Heat pump for hot water in summer - and get air conditioning as a bonus.

MikeAmann

Is this really now doable?
I have an oil-fired boiler with an indirect HW heater, and that is great for the winter since the hot water is essentially free. But no so great for the summer.

My house has non-functioning central air - the dog next door used to lift his leg and pee on the outside condenser unit which rotted the coil. That unit is gone, but the evaporator coil/fan/air handling unit is still in the attic and the ducts to the rooms are still in place (blocked off).

From what little I have read on heat pumps, I have to ask, "can a heat pump be fitted so that it will make hot water (stored in the indirect tank) and also connect to the coil/fan/air handling unit for air conditioning?" I read that the heat pump pulls the heat out of the surrounding area (and also dehumidifies), so would this be installed on the main floor?

Winter = boiler supplies heat + hot water
Summer = heat pump supplies hot water + air conditioning

My house is an 1100sf brick ranch, 1 level plus unheated basement and attic.
I am in southern CT.

Please educate me. Thanks.

Hot_water_fan

It seems the manufacturers of the Home Depot/lowes variety really do not want you to connect their HPWHs to ductwork . It’s a similar concept to using a water to air heat pump, but instead of ground loops, the indirect would be your heat sink. It’s totally possible. Economical is another question. 

MikeAmann

I wouldn't be able to do ground loops - LEDGE everywhere.
Okay then, water to air heat pump.
Do have any links that I can do more research?

Larry Weingarten

Hi, I think one of the first things to look at is how much cooling you can realistically expect to get from a heat pump water heater. I'm not sure it's enough to make a noticeable difference unless your hot water usage is surprisingly high. I bet @Jamie Hall will know what math to use

Yours, Larry

pecmsg

Can it be done.........YES, supermarkets do it quite often.

Is it feasible? for a 40 - 50 gal heater no return on investment.

A HPWH is the way to go.

Hot_water_fan

@pecmsg agreed. AC + HPWH seems way cheaper than a combi system. 

@MikeAmann something like this: https://www.nordicghp.com/wp-content/uploads/2021/11/002002SPC-02-R-09-HAC-P-L-SDET-60Hz.pdf 

Jamie Hall

Well, I could do the math... but without even doing that there is a huge mismatch between the heat required for you domestic hot water and the heat required to be removed to cool a house.

Not to say that there would be no theoretical advantage to utilising the cooling effect from an HPHW in the house. There is -- but it's inherent in the way they are built (the evaporator usually just sits on top with a fan)). No point in getting fancy.

MikeAmann

Thank you all. Big mismatch, I agree.
Maybe a better question is this:
Suppose I have a central air conditioning system. Normally the heat removed from the house gets wasted to the outdoors via the condensing unit. Is there a way to instead use a portion of that heat to make hot water?

ChrisJ

Now hold on. Before we all go assuming his domestic hot water usage would never be high enough to cool the house...


How many teenagers do you have?

JakeCK

It might work, and be economical if you have a super insulated house and have the hpwh sitting in the middle of the kitchen. And of course 2 or 3 teenagers in the house. 

Hot_water_fan

Normally the heat removed from the house gets wasted to the outdoors via the condensing unit. Is there a way to instead use a portion of that heat to make hot water?

Indeed there are heat exchanges for this but their brand names escape me. 

ratio

IIRC Mitsubishi has a few hydronic heads for their City Multi system. If you get the heat recovery model, you can pretty much pump heat around. Not sure you can get the outdoor unit in single-phase, though.

It seems they have a water source model as well (see here). That's pretty neat.

Hot_water_fan

@MikeAmann I think the cooling load of a 1100 sqft house in CT and it’s DHW usage during the summer is probably pretty close - say you use 15 gallons a month. That’s around 15*.8*138,000/30 = 55,200btu per day. That’s probably pretty close to your average cooling daily usage. Probably not enough for the hottest day, but warm days sure. 

MikeAmann

I'm single and live alone. No children (there is my savings right there).
How many teenagers do I need? If they were female, legal, and HOT..... Hmmm?

I am just trying to make use of things that I already have, like the central AC.
I was reading how service people work on HPWH in the basement.... and freeze.
Things like that get me thinking. Sometimes that's good, other times not so much.

ChrisJ

Hot_water_fan said:

@MikeAmann I think the cooling load of a 1100 sqft house in CT and it’s DHW usage during the summer is probably pretty close - say you use 15 gallons a month. That’s around 15*.8*138,000/30 = 55,200btu per day. That’s probably pretty close to your average cooling daily usage. Probably not enough for the hottest day, but warm days sure. 

20 gallons of water heated from 55F to 120F uses approximately 10,847 btu.

TonKa

https://www.hotspotenergy.com/residential-heat-recovery-water-heaters/

Jamie Hall

Going to need some ball park figures here, but it's an amusing diversion...

Let's suppose, for the sake of argument, that you have a more or less average family in a more or less average house in a climate something like central Connecticut.

Some ball parks. Hot water usage. Oddly, there are figures on that -- and one can generally figure that the usage will be around 20 gallons of hot water per day per person. So we need a heat input there of 100 F times 20 gallons of water which works out to somewhere around 15,000 BTU per person per day. Typical family of 4 -- call it 60,000 BTU per day.

Now heating and cooling load. Suppose it is a more or less typical modest house with a design day heating load of 60,000 BTUh at 10 F outside. That works out to a heat transfer rate of about 1,000 BTUh per degree of delta T, interior to exterior. (the heating and cooling numbers are slightly different, but let's not get picky).

Now in the summer time, being all modern, we want AC and a 20 degree differential. We need some 20,000 BTUh cooling capacity. Can the heat pump water heater help? Well... um... a little. With the demand we are putting on it, it will provide around 2500 BTUh cooling power. That's not entirely trivial -- somewhat over 10% of the load.

But let's look at the heat -- which is more like 6 months of the year than 6 weeks. We need to add our heat pump water heater's demand for heat onto our heating load. Less of an impact (and the colder it gets out the more irrelevant it becomes) but it's always there.

There's no free lunch...

Just musings, and I need more coffee.

MikeAmann

Thanks all. I will let this idea die.
But I might look into a hydronic solar collector on the roof tied into the IWH for summer use only.

When the time comes, I assume that a new evaporator coil/fan/air handling unit with the refridgerant of the current time can be added to my existing central AC system? All of that is still in place - just the condensing unit is gone.

MikeAmann

TonKa said:

https://www.hotspotenergy.com/residential-heat-recovery-water-heaters/

OMG, this site has some great ideas. Let's see how long it takes me to get out of this rabbit hole....
https://www.hotspotenergy.com/heat-recovery-air-conditioners/
https://www.hotspotenergy.com/pool-heater/

MikeAmann

It looks like hotspotenergy already incorporated this idea into a mini-split AC:
https://www.hotspotenergy.com/air-conditioner-water-heaters/

ethicalpaul

Don't worry about how much cooling you will get from a heat pump water heater. It's not much.

Worry instead about how you can avoid heating up your basement/house in the summer by using a boiler to provide your hot water.

I can't recommend enough today's heat pump water heater tech. If you are using oil, propane, resistive electric or even natural gas to heat your water, just go buy a heat pump water heater right now. Your state might even pay you, mine did. It's so cheap it's almost free.