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Hot water coil placement

nhbean | Posted in Mechanicals on

I am looking at adding a hot water coil to our heat pump system for supplemental/emergency heating, and would like advice on size and placement of the coil.  We are in climate zone 4, with a design temperature of 5 degrees F and a Manual J heat load of just over 21,000 kBtu/hr.  I have a Westinghouse 95% natural gas tankless water heater with a built-in pump (WGRGHNG150) that can be used to supply the coil – 150,000 BTU with a 10:1 turndown ratio.  There would be about 60′ of pex piping to run from the hot water heater to the coil and back, which I believe would put the pumped supply around 2GPM. 

Currently the HVAC system is composed of:
– A 2 ton variable speed Bryant air handler (FE4ANB002000)
– A 3 ton variable heat pump (288BNV036000)
– The evolution controller (SYSTXBBECC01-A)

It’s split into four zones. The system-reported static pressure is 0.28, and the available CFM range is 300-1050.  The location of the air handler means it has a very short plenum – the first takeoff is only 16” downstream of the air handler.

The options I am considering are:

1. Adding the coil in the plenum upstream of the air handler. This would mean a small coil (the plenum starts at 11”x16” and transitions to 11”x21” that would be very close to the takeoffs).  

2. Adding a 20” x 20” return plenum and placing the hot water coil into there.

Option 1 is the recommended placement, but I am concerned about the already-short plenum and not getting enough delta T from the smaller coil to keep the hot water heater in condensing mode.  Also, the hot water heater will run a cycle every 6 hours to prevent stagnation in the coil, which means we’d be dumping some heat in the HVAC during the cooling season.  But the heat pump and hot water coil could operate in tandem. 

Option 2 would mean locking down the heat pump while the hot water coil is in use (to avoid pressure issues in the heat pump exchanger).  However our economic balance point is so far tilted towards natural gas that it is always the more economical option.  I would probably do 4’ of return, the hot water coil, and another 4’.  In each of the 4’ sections I would cut in a 20×30 filter return.  For the heating season I would put a solid sheet in the close filter box, so the the draw would be across the coil, but in cooling season swap it for a prefilter, effectively bypassing the heating coil.  I would probably use a full 20×20 coil, which as I understand it, would result in a lower delta T.

I’m leaning towards option 2, but would like to solicit feedback before I move ahead.

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  1. Expert Member
    Dana Dorsett | | #1

    If it's only for backup heat you wouldn't really care if it stays in condensing mode, but you DO care if the return water is warm enough to cause the tankless sense it as an error, spit an error code and stop, which isn't uncommon for tankless water heaters. These things are NOT designed to be hydronic boilers, though they can be used that way if you know what you're doing. Read the manual VERY carefully, and talk to tech support before embarking on this path. Odds are pretty good using it for space heat will void the warranty.

    >"But the heat pump and hot water coil could operate in tandem. "

    Not really- the operation has to be either heat pump OR hydronic coil, not both. To have any efficiency the heat pump's incoming air temp at the coil has to be under 80F, well below the output temp of a hydronic coil. If the hydronic coil's incoming air temp is too high (which it would be if the heat pump were running) it won't be able to emit enough heat (even at the max temp of the water heater), causing the tankless to short cycle (or just spit an error code & stop.)

    A 3-5kw auxiliary resistance heat strip downstream of the heat pump coil is probably the better bet as a backup. Heat strips CAN run in tandem with the heat pump (it doesn't matter how warm the incoming air is out of the heat pump in to the strip), and don't suffer efficiency hits or damage even when short-cycling. A 5kw strip is ~17,000 BTU/hr, a large fraction of your design heat load, and would be enough backup to fully heat the house on it's own more than 95% of the time as well as covering any shortfall during record breaking cold snaps.

    The Bryant air handler is probably already set up to control auxiliary strip heat too.

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