HW Reheat Coil

[MechGuy22]

Hi Everyone.

I am adding a chilled water outside air unit to a exisitng project. This unit will handle around 3500 CFM of outside air. The unit will be equipped with a hot water re-heat coil from the factory so all we need to do is get some hot water to it. The problem is that the only hot water available is from a large bank of exisitng water heaters which is being used for potable domestic water. I am really hesitant to circulate water from the hot water heaters directly to the fan coil becuase it is potable water so I was thinking of running it through a heat exchanger. Has anyone on here used potable domestic water for reheat and does anyone know of a good heat exchanger for this purpose?

 

[DRWeig]

Well, my first thought is "more info please."

No, do not use potable water. Someone may correct me, but I think you might need a double-wall heat exchanger to keep from contaminating the potable water in the event of a tube failure.

Have you calculated that the bank of water heaters can meet the heating load at design outdoor conditions?
The water heaters are set for what temperature? Your heating coil is rated at how many BTUH at what water temperature?

Best to you,

Goober Dave

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[MechGuy22]

Thanks DRWeig

This is really conceptual at this point, I still have to go through and fixture out GPM etc. I am looking for a really small temperature rise and plan on delivering the air directly to the space.

 

[willard3]

Water heaters are not boilers.....the safeties and controls are different.

 

[macmet]

I believe Goober Dave is correct about the double walled heat exchanger. When I was in the district energy world any HX used for potable water had to be double walled. I never did look into whether that was a company policy or a code requirement, but seemed reasonable to me at the time.

 

[MintJulep]

Don't do this kludge.

The cost of the heat exchanger, circulation pump, valves and controls that you'll need won't be that much less than just buying the small proper heating service boiler that you'll need.

Or get an air handler with electric or gas-fired reheat.

Or even better, don't bother with reheat at all. Do you really need it?

 

[sprinkler1000]

You talk about existing water heaters.

What is the primary source of heating? Are they electric?
If so, and you dont have primary water heating, consider installing electric resistance instead of the reheat coil in the air handler.

 

[317069]

from where the existing water heaters get their water, there should be a cold water line to feed them, if you want to use a boiler, you can get a branch from there.

 

[MechGuy22]

So I found a diagram on the manufacturers site that depicts the HW coil being served from the water heaters and the same water heaters serving domestic water. We just discussed it internally and decided to go without the reheat but this has served as a learning experience. I appreciate all the opinions. Let me know what you guys think of the diagram though.

 

[NegativeZero]

that is for a small residential ac unit with reheat. I think its insane.

 

[Drazen]

Ah, the schema you presented should be banned...

 

[ChasBean1]

To put it in practical matters, if you want to cool and dehumidify 3,500 cfm in a 91°F db / 74°F wb environment and then reheat 10°F (I’d suggest 15-20°F for design reheat), it would be about 37 MBH of reheat. Compare that with your domestic water heaters’ capabilities.

 

[cdxx139]

or make it a DX unit packaged unit and add a hot gas reheat coil.

knowledge is power

 

[EnergyProfessional]

Running domestic water through the coil will corrode it. normally the boiler water is a closed loop with de-aerator, chemical treatment etc. for that reason.

However, if you use a proper heating loop, I believe a double-wall HX is needed (you don't want those chemicals in the drinking water in case of a leak).

If you have very little heating load, your tankless water heater may cycle too often depending on how low it can turn down.

 

[Drazen]

if he has no load at air heater side, dead part of circle could become terrifying legionella nest in case of connecting piping being of any length. he could solve it by introducing sanitation cycle into this part of circle, so in cooling season occupants would have to take off all clothes quickly during each sanitation cycle.

that is one exotic scenario that first comes to mind, but there are likely others as well.

 

[EnergyProfessional]

Drazen: really good point I haven't thought off.

I read some older residential HVAC books that recommended the "stealing heat from waterheater method". Obviouly the authors of thsoe books must not have ahd any clue about codes.