I have a heated slab in my off grid home and right now it i have a propane fired Bradford-White water heater that handles both the slab and the domestic water. The floor is 3 100' loops and i circulate it with a Taco 007e. My question is what is the best, most efficient way to heat both the slab and my domestic water when this Bradford-White poops out.
water heating question
- Thread starter panhead
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OzSolar
Whatever you did, that's what you planned.
If you could elaborate on what you mean by best and most efficient that would help us answer your question.
"Easiest" would be to go back with exactly what you've got since it's already working for you.
Speaking from experience, solar and space heating don't go well together. Short, often cloudy days followed by long cold nights don't result in any sort of reliable or useful heating from solar. Don't even start down that road in other words.
Cheapest might be a HP water heater but you'd probably need two or more of them to match the BTU output of your current LP water heater. No doubt that one of them would work fine in the summer for your potable needs but I'm sure you'd struggle to produce enough kWh with your off grid PV to use them for space heating in the worst 6 to 8 weeks of winter.
For sure the cheapest to use would be a wood boiler but the upfront costs combined with effort of collecting firewood and feeding the boiler make it the least attractive.
doing it different
Solar Enthusiast
Not totally sure a heat pump hot water heater to heat the floor makes sense if you are heating a slab and the heat pump is in the same area of the slab. Like in a basement... the extra heat needs to come from somewhere else.... like outside.
seems to only make sense if you're trying to do domestic hot water....
I could be wrong though....
seems to only make sense if you're trying to do domestic hot water....
I could be wrong though....
A heat pump water heater will not likely work, as the heat it attempts to capture and concentrate will be taken from the same space you are trying to heat. There are three approaches (that I can think of) possible, using solar:
1) Install an air-to-water heat pump, which has the compressor located outside the home. This is at its heart the same technology as a heat pump water heater, but you are capturing heat from outside the space (so it can be moved into, and then released, inside that space). These are common in Europe ut much less so in the US (yet, anyway). But they are available, albeit at a cost.
2) Purchase a buffer tank with electric heating elements. These are much less expensive, but they require an extreme amount of power (I have one waiting to be installed that takes 6 KW). This is practical only if you have much unused solar capacity, and even then only on sunny days.
3) Use a solar heat collector (flat plate or evacuated tube type). These were once common, but have fallen out of vogue as heat pumps have become more common.
Hope that helps.
chrismarnay
New Member
You can do a direct replacement of your WH with a heat pump model. B-W makes one, but it's a bit noisy, and noise is something to consider. LG makes a really cool one, but there aren't many around yet. I would avoid Rheem. I just bought an LG to replace a Rheen that was only 10 yrs old and operated at only moderate duty, way less than you're considering. So, put your HPWH where the noise won't be a problem, and you might also think about water inlet temp. If there's a way to raise it somewhat, it'll help. I switched from propane to a HPWH two years ago. One problem I've had is I didn't take the need for additional overnight electricity storage seriously enough. We heat with wood, and we're in a way warmer climate, northern CA. We switched water heating and cooking to electricity, but didn't appreciate how much additional overnight usage would affect needed storage. I'm now looking into doubling the battery bank. Since batteries are expensive, this issue could sink your project.
He's using it for space heating, via a slab. So the heat he would be capturing is taken from the same space he's trying to heat. Unless he uses the electric heating elements instead of the heat pump, it's not like those BTUs will be magically created.
I've done a lot of data logging with my HPWH. I see a lot of talk about environmental temps when running a HPWH. I can see a clear improvement in the degree F delta/derivative when I open my garage door and its warmer outside. However what I've noticed that causes the biggest change during cold weather is the incoming water temp. Our water in the winter months is 20-25*F lower than it typically is in the summer. The recovery for that extra differential makes a huge difference in run times. Not much you can do about that though unless you have one of those recovery drain/main lines to recover some of the heat that goes down the drain.
If you're going to use this to heat, you're going to need to externally route the supply/return for the unit. They make kits for this. If you're in a northern climate, you're going to get a bit cold though!
If you're going to use this to heat, you're going to need to externally route the supply/return for the unit. They make kits for this. If you're in a northern climate, you're going to get a bit cold though!
efficientPV
Solar Wizard
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Most HPWH are in the 600W range. COP will be much lower in the winter, probably closer to two. Regardless of what number you use, the standard water heater will be lass than 1500W of heat, not much for a slab. A much larger unit is needed than a standard water heater.
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