging from your belief that temperature setback doesn't save energy, the analogy to driving a car, the description of high efficiency gas heating systems, etc. Please don't propagate myths.
Setting Back Thermostats
Michael Blasnik,
I agree that setting back the thermostat should save Energy in an average or poor performing home.
However ... something tells me that the savings are not as great as estimated in this Blog
https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/getting-ready-winter#post_comments
Curious about your take?
I am just guessing that setting back the thermostat may not have much impact in a Low Energy Home (similar to a passivhaus)
Your Thoughts?
setting back thermostats
I agree. Temperature setback is most effective in relatively inefficient homes since the savings depend on the indoor temperature dropping. In a very efficient home, the temperature drops so slowly that there is little savings potential. Also, if the efficient home has a properly sized low capacity heating system then the recovery time from setback can be so long it can't really work.
The simplified calculation in the linked posting over-states the savings even for inefficient homes because it assumes that the temperature drops to the new setting immediately. Still, you can achieve savings of 5%-10% of heating loads from temperature setback in many older homes.
Reply to Michael
http://www.cchrc.org/App_Content/files/FinalReportRHE.pdf
http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12530
http://optimalbuilding.com/files/7_reasons_why_oversizing_is_a_bad_idea.pdf
http://arkansasenergy.org/business_development/energy/files/Clearinghouse/ACF7893.pdf
I can give you more info. These are not myths. What is being said here about over sizing heating systems is dead wrong. This is why energy efficiencies are not going to work, to many low cost solutions to a high price issue.
Now set back. Yes it will heat up quicker, but it will also cool down quicker causing the system to short cycle. That is a major waste of energy, since the home will not benefit from the quick heat up/ cool down. Next your upfront cost of over sizing has to be included into the cost it will take some time for pay back.
Here is a link for the corrosion
http://www.hvacoracle.com/cgi-bin/publish.cgi?p=99&s=heating
Set back is a simple lousy answer to a really big issue and if that is what energy efficiency has become we are in big trouble.
reply to Kevin
I did not dispute that furnaces are often over-sized and I also pointed out that you can save equipment costs by installing a smaller system. I also said that AC oversizing is likely to be more of a problem due to moisture control, although some recent studies even call that into question. My main point about sizing was that there is little or no energy savings from 'right sizing" of modern furnaces and you provided no data to dispute that in any of your links.
Your first link to the Alaska study just documented that systems are indeed sized much larger than loads. There is no data on energy savings from right-sizing. The second link just includes a recommendation to properly size heating systems when you replace them. Again, it provides no data about energy savings from sizing. The third link is just a series of claims with no data or references. The fourth reference mentions some research about air conditioner over-sizing done by Proctor Engineering Group -- where I was the director of evaluation for four years and worked closely with John Proctor on several of the most cited research studies on air conditioner and duct system field performance and sizing issues. Still no data on energy savings. Your final link about corrosion makes no claim about over-sizing being a cause of it.
So none of your links actually has any data to show energy savings from proper sizing. I am not saying that people should oversize systems -- I am saying that the energy penalty is overstated, dramatically.
You also continue to show that you don't understand how temperature setback leads to energy savings because you apparently don't understand the basics of heat transfer or heating system operation. In the vast majority of homes, lowering the thermostat setting when you are away or sleeping is just plain common sense and can provide significant energy savings (backed up by measured data). Your inaccurate claims are counterproductive.
Reply to Michael
OK It seems we have a different of opinion. I will try and state my case and if I am wrong I will admit to the fact.
The main issue I have is the statement that over sizing systems has no effect on savings or the efficiency of the equipment. Almost all manufactures will tell you that statement is untrue and can cause a voiding of their warranty of their produce. Now nowhere in this blog can I find a definition of what over sizing means, is it 10%, 20% or 300%? That statement is too vague when we are discussing energy efficiencies. I can install a 399K BTU Energy Star rated piece of equipment in a 2000 sq foot home and say, based on this blog; it will save just as much as a smaller unit, because there is no data to say it won’t. That is just obscured and completely wrong. I have seen this type of installation many times over and many in the heating and cooling industry will tell you the same and we are not talking isolated incidents.
As for my knowledge and understanding of heating or lack thereof, all I can say is this. When talking about heating and cooling of a building we are not concerned with heating the structure as it could careless as to what temp the building is. It is the people in the building that we are trying to make comfortable. Knowing this we must first understand how the body works. The average human makes about 400 BTU’s an hour sitting in a chair, but only needs 100 to continue living. So it gets rid of the 300 BTU’s it creates. That is all that matters when design heating and cooling systems, anything else is just window dressing. This fact has been lost over the years and I can not understand why, buildings do not pay utility bills people do.
Your set-back argument is over 30 years old and has done nothing to save money, other than to give people a false sense of savings when in fact they have not saved on dime. When you oversize a heating system, improperly install the equipment and try to defy the laws of nature you pay up front and continue to overpay over the years. All that setback does, in this case is slightly lower your waste to begin with, but you have already overpaid to start.
Now here is someone you should talk to, his name is John Siegenthaler
http://www.healthyheating.com/Siggy/Siggy.htm
Here is another site that can help you.
kevin
This will be my final reply. I am not saying that you should drastically oversize equipment or that there will be zero impact on efficiency if you install a 399 kbtu heating system in a small home. I am saying that, back here in the real world, there is no data or even good theory to indicate that heating energy usage will vary significantly from heating system sizing over the typical range of residential system sizes. I am in favor of avoiding over-sizing because it is cheaper and may provide some other benefits. For air conditioning, a recent study in Wisconsin found zero difference in cooling energy usage between a 2 ton and a 3 ton unit (swapped in the same home). Still, I think the smaller unit is the better choice.
Your discussion of keeping occupants comfortable certainly has some truth, but air temperature is still the key way to accomplish that. Claims about energy savings from radiant floor heating have not been proven. In fact, a study in Canada found no difference in indoor temperature settings between homes with radiant floor heating and forced air heat (the radiant floor homes actually had slightly higher thermostat settings)-- see http://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/01-106-e.html
Your complete misunderstanding of the value of temperature setback is remarkable. I actually do have measured data showing significant heating savings in thousands of homes from programmable thermostat installations (which includes the diluting effect of people not using them). You, on the other hand, can remain firmly attached to myths. I hope you don't spread your bad advice to many others.
Reply Michael
I understand you are afraid of the future since the past has done such great things for us. You do know that set-back is not a new idea? People have been doing set-back for generations, only difference is today we rely on programmable ones, I blame being lazy, how about you? My Great Grandfather was turning the T-stat down decades ago and yet this is the best you have…”I have 1000’s of data to provide I am right”.
Nothing gets me more than a so-called self proclaimed educated real world “Professional” spouting off what someone else told them to be true. Do the math yourself and stop with your outright lies. I told you that just about every manufacture will tell you that over sizing any heating system is not good, yet you can give me NO “empirical, measured or any other type of data” to prove that wrong. I can get each and every one, no matter which you want to choose, to tell you that you are completely and totally DEAD WRONG!
If you truly do understand heating or A/C for that matter, then you would know that doing a heat loss/heat load is by no means an exact science, NEVER said it was. However it IS better than your “Hey Rocky watch me pull a rabbit out of my hat” sizing method. Where do you get off spending these lies and not have FACTS to back them up? You ask me for data, are you kidding me? You want data?
Modulating Boilers, Two stage Heating Furnaces, Outdoor reset, Multi stage Boilers, Two stage A/C units, Multi stage A/C units. Go ahead Google just one, then tell me over sizing has no ill effects. If over sizing was no big deal as you would have many to believe then why in the world would we be install these units. Better yet, why would there be over 100 equipment options for people to choose from, FROM the same manufacture? You make it sound as if we should just have small, medium, large and super size. Sorry Heating and cooling is NOT McD’s, that is a stupid simple uneducated answer.
PS: The next time I get called at 3AM to a home to look at a failed heat exchanger in an oversized high efficient furnace, that the local fire department shut-off from high CO levels, I will forward your name to the installing company to be their “Expert witness” to say it wasn’t true.
Signed
Kevin A Gerrity
Kevin, you're attacking the wrong guy
Kevin, apparently you don't know who Michael Blasnik is. He is one of the most well-respected energy researchers out there. He doesn't base his conclusions on commonly held beliefs, but rather on data.
Michael sometimes surprises people by presenting data that shows that some long-held beliefs turn out not to be true. His investigations are exactly what the industry needs.
I have been reporting on Michael's findings for many years. I, too, assumed that right sizing was a sacred cow. But many good studies (the ones that Michael cites) have now shown that oversizing HVAC equipment doesn't cause the penalties we thought.
It's important for all of us to listen to the best experts in the industry and to learn from new data.
Kevin, the only argument that will sway Michael Blasnik will be based on data from well-conducted research studies. It's clear that you don't have such data, so it's time to let go of this debate.
setbacks
Kevin, since you are not the only one who has this misunderstanding about the benefits of setbacks, let me jump in and see if I can clear it up. Since you're in the HVAC industry, you no doubt know that the amount of heat loss/gain by conduction is determined by the temperature difference. The higher the delta T, the more energy you have to use to maintain it.
Let's do a little number crunching. Say you have a house with 5000 square feet of building envelope, and it has an average U value of 0.2. If you keep the house at 70 degrees for 8 hours at night with the outdoor temperature at 30, the furnace has to add 40,000 Btu/hr or 320,000 Btus of heat through the 8 hour period. If it does so at 80% efficiency, you pay for 400,000 Btus.
If instead you set back the temperature to 60 degrees for 8 hours, the furnace adds 30,000 Btu/hr or 240,000 Btus of heat for the night, and you pay for 300,000. When you get up in the morning and turn up the thermostat to 70 again, if the temperature is still at 30, now the furnace has to add 40,000 Btu/hr again, just as it would be doing if you'd never set it back.
There's no penalty for trying to bring the temperature back up after the setback, the unit has to run a bit longer to do it, but the heat flow is the same as if there'd never been a setback. Meanwhile, you've saved 100,000 Btus by setting the T-stat back 10 degrees for the night. It all depends on delta T. It's just basic physics at work here.
Thanks Everyone!
I want to thank everyone for taking part in this discussion. It's important that we continue to have these discussions and that they remain productive. It's also very important to have an open mind and be skeptical as opposed to having a closed mind and skeptical. A good scientist is open minded, unbiased and skeptical.
The best foundation for good beliefs in the physical and natural world is repeated observation. There are a few key terms in that last sentence. I'm only talking about the physical and natural world, not the supernatural, which requires a whole different kind of belief. Repetition in science doesn't mean that you have observed a specific phenomenon two or three times, or a even a few dozen times. It means that you have observed things hundreds or thousands of times and have cranked those observations through scrutinizing mathematical equations in an unbiased way to support or fail to support a hypothesis. Observations and statistics can be biased, both intentionally and unintentionally.
I understand how personal observation of a few instances can have a very poignant effect on a persons belief system. But what we really need to challenge ourselves to do in these sorts of situations is to stay open minded and look for empirical trends.
Keep up the good discussion!
reply Martin Holladay
The issue I have is the vague statement that over sizing heating equipment is not detrimental to the efficiency of the unit. My reason is that the people who make these units (Trane, Lennox, American Standard, York, Buderus, Burnham, Viessmann, etc…) ALL will tell you that is wrong. They have the data and research to back this claim up along with the engineering to prove it. Also they have pictures of furnace and boiler heat exchangers that are less than 5 years old that have been rotted by flue gases because the systems where over sized. There is not data or research on this because those doing the data and research collecting are not asking the manufactures what problems they are finding with their units. Just call one of them and ask if they have ever been include in a study that shows the practice of over sizing has no effect on their equipment, but add when installed in homes not science labs.
The other issue is heating equipment is sized for the average lowest temperature of the region it is installed. That means you only need the systems full capacity for about 10% of the heating season. The rest of the time your system can be oversized as much as 1000%. Your furnace does not understand the difference between a 60 degree day and a -5 degree day; it just knows how to come on to full capacity. Now if you use the same sizing method in Tenn. as in Alaska, but do not consider Alaska is colder you will get a call on day one that you undersized the system even though in Tenn. it would be consider oversized. That is basic common sense.
From the article:
“but Blasnik says that there is no evidence to support that an oversize furnace is detrimental to the efficiency of the unit. Further, oversizing can compensate for duct design and installation issues. Of course, that’s not a good reason to settle for sloppy workmanship and oversizing furnaces. The take away is that it is not worth the investment to replace an properly functioning oversize furnace in an existing home.”
This is what Michael said and nowhere does he qualify the definition of what he means by over sizing, that is huge and as I have said over and over again wrong. If he was correct then we in the heating industry would not be installing 2 stage furnaces and save our client 20-30% on their fuel bills. If he was correct then Outdoor reset would have no measurable effect on the consumption of fuel. If he was correct then if we installed a “right sized” modulating condensing boiler with outdoor reset people would not lower the money they mail into the utility company by, in some cases, 65%.
Sorry thermostat turn-down at night, turn up in morning is very old and the industry has moved on from that.
Reply Allison Bailes III
I do not have a misunderstand of set-back. I just choose to do it at the source where the energy is create and not where it is going to be used. You see turning the thermstat up and down will save you money, however it will not lower the amount of waste you have to create that energy (heat).
I will use your examlpe to explain: A 5000 square foot home, with an average U-Value of 0.2 that the desired inside temperture is 70 and the outside temperature is 30, that is a difference of 40 degrees. Now the home will cool down at a certain rate.
Now let's change only the outside temperature to 10 degrees. Now your difference is 60 degrees and the home will cool down quicker. This will cause your system to come on and run longer then if it was a 40 degree differential.
The issue is your 80% system was not sized to heat for a low of 40 degree, or 10 degrees. It was sized to heat your home on a worst case scenario of 0 degrees outside. Your 80% unit operate as if every time it comes on that the outside temperature is 0 every day of the heating season. That means if it is not 0 degrees outside you are wasting energy and the amount could be as high as 90%.
So in your example we could at some time create 200,000 Btu's and only use 20,000 btu's to heat the home. I haven't even added in the lose from the duct system not being properly installed and sealed.
You have to unsderstand your heat or cooling system is only as efficient as the poorest installed componant. That includes the unit, duct work and the registers.
Now I will give you some thing to think about. Have you ever gone into a "New" building and saw people with electric heaters under their desk? Ask them why the have those heaters there. They will tell you that their feet are cold. It is because air is not a very good heating source.
reply Rob Moody
That is why to date no one has dis-proved the law of nature that warm air rises and cooler air falls. No matter how many times they install all the register in only the ceiling or floor of a building. Or how they don't consider a lower temperature in a floor or ceiling to be as much as of a concern when insulating a home. Just because it is not directly exposed to outside temperatures.
One last thing, science defines energy cold as below absolute zero. That can be seen as everything else has heat, to put it simply.
http://en.wikipedia.org/wiki/Absolute_zero
kevin, you better back up.
thats it kevin, i am tired of this. Michael is a great man, next time you come on geen building advisor, you better not leave a comment! game over big boy.
Kevin, your arguing is
Kevin, your arguing is getting way overemotional. You claimed: "The main issue I have is the statement that over sizing systems has no effect on savings or the efficiency of the equipment."
Michael backed up those claims with data: "recent research in Wisconsin (by ECW) and Florida (by FSEC) both found that changing air conditioner equipment size did not lead to any measurable energy savings and no significant differences in comfort.").
Your experience in the field is anecdotal, not evidence. Any one of a thousand HVAC people with similarly long careers can come back with different opinions.
As for your argument that drastically oversized equipment can lead to early failure, that's not a point Michael ever seems to address. OTOH, you never back it up with anything more than anecdotes and claims that the manufacturers support you on phone calls. Not evidence. Opinion. Maybe right, maybe wrong, but it's just opinion to the rest of us, so it's not convincing, and your emotions are making you less credible to much of the audience.
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And, "Anonymous", keep your stupid threats to yourself in your momma's basement.
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