Replacing A/C units with Heat Pumps

[BillO]

I have an older coach with 15 year old A/C units and a Webasto diesel furnace (predecessor to Aqua Hot, I believe).

During last winter in South Texas I relied on the Webasto during December and burned nearly 70 gallons of diesel in that single month. After that I picked up a couple of electric space heaters to offset the furnace. While the resistance heaters did keep the furnace from burning up another one-half tank of diesel for the rest of the winter season, they weren't very satisfactory either.

Meanwhile, a Tiffin owner next to me said his heat pump units covered most of his needs with his Aqua Hot only firing occasionally.

Consequently, I'd like to replace my aging roof A/C's with heat pump units to get a little more effective heating at moderately cool temperatures, while still using the Webasto if it gets really cold.

I understand the issues of heat at the ceiling, etc. with the heat pumps and figure I can overcome that.

However, I have a thermostat wiring issue that is a more significant issue. Since the coach was only wired for A/C units it doesn't have an extra wire from the Tstat to the roof-top unit to control the heat pump side of operation.

Carrier made a unit with the temperature control in the overhead unit which would have allowed me to use the existing Tstat wiring to get control back to the furnace, when needed. Unfortunately, Carrier has apparently decided to pull out of the RV business. Neither of the other logical makers -- Dometic and Coleman -- seem to offer any similar option.

Has anyone replaced pure A/C units with heat pumps and overcome this issue without tearing up the inside of the coach to add control wiring?

As a footnote to an already lengthy post I did try to contact RVP with a concept for using one of the fan control wires (involving wiring both fan controls to one wire at the Tstat and separating them at the overhead with a SPDT to move A/C fan control to the overhead). I never got past the first step before the fellow that answered the phone went ballistic and flatly said "impossible".

[tbutler]

It is not quite the same issue but we did have our heat pump replaced on an old motor home. We didn't do this as a do-it-yourself project, we took it to a dealer in Sacramento, CA. We found that the new heat pumps, Dometic, required a new thermostat. That thermostat took different wiring and they replaced our wiring. We never saw any evidence that replacing the wiring had torn up anything. I don't know how they did it as we left the coach with them. It may not be possible with your coach but it was with ours.

[BillO]

Thanks for the reply and encouraging note.

I did look at the Dometic and it looks like they use something like flat telephone or ethernet wiring to connect to their Tstat. It may be that a good technician can snake that through the walls a bit easier than conventional wire.

My coach -- a Vogue Prima Vista -- is built a little differently from some which is part of my concern with getting extra wiring into it. It has a Crown bus frame/chassis with fiberglass walls on each side of the metal cage. Between the two walls the space around the metal frame is filled with structural foam and from all that I can see there isn't any clean wire chase from A/C or Tstat.