Finally got my heat loss numbers…ouch!!!
bobhol
| Posted in General Questions on
I thought I was on the right track but I am not so sure now …42000 btu is the heat loss number .It is a 1200 sq ft slab on grade 1.5 storey .Total living space is 1900 sq. feet .It is 2by 6 blown in fiberglass with 2.5 inches of EPS on the exterior .We do have lots of triple pane large high windows on the lake side which is a north east exposure …,that one large combo living,kitchen,dining room is a 17000 btu heat load .I was hoping to heat it with a Fujitsu mini split on each level …Is that still a viable option ? I am in Peterborough Ontario which is a zone 6 …thanks,Bob
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
That seems awfully high. Is the slab insulated? For my house, currently being designed, the preliminary heat loss calc. Is about 12000 but/ hour, zone 6, design temp is around zero f. It is one floor, about 1800 square. Ft.
I just talked to my builder and found out a few things ...he sent me drawings used for the heat loss ...first thing I noticed was 2.5 inches of EPS on the exterior( I will edit my above post from 5 inches to 2.5)...second thing I noticed was window area was greater than the concept drawings he sent me ,probably cheaper to join three windows rather than have 3 separate windows with insulated wall between them .There are 2 banks of those on the north wall .I will have to cut back on window space and try upping the insulation slightly ...maybe XPS rather than EPS ..his ceiling specs show r75 and the wall specs show r24 with r10 exterior sheathing....slab is insulated with 3 inches of EPS......ANY AND ALL COMMENTS WOULD BE APPRECIATED ,thanks,Bob
Your envelope design can be retooled for lower energy demands by thickening all insualted elements. Air sealing and dense packed cellulose can markedly improve performance. Are you using a heat loss formula that takes infiltration/exfiltration values into consideration; and are you aiming for a "very tight" envelope. Particularly considering you're using force air heating.
I would be concerned with mini-splits as the sole heat source in Peterborough, no matter what the BTU's are. Heat pumps are the poorest performers and least efficient at the times you need them most: very low temperatures. I would design in a back up heating solution, such as a small wood stove centrally located on the main floor or an electric heat source per room.
Bob,
I am not seeing any Fujitsu units that can handle Peterborough 99% design temperatures with those heat loss numbers. The XLTH series claim they work down to -15F (-26C), but they only produce 18k BTU/h at 47F. I can't find anything telling me what they will put out at lower end temperatures. In any case, two units is still only 36K BTU/h.
http://www.fujitsugeneral.com/wallmountedRLS2H_specs.htm
XPS will only net you R3 upgrade.
Without seeing your plans, heat loss calculations and glazing details (area, R value, etc) I would say that any single upgrade/change will not put you in range of the above Fujitsu units being adequate.
Does your builder/designer know that you want to heat with two heat pumps?
cheers
A few things going on.
First off, this:
https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
The specified exterior foam is inadequate for Zone 6 on a 2x6 wall if it is EPS. As XPS it is ok.
Secondly, you'll get a higher quality of feedback if you can find the design temperatures and assumed exfiltration on the heat loss calculation report. Posting the floor plan wouldn't hurt. There is a lot of data missing to try to point out where the issues are. However the glaring one is the NE wall of glass.
Thirdly, Lots of NE exposed glass would be indicative of design desires conflicting with energy goals. You're in a tough site with NE views in a cold climate; there will be some performance compromises if you cannot determine how much northerly glass is a lot and how much is enough. Has your builder completed a house with these design goals before? Are they designing it? Do you have access to the designer and are you using a third party energy modeler?
At 1,900sqft, you're certainly in the size range where 20kbtu/hr or less is feasible and dual minisplits becomes viable but it seems from here that someone in the process is not making the needed adjustments to get there.
Thanks for the feedback everyone ...I have a propane fireplace as supplemental heat and the building inspector is mandating a heat source in every room so we are installing minimum baseboards.I agree that the glass wall is the main heat loss so we will have to cut back on window size .The comment on the exterior cladding is well taken and I will look into that ...my builder is very receptive to building a "pretty good house "so I will also see what he suggests...I am aiming for mid to low 30,000 btu...keep the suggestions coming ,I hope to break ground in september .This heat loss was a long time getting done but everyone is busy ..I will find the design temperature and assumed exfiltration and post it ...regards,Bob
The design temperature is from -11f to 72f or Design TD is 83F.The infiltration method is F280.Air changes/hour 0.48....this report is very complex and more than a bit confusing for me .It was produced using a program called Wrightsoft .I think I will see about making the windows smaller by about 20 per cent on the north and west walls and talk to the builder about XPS and 4 inches total on the outside....any other suggestions?
Jason...thanks for the input....I was counting on mid 30's for the heat loss and supplementing the mini splits as needed with the propane fireplace in the cold snaps...I see you are from Peterborough as well ...Needless to say I am disappointed with the numbers but resolved to find a solution ,regards Bob
BOB,
Your biggest problem is the assumed infiltration loss. The assumption is WAY HIGH and suggests a VERY LEAKY house that would have a blower door test of 6+ ACH @ 50 pascals. With minimal attention to air sealing you should be closer to 1.5 ACH at 50 Pascals. Ask your builder to do a blower door test and ask for a guarantee that it's below 2 ACH @ 50 pascals then ask that the infiltration rate for the heat loss calculations be 2/12= 0.1666ACH .
Thanks Jerry...I hope I am reading the report right ..I will get in touch with the gentleman who created it and get some clarification tomorrow.If you are correct and I am able to get the actual air exchange to 2 or below on paper and in reality ,in the grand scheme of things how would that affect the final heat loss number?
Bob,
The actual infiltration is a fraction of the test number because the test artificially creates a greater pressure difference than actually will occur. The conversion factor is between 1/12 and 1/20 and is just a guess. Since you said your house is 1 1/2 story I used the 1/12 factor and applied it in reverse to the data you gave to come up with the conclusion that the leakage assumption is bad. With the 2 ACH test data the heat loss due to infiltration should be about 1/3 of what was calculated. BTW the proverb "garbage in = garbage out applies. I'd be very suspicious of all the heat loss calculations and ASSumptions. For example were the u factors of the windows you are paying for actually used or did they use something else? Triple pane windows should be u=0.2 or less
Bob,
If you have any time, at all, it would likely be easy for you to take a larger role in your own energy design, and greatly increase your comfort level. We are right in the middle of the Information Age, and that means that many people can in some ways become experts, essentially, in subjects that they always thought could only be handled by professionals.
On the internet, look up and download BEopt, from the US National Renewable Energy Lab. It is free, powerful energy design software, and I am assuming it is available to Canadian citizens. About the hardest thing you have to do to use it is to download the weather file for your area, again free. After that, it is just a fill-in-the-blank process, with you providing input such as wall R value, window U value, estimated infiltration, etc. It sounds like you are alreadyy familiar with materials, R values, U values, infiltration, etc., and that's about all the prerequisites you need to use BEopt. It is extremely easy and quick to change variables to see the effect on overall heating/cooling demand, not to mention hot water demand, lighting, etc.
I used BEopt to accurately predict what my heating/cooling demand would be after I completed a huge energy retrofit of my house. It's a very long story, but I went from heating demand of about 40 kBTU/hr, using a central heat pump with ducts, to about 14kBTU/hr., using two small, ductless mini-splits for 1 1/2 stories, 1500 sq. ft., 12F design temperature.
It is useful to note that sometimes experts and professionals get hung up in their traditional ways. Sometimes, and in some ways, the Information Age can possibly make you as much or more of an expert than "the expert." When I finished my deep energy retrofit, I brought in 3 HVAC contractors to give me an estimate on pricing and capacity for a new heating/cooling system. I already knew what I wanted and needed, and knew that I was going to personally tear out my old, existing central HP and duct work and install mini-splits. In short, all the traditional contractors were astonished with what I had done, and did not believe that the minis would be adequate. They were wrong. When the HVAC contractor I finally chose first arrived for the estimate, I asked him while we were still outside what he thought I needed for heating/cooling. He estimated "about a 3-ton unit." I live in a neighborhood full of 3-ton central units. Even after I told him what I needed and wanted, he wrote the following disclaimer in his quote: "Customer chose design and capacity of units. Rooms will not all be comfortable and there will not be adequate heating/cooling distribution." This contractor is a very good and established contractor, and a good person, but, again, he was very wrong in this case.
You have received some very good information from the people above. As examples, Matthew is "right on" in saying you should be able to achieve less that 20kBTU/hr. I feel comfortable in agreeing with Matthew because I have done extensive energy modeling using the Energy Plus software (also free, but much harder to use than BEopt). Jerry gives you important information about infiltration. Again, hopefully, my contribution is that you can readily see for yourself what you can achieve using BEopt, or something like it.
Bob,
It seems likely (very likely) that Jerry is correct and that the assumed air exchange rates used for your heat loss calculations are too high. This should hopefully resolve your heating issue. If the estimates in post 11 are accurate, cutting 42,000 btu/h by 1/3 will significantly change things. Even being conservative and cutting it in half will be dramatic.
The bigger concern I would have, was this a simple input error or is your builder using that high of an infiltration number on purpose?
Even the most basic of standards, R-2000 requires 1.5ach50.
http://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/oee/pdf/residential/personal/new-homes/r-2000/standard/current/R2000-standard.pdf
Since you are doing research on this site, and asking pertinent questions, I will assume you have a firm understanding of the importance of air sealing. If your builder is in fact using design assumptions of 6ach50 (or even half that), I would ask them some hard questions.
With expectations that low, however, I am starting to think it might just be an input error. Or we are missing something. Even with no attention to air sealing, I am not sure a tract house in Canada would come in at 6ach50.
And yes I am also in Peterborough. Glad to see I am not the only one interested in high performance homes around here! Looking forward to your report once you get this thing built.
cheers.
Jerry,Sonny,Stephen Flitch ,Matthew and Jason...thank you all for your attention and advice .You cannot know how much it is appreciated .I was disappointed to say the least with the report but have now picked through it and have resolved to get it right.Your suggestions and words of wisdom are invaluable .I have been a follower of this forum for quite a while and recognize the collective knowledge and expert advice available.....Jason please feel free to contact me ,I am in the phone book and work at the local Honda store...any other thoughts from other readers ,please post your comments...,regards Bob
I was able to get through to the fellow who generated the report.....nice man but overworked for sure ...he had 4 screens open to work on when I called .when I explained my concerns he had the right answers.My specs called for r16 exterior foam but his program only allowed r10.He adjusted the inwall value of r24 up to r30 to compensate...the windows were calculated using a generic dual pane and my specs were for triple pane ...he changed that...I asked about ACH info but his program has a tight house or very tight house settings only...I asked for the very tight setting ...after accommodating me with these changes,he ran the numbers ....it came to 32000 BTU heat loss ..much better ...I am going to let the builder know ..lol..as he emailed me the new test but not the builder.I plan to tweak the window sizes on the north east wall and with any luck I should be able to get under 30,000...I guess I will be much more involved with this project from here on. I didnt want to be a pest or nuisance but after this I will be watching much more closely...again many thanks to all the respondents,Bob
Who makes the heat loss software you're using? The 2 most common Manual J programs are from Elite and Wrightsoft. Most users rely mainly on the default components already in the software and may also draw from a default building template, rather than start a new building from scratch.. This can sometimes be problematic when the default includes things like high rates of exterior duct leakage, or other poor properties.
Both programs also allow users to 'build' custom components and specify different leakage rates. I would suggest that you get a copy of the inputs page for the heat loss calculation and verify that everything is as you intend. Usually a quick skim for the really high btu items is enough - eg duct loss when there should be none, oddly high window loss or infiltration numbers, etc.
Bob,
I hope you are not paying for the heat loss calculations as they are simply INCOMPETENT! The change from r24 to r30 in cavity insulation is no where near enough to compensate for the change from r10 to r 16 outsulation. The result is the heat loss through your walls is being overstated by at least 10%. What framing fraction is being used in the heat loss calculations? Hopefully your builder is framing 24"OC and using OVE which should cut the framing fraction to 20% or less. If the software is assuming 16" OC traditional framing and a framing fraction of 25% or more while the builder is doing the smart economical and energy saving right thing and framing with 24" OC OVE that will represent another 10 % overstatement of the loss through the walls. What was the ACH number used for the "very tight house"?
Bob,
I have a very basic question, why a 1 1/2 story design? This type of structure poses several significant design and building challenges if a well insulated,air tight energy efficient structure is the goal. It is much easier to achieve these goals with all vertical walls and a flat ceiling plane such as a two story design offers. Your original post mentioned an r 75 ceiling that is easy with a flat ceiling and a vented attic above (24" of loose cellulose gets it done) . But sloped ceilings need great care in design and execution to avoid moisture related durability issues.
Hi Guys...the program is Wrightsoft...I was told that r16 on the outer wall wasnt an option in the program so he increased the inwall insulation...when I asked about ACH numbers again he said the options were tight house or very tight house ...as to 1.5 stories ,the house fit on our footprint as we were limited and we have a peak roof height restriction and also we like the look of it...we are tearing down an original cottage and have to follow township regulations on a new build ...thanks for the input..,Bob
Bob,
If the only "knob" on the program that can be turned is the cavity insulation it'll need to be raised to r 40 to approximate r16 over a 2x6 wall with 25% framing fraction. or r38 if the framing fraction is 20 % His raise of r6 does not account for the reduction in heat flow through the studs that added outsulation provides, he only has accounted for the improvement where there are no studs.
Thank you Jerry!!!The math skills on this forum amaze me !!
I use Wrightsoft every day and know several other designers that do the same. I have never talked to a user of this mult-thousand dollar software (full-suite) that uses "generic" or "templates" in their design work. The point is in the details.
The heat loss was high and the designer was sloppy from the details imparted here. DIY heat loads for modern construction with ERV, triple-glazing and rigid foam in cold climates is not for the amateur or the harried professional.
If you are building your home around your heat source you need a proper ACCA Manual 'J'.
If your builder will agree to a blower door and a specified ACH, good luck with that, then that number can be plugged into the Wrightsoft program and all your worries are over...
Morgan,
As I said previously "GARBAGE IN =GARBAGE OUT". If you can't do heat loss calculations without software, you shouldn't be doing them, or call your services "professional" and simply have no right to bill for them! What Bob has encountered is simple incompetence aided and abetted by a high priced tool used ineptly by someone lacking fundamental understanding of heat transfer.
Bob,
I downloaded the demo version of Wrightsoft to find out how to get it input correctly. The framing spacing is an input and needs to match what you are using either 24" OC or 16" OC They do not allow over r30 cavity insulation so the best option is choose r25 then select r12 exterior rigid AND r4 interior rigid This will get as close as needed to your actual wall with r16 rigid. Then for the windows you'll need to have him use "custom values" and check NFRC rated and input the SHGC and U values from the window maker. The program's allowed standard selections do NOT allow a u lower than 0.27 which is ridiculously high for a modern low e coated triple glazed window. To accomodate your r75 ceilings you'll need to use both ceiling and roof insulation inputs as the program limits ceiling insulation to r56 so select r50 for the ceiling and r25 for the roof which has the same effect as r75 ceiling. The infiltration should be about right with the "tight" setting as this results in 0.13 ACH which would mean the blower door test was 1.5ACH or better @50 pascals Hope this helps and thank MORGAN for his helpful input!
Again Jerry many thanks ...your knowledge and patient explanation are starting to get through and I am understanding better what is going on ...at the least I will be able to talk to my contractor and make him understand my concerns ...and ,yes, I will have a blower door test run to make sure I am as close to 1.5 ACH as he can get ...that will be his goal ..regards,Bob
Bob,
In some of the above responses you are getting an example of what I noted in #12:
"...sometimes experts and professionals get hung up in their traditional ways." There has been much talk about the Wrightsoft software, and perhaps it is the most widely used software for Manual J calculations, but that "multi-thousand dollar software" (#22) is not needed. The free BEopt software integrates the calculations involved with a Manual J anyalysis. BEopt performs calculations based on the Energy Plus engine, probably the most detailed and comprehensive computer program ever developed for energy analysis. Even more interesting is the fact that BEopt does not suffer from all the problems and work-arounds needed to use Wrightsoft, as noted in response #24.
There's a couple of funny (humorous) things going on, here, worthy of comment. First, BEopt probably cost multi-millions of US tax dollars to develop, and we probably paid and are still paying too much for it in tax dollars. But it is "free" to the user, extremely powerful, easy to use, does much more than Manual J, and it's accurate, so I find it a little funny that it is apparently not widely used by "experts." Second, it's funny to think that maybe it's not more widely used because people (including me, sometimes) don't believe anything useful can come from the US government, and perhaps they don't trust the program.
Again, you have received some good responses, and I am not saying any are wrong; I'm just saying that it doesn't need to be as complicated and expensive as it appears to be, based on the responses given.
Complicated is the optimum word ...my builder is hard to get a hold of ,as to be expected I suppose in the middle of a busy season ...anyway we made contact and because of some ambiguous labeling on the house drawings ,the values were misread by the fellow doing the heat loss .His outsulation value was R20 but labeled R10 ,so that was taken as the value in the heat loss report .Total wall insulation is r44 less heat loss through the framing at 16 inch centers.Windows were taken as generic double pane rather than the high performance triple pane we had discussed and ,in fact, he is providing ...another oversight...He is committed to building a pretty good house and seems up to the challenge ...I feel less stressed after the conversation for sure but will remain more diligent and involved ,as suggested ,from here on in ...I only have one chance to build it right!!!regards,Bob