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Hydronic heat supplement to Heat pump Home heating System. Anyone have experience putting a heating coil in the ductwork of a forced air heat pump system?

Background. The existing HVAC system is a heat pump with auxiliary electric heat strips. I am considering supplementing the system by installing an Aquecoil HHU-AE-2 water coil (inbuilt circulating pump) to my ductwork, fed from my home water heater. My options are: 1. install using the manufacturer recommended direct connection to my water heater. The possible risk is growth of bacteria in the coil over the summer when the Aquecoil pump is idle. The manufacturer states that normal movement of water in the house over the summer as showers, faucets and appliances are used will induce a current in the Aquecoil loop and prevent stagnation and any growth of bacteria. 2. Install two closed loop systems with a flat plate heat exchanger to isolate the water in the Aquecoil from the water heater.

Existing systems. 1. Goodman 3.5 ton compressor. 2. Coleman Evcon Air Handler AH14 with electric two stage auxiliary heat strips. 960 atr cfm. 3. Gas water heater is Ruud PR75 (75 gallon capacity) and is located hard by the HVAC air handler. The pipe run from the existing house hot water feed to Aquecoil will be a maximum of 5'.

General configuration Heat source using house water heater 1. Output side. T-fixture from existing hot water pipe, ball valve with bleed port, swing check valve, 2. Return side. Swing check valve, ball valve, return pipe to water heater cold water input. 3. Add scald valve assembly to house hot water supply.

Questions. 1. To what extent will this additional heating loop affect water pressure to house appliances when in operation, if at all? 2. To what extent will the Aquecoil affect air pressure in the HVAC ductwork? 3. Open or closed loop system?

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    You can't use potable water for heating loops. So that's a codevio unless somebody's done some really exceptional engineering work. Your first stop is the permit office to see whether this rig can be permitted. You want to litigate that conclusively before you buy any hardware.
    – Harper - Reinstate Monica
    Mar 19, 2017 at 18:56
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    My local planning dept OK'd the Aquecoil configuration and materials (stainless steel and copper/brass fittings) but I am consider ing a closed loop configuration with a flat plate heat exchanger. My HVAC guy confirmed Aquecoil's claim that sufficient convection currents are created by the temperature delta arising from air conditioning in the summer months but I will add a timer and run the pump for a few minutes every night just to prevent any stagnation - or drain the coil every Spring and refill in the Fall. I'll also add an outdoor thermostat to limit the pump's operation to temps <40F.
    – Bendite916
    Mar 23, 2017 at 19:34
  • Did you install it? Does it work well?
    – Samuel
    Jan 11, 2022 at 13:10

1 Answer 1

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Better idea: flip things on their head

You need hot water to run a hydronic heating coil efficiently, far hotter than you'd ever set your water heater to (water heaters struggle to reach 180°F which is the standard temp a hydronic boiler is set to). However, your hydronic load probably pales in comparision to your domestic hot water load most of the time, and your current gas heater is not all that and a bag of chips regarding efficiency (EF=0.50).

So, the solution I propose is to use a hydronic boiler to drive an indirect hot water heater tank in addition to the coil in the air handler. An indirect tank the size of your existing heater with a 150kbtu modulating-condensing boiler can give you practically infinite hot water, will have far fewer standby losses (~350-400Btu/hr for a Bradford White SW-2-80-L based on the AHRI figures of 0.6°F/hr and a 71 gallon potable capacity) compared to the kBtu/hr range standby losses of a conventional gas heater, and will be more efficient than your existing heater overall with a computed EF in the 0.7 to 0.8 range depending on the thermal efficiency you can milk from the hydronic setup (good modcons get into 95-96% thermal efficiencies, and indirect tanks are quite efficient as well), comparable to a good condensing gas tank-type heater.

Atop that, you'll be running the coil in the air handler more efficiently due to the improved delta-T available over what a domestic hot water heater can produce, and you have a solid platform for future desires (such as zoned air handling or radiant heat).

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