Heat pump hot water heater ducting

jsands

Greetings all,

I live in WA state. I have a Bradford white 80 gallon heat pump/hybrid water heater. It is located in a corner of my large 3 car garage (my garage is not heated). In the summer it works great. When it gets cold, like now/ in the winter, it cant heat water hot enough, can’t recovery fast at all, and it runs nonstop (when in heat pump mode) and blows cold air into my garage making my garage even colder. 

I want to duct it with the available ducting kit. 

My idea: about 40’ away from garage (up in the attic) I have a 20’x20’  conditioned attic storage space. Access is via a pull down ladder and it has two home hvac vents that heat and cool the space year round. I was thinking of pulling the conditioned/warmed air from this storage area INTO the water heater via 8” ducting- nice supply of warm air. And then, duct the cold air OUT through the exterior wall of my garage so it blows the cold air outside. 

Is this a good idea? Any foreseeable problems with this plan? I have read about vacuum effect and disrupting the pressure of the house. I figure if the garage is not conditioned and the heated  storage area is up in the attic then it shouldn’t cause a big air flow vacuum issue. But I’m looking for expert advice. 

Thank you so much! 

Jarret 

Zman

Heat pumps simply steal from Peter to pay Paul. If Peter has excess it is not an issue, when Peter is a little short, well that’s a problem. They work great when they are outdoors (and the climate is not too harsh) and when you need cooling anyway. The rest of the time, you are heating a space only to have the heat pump steal the heat. I don’t think I would go to the expense of installing ductwork to enable the thievery. Just run it is electric mode and carry on.

jsands

Thanks for the quick reply. Would it change your opinion if I told you that I run my house mostly off solar and battery and that when I run my water heater in electric mode it pulls nearly 5000 watts, which is nearly half the capacity of my inverter. When I run it in heat pump mode it’s only 1200 watts. It’s important to me to be able to run it in heat pump mode, as that allows me to also charge my EV without worrying about overloading my inverter.

the attic storage area is getting heat anyway, but no one is up there and the thermostat is in the main living area, so pulling heat from there may cool that down a little but it’s just storage so I figure it doesn’t matter much. 

Hope that makes sense. 

Thanks again! 

Jamie Hall

No. The heat pump, as @Zman says, works by extracting heat from a source and moving it somewhere else where it's wanted. Now if the source from which you are extracting heat is part of the place to which you are moving heat, you're going around in circles -- and, physics being the lady she is, losing some every time around.

Now in your situation it's a little more complicated, and in your proposal a lot worse: you have a conditioned space with an HVAC unit keeping it warm in the winter. You propose to extract warm air from that space and run it through the heat pump to provide heat to warm some other space and dump the now cooled air outside. What could go wrong? Think about it. That air which you pull from the warm space has to come from somewhere... which is, ultimately, outside. So you use energy in your HVAC system to warm it up. Then you take that air and extract it and remove heat from it, using that heat to warm another space -- and dump the now cooled air back outside.

The efficiency will be horrible.

Much better to provide intake piping for the Bradford White from outside, and exhaust piping to outside. If the outside air is too cold to run the Bradford White -- which it probably will be -- you need a different heat pump with more capacity.

jsands

That makes sense. Can’t fight physics. Thank for the detailed explanation! 

Zman

What is the heat source for the house? What type of fuel? What you are attempting to do is just stealing heat from the house and putting in your water. You then have to reheat the house. All of this comes with efficiency losses in the process.

jsands

Main house is heated with a high efficiency heat pump. Propane back up when it gets super cold. My garage, which is not heated, and where the hot water heater is located gets too cold to efficiently heat the water. I do understand a different water heater may be the way to go (put more money). 

Where my physics understanding failed me, was that I lost view of the big picture. 

My original thought was that My living area will be heated by my main HVAC based on the thermostat (68 degrees). I have a separate/enclosed heated storage room in my attic that just happens to get heat vented in there (cold air in the summer). I didn’t realize stealing that heat to provide to the hot water heater would be so inefficient. I didn’t think my main hvac system would run any more or less given that the storage areas temperature won’t trigger the thermostat in the main living area. 

I could just let the hot water heater vent cold air into the garage, but it does make the garage cold, so I thought if I vented it outside my garage wouldn’t be so cold. 

I do appreciate the insight and opinions. 

Zman

This is the untold truth about heat pump water heaters. If you cool your storage area with the water heater your house heat pump will absolutely work harder to recover that loss. Heat alway wants to go to cold, if the storage area gets cold, the heat from the house will replace it.
Unless you either live in an environment with long cooling seasons or you have an inefficient boiler room with residual heat to spare, self contained DHW heat pumps make zero sense. They just become an expensive and overly complex resistive electric heater.

EBEBRATT-Ed

When it comes to energy nothing is free. Steal warm air from a house and then you use more energy to heat the house.

Heat pumps are not viable in all locations.

Hot_water_fan

Unless you either live in an environment with long cooling seasons or you have an inefficient boiler room with residual heat to spare, self contained DHW heat pumps make zero sense. They just become an expensive and overly complex resistive electric heater.

The Caleffi Idronics journal covers this well: if a house is heated with a heat pump, then no, the COP is not 1. It's not COP = 4, but it's not resistance level.

Larry Weingarten

Hi @jsands , A few thoughts. I wouldn't vent the HP outdoors unless you take the supply air from outdoors. This is because if you vent it out, cold outside air will have to come in from someplace, cooling your living space. Keeping the pressures balanced is a good thing! Pulling air from an attic space could be okay if that space is air-sealed from the living space and it has a dark roof. The dark roof and no insulation on the underside of the roof would mean you're harvesting sunshine, at least during the day. Also, starting with the warmer attic air, you may not be cooling the garage down all that much as the outlet air from the HP should be warmer.

Yours, Larry

Zman

Hot_water_fan said:

Unless you either live in an environment with long cooling seasons or you have an inefficient boiler room with residual heat to spare, self contained DHW heat pumps make zero sense. They just become an expensive and overly complex resistive electric heater.

The Caleffi Idronics journal covers this well: if a house is heated with a heat pump, then no, the COP is not 1. It's not COP = 4, but it's not resistance level.

On the days he is describing the house heat pump is nowhere near COP 4, probably closer to 2. Calculating the net loss after 2 heat pumps would be dizzying. I think we can agree that "the juice is not worth the squeeze".

JMWHVAC

Yes, DHW HP is not a "green" fix-all. Now if you had a woodstove heating your garage....! In cold climates, most situations they do not save energy in winter. I have done a number of systems (including my own) with a DHWHP for summer and when winter rolls around the water is heated by another source, the same source that heats the house.