What should I replace my heating and cooling system with?
Hi all,
I’m looking for advice on the ‘best’ solution to my heating and cooling (and hot water)
– Zone 5b
– Loads from coolcalc and manual D/J
Heating loads
– Main house: 28k to 36k, 900 cfm
– Sunroom addition (can be closed off): 8k, 150 cfm unducted not planning to directly heat (current situation)
– Office addition 5k/3k, 180 cfm. Unducted, can probably be heated via staircase.
Cooling:
– Main house cooling load: 24k to 36k, 1200 cfm
– Sunroom addition: 8k, 450 cfm unducted not planning to directly heat (current situation)
– Office addition 5k/3k, 350 cfm. Needs some cooling cooling.
– Existing Heating: 115k 95% variable speed furnace (over 10 years old?)
– Existing Cooling: 2 ~1 ton old minisplits (relatively expensive to run)
– Existing water heater: standard low water heater at the end of its life
Goals:
– central cooling that is highly efficient
– Efficient blower to run in circulation mode for filtration and even out temperature
– Correctly sized heating and cooling system to eliminate rapid turn and off
– Solution for office cooling
– I’ve thought about mini splits or just an electric heat pump but everyone says the climate is just a little cold with no backup heat and that minis are the most expensive option.
– There is a lot of SW glass for passive heat in the winter but its problematic for summer
I’m currently looking an HVAC contractor to try to redo all 3 but relying on ideas but I’m not getting great proposals. Things like: Won’t do a new manual D/J, won’t use the old one, want to upsize lots, propose single stage SEER 13 equipment, etc.
I have found one contractor who will do whatever I want but he doesn’t really know what would be best from an efficiency perspective. I have ideas (heat pump) and this contractor (hydronic) has some ideas but I’d love to hear what some other folks think would be ideal. Open to all options.
Thanks for any ideas!
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Replies
Jill,
If you care about operating costs, you would need to tell us your electricity rate and the local cost for natural gas or propane.
The contractors who are telling you that Climate Zone 5 is too cold for ductless minisplits are not telling the truth.
Climate Zone 6a here. We had close to two weeks of approximately 0-5F. My ductless mini splits worked great, they easily kept the house at 68F.
Running a heating/cooling air handler just "...filtration and even out temperature..." is extremely inefficient, independ of how efficient the blower motor is. An HRV (heat recovery ventilator), set up to use recirculation would be much more appropriate.
The "...everyone says..." experts on what ductless mini-splits can't do are completely out to lunch. It's possible to heat with minisplits even a -25F outdoors (even though that's below the extended capacity table ranges). In zone 5B it's not only possible, mini-splits can hit the seasonal efficiency performance of ground source ("geothermal") heat pumps, if sized optimally.
And this is NOT "new news"!
Several years ago the NEEA did field monitoring of mini-splits in the northwestern US, covering zones 4C to 6B. The "Inland Empire" and "Boise/Twin" regions were zone 5B, the "Eastern Idaho" group was zone 6B, where even now 10-12 year old mini-split designs not only functioned well, the average coefficient of performance (COP) was nearly 3. See Table 24, p35 (p52 in PDF pagination.
http://neea.org/docs/reports/ductless-heat-pump-impact-process-evaluation-field-metering-report.pdf?sfvrsn=18
The typical nameplate HSPF (heat season performance factor) of those units of old was about 10.0 BTU/watt-hour. Current model ductless mini-split designed for cold climates are now testing in the HSPF 13s & 14s. If sized optimally you should be able to beat the efficiency performance of the field survey units monitored by the NEEA in 2010-2011 time frame.
One of the advantage of being a "-B" zoner is that less capacity & efficiency is given up to defrost cycles. Another is that since there is effectively zero (often negative) latent cooling load, SEER 25+ units can be used without indoor humidity levels ending up in the "clammy cool" range. There are several ductless mini-splits in the SEER 30 range now (more than 2x as efficient in cooling mode as an SEER 13 split system AC or heat pump.)
HVAC contractors rarely do Manual-J calculations, and only a fraction of those who will perform them correctly. Hire an engineer or RESNET rater to do an AGGRESSIVE Manual-J/D analysis on your place.
Since you have a heating history using the fossil burner it's possible to use gas bills against weather history data to get a fairly decent ballpark on the whole house heating load, and it's not very time consuming. See:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
In a zone 5B climate with "...a lot of SW glass for passive heat..." the fuel use load calculation may hit somewhat to the low side, but not more than 20% unless the house had been meticulously designed for solar tempering, including optimizing the specifications for the low-E coating. Run the fuel use load calculation, see how it stacks up against the CoolCalc freebie numbers. The fuel-use load numbers will almost certainly come in substantially lower, no matter what reasonable HDD base temperature is used.
This is fine equipment . This company out of Canada gets what we are all trying to do . Pricing on equipment is not outrageous , actually it's very reasonable . You can zone with a single air handler w/o using bypass ducts or any of the other feeble attempts at doing a good job without really doing what is necessary to do it right . NEW HE-Z zoning series . Dependent upon what type water heater you have you may use a hydronic coil within this unit also for heating . Dx or hydronic coils for cooling . They will design system based on your plans and heat / cooling load calcs .
Martin and Dana might find this interesting if they have not already been made aware . We purchased equipment for a remote project , 3 HE-Z 4 zone handlers , 3 air purifiers , all supply ducts , return frames , heating and cooilng hydronic coils totalled 12,000.00 , this was a large home . Have a look , http://www.hi-velocity.com/
Reply to Martin
Rates:
Electricity
Winter: ~$0.10/kwh.
Summer: ~$0.10/kwh similar tier 1 pricing. i'm not sure about tier 2 pricing as I don't currently hit it.
Gas: This past year, $0.47 to $0.50/therm winter and summer.
Minis and winter: Yeah, I'd don't feel like I'm getting straight info.
Richard,
That's really interesting equipment but I already have ducting so I'm guessing it isn't a good option to run different ducting. But thanks for sharing the link.
Dana,
Thanks for the long answer. I think I understood most of that.
First, I did have someone from a company that does energy ratings do my manual D/J. He came up with total loads around 48k for heating and cooling. The problem, as described above is that a good chunk of those loads aren't ducted.
As for running the furnace fan for filtering, my thermostat has a setting that turns on the fan for about 10 minutes an hour at some speed less than max. It doesn't seem to increase my electricity bill much. I'm told that is because my furnace has a variable speed motor. Unfortunately, my seasonal allergies are bad. Running the fan on recirculation seems to help.
Are mini splits really better than getting a high end variable air conditioner? The contractors I've talked to want really high install costs for multiple mini splits. I was wondering whether a nice high end heat pump like the Trane XV20i would be a better option given I already have ducting.
Looking around at available mini splits that meet 48k btu load, I only see SEER 19. That seems pretty much the same as "full size" high end heat pumps for ducted systems. For example Trane XV20i or Rheem RP20 etc. What am I missing?
Can you make a BEOpt model? It's free and fun to use. Once you build your house you can tinker with systems.
Jill,
Your energy costs are low. For you, burning natural gas is significantly cheaper than heating with electricity.
Natural gas at 80% efficiency: For 100,000 BTU, you pay $0.63
Heat pump with a COP of 3.0: For 100,000 BTU, you pay $0.98
Heat pump with a COP of 2.0: For 100,000 BTU, you pay $1.47
So when it comes to cost, gas wins.
Martin: sure. But gas won't be cheap forever and doesn't play well with solar (planned none yet) and supports fracking (a problem in my area).
So no opinions anyone on a central heat pump with inverter technology like Trane XV20i?
If not, anyone have any experience with the Trane modulating furnace? Or another modulating furnace that has a good turndown?
Most of the modulating ducted heat pumps don't have extended temperature capacity tables below -20C/-4F, and many don't even go that low. They also don't have turn-down ratios more than about 2.5:1 (at minimum speed they're still delivering fully 40% of what it would deliver at maximum speed, no matter how many incremental steps it has), so VERY accurate load calculations and careful sizing is required for them to hit their high HSPF and SEER efficiency numbers.
The ECM drive air handler for a ducted heat pump uses 10x the amount of power of a HRV in recirculation mode. So even running a 100% duty cycle on the HRV uses less power than cycling the heat pumps air handler for 10 minutes per day.
Jill,
I wasn't trying to talk you into burning gas. I was just doing the math. Your original question implied that you were considering different options -- not just all-electric options.
Here at GBA, we almost always advise readers to heat and cool with either ductless minisplits of ducted miniplits. This solution is hard to beat.
You wrote, "I've thought about mini splits or just an electric heat pump but everyone says the climate is just a little cold with no backup heat and that minis are the most expensive option."
As I pointed out in an earlier comment, your climate is not too cold. What you need to find is a contractor who can install ductless or ducted minisplits at a reasonable price.
Dana:
You said "The ECM drive air handler for a ducted heat pump uses 10x the amount of power of a HRV in recirculation mode. So even running a 100% duty cycle on the HRV uses less power than cycling the heat pumps air handler for 10 minutes per day."
I have been looking into this because I will have a zero clearance wood-burner and wanted to circulate the heat most effectively and economically. The blower add on for the stove that connects to the HVAC system is not cheap, and uses 160 watts. I saw an old study of 1/2hp variable speed ECM motors in air handlers and when in the "on" position, they did 400-500cfm and drew a median of 100 watts. Some were less than 50 watts. This is a 2004 study, so I am assuming it is even better now. http://aceee.org/files/proceedings/2004/data/papers/SS04_Panel1_Paper23.pdf My Panasonic ERV lists consumption anywhere from 20-84 watts for the 100cfm of air. I think the variable speed blowers can be competitive.
Jill, I wrestled with the same question a year ago. I tried to make minisplits work for me but in the end went with a heat pump, air handler system. american std platinum system similar to the Trane you're looking at. No regrets so far. With a 2200 SF house, & not wanting minisplit wallwarts the minisplits didn't win out.
Add some PV panels and go net zero.
Kevin: My typing fingers are faster than my brain- that was intended to be "...per hour.". not per day! (DOH!!)
Dana,
Adding an HRV seems tough. I don't have any ductwork for it. I'm not sure what that would cost but thousands I assume before the equipment. My contractor kind of lauged at the idea. While I'm interested in efficiency, I'd guess that is too much capital spending. Do you have a low cost suggestion for adding an HRV to a house? I looked into adding it to the furnace but that doesn't solve the problem you mention about high fan power use.
I did some reading over the weekend.
I don't think my recirc blower is using as much power as you suggest. Or maybe I don't understand something. I looked up my continuous fan wattage in a manufacturer pdf and it says 100W for continuous fan speed operation and says it produces 660 cfm. I can probably ask my electrician to confirm the power draw.
Let's see if I understand the math:
Running the variable speed furnace fan on the lowest speed for 10 minutes an hour 240 days a year
100 watts draw per manufacturer.
/ 1000 watts per kw
= 0.1 kW power draw
x 1/6 hour per cycle (10 minutes)
= 0.016 kwh per cycle
x 24 cycles per day
= 0.4 kwh a day
x 240 days (during heating season, I don't usually run in recirc) per year
= 96 kwh per year. At $0.10/kwh that costs me ... $10/year?
while delivering
660 cfm x 10 minutes/cycle = 6600 cf per cycle x 24 cycles per day x 240 days per year = 38M cf per year
In comparison this hrv article says
https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/our-top-efficiency-heat-recovery-ventilator
35 W power draw per GBA
/ 1000 watts per kw
= 0.035 kw per hour
x 24 hours per day
x 240 days (to make my comparison apples to apples)
= 200 kwh per year. $20/year. (~the "same" cost)
So then in terms of airflow:
100 cfm
x 60 minutes per hour
x 24 hours per day
x 240 days per year
= 34.5 M cf per year (~"same" cfm)
Now, I'm a little suspicious of the idea that the variable speed fan blower delivers 6.6 cfm per watt and the hrv only ~3 cfm per watt but then the furnace isn't blowing through a heat exchanger? I don't know how to confirm the airflow. But even if it draws 200W an hour or delivers only 300 cfm (but not both), it isn't very different than the HRV.
But to get the hrv option, it sounds like I'd have to spend $6000? Hmm, that GBA article is for a new house. I'm sure it's more in an old house. That's my air conditioning budget.
Now, I understand that the HRV solves other problems and I'll have to think about that. But in terms of my desire to run my air through a MERV 13 filter, I'm not seeing why adding an HRV wins.
What am I missing? Is there problem with my math?
Dana,
Regarding low temp heat pumps:
Ok, I thought you might say that. I'm having a really hard time finding performance data, especially low temperature option for the inverter type heat pumps. I did find one for Amana and it puts out only 15k btus at 5 F for a 60k btu model. But so many of the manufacturers don't share a pdf for a details minded person to look at. It sounds like you think it is a bad idea for my climate.
Regarding modulation:
Yes, I had originally thought a new modulating furnace but the models I could find data for are disappointing high in lowest modulation (similar problem to my 2 stage furnace). Gnarr. You'd think they could work like a boiler and put out lower temperature air or something. The best I found in modulating furnaces (again, where is the pdf with the details) was a consumer brochure for Trane that said 40% of total capacity.
It's really frustrating. I had my windows replaced a years back and have good insulation and the furnace runs so very little unless it is below zero. I'd like to replace it with more properly sized equipment but it seems like a lost cause!
Ok, recapping a bit:
Central heat pump: no one thinks these work for cold temperatures.
Mini splits: will work in terms of cold temperatures but expensive to install and will increase heating operation costs
Hydronic: No one said anything about hydronic as a the solution so I guess no love there. Contractor has some love because of its ability to deliver low btu heat.
Modulating furnace: none of them modulate low enough.
It seems like these things make it hard or expensive:
- mini split the whole house
- use tankless to heat a hydronic coil or radiators
- use an inverter driven central heat pump for heat (requires backup heat)
- adding an inverter driven AC unit
It seems like the "easy" button is:
- Replace current furnace with smaller furnace (sizing?) with similar variable speed fan
- Add air conditioning when furnace is replaced. Looks like 2 stage SEER 18 is the best I can do.
- Replace existing office mini split with heat pump SEER bajillion single head mitsubishi or LG to provide heat and more efficient cooling.
- Continue to use furnace for filtration and recirculation
- Continue to not have an HRV (separate problem)
- Add a tankless natural gas condensing water heater to replace standard water heater.
So in terms of something for my HVAC contractor to bid it seems like there are two proposals to request with 2 optional items. All of them include a new tankless water heater just for DHW:
A: Keep furnace, keep mini
Keep existing furnace and add 2 stage air conditioning to it
Keep old mini in office
B. Keep furnace, new office mini
Keep existing furnace and add 2 stage air conditioning to it
Install new mini in office
C. New furnace, keep mini
New 95%+ 2 stage furnace (variable speed bower) and air conditioning coil
Keep old mini in office
D. New furnace, new mini
New 95%+ 2 stage furnace (variable speed bower) and air conditioning coil
Install new mini in office
Forget about all the fancy options.
Any final thoughts? I'll probably ask another question about equipment selection separately.
I think the final straw on the expensive install, more money to operate mini split option was the solar tariff announcement as that likely substantially delays installing solar.
Thanks for all the help understanding all of this. HVAC is ridiculously hard to understand.
Jill,
I noticed that many of your options included these words: "Keep furnace, keep mini."
This raises a basic question: Why are your replacing your equipment?
Does your equipment work? If so, there many be no need to make changes.
Jill,
These ideas may have been mentioned earlier in this threat. But in any case, I also would consider these potential projects (assuming the furnace still works):
- Air seal the structure to reduce air infiltration and stack effect
- Increase insulation where practical and cost effective
- Run blower door test to collect data
- Reevaluate HVAC options