DHW with an Air Water Heat Pump

CDHE

I am setting up a Chiltrix CX50 (4.75 ton AWHP) in my studio building (at 6,300' altitude), and am confused about sizing of an indirect water heater. In the near term, our only needs will be for hand washing in a 1/2 bath. Down the road there might be a bath/shower and efficient washing machine. All equipment is to be set in a crawl space with a rather narrow (22"W) opening.

I had been considering installing a Triangle Tube Smart 40 since it fits in the opening, and feels large enough to avoid future remodeling of the system. I ran my design past the Chiltrix' staff ME and got this response: "we would not pair a DHW40 to a  CX50, it would not perform well due to big capacity mismatch. It would need to be DHW80 if we supply it".

I imagine that the ME's response might be designed to keep their involvement in problem solving customer's systems to a minimum, but surely there must be a way to get to the desired result…..

Any suggestions? Thanks in advance.

EBEBRATT-Ed

I would think Chiltrix is overestimating your hot water usage but a HP is not going to output very hot water to heat an indirect. Certainly not 180 deg like a boiler can.

I think they are upping the tank size to give you more capacity.

Upping the tank size will not give you hotter dhw just increase its volume

I looked up the Chiltrex and it can output 95 degree water. The BTU output depends on the outdoor temp so the BTU output can range from about 25000btu to 75000 btu with 57000 btu selected at an outdoor temp of 45 degrees.

Because the HP outlet temp is low they want a larger storage tank

The ideal set up would be an indirect with electric back up but I don't know if they are made.

I would go back to Triangle Tube with the heat pump capacity chart and see what they think

There are also HPWH though I am not fond of them.

CDHE

Thanks for the advice. This 50 gallon unit seems to fit the bill as far as having electric back up and press-fitting into the crawl space:

https://www.bockwaterheaters.com/mwdownloads/download/link/id/78/

EBEBRATT-Ed

I thought someone made them. If you have a 30 amp two pole circuit it would probably be a good idea. If you don't use much HW the heat pump may do it.

Hot_water_fan

For a space like that, I’d strongly consider a plain electric resistance tank. Sometimes efficiency just costs too much.

DCContrarian

I'd just go with a regular HPWH.

I'm not fan of the way Chiltrix implements hot water. When you're in cooling mode it doesn't scavenge the waste heat. When the water heater calls, it turns off cooling mode, switches over to heating mode, heats the water and then switches back to cooling mode.

With an 80-gallon tank, if you need to warm the water by 50F that's 33K BTU. The CX50 is rated at 50K BTU/hr so that's about 40 minutes without cooling. I think I would notice that. In the heating season you're without heat while the water heater runs. If you look at the performance curve of the CX50 it really drops off as it gets cold, at 23F it's about 70% of what it is at 47F. So you'd be looking at about an hour without heat.

With a regular HPWH, in the cooling season it cools and dehumidifies your house. If you're heating the house with a heat pump, it's a controversial question whether it's more efficient to pull the heat from outside in one stage with an external heat source water heater or in two stages, with a heat pump heating the house and a HPWH using that heat to heat the water. There's a long thread about it here: https://www.greenbuildingadvisor.com/question/where-is-the-most-efficient-place-to-get-the-heat-for-hot-water-from

Where there is agreement is that if there is a difference it isn't much. If you cool in the summer the cooling provided by the HPWH will more than make up for any difference.

hot_rod

do you have room for a hpwh? They are pretty tall.

The hpwh pulls heat from indoors, unless you duct it outside. A2Whp pulls from the outside.

Here are the calculations for the cop of a hpwh, when pulling the “heat

“ from a space that is heated by a heat pump. Called cascading heat pumps.

https://idronics.caleffi.com/sites/default/files/magazine/file/idronics_33_na.pdf

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EBEBRATT-Ed

The OP mentioned "crawl space" and a 22 " door if I am not mistaken a HPWH won't fit.

CDHE

Chiltrix is already set in place, so that ship has sailed. And I was not planning on cooling the building (shady Colorado site). But one hour without heat on a windy winter's day…yikes! Crawl space is 56" tall. Perhaps a separate electric water heater does make sense.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1804235#Comment_1804235

I think you may be looking at $2- 300. per year for DHW costs with a tank. So , based on your KWH, it may be the best option.

There are some 2 piece water heater heat pump systems. Niles I think is one brand.

If the HP has a desuperheater, certainly use that with any tank you chose to scrub heat into a tank when in cooling mode.

Dave H_2

The biggest factor when selecting an indirect tank to be used with a AWHP system is the surface area of the coil. The larger the better.

The reasoning behind not using a standard indirect tank is that they don't need the surface area because you are working with a large delta T (180F boiler water vs 120F tank).

With a AWHP, you are looking at much lower water temps that the unit can make, so in order to get the heat transfer desired, you need much more surface area

CDHE

This from Chiltrix' site about coil surface area:

 if you select an indirect tank from a different supplier that's fine, just make sure it has at least .35 ft^2 of coil surface area per gallon with at least 1" ID, and can support a backup element

EBEBRATT-Ed

The above is true but it also depends on HW usage