Ducting HPWH to Central AC

241comp · May 8

I have an AO Smith heat pump water heater located in an unfinished basement, which is designed to allow a connecting n to 8" round duct. I have a central air conditioner installed in close proximity. Is it reasonable to duct the (cold air) outlet on the HPWH to the return duct on the AC, provided there is also an open outlet that will exhaust air when the central AC is not circulating air? This would potentially increase the cooling capacity of my AC by about 10-15% and would provide some cooling when the AC is in circulate mode. Here is a general layout of what I am thinking:

diagram.png

DCContrarian · May 8

If the AC has a return nearby just terminate the duct next to the return.

EdTheHeaterMan · May 8

https://forum.heatinghelp.com/discussion/comment/1801233#Comment_1801233

Next to, not into?

Screenshot 2024-05-08 at 11.02.08 AM.png

Won't that just dump the cold air next to the duct, not into the duct. How will cold air next to a return benefit the rest of the home?

I believe that the original design is much better, the cold air actually mixes with the air in the duct that flows into the rest of the house. Operating An ECM variable speed blower constantly is the only way I would recommend this. By running a PSC motor continuously is a bit pricy, while the ECM on constant only operates at about 15% of the regular cooling speed and a substantial savings.

Screenshot 2024-05-08 at 11.09.44 AM.png

If course you will close the duct in the winter as not to add load the the heating system.

DCContrarian · May 8

By "return" I meant an opening, not just the duct. What I'm suggesting is an air gap between the HPWH exhaust duct and the return so that if the HVAC isn't running the air has some place to go.

"If course you will close the duct in the winter as not to add load the the heating system."

So where does the heat come from in the winter then?

EdTheHeaterMan · May 8

https://forum.heatinghelp.com/discussion/comment/1801271#Comment_1801271

I knew that… I was just bustin' on you. LOL 😈😈😈

As far as where the heat comes from in the winter… "the thermostat"…. Everybody knows that! Turn it up … you gets heat… turn it down you gets cold.

241comp · May 8

The design I have in mind leaves an open vertical path for the HPWH outlet air. My thinking is when the circulator is off, this opening be the lowest resistance path and the air will simply blow straight up into the room (basement) as it does today, unducted. When the circulator is on, the circulator will pull air in from 8" round duct which will be the HPWH outlet cold air when it is running (and room air otherwise).

As for heating, we have hydronic heat - our central air handler is for cooling only.

pedmec · May 8

Having cold air entering your return can have negative effects on your heat pump system. Depending on how much of the air is entering the return and at what temperature you are reducing the load on the heat pump system causing the back pressure on the refrigeration cycle to drop the saturation temperature. if the air is cold enough it can cause the saturation temperature to drop below the freezing point and the heat pump to freeze.

ethicalpaul · May 9

There's really not enough cool air (it's certainly not cold) exiting a heat pump water heater to have a noticeable effect or to make the cost of the ducting worth it, IMO

Hot_water_fan · May 9

I think the manuals say not to duct

ethicalpaul · May 9

my Rheem says you can duct it

241comp · May 9

Thanks for the feedback, everyone. My AO Smith HPTS-80 allows ducting and specifies a maximum of 300' of 8" rigid duct (100' of flexible). I would have about 8' plus 3 elbows. The capacity of the heat pump is around 3800btu - I wouldn't expect that to drop the return air temps enough to cause problems.

The output is enough to noticably chill my lower level and my 2.5T AC is marginal for cooling my home (on the hottest days of summer). If this were simply a cost saving measure I think I would skip it, but I see this more as a way to provide a slight capacity boost and even out the temperatures somewhat between floors.

I think I will move forward with this and I'll report back any interesting results.

Hot_water_fan · May 9

Allows ducting into a central system?

Have we established the current AC is too small? Adding more capacity might not fix anything.

pecmsg · May 9

Try ducting the incoming air and positive pressure the basement.

ethicalpaul · May 9

Allows ducting into a central system?

You didn't say that the first time 😅

241comp · May 9

https://forum.heatinghelp.com/discussion/comment/1801357#Comment_1801357

Definitely not specified for ducting into a central system, or I wouldn't have created this post. 😁

I have not definitively established that the current AC is too small. Current system runs around 100% duty-cycle on hot days, and temps slowly climb above the target temp. Ducts are large enough to accommodate a 3.5T system (according to HVAC places that have come out and quoted me a replacement system). The current air handler says it is for 2-3T. Will adding 4000btu of intermittent additional cooling capacity actually help? I have no idea, and I don't really know how to figure that out but I'm open to any recommendations on a technical approach to that. But it feels like it might.

241comp · May 9

https://forum.heatinghelp.com/discussion/comment/1801362#Comment_1801362

Can you help me understand this a bit better? Are you advising ducting the intake to eg. the first floor to pull in warm air and create a positive pressure in the basement, which would force cold air from the lower level to go somewhere (up the stairs, perhaps)? There is a door separating the lower level from the upper levels so might this primarily push air outside?

Alternatively, perhaps both the intake and return for the HWHP could be ducted to the first floor. This would potentially add some noise to the living space, but seems like a reasonable thing to do (during the summer). This also has the advantage of not requiring the circulation fan to be running in order to benefit.

PRR · May 17

heat pump water heater

How much hot water do you make?

At my house it would only be a few minutes a day, as opposed to the A/C which can run 18hrs/day.

EdTheHeaterMan · May 17

@PRR When I turn on a 40 Gallon gas water heater for the first time, it takes about 30 minutes to heat the tank of cold water, before the gas valve shuts off. Did the same test on an electric water heater after installing one, It took 4 hours for the bottom element to be satisfied. I think those elements can get pretty hot if they are not submerged in the water. I then timed a 32 gallon oil fired water heater and that took about 18 minutes from cold start to satisfied aquastat to shut the burner down.

I never installed a heat pump water heater, but I would imagine that without the backup electric elements operating, that the 120° to 140° hot gas from the compressor will take much longer to heat the water than any of the above conventional water heater tanks. With that in mind, and the fact that you may not always be starting from a completely cold tank, I have a feeling that your few minutes a day, may actually be at least an hour or maybe even more that an a few hours of heat pump operation. And if @241comp gets really intuitive with his family shower timing and laundry, he can make that water heater compressor operate at the hottest point of the day where that additional cool air from the condenser coil will actually have the most benefit.

So to say that there will be little to no benefit, is premature at best. IMHO

And if someone else wants to experiment, where it does not cost us any $$$, and is willing to post any positive results for our benefit. then I say "Don't stop the experiment…. Give words of encouragement."

@241comp, Just try to keep the necessary data, Like Degree Day information, electric usage, operating times of the HPWH from before and after the duct install. Then post your results.

ethicalpaul · May 17

A family of three or 4 might use 60 gallons of hot water per day, or so I read.

That's 500 pounds of water per day

Bringing 500 pounds of water from 50 degrees to 130 degrees is 40,000 BTU per day, right?

Now someone who knows AC can take over. How much cooling is 40,000 BTU per day? That's assuming heat-pump-only mode which I run in, but if someone uses their resistive elements, that takes away from this amount.

But the whole ducting into the AC system seems like a problem to me. I can't picture how you do it without messing something up.

Ducting HPWH to Central AC

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