dual fuel kit questions

[hq4ever]

A friend has a heat pump with a 80% efficient Natural Gas furnace. All of the low voltage wiring converges at a dual fuel kit that is mounted on a nearby return air duct. I have some questions about how this should work:

1) What is the purpose of the dual fuel kit?
2) Will the heat pump continue to run when the stat calls for auxilliary heat and turns on the gas furnace?
3) I understand that the outdoor unit switches to A/C in order to defrost the outside coil. On regular A/C systems, homeowners are always warned not to run them if the outside temp is cold. What is different with a heat pump that allows it to run as an A/C in very cold temps without damaging the compressor?

Thanks so much for any replies to my questions.

[BaldLoonie]

1. Interface between stat, heat pump & gas furnace
2. Better not
3. Most HPs have accumulators to catch liquid floodback, crankcase heaters, plus it isn't in defrost long. Defrosts are hard on the heat pump, no doubt about it.

[hq4ever]

Thank you for your reply Baldloonie.

One more question then...Since a regular heat pump stat energizes Y and W when bringing in auxiliary heat, is it the dual fuel kit that makes sure to that the heat pump doesn't run? Is that the kit's only function? If so, seems like there could be a setting on the stat to not energize Y when there is a call for aux. heat. Then you could get rid of the kit.

I'm really just trying to understand what this kit does. My friend has had so many problems with her system, and the HVAC service people sent out from the waranty company have just been changing parts (including 2 compressors and then the entire outdoor unit!).

Thanks again.

[BaldLoonie]

Depends upon the kit. Most HP stats energize only Y. If they energize W, that would be jumped to Y. Most kits if they get a W2 signal, the call for backup heat, know to shut off the heat pump when energizing the fire. Some kits and the integrated stats like the Vision Pro, don't bring on backup. It's either pump or gas depending upon outdoor temp.

[Black Adder]

Used to use fossil fuel kits, but since the advent of the "Vision Pro" tstats, we just use an outdoor temp sensor to shutdown the H/P and bring on the furnace at lower outdoor temps. Has really made our job much nicer and I find the tstat is more reliable.