[Trainer]

A Foretravel Forum member has a unique basement A/C system on his older coach.  His board is fried such that a board rebuilder has pronounced it un-repairable.

The following was posted in response to his request for help and ideas.  Hope you all will find it interesting if not indeed helpful.

"Tyler, I owned a commercial HVAC company for years. We serviced transport refrigeration as well. I am (unfortunately) very familiar with overly complicated control circuits.  

Basic AC systems are very simple. First, the components have to work; meaning the compressor has to run and compress the refrigerant, the condenser fan (outside) has to run and move air, and the evaporator fan (inside) has to run and move air. The compressor will have capacitor and/or a potential relay that will fail. The fan motor may have one as well. These can and should be checked and replaced if needed. I usually replace them anyway since the get weak with age, are inexpensive, and will fail at the most inopportune time.  

Next you need a control circuit. The simple circuit is a contactor (relay) to actually connect 120V to the components upon demand, and a thermostat to control to contactor. Typically, the compressor and the condenser fan are controlled by one contactor and the evaporator fan by a second one. This allows the evaporator fan to be on for circulation without cooling. 

In your case, I would buy one double pole 20 amp contactor for compressor and condenser fan, one single pole 20 amp contactor for second compressor, and one single pole 20 amp contactor for evaporator fan. All should have 12V coils (for control voltage). You then need a 2 stage cooling battery powered digital thermostat. You will have to pick up 12V to power the control voltage circuit. You have the original wiring schematics so that should not be a problem. Once installed, you have a simple control circuit with readily available components.

TOM"

[Guy]

That is some great advice. It is almost exactly how our basement air units are controlled in our 1984 Newell. I recently had to replace a 2 pole GE mechanical relay at a cost of 16.50 on Amazon. Our control box has a 12 to 24 volt control transformer, meaning our relays have 24 volt DC coils. I don't know Newells reasoning for this, but it has worked well in all three basement control boxes for 35 years.

[Richard]

Yep!

[77newell]

I thought there was one additional function of the control board beyond just starting and stopping motors as described. On my basement ACs, when the thermostat calls for cooling the evaporator fan immediately tons on followed 3 minutes later by the compressor and the condenser fan. I had understood, perhaps wrongly, that the delay was necessary to prevent a compressor problem due to trying to start the compressor against a high outlet pressure that is caused by possible liquid refrigerant trapped between the compressor and evaporator if the compressor is started too soon after stopping. Otherwise there would be no need for control boards in single thermostat single compressor systems, and yet everyone of those I own, rv or house, has this time delay. What am I missing?

[Richard]

Jon,

When I built a prototype alternate board I discovered that the delay function is in the thermostat logic. The tstat sends separate signals for the the evap fan, cond fan, and compressors. The one situation that would require some noodling is how to get the cond and evap fans to run at two different speeds depending on whether one or two compressors were running. I ended up using six relays instead of four to get high and lo fan speeds. I need to do some research and see if the tstat power supply can power the relays directly.

Also to Guy's comment. House systems are 24V, and of course much more common. They may not have been a 12V tstat with the functions Newell wanted available when your coach was built, so it would have been pretty easy to run a transformer and use a 24V volt system.

[The Soft Boulders]

Hello everyone,

The coach in the OP is ours, the air conditioner is a RVAC model 2702 and the board is a 305-2547. Apparently our coach is the only one that Foretravel built to have the unit and the basement air system. The unit was also set-up to run off of a pair of engine mounted compressors and get power from the 1500 watt inverter to run the fan motors so the generator did not need to be ran while driving. Although I believe that system has been removed there are references to it in the schematic.

I would like to try and keep the system and not go to roof units if possible and am trying to see what the options are for repairing the unit we have. We thought that replacing the board was the best route but have learned that they are quite scarce. Then we thought repairing ours would be the next option but were told that our board was too baldly burnt to fix.

A few of the folks on the Foretravel forum have suggested that building a sort of control unit that would replace the phb is possible and that it may not be too difficult to achieve. I have a basic understanding of electricity and components but am by no means fluent. I have been reading around on this forum and have found a lot of excellent information but didn't want to post until I felt a little more knowledgeable about what the Foretravel folks were suggesting.

Thanks to everyone for your help and thanks to the OP for taking the time to start a thread for us.

[Richard]

Where are you located? I may be able to help.

[The Soft Boulders]

We are in Tulsa, Ok

[folivier]

Welcome Soft Boulders! Hopefully you can get this sorted out, Richard can be very helpful.

[Richard]

Ok. The rvac 2702 is what we have. Your comments about engine mounted compressors have me concerned since the 2702 is hat we have has no provison for that. It does have a provision for running off an inverter.

What kind of thermostat do you have?

Can you wait until October? We are on our way home from the PNW.