Better option than a heat-pump water heater for a boiler?
user-2901637
| Posted in Mechanicals on
I have a home in Zone 4b that currently uses propane (no natural gas service available) to fuel a 10 year old mediocre boiler (hydronic baseboards) and mediocre tank water heater, both in the garage. I’m considering replacing both the boiler and water heater with two electric heat pump water heaters (2.5 to 2.75 EF) and wonder if there are better options I haven’t considered. Heat pump mini splits and a wood stove currently handle part of the heat load, but don’t take care of the whole house. PV is scheduled to be installed soon and will handle the total load for these units.
Thanks in advance for any wisdom!
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Replies
Jane,
If you want, you can replace your existing propane-fired water heater with a heat-pump water heater located in your garage.
However, it doesn't make any sense to try to use a heat-pump water heater (or even two heat-pump water heaters) to supply hot water for a hydronic space heating system. A heat-pump water heater doesn't have enough capacity for that function.
If you like hydronic distribution, stick with a boiler.
If you want to heat your house with electricity, you can abandon the hydronic distribution system and add another ductless minisplit or ducted minisplit to heat the rooms of your house that aren't now heated by your existing ductless minisplits.
Wow! Thanks for the quick response Martin! Not to be stubborn, but I'd really like to figure out how to keep the hydronic system (main heat in master bath and multiple lesser used rooms) and get off the expensive propane. Do you have any experience with the mini boilers I've heard about recently? It doesn't make economic sense to me to replace with mini splits when spare bedrooms are rarely used (although when used it's generally during winter months). And I'd also really like to take advantage of our soon-to-be PV system.
Every heating system starts with a room-by-room heat load calculation, and if you intend to use the existing radiation, you need to figure out what the average water temperature needs to be do deliver that design-day heat using that radiation. At 130F AWT (135F out, 125F back)typical fin-tube baseboard delivers maybe 250 BTU/hr per foot.
Heat pump water heaters don't have a lot of output in heat-pump only mode. Even if your radiation can deliver the heat, you also have to be sure that the heat source can put that much heat into the system.
I understand that the system will not be able to always run in heat pump only mode but will need to kick up into the "hybrid" part and use electricity, which is part of the reason for the PV system. I didn't mention the hybrid (are there heat pumps who's that aren't hybrid?) portion, but it would handle loads in the swing seasons of fall and spring without kicking into electric mode, I think. Based on some whole house calculations (using REMRate) we need less than 1000 BTU/hr to satisfy our needs after the mini splits are factored in, although I want to be sure we have the capacity for those really cold days/nights we get sometimes (7 last night). The units I'm looking at have input ratings around 4.5 MBtu/hr and energy factors of 2.75 per AHRI.
Again, looking for the most efficient, electric, water based method to tie into the existing system.
Thanks for your thoughts and input!
Jane,
1,000 BTU/h is only 293 watts. If all you need is 293 watts of heat, buy the smallest electric space heater at WalMart that you can find (for $39). Or just leave 3 big light bulbs burning.
My guess is that your math is off, and that your design heat load is more than 1,000 BTU/h.
The 1KBTU/hr is the calculated shortfall of the mini-splits (and not the whole heat load), but that too is a pretty squishy number, "in the noise" of other calculation errors for most homes.
There are no heat pump water heaters that put out 4,500,000 BTU/hr (that's more than 10x the design heat load of my not-so-superinsulated antique house!) The Air Tap ATI-xx series put out about 9000 BTU/hr in heat pump only mode, the Stiebel Eltrons about half that, the GE GeoSpring about 6000 BTU/hr.
The EF numbers are based on environmental parameters that you won't actually see when drawing heat out of a garage when it's +7F outside. Efficiency drops pretty dramatically when the room temp is below 50F, and they all have a minimum operating air temp.that you're very likely to hit in the garage if pulling 5000-10,000 BTU/hr out a garage. The AirTaps are set up for a minimum ambient of 32F but they may be programmable for lower, but water heaters are not set up to auto-defrost, and running it at a high duty cycle in ambient environments below 40F will surely frost up the coils.
Bottom line, it's better to go with a purpose-designed heating solution rather than try to make a duck produce goose eggs, even though they have many functional similarities.
But getting a real handle on the design heat load relative to the amount of baseboard in each room is still on the critical path to a satisfactory solution.
If you had heated floors I would see some merit in maintaining hydronic distribution to the rooms that need supplemental heat, but if there are just baseboards, I would consider abandoning the hydronic altogether and use electric baseboards, wall heaters, or even space heaters in those rooms. You can't really get water that is hot enough for baseboards out of typical heat pump water heaters like those you are discussing or even a Daikin Altherma. You would have to go with larger radiation to utilize the lower water temps available from heat pumps.
Unless your electricity is really expensive, it's probably on par with or cheaper than propane. An electric boiler would do the job, if you really want to stick with baseboards, but a couple of small electric heaters would probably be cheaper to install, operate, maintain, etc.
What do electricity and propane cost there?
I agree with David. If you only need it for the coldest days, the heat pump isn't running with high COP on those days, and using electric heat wouldn't be bad. And electric baseboards are a fine substitute for that purpose. Electric radiant panel heaters are another good option.
An electric boiler solution would probably be substantially cheaper to install than ripping out the existing baseboards and replacing them with electric baseboards. The electric baseboard solution allows you to easily micro-zone it though.
But you still have to do the heat load math & radiation math in a more rigorous fashion to correctly size an electric boiler. You can still get over 200 BTU/hr per foot out of baseboard at domestic hot water temps- it's not insane even though it's not designed as low-temp ratdiaion. But there has to be enough baseboard to supply the requisite amount of heat at those low temps, and enough of a heat source to get that heat into the system.
Typical baseboard systems were designed for 160-200F water, but rarely actually needed more than 140-150F water even on design day, overdesigned due to crummy rule-of-thumb practices. If most of the heat is being delivered by the mini-splits, you probably don't NEED very high water temps to get there, but just winging it with a dedicated in-line water heater isn't a great idea, and would be more expensive than an electric boiler solution to do it right (and code-legal.)
You could wing it using rules of thumb, sizing the boiler to the existing radiation. Fin-tube has very non-linear output below water temps of 115 F or so, but if you sized the electric boiler such that it delivers at least 200BTU/hr per foot of existing baseboards it should at least behave OK as a system, provided that's really enough heat (it could be way too much heat, or not enough- without heat load calcs it's hard to guess.)
>>An electric boiler solution would probably be substantially cheaper to install than ripping out the existing baseboards and replacing them with electric baseboards
I think it really depends on the setup. An easily accessible electrical panel and a couple of easy, short runs to bedrooms would be a quick job for an electrician. I would expect to pay over $1000 for a small Electro-Boiler or Thermolec, plus the technicians's time and other materials to pipe it in,
Of course, if there's a slab floor and cathedral ceilings and no way to run new wire, the boiler wins easily.
Thanks so much for all the thoughtful responses! Not a simple setup so the wiring, etc. to install electric baseboards or similar would be a costly prospect. I will look for an electric boiler to replace the current propane unit (after running heat calcs).