Difficulty in Properly Sizing Equipment - 2140 Sq Ft New Construction Spray Foam House
Comments
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Just for fun, I also did an analysis of what it would look like if you did two heat pumps, each one rated at 25,000 BTU/hr. I arbitrarily split the load in half for each zone.
You can see it here:
Weighted COP comes out at 3.4. The backup heat runs one hour per year, less than one kWh.
This is the heat pump I chose:
https://ashp.neep.org/#!/product/34581/7/25000/95/7500/0///0
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I would suggest that the installed cost of a HyperHeat heat pump and a solar array to power it would be less than the installed cost of an oil boiler, radiators, and air conditioning.
@DCContrarian at least in the original post, heat pumps are the plan. I think the true decision is boiler or not. I’d suspect electric resistance in the kitchen and baths would come out many thousands cheaper lifetime. Could ever use electric baseboard in the rooms too as supplemental. Especially with the space constraints detailed - an oil boiler is a really expensive toy here and doesn’t provide any benefits as far as I can see.
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"To really get a picture of the energy usage you have to analyze a specific piece of equipment, in a specific house, in a specific location."
I said that because intuitively it seems to me that sizing is important. But I just reran the numbers with different heating loads:
39,000 BTU/hr (original calculation): COP = 3.08
20,000 BTU/hr: COP = 3.10
60,000 BTU/hr: COP= 2.95
So it turns out that so long as you get the sizing within a pretty big window it doesn't really matter that much. So I'll amend my statement to say you have to analyze a specific piece of equipment, in a specific location.
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As an overview for this thread, I think several points can be considered:
@2Luckysat 's home design is going in the right direction. The foam insulation must be done properly to construct a home that will last a long time and not have mold issues. The Anderson 400 series windows are beautiful, although more efficient windows might be considered for the best air sealing (this is key to low heat loss) so the house becomes more energy efficient and tighter. These steps would greatly help in a heat pump only application from a comfort in cold weather standpoint so it does not feel cold near windows and walls; it would also substantially reduce the heating and cooling costs. I suspect the air changes per hour are one reason why the heat loss is as high as calculated.
The addition of properly sized radiant heat and baseboard with a boiler will make @2Luckysat 's home quiet, comfortable, and cozy independent of the windows and construction. This is easy to install during the construction phase, but much more difficult in an existing structure. Baseboard is designed to add heat near windows to break the cold feeling from radiating heat to the cold outside and from being exposed to the low R-value that windows typically offer. Another option would be to add hydro air to the duct work after the fact if comfort is not satisfactory with a heat pump only solution.
If the boiler is run during cold weather, the annual COP of the heat pump will be much higher. This takes the strength of both systems (the heat pump operating in the shoulder seasons when it can be most comfortable and efficient, and the boiler when it's cold outside and can run more affordably than electric cold climate heat pumps when defrost cycles play a more significant role below 40°F). Further, if energy prices change in the future and favor one system over the other, that system can run primary at any time. If one system goes down in the middle of the winter, the other can heat the home.
If power outages are a concern, a small generator can run the boiler, preventing pipes from freezing and making the home comfortable for a low cost. A heat pump only solution requires a much larger generator that will sit idle for many hours a year.
Peace of mind and comfort are affordable choices with a boiler regardless of the outside weather and power interruptions.
Roger
President
Energy Kinetics, Inc.0 -
Further, if energy prices change in the future and favor one system over the other, that system can run primary at any time. If one system goes down in the middle of the winter, the other can heat the home.
If power outages are a concern, a small generator can run the boiler, preventing pipes from freezing and making the home comfortable for a low cost. A heat pump only solution requires a much larger generator that will sit idle for many hours a year.
@Roger these two points are your strongest for a secondary heating source. However, you don’t need a boiler to have a backup heat source - a furnace handles that just as well, using less space which seems highly important in this situation.
The radiant comfort argument falls flat though- you don’t need a boiler to provide that in select areas of a house. For a house with only a ~10kw heat loss, installing electric baseboard + infloor heating could work out to be substantially cheaper with the same comfort. Plus less to break and less space required. Simplicity, all else equal, seems better.
I think the COP discussion is getting too in the weeds. Even if it’s cheaper to operate for part of the year, installing an expensive boiler auxiliary system to save $100-$300 ish (guess) a year is a waste IMO. That’s what? A 50 year payback?
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My sense is that comfort in cold weather is a primary reason why hybrid systems are so popular (see the frequency of hybrid system operation in the reference I posted earlier), that is coupled with affordable operation making it an easy decision in this scenario. Electric in floor and strip heating should not be considered for it's expense and high emissions in this case.
President
Energy Kinetics, Inc.0 -
Hybrid doesn’t equal boiler - furnaces count too. Plus selective electric radiant is absolutely viable - is it more expensive to operate ? Absolutely. Is it so cheap to install that it doesn’t matter? I’d argue probably.
The hybrid discussion is interesting. Obviously, these are homes already built. So these are existing houses which are adding heat pumps to furnaces. However, this house is on paper, so I think what existing homes are doing matters less. It could be that new homes are finding less need for dual fuel systems, I don't know. We aren't comparing apples to apples, we are saying old homes like hybrids (some of which already have a furnace) so new homes must also like hybrids and I don't think we can say that confidently.
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Let's say for the sake of argument the breakeven point between electricity and oil is COP 2.5. If you go back to the spreadsheet I linked to at the end of the first page of comments, you'll see that the heat pump I use in the example has COP 2.5 at 25F.
In Suffolk County there are an average of 413 hours a year when the temperature is below 25F. In our hypothetical house, for those 413 hours you'd need 14,108,351 BTU, or about 1/6 of the annual total. If you got those 14,108,351 BTU's from the heat pump in the example you'd consume 1,916 kWh of electricity. If instead you got them from a heat source with a COP of 2.5 you'd consume 1,653 kWh of electricity (or whatever your fuel equivalent is).
So by switching from the heat pump to an alternate fuel whenever the COP is below 2.5 — outside temperature below 25F — you'd save 262 kWh per year. At 20c per kWh that's $52.40 per year.
To do so, you'd have to install a piece equipment that costs in the same neighborhood as a nice reliable car.
All of this is easily seen if you look at the spreadsheet I created.
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Interesting discussions! I always say most homes need a fire for heat in my state of PA. The exceptions are small apartments/houses, or even midsized ones that are very well insulated. In those situations it may not pay for a hydronic system or a furnace. (Natural gas will cost less than oil or propane, so what fuel is available makes a difference also.) Years ago a family member built a small house (maybe 1000sq ft) and I said it is not worth doing carbon fuel heat. We did a heat pump with backup heat and a low efficiency system at that (by todays' standards). No regrets. The occupant is happy with upper 60's house temp and electric bill is amazingly low.
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@Hot_water_fan "For a house with only a ~10kw heat loss, installing electric baseboard + infloor heating could work out to be substantially cheaper with the same comfort. Plus less to break and less space required. Simplicity, all else equal, seems better."
I plugged into my spreadsheet the following scenario: what if you had electric resistance heat under 100 square feet of the house, set to run whenever the heat was on, with a surface temperature of 80F? That gives 20 BTU/hr/sf, or 2000 BTU/hr, and would consume 586 watts.
It would run about 5700 hours a year and consume about 3,000 kWh per year. The total electricity consumption would go from 7,962 kWh per year to 10,010 — the heat pump would run somewhat less, saving about 950 kWh per year, so the net change is about 2050 kWh. At 20c/kWh that's about $400 per year in additional electricity.
If you account for all costs that's probably the cheapest way to deliver that level of comfort.
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@DCContrarian i think it is too, unless baseboard is also desired and used in every room. Especially if a utility room is being built to house (at least partially) a boiler. Totally different calculus than a house with an existing boiler.
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There is a real joy to a combustion-free house. You can just build tighter. It's quieter, cleaner, more comfortable.
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The comfort argument worked before the advent of EVI (aka hyper heat or cold climate) heat pumps. Old heat pump's output tanked bellow freezing and they would deliver at best lukewarm 85F-90F air which you don't want near an occupant.
The hyper heat units put out nameplate BTUs in cold climate, some even down to 0F. To get rated output, since the blower is fixed, these need to blow pretty hot air on those 0F days. That means you can get 110F+ output air out of the unit when cold. Also since most of them are modulating, they deliver some heat all the time so you don't have that typical blast of hot air followed by long wait times of the typical oversized gas furnace. Combine this with modern air tight construction and there are simply no comfort issues. The temperature inside is just even, you don't even notice the heat pump running.
As for backup, most will run down to -22F, so maybe in zone 6 or 7 you might want some backup, it zone 3/4/5 it is simply not needed. Even in cold climate, resistance heat backup makes the most sense as it is simple to install and for the couple of hours in the year it needs to run, the operating cost is noise.Hyper heat units don't need to switch over to full electric backup like older heat pumps when it gets cold. Since they can deliver some heat with a COP over 1 even during those polar vortex days, the heat pump and backup can run at the same time so a fair amount of your space heat will still come from the heat pump.
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I thought about going down the rabbit hole and calculating how much energy it would take to use a combustion boiler for the heated floors in my example, but I've done enough. Let's just say it's not zero. And it's more than the heat pump alone would be. So if what you want is heated floors, the hypothetical $400 annual cost I give in the example also has to be compared to the fuel cost of whatever the alternative is going to be, which is going to be something too. Less, because resistive heat is expensive, but not zero.
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I apologize in advance for contributing to a fully hijacked thread…
There's real joy in a quiet, warm home, with nearly endless hot water and affordable energy bills in cold climates. There are different ways to achieve these results and cold climates present unique challenges. Hybrid solutions are exceptional for the state of today's technology as there are not many passive houses out there.
In the meta analysis I cited earlier in this thread, there are a few important points to help clarify comparisons between technologies.
- Cold climate heat pump utilization: "Some home homeowners indeed used their heat pumps dutifully even during the coldest days of winter, but most dropped out at some point as the weather got colder, or never even turned on the systems at all for heating purposes." … "The data curves for the three field studies show that actual electricity consumption was only a small fraction of what would be expected with full heat pump utilization. Note that the actual electrical demand curves are relatively flat below 30 deg F. This indicates very low heat pump utilization below 30°F. Since heat pump power demand increases dramatically as the outdoor temperature drops further, due to increasing heat load plus decreasing heat pump COP, this means further that the homeowner percentage drop-out rate is increasing as the temperature drops."
- The field achieved COP of heat pumps is 2.0 to 2.5 near freezing (30°F); this actual field performance is substantially below manufacturer ratings which have been referenced elsewhere in this thread.
- EVI heat pump output is published to be between 73°F and 91°F for this top performing 26.3 SEER model (see p. 230). This is running at high speed with leaving air velocity of 8.9 ft/sec, so it may not feel cozy and it may not be perceived as quiet. Possibly this and item 2 contributes to item 1; the studies confirm the high rate of homeowner heat pump drop out in cold weather, but they do not positively identify the root cause.
- The most common heat pump refrigerant today, R-410A, will be prohibited on January 1, 2025. A2L (slightly flammable) replacements like R-32 and R-454B (70% R-32) are not currently code compliant unless a state law exempting them has been passed, but they do have the advantage of lower GWP than R-410A.
These issues need to be resolved to greatly increase broad voluntary homeowner use of cold climate heat pumps during freezing temperatures.
President
Energy Kinetics, Inc.1 -
2. The numbers I used in my example are not manufacturer ratings. They come from actual measured performance from NEEP.org, a non-profit testing group.
3. The heat pump used in my example achieves its rated capacity with a temperature delta of 36F, or an output of 108F when the room is at 72F. But it's always important to look at specific equipment in a specific location.
4. R32 is coming. R401A will be available for existing equipment for a long time, it will gradually get more expensive over the next 25 years.
To point #1, maybe 40 years of consumers being told "heat pumps don't work below freezing" has had some impact?
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A few things I believe to be true, open for discussion:
- Hybrids are great if the fossil heating appliance is free (ie already installed). This boiler would not be free.
- The hybrids that make more sense are furnaces, not boilers, when installed in a new house so the ductwork can be used and less hardware is required.
- Hybrids are best when fuel 2 is cheap, unlike oil.
- Hybrids make great sense when budget options are used - combining a cheap furnace + cheap heat pump makes more sense to me than expensive furnace/boiler + expensive heat pump.
- Hybrids seem better matched to existing homes with high heat loads and other constraints (like 100A service). New houses benefit less, as install is easier and total available savings are lower.
- If radiant options are desired for subjective comfort, if the loads are small enough, electricity can make more sense than installing a boiler.
- When AC is already planned, concerns about refrigerant are immaterial. The refrigerant is coming either way.
- I typically think domestic hot water shouldn’t decide a heating and cooling system unless the situation is unique.
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NEEP uses manufacturer reported ratings; field performance data in the meta analysis reflects field performance. One would think that someone who took the time and effort to purchase and install a heat pump would be excited about it, use it, and continue its use if satisfied. There are a variety of reasons why someone would elect to use a particular heating system if they can choose between available options.
President
Energy Kinetics, Inc.0 -
I have a hyper heat wall mount at the cottage (zone 6, so real winter). Even when it is -25C (-13F) up there, the heat pump delivers above 100F air.
Now there are many ways to improperly install one, but it is also not that hard to get it right.
Needed heat+AC of a multiunit project a while back. The BOM cost of the cold climate ducted heat pump was less then furnace+AC, didn't need gas connection and also saved a couple of bucks CO sensors. Bonus, the indoor unit is much smaller and requires no clearance so it takes up much less space (I'm in an urban area so sqft cost is very high). The places are sub metered so I know how much energy is used, the winter cost bump is not much above what would have been the gas meter fee. This is slightly above code construction, nowhere near passive house. Simple economics says a fuel burner is dead if you need AC.
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We may have lost our OP on this one. Let's get back on track. Thanks!
Forum Moderator
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Not to get off topic, but is the house already built and insulated with spray foam? Spray foam seems like an ideal insulator because it fills every void and can totally seal everywhere it's sprayed or injected. But foam has huge downsides:
Because it seals so completely, it's very easy for moisture to get trapped. Even the smallest leak in the building envelope can result in wood rot and mold.
Unless the electricians install all wiring in conduit where the foam will be installed, then the wire is completely glued inside the foam. This can lead to many problems, including damage to the wire insulating jacket (due to incompatibility with the chemicals in the foam).
Plumbing is also encased in the foam. If there's ever a leak, good luck.
It makes it extremely difficult to ever perform any future repairs or remodeling.
Many times, banks will refuse to write a mortgage (or remortgage) on homes containing spray foam.
Many times, insurers will refuse to insure a home with spray foam.
Spray foam can substantially reduce the resale value of your home, and wise buyers will not purchase a house with existing spray foam.
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I know we've already been warned about thread drift, but my radar went off when the OP described his house as a "spray foam house." As opposed to say "super-insulated" or "built to latest codes."
Spray foam is the most expensive form of insulation. If your house is being built to modern codes, it's going to take a lot of it. This article describes how some contractors are getting away with skimping on the amount of foam they use by claiming foam is more effective than other insulation:
The number one thing you can do to make your house comfortable and economical to operate is insulate it well. During new construction superior insulation and air sealing adds almost nothing to the cost. I like to say "an ounce of insulation is worth a pound of hydronics."
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I am going to chime in from the repair tech standpoint. Heat pumps are wonderful examples of engineering, however I can't envision myself outdoors working on a condensing unit in subfreezing temperatures. It doesn't take long for you to be overwhelmed by the cold (my northern counterparts are laughing at me now!). But outside in the elements, rain, snow, cold it may be difficult to focus on the repair of a complex electronic appliance. Just my two cents. Ron
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