Weird RV Air Conditioning/Heat Pump Project

[Ted]

I decided to make a separate thread for this to hopefully get the attention of folks who know more about HVAC but may not read my RV thread.

The RV has three air conditioners - two roof mount units (which are also heat pumps) and then the driver's area air conditioner, which is engine driven (or at least was). Because the engine is in the back, the system is complicated and not a good design. One of my lines failed last year, so I removed the 80-some feet (total) of AC hoses and vowed to make a better design.

The convenient part about having an RV is that I have a generator, an inverter, and essentially a whole lot of electricity available. So I've ordered a NOS R22 115VAC compressor rated for 13k BTUs. My plan is to mount it up front (near the evaporator core) and then run some much shorter lines, a new receiver/dryer, some 115VAC wire from an outlet that's already up front in that area, and then have electric AC. We tend to run the generator while driving anyway as we use a lot of electricity going down the road (we usually need the roof top AC/heat pumps going for comfort plus we're often running the washing machine, microwave, etc.).

Although the compressor is rated for R22, it should work fine with R134a, just perhaps a bit less efficient and I'll need to put the correct oil in. I doubt this should work much differently from standard R12 to R134a conversions, which I've done many of over the years.

One idea that came to mind was adding a reversing valve so that I could use this as a heat pump as well as an air conditioner. This would be nice as the heating up front is not "great" by any stretch, largely because the 40' worth of heater hoses end up making the coolant lose a lot of heat on its way to the driver's area.

The biggest question I have with this (maybe someone who knows more than I do about HVAC can answer) is how would an automotive expansion valve handle a reverse flow for a heat pump setup? I'm not sure whether it would still work in the same sort of manner or if it would get weird. Any thoughts?

 

[-KLB-]

So do you even have an expansion valve, or is it one of the orifice tube type systems? A lot of vehicles went to a combination orifice tube and two stage strainer. Reverse flow might just blow them out of their holder and into the tube that is normally upstream, while dislodging any brass shavings and sludge that the strainers had caught.

 

[tspear]

And here I thought @Ted was starting a home built plane and was going to out in a heat pump. It made no sense...

Tim

Sent from my HD1907 using Tapatalk

 

[Ted]

So do you even have an expansion valve, or is it one of the orifice tube type systems? A lot of vehicles went to a combination orifice tube and two stage strainer. Reverse flow might just blow them out of their holder and into the tube that is normally upstream, while dislodging any brass shavings and sludge that the strainers had caught.

Good question. Yes, the setup has an expansion valve. I had expected it to have an orifice tube, but it's a proper expansion valve.

 

[flyingbrit]

One idea that came to mind was adding a reversing valve so that I could use this as a heat pump as well as an air conditioner. This would be nice as the heating up front is not "great" by any stretch, largely because the 40' worth of heater hoses end up making the coolant lose a lot of heat on its way to the driver's area.

It may not be as "simple" as adding a reversing valve. A problem with heat pumps is that the outside coil will often be below freezing, so frost forms. If there is too much frost the air flow and efficiency go way down. So a typical heat pump will have a defrost cycle where the unit is put in cooling mode so the outside coil heats up and melts the frost. This requires a) something to sense frost build up, e.g. an air pressure sensor on the down wind side of the outside coil plus b) auxiliary heat to override the cooling of the inside coil and prevent cooling when heating is desired.
That said, my experience is with residential heat pumps. No idea if RV units work the same way.

 

[Ted]

It may not be as "simple" as adding a reversing valve. A problem with heat pumps is that the outside coil will often be below freezing, so frost forms. If there is too much frost the air flow and efficiency go way down. So a typical heat pump will have a defrost cycle where the unit is put in cooling mode so the outside coil heats up and melts the frost. This requires a) something to sense frost build up, e.g. an air pressure sensor on the down wind side of the outside coil plus b) auxiliary heat to override the cooling of the inside coil and prevent cooling when heating is desired.
That said, my experience is with residential heat pumps. No idea if RV units work the same way.

That's a very good point, and I think that right there makes the idea unworkable (at least in version 1.0). I am going for a setup with minimal controls. I can confirm that RV heat pumps (like our rooftop ones) will do the cycle you mention, just like home ones. I should've thought about that.

Back to normal AC it is, which will be just fine. For extra heat in the winter it's simple enough to just put a little space heater near the driver's footwell (we already have several) and power that off the generator for extra heat beyond what the coolant-derived heater can do.

 

[tspear]

@Ted

Why not install one of the RV ceramic heaters? They are likely much safer.

Tim

 

[Ted]

@Ted

Why not install one of the RV ceramic heaters? They are likely much safer.

Tim

Admittedly we haven't put much effort into the heating aspect, but a tiny space heater up front while I'm driving is sufficient and I'll know if it sets my leg on fire. The rest of the coach (thus far) has always been kept comfy with the heat pumps while driving. However, we likely will go for something better/something else next winter if we find ourselves doing some more trips.