Dual Fuel System

[Ralphs1]

Is there any reason the evaporator cannot be put on the return side of a 90% gas furnace to allow a Heat Pump to operate down to economic balance point. Then staging 1st or 2nd stage of the gas furnace simmilar to a air handler with resistance. There are no more pilots so the heat exchanger should not sweat durring A/C operation.

 

[rogzog]

Just a thought... you should insulate those ducts and plenum. Interesting setup.

 

[sterl]

You base plenum might encounter some condensation during AC season, right after the thermostat is satisfied...You might want to run the fan a few seconds after.

nice setup

 

[Ralphs1]

Fan runs all the time for the air cleaner. So far the system has operated great. We will see how it works durring the summer. Hopfully the heat exchanger will not sweat. Still tring to get mfg. to approve but they have not told me no yet eather.

 

[tbedford]

Replying from Vancouver Island, west coast of Canada. In our temerate climate, I see many installations as shown and I am happy to reply based on my experience. Success depends primarily on correct sizing and installation of components.

Coil location needs to be downstream of the gas heat appliance in the remote chance of simultaneous operation of both in heating mode. The scenario (coil upstream of gas HX) drives the refrigerant pressures high, causing potential damage and expensive repair tom the heat pump. Programmable thermostats c/w dual fuel controls are often used to prevent this. Verify fan amps at 100% fan speed and compare to measured total fan static pressures.

All downstream duct needs to be insulated to prevent condensation during cooling season...even in a cool basement

The photo appears to have a common drain between the combustion condensate and the A/C condensate. This can allow products of combustion into the supply air stream under certain conditions. For the peace of mind and the inexpensive price of PVC, the drains and p-trap should be separate. Remember also, during heating operation, the heat pump will periodically go into a defrost mode (air conditioning without the outdoor fan working).

If the fan motor is a variable speed, there needs to be a supply sensor or other control interlock to ensure approx. 400 cfm per ton in the AC mode. The manufacturer will have this in his control scheme somewhere. Please verify.

In order to obtain the high SEER's noted, the installation also requires a thermostatic expansion valve and therefore the compressor requires a hard start kit. This is not seen in the photo, although it may be in the coil cabinet.

Hope this helps you,