All electric house and owner education
I’m thinking of going all electric and replacing my gas hydronic heat with mini splits. I’m in Colorado, zone 5A. The design temp is 0F, but rarely it gets down to -20F (which is a concern if it’s below the minimum temperature for heat pumps). My heat load on a design day is right around 20,000 BTU. An online calculator and calculation based on fuel consumption agree pretty closely.
I think three mini split heads could handle the load, but the kitchen, one bathroom, and one bedroom would be far or isolated from the mini split heads. I could put electric baseboards in those rooms for backup. I might also need more electric baseboards, because if it gets below -13 ~ -15F do the mini splits stop functioning all together? I don’t want to leave the old hydronic system, because at some point it will get too expensive to maintain for backup use, and the connection fee for gas is $16 a month, so I’d rather not pay that.
My concern is, if I put in electric baseboards, how will future owners or tenants know to use the more efficient mini splits rather than the wasteful electric baseboards? Of course I could tell them, but I don’t have high confidence that the information will be passed on to the next next owners (or their tentants if it’s rented out). I don’t want to install a system that’s going to result in more energy usage in the future, even if I’m not the one paying the bills.
Mini splits seem to need a little more thought put into using them than most people put into their heating system. For example, to heat the small bedroom efficiently the door would have to be left open so it gets heat from the mini split in the other room and from downstairs, instead of just turning on the electric baseboard. And the mini splits should be left running all the time instead of using a setback and them having to go to full power to recover. Most people don’t understand or think about things like that.
How do you educate future residents to make sure energy saving systems are used to their full potential?
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Replies
Matt V,
I'll be watching this thread with interest. The increasing complexity of houses poses similar problems for other systems too. Since full-time mechanical ventilation was required by our code I keep hearing new house owners say they have turned theirs off to save energy. I also wonder how owners are supposed to find out which layer of their assemblies is the air-barrier now it has migrated from always being the poly to a variety of other materials and locations. Good topic.
Or maybe even educate current residents. I'm curious if you know that switching to 100% electricity will be better or worse for the environment than a mix?
I wish minisplit manufactures offered better integration with backup heat options. The ideal setup would be that the minisplit thermostat controls the baseboard heat. I don't know if any of the traditional cold weather wall units integrate with backup heat.
Ducted systems address the conditioned air distribution issues of wall mount heads.
The ducted Fujitsu units can tie to auxiliary heat and will kick in when room temp falls is 6F below the setpoint and will shut it off as the room temp rises to 2F of the setpoint. This could be baseboard with the right relay setup or a duct heater insert. Duct heaters, while common in commercial use, are pretty rare in residential use.
An alternative to baseboard for backup is in wall electric with a fan. It wouldn't take up so much wall space that limit placement of things like couches.
Jon - It is pretty easy at this point to source your electricity from non carbon sources.
The 11.3 HSPF system that is going in should operate at around $10.88/mmBTU vs $15.05/mmBTU for a 95% gas system ($.123/KW composite time of use, $1.43/therm). So it at least makes sense from a dollars standpoint based on the spec sheet data.
de-mountable electric wall heater that plugs in: https://www.convectair.ca/en/products/120v-plugin/apero
I’d be nervous using any plug-in electric heater as a long-term heat source in a house. The reason is because these tend to be high amp draw devices, and the typical receptacles used in residential settings get loose in a relatively short period of time and then you get hot plugs and melted cords. I’ve seen things get so bad the plugs are black and charred.
If you plan to use plug-in heaters routinely, at least use “spec grade” (they will have that term stamped on them) receptacles. These are the commercial standard receptacles and while they cost a bit more, they are FAR more reliable and will stay tight much longer.
Also, don’t even think about using the poke-in backwire connections on any receptacles running heaters. My personal recommendation as a consulting electrical engineer is to never use poke-in back wire connections for anything, ever, period with no exceptions. I really think those poke-in things shouldn’t even be allowed by code.
Bill
In mild weather heat pumps are better than burning natural gas, assuming the fuel at the power plant is natural gas or something cleaner.
Let's say most of our electricity comes from burning natural gas. If the efficiency is 40%, you get 40,000 BTU of electricity per therm of natural gas. With a heat pump with a COP of 4 you get 160,000 BTU of heat in the house per therm of gas burned at the power plant. That's reasonable in mild weather, usually at the 47F spec.
My hydronic heating system is probably around 70% efficient, so 70,000 BTU of heat per therm of natural gas.
On an extremely cold day the COP might drop to around 2, so now you get about 80,000 BTU of heat per therm of gas at the power plant. That's still better than burning gas directly in my boiler, but not as good as a high efficiency condensing boiler.
Backup heat would be used only occasionally if the system is well designed, so the relative inefficiency should be small. Obviously heating the whole house with resistance electric would be pretty bad, and that is what prompted my original question.
In Colorado we're already at 28% renewable energy (mostly wind), and that seems to be growing. The rest is natural gas (33%) and coal (39%). I'm not sure how much worse coal is for the environment than natural gas. I also pay $0.01/kwh extra for "100% wind" power, of course that is somewhat theoretical since it all comes from the same place anyway.
https://www.xcelenergy.com/energy_portfolio/electricity/power_generation
Although the design temperature is 0F, for most of the winter it's a lot warmer than that, maybe in the 20s overnight. And it's often above freezing during the day. So a heat pump will be working fairly efficiently for a large part of the heating season.
Alternatively I could upgrade to a ~95% efficient condensing boiler, but the overall efficiency would be less than 95%. My boiler lives in a utility closet off the back of the house, where most of the waste heat from the hydronic system is probably lost to outside.
>”Let's say most of our electricity comes from burning natural gas. If the efficiency is 40%, you get 40,000 BTU of electricity per therm of natural gas. With a heat pump with a COP of 4 you get 160,000 BTU of heat in the house per therm of gas burned at the power plant. That's reasonable in mild weather, usually at the 47F spec.”
Not exactly. When you burn natural gas (or any other combustible fuel) and use it to generate electricity, you have efficiency losses in the boiler and other systems at the power plant that result in you not getting the full thermal energy of the fuel converted into electrical energy. You also have line losses in the power lines that mean more power had to be generated than you actually used. Natural gas pipelines have friction losses and pumping stations so there is some loss there too, but in a different way.
The ultimate effect of this is to require more gas to be used at the power plant than you’d think to run your heat pump. My guess is you’re probably burning about 2-3x as much gas as you’d think, so your heat pump is somewhere between 100% and 50% more efficient in terms of heating your house than if you used natural gas in your furnace, from a strictly energy use standpoint. Natural gas burned in your furnace for heat essentially converts all the energy in the gas to heat, minus whatever the losses of the furnace are (which will be less than if the gas was used to generate electricity). You still come out ahead with the heat pump, just not as much as you’d thought.
If you’re trying to save money, the heat pump still comes out ahead, but not nearly as much, especially in very cold weather. In Colorado, where solar can work well and I see lots of LARGE rooftop installations going in when I visit family near Aurora, heat pumps have the added benefit of allowing you to use your solar system to help heat your house with zero emissions. That’s another thing to think about if reducing emissions is your goal.
Bill
I can't find good data about the overall efficiency of natural gas consumed at the powerplant to electricity used at my house (and of course it varies, but I'm just looking for an overall average), but I think 40% overall efficiency is reasonable. The power plants themselves might have 60% thermodynamic efficiency.
This article says 140 TWH of electricity per TCF of gas in 2015, and I guess that's a US average.
https://www.forbes.com/sites/judeclemente/2016/04/10/u-s-natural-gas-electricity-efficiency-continues-to-improve/
It's not clear if that includes transmission losses, but that should only be a few percent. ("The U.S. Energy Information Administration (EIA) estimates that electricity transmission and distribution (T&D) losses average about 5% of the electricity that is transmitted and distributed annually in the United States")
In more familiar units that's 14 KWH per CCF of gas or 47770 "BTU of electricity" per CCF of gas. So that's actually better than my assumption of 40% efficiency above.
If we compare to a 95% efficiency heating system, the calculation from CCF is as follows:
1 CCF x 0.883798 therms per CCF x 100,000 BTU per therm x 95% efficiency = 83961 BTU of heat per CCF of gas burned.
Apparently we get pretty weak gas here, the therms to CCF conversion is straight from my bill.
With full strength gas that would be closer to 95,000 BTU per CCF of gas.
If the worst case COP is 2.0, heat pump heating still comes out ahead of even a high efficiency boiler (but it's close). For most of the year the COP will be a lot higher than 2, and my current heating system is a lot less efficient than 95%.
I'll wait for the experts to chime in, but based on what I recall reading here in the past:
Fujitsu has a model with design temp down to -15F. Based on real world experience, they will run colder than that. Your rare -20 conditions probably don't last very long, and would thus be unlikely to challenge the system.
Keep a couple space heaters around in case of emergency if it gives peace of mind. Those are seem unlikely to get used by a future owner/tenant. Make it even less likely by ensuring the room is comfortable with the mini-split--perhaps you need a ducted system to reach those remote rooms. I think that's a valid question more generally, as Malcolm indicated. But in this case, if you make the easiest option work well and make the less desirable option less easy, it should take care of itself. I.e., why would people reach for a space heater when a thermostat is right there on the wall. The same could be said for an electric baseboard if you go that route, just have the most basic model with the lousiest controls.
Does code require a heat source in each bedroom?
Ducted would be ideal, but I don't have any good place to run ductwork or hide the air handler (I know they are small). I'm already trying to find a place to hide some ducts for a HRV.
This just addresses part of your question...I'm going to install springed 'self opening' hinges on bedroom doors to encourage the doors to be wide open when not closed. It's a small thing, but might encourage correct behavior.
Have you run the math on the room by room heat loads against the radiation in those rooms to estimate water temperature requirement on design day?
What is your 99% design temperature?
How are you heating the domestic hot water?
I have not done a room by room calculation. But based on experience I know that there are insufficient radiators on the main floor to maintain 68 degrees on a 0 degree F night with ~180F water from the boiler. An air to water heat pump would require much larger radiators to work with lower water temperature.
The 99% design temperature is 0 degrees F. But we tend to get wide temperature swings, and I suspect there is more range of extreme outlier temperatures than a coastal area, for example.
Domestic hot water is resistance electric right now. I need to change that, which is one of the things that prompted this question. If I can eliminate the gas boiler that frees up space in the utility closet for a hybrid water heater. If I'm sticking with gas, a gas water heater might be a better option.
"My concern is, if I put in electric baseboards, how will future owners or tenants know to use the more efficient mini splits rather than the wasteful electric baseboards? "
The way I see it is a free country and the next owner of your house will do as they please.
If the next owner decides to make poor choices it will cost them money. They may not like your minis if so they will install a system they like.
In fact the previous owner of you house would not likely approve of your choice to remove a system he paid a premium to have install, that many consider the most comfortable type of heat in any home ever. In dollar terms assuming you are on city gas, in most towns operating the gas cost less dollars than the minis will.
I am guessing you want to make the “green “ choice and cover the roof in solar panels and hang mini splits on all the wall and you will love them but the next owner may think they are ugly and rip it all out.
I once made the mistake of see a house after I sold it, needless to say I did not like their choices but they do.
Walt