Harvest hot water heatpump system

[begreen]

I just saw this mentioned by a homeowner on Reddit. It appears to be California only. Does anyone know much about this system?

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[woodgeek]

Meh. It uses a HW thermal battery to store BTUs pumped during the day (cheap kWh) for use at night (expensive kWh). The same battery is your HW tank.

Seems sensible enough, and CA appropriate.

If you heat the HW too much, you get COP losses, so the swing is likely small. The Air handler has a hydronic coil, so low service water temsp are not much of a problem. Say you swing from 100*F to 140°F, and the tank is 100 gallons. 800 lbs* 40°F = 32,000 BTUs.

My house needs about 10x that much overnight in winter, and I don't have TOU pricing. So useless for me.

For a well insulated CA house with TOU and cheap (solar) daytime rates... sure. But if you're only using 32 kBTUs at night, you are using <2 MMBTU per month, and your energy bill for a conventional system is pretty cheap already.

Meh.

 

[DBoon]

I do something similar to this idea using an oversized 180 gallon geothermal heat pump water storage tank, but agreed that it can't supply continuous heat for more than a few hours. Mine could store about 3-4 hours of heat on a "normal" winter day. But pushing the water temps higher to increase capacity reduces the COP a lot on the geothermal heat pump. Since I have the water supply coupled to radiant floor heating, I can scavenge really low temperature heat and reduce the tank temperature to 70 degrees or so before I recharge it again to 110 or so, giving me about 58 kBTUs (and ridiculously great COPs on the heat pump the first half of the recharge). Since I run the wood stove most of the colder winter days, this provides enough heat to augment where the wood stove heat doesn't circulate really well. Again, this is more of a dork experiment than any kind of cost-saving move.

Summary - what Woodgeek said, basically.

I'm somewhat experimenting in anticipating a future of TOU rates in which the local utility doesn't want my daytime solar and I want to charge vehicles or store heated water with cheap daytime solar PV in winter to mitigate nighttime expenses. For me, realistically, this is 15 years away.

 

[Whitenuckler]

When I had my last mini split work down this spring I mentioned to the tech that I had heard of a heat pump that heated
water. The reason I asked is I still have all of the water rads in place from the previous Oil Boiler. He said yes, there is a
system available but it is very expensive. He said 20,000CAD? I didn't look up the details, so not sure about a storage tank.

 

[EbS-P]

Federal efficiency incentives are as good as dead after this year. I don’t see this taking off.

 

[woodgeek]

When I had my last mini split work down this spring I mentioned to the tech that I had heard of a heat pump that heated
water. The reason I asked is I still have all of the water rads in place from the previous Oil Boiler. He said yes, there is a
system available but it is very expensive. He said 20,000CAD? I didn't look up the details, so not sure about a storage tank.

The issue here is the 'lift' needed. Most radiators assume water at 160-180°F, and making a heat pump pump to that output temp really kills the COP. My air to air heat pump pushed heated air into my space at 85-90°F in the winter, for comparison. Circulating 100°F water in your radiators is not going to do the job.

 

[Whitenuckler]

The issue here is the 'lift' needed. Most radiators assume water at 160-180°F, and making a heat pump pump to that output temp really kills the COP. My air to air heat pump pushed heated air into my space at 85-90°F in the winter, for comparison. Circulating 100°F water in your radiators is not going to do the

I'd be better off just getting another 40 gallon electric hot water tank if I wanted to get them working again. I think one or two more
minisplits would be good instead. I like the cooling.

 

[EbS-P]

I'd be better off just getting another 40 gallon electric hot water tank if I wanted to get them working again. I think one or two more
minisplits would be good instead. I like the cooling.

Once you add solar it’s just heat pump or freeze Well not really it’s heatpump or Wood stoves. I try to keep the resistive strips off.

 

[Whitenuckler]

Once you add solar it’s just heat pump or freeze Well not really it’s heatpump or Wood stoves. I try to keep the resistive strips off.

I'm not sure if I'll ever get solar panels here. I don't want to get a contract and have someone else own them. I don't have the money available to invest in them. I am always thinking of being able to go without utility though. I have a couple of generators and the pellet stove for heat. I would really miss the cooling though if I lost power in the summer heat wave. Luckily, I set up an unused spot in the basement as my beer zone and an air bed. Seems to only get about 21C so far down there. I have convect heaters in all bedrooms plus others. If the weather is really cold, and the heat pumps crash, I will still have good heat. I plan on running the pellet stove all winter