HW Reheat Coil
HW Reheat Coil
(OP)
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?
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?





RE: HW Reheat Coil
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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RE: HW Reheat Coil
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.
RE: HW Reheat Coil
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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?
RE: HW Reheat Coil
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.
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knowledge is power
RE: HW Reheat Coil
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.
RE: HW Reheat Coil
that is one exotic scenario that first comes to mind, but there are likely others as well.
RE: HW Reheat Coil
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.