Nutty idea?
blackfeet
| Posted in Mechanicals on
Can an 80 gallon heat pump water heater be used for staple up Pex radiant heat in a very well insulated 800 sf space? Or is that just insane? Climate zone is 4 marine. Near Seattle . R60 in ceiling. R23 walls. 2ACH or less. Conditioned crawl. Advanced framing.
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Where is the heat pump located? HP just move heat, so it would have to be outside.
It could be either in the rather tall crawlspace. Or the attached unheated garage. If the crawl was conditioned, would this defeat the purpose? Meaning, the HPWH takes all the heat out of the insulated crawl? This is new construction. I could just vent the crawl and install floor joist insulation. R38 to meet energy code.
I and just trying to get away from A2Water heat pumps like Spacepak because they are so pricey.
Unfortunately that concept wouldn't work if you place the water heater within your conditioned space. I don't think it would work in the garage either unless you can find a heat pump water heater that will operate below 0F. I think most of them are only rated down to around 40F or 50F. Heat pump water heaters aren't really designed to heat spaces like this and you'd essentially be trading the Spacepak air to water heat pump for a water heater that is an air to water heat pump. If you were in a milder climate it might work. Heat pump water heaters produce much less heat than air to water heat pumps designed for heating residences.
What you are proposing sounds like a perpetual motion machine and not a workable idea, in that the HPWH does not generate many BTUs.
What it does is move BTUs from the air and put them in the water. The BTU in the air must be provided by another source like a gas furnace or heat pump that collects out door BTUs and moves them indoors.
Most HPWH need a source of BTUs over 50°F.
Walta
Besides this not really working (a heat pump scavenges heat from it's surroundings, it doesn't "make" heat, so it can't heat up a space it's within), a heat pump water heater likely won't have sufficient BTW delivery to heat an 800 square foot space regardless of the location of the water heater.
There are tank-type water heaters (and maybe some on-demand ones too) that are rated for use with hydronic space heating systems like this -- I'd suggest you look into one of those.
Bill
Thanks everyone. At least I know now to focus back on the Spacepak or some other A2W heat pump designed for heating purposes. The Chiltix comes in at a lower price point. I will call them.
If you have sufficient S exposure, it may be (much) more cost-effective to invest in solar PV panels and use a (much) less expensive heating source - perhaps even simple resistance heating. With the such a small, well-insulated space and moderate heating zone, your maximum heating demand must be really small and you'll get about 3412 BTU/hr per kW.
Thanks. We will be having solar and have room for about 8KW. But....the new Washington Statte energy code is not friendly to resistance heat. We will look at mini splits as our alternative as they are cheap, efficient and DIY friendly.
Washington state means cooling. With an efficient structure, you are much better off to invest your effort in a proper ducted mini split install. This gets you heating, cooling, fresh air distrubution(if you connect the ERV to it) and filtering for much less than anything else.
Wallmounts + radiant +ERV with ducting is essentially installing two complete heating systems.
For floor heat in selected areas, use resistance matts.
The Rheem Professional Prestige can do with an ambient temp down to 37°F, so you could use that one in the garage, maybe the outlet ducted to outside and a grille in the garage wall to let air in. 37°F is not that low so you would have to think "hybrid" and have another heater to kick in below. Also you need to know the heat load of the building at say 40°F as a practical limit - so that the heat pump can actually heat the house with a bit room to spare. Even if you have to swich over to direct electric heat, the pv will reduce the consumption from the grid. Maybe this is still viable when averaged over the year. Depends on your climate, how much sun you get in the winter for the pv etc. Maybe you have other means (wood stove etc.) to take over for that winter period?
Still, this is - as you wrote - kind of nutty - not standard at all. But maybe feasible...
Typical HPWH has a 4500BTU heat pump. The heat pump is small enough that if you have the wrong type recirc for regular use, it will run all the time causing early failure.
I guess you could heat something with it I guess but won't be 800sqft. It also won't last long.
blackfeet,
You might look into the dual format window air conditioners that are basically able to run the coils in either direction. If the insulation levels are that high for that small a space and your windows aren't single glass it might be a good and cost effective choice. It would also negate having to mess with your presumably good floor insulation. Staple up PEX without spreaders and tight insulation below would be low on my series of heat choices. Plus side is you get AC.