12.04.2010

RV Heating and Cooling, or How I Learned To Stop Worrying and Use a Multimeter

Look, I'm no tech wiz. I've learned along the way to do a bit of electrical and mechanical work myself, but when it comes right down to it, I'm usually afraid to even touch things like "complicated" heating and cooling systems. I mean, the little experience I have with things like replacing starters, water pumps or head gaskets in cars, or wiring up a new electrical outlet, new thermostat, etc - these things are small potatoes when you start thinking about the dangers of working with a live AC unit or something that burns propane. The dangers there are real - explosions, electrocutions, or worse.

In 2009, just as the New Hampshire winter was setting in, my furnace decided to quit on me. The thermostat said "turn on" but the furnace did nothing. I tore apart my couch, pulled the furnace out and started applying low amperage 12 volt to various things that looked like they should take it. Of course, this produced no results. I decided the blower motor must be dead and gave up. I spent a few months running an electric heater to keep warm and quickly found my pipes freezing up.

In December, I picked up and moved to Florida. Problem solved - don't actually need a furnace there.

While in Florida, however, my AC unit decided to quit on me just as the summer heat set in. Thankfully, a long-time family friend was staying in the same campground. He, having been an HVAC specialist a good portion of his life and even teaching classes at a college for it, knew a little something about what to look at and how to test it. He brought over his multimeter and started poking at things with what appeared to be little fear of electrocution. His determination was that the "running capacitor" was shot. I pulled it and ran down the street to get a new one but, after they tested it and said it was fine, I decided that rather than bother him to come over and keep testing for me, I'd get my own hands dirty and bought myself a multimeter.

Best tool investment I've ever made.

I tested this and that. After a few minutes of poking the probes at different terminals, I found that the problem was definitely BEFORE the capacitor, not in it. No voltage ever made it to the capacitor when the thermostat said "turn on". After scouring the internet for info on how an RV air conditioner works, I found myself probing the control board. Lo and Behold - it was a simple relay switch in the control board. $80 and 2 weeks later, I had a working AC again. Thank God as my interior temps were soaring to 100*F almost every day!

This taught me a valuable lesson - if you expect to have a shot at fixing things in your rig, you not only NEED a multimeter, you simply CAN'T be afraid to use it. A multimeter allows you to test for three things that will almost always narrow down the problem - voltage (both VAC and VDC), amperage and resistance (ohms). 9 times out of 10, simply testing for voltage at some connection point will tell you exactly where the problem is.

After moving here to Albuquerque, I knew I had to try to fix my furnace. Winter was coming and temps were already dropping to the upper 30s at night. Since I had found the problem with the AC and replaced a cheap part rather than the whole unit, my optimism was up and I was pretty sure I could find the problem. Firstly, let's outline the basics of how a standard RV heating system works so you can follow along.

1 - The Thermostat:
When the thermostat registers an interior temperature cooler than the requested temperature, it sends DC voltage to the furnace. When it registers warmer than the requested temperature, it stops sending that voltage. It's that simple.

2 - The Furnace On/Off Switch:
I'm not sure if all furnaces have this, but my Suburban does. Two wires come from the thermostat, and one goes through a simple on/off rocker switch. When off or malfunctioning, the voltage from the thermostat will go no further than this switch.

3 - The "Time Delay Relay" / aka "Klixon" Relay:
This relay controls the running of the blower motor itself, turning it on and off based on the DC voltage signal from the thermostat. It allows 12VDC to run from your main house battery/converter to the blower motor when the thermostat is sending a signal, then will actually wait a certain number of seconds before stopping that 12VDC when the thermostat has stopped sending voltage.

4 - The "Sail Switch":
This switch is mounted on the side of the blower motor, with a "sail" piece inside the blower motor itself. It simply monitors the operation of the blower motor. When the blower motor turns on and starts blowing air, the "sail" part of the switch gets blown in a certain direction and the switch turns on, allowing the voltage from the thermostat to flow to the control board. If the blower motor doesn't turn on, or isn't blowing the air strongly enough, the sail never gets pushed to the "on" position, preventing the burner from ever starting.

5 - The Control Board:
This is where all the rest of the magic happens - opening the "orifice" to allow propane into the burning chamber, ignition of that gas, overheat monitoring, etc. I wont go into details because, well, I don't know the details.


Very, VERY simple system, at least compared to what I expected. So how did I test mine?

Firstly, I made sure there was voltage coming from the thermostat. I turned on the heat so the thermostat would tell the furnace to turn on, then I took the multimeter and put the black/negative probe on the 12VDC ground wire and the red/positive probe on the wire coming from the thermostat. Yep - got voltage. That means the thermostat is fine and there's no break in the wire between it and the furnace. Next, I checked to see if the on/off switch was working correctly by doing the same thing, but putting the red/positive probe on the connection to the relay switch, which is the next connection point after the point I was testing. Yep - I got the same voltage there, so that's fine. Then I checked to see if there's 12VDC running to the relay. I put the positive probe on the relay terminal and registered a clean and steady 13.1VDC. When I checked to see if the 12VDC was coming out the opposite terminal, however, I got nothing. Looks like the relay's dead.

I ordered a new relay from the internet (less than $20) and installed it. Turned on the thermostat and on pops the motor. Good. Fixed everything for $20. A new furnace, which I was convinced I would have to buy back in New Hampshire, would have cost over $250.

Moral of the story? Buy a multimeter and learn how to use it, then learn about how RV heating and cooling systems work, especially the ones in your rig. They're much more simple than we realize, and they're often VERY easy, and very CHEAP, to repair.

Afterall, when you're RVing these days, you've gotta save EVERY PENNY you can.

2 comments:

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