How much room heat do Heat Pump Water Heaters need?
Ine
| Posted in Green Products and Materials on
These seem like a great idea if one needs to heat water with electricity but will a heat pump water heater be effective if enclosed in a small, unheated mechanical room in a basement kept at a low room temperature? This is in a heating climate and we don’t need air conditioning. My logic (and it may be faulty) tells me that, eventually, the room would cool to a point where the heat pump would quit working efficiently.
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Replies
Donald,
You're on the right track with your questions. Heat-pump water heaters are best used for homes in hot climates with a significant air-conditioning load. In Florida, a heat-pump water heater can help cool a house. If your climate doesn't require air conditioning, there are probably better ways to heat your domestic hot water.
As your question implies, a "small, unheated mechanical room in a basement kept at a low room temperature" is probably the worst possible location for a heat-pump water heater — especially since you live in a climate where air conditioning is not required.
Thanks for the confirmation Martin. I'm guessing then, that the next best way to heat water if I have to use electricity is with a Marathon water heater.
Donald,
There's a third option: an electric on-demand (instantaneous) water heater.
Donald,
Don't rule out a HPWH, yet. While Martin is correct in that the BEST place for a HPWH is in hot climates, HPWHs can save energy and money in a broad range of climates., especially if you have to use electricity to heat your water. In order to figure out if it makes sense for you, you need to share a few things:
1) Are your basement walls insulated, or is the floor over the basement insulated (or neither)?
2) At what temperature do you keep your basement in the winter?
2) What heating fuel do you use?
3) What price do you pay for your heating fuel (including all taxes, surcharges, etc.)?
4) What is the price of your electricity (including all taxes, surcharges, etc.)?
4) What is the efficiency of your heating system (include distribution losses, such as duct losses, if applicable)
5) Where are you located?
John,
Here are the details as far as I know to this point. This is new construction at the rough in stage so I don't have all the answers yet.
1. ICF basement no insulation in floor above but 4" XPS under slab with tubing for radiant installed.
2. Probably low 50s. Radiant is there to keep house from freezing when we aren't around to keep fire going.
2.Wood for primary heat, electric tankless for the radiant loops in basement.
3. Electricity at the site is primarily from hydro and is nominally .0625/KW but the real cost with all the extra charges is about .125/kw. The wood is cut on site and is free.
4.Wood burner is on first floor. No ducts.
5. Upper Peninsula of Michigan right on the line where 8000HDD starts on the map.
Martin,
I have considered an electric tankless water heater. We have a tankless in our current house: Bosch natural gas 125 HX that I put in 5 years ago and my wife is less than happy with. Our well installer and local plumbers seem to think that our well water may cause mineral deposit problems with a tankless unit. The Marathon unit should eliminate both these issues. What I'd really like is an electric combi unit that could do domestic HW and the radiant floor but these only seem to be available in Europe.
Thanks to both of you for your input.
Donald,
First - since you're heating with on-site wood, why not heat your water primarily with wood as well - http://thermacoil.com/index.html?
If that's not an option, and you have to use electricity, then the heat pump water heater is definitely the way to go. Here's why: you're still heating the water with wood! Your woodstove will heat the air, and the HPWH will "pump" some of that heat into the water tank as needed. In a sense, it's no different than the diagram from the link above...it just takes energy to do it with a HPWH.
With the HPWH, your ($0.125/kWh) electricity use for water heating will be roughly cut in half, and your (free) wood use will go up in keeping with the amount of hot water you use during your 8,000HDD.
The small mechanical room needs to be slightly modified. You'll need a louvered door (instead of a solid door), in order to circulate warmer air to the HPWH, and you'll need to duct the cool "exhaust" air from the HPWH to a larger room in the basement or first floor. No big deal. As with any duct run, keep it short, smooth and fat. I think most HPWH's exhaust in the range of 150-250cfm...get the specs from the manufacturer and size the duct accordingly.