From: John De Armond
Subject: Re: RV Roof A/C units
Message-ID: <393F12B8.DD6BBA3B@bellsouth.net>
Date: Wed, 07 Jun 2000 23:27:53 EDT
Newsgroups: rec.outdoors.rv-travel

"GEORGE R. COONEY" wrote:
> 
> Anyone know where I can get some ideas as to what might be causing
> my RV Roof A/C units to dump a 20 amp breaker when the air temp
> gets up a little over 100 degrees? The front one started it a couple
> of years ago and I measured the current draw. It started out at 15
> amps and slowly climbed up to 20. The rear one held steady at 15
> amps no matter what the temp. Now the rear one is doing the same
> thing. I swapped all the caps, relays, etc when the back one was
> working ok and that had no effect. At one time I thought maybe it
> was an overcharge because someone has put valves on the front one.
> The back one has no valves. But now that the back one has started
> doing the same thing it can't be an overcharge. Any ideas appreciated.
> Thanks.

If you're where the sun is shining on the condenser (Arizona, duh!),
the behavior is probably normal.  Once the condenser temperature
reaches 150 degrees or so, an R-22 system is pretty much lost.  The
condensing pressure is so high the compressor can't deal with it.
The critical temperature for 22 is about 205 degs.  The pressure
rises dramatically as the temperature approaches the critical temp. 
Here's an example.  At a condensing temperature of 120 degrees
(pretty normal around here for 90 deg weather), the pressure is 260
psi.  At 150 degrees, the pressure is 382 psi.  Try to contemplate a
single stage compressor trying to pump 70 psi gas all the way up to
almost 400 psi.

This is why R-22 is never used in automotive systems.  I was quite
surprised to see it in roof-top units.

The only way to know whether this is the problem is to measure the
condensing temperature and pressure if there's a test fitting.  A
thermocouple stuck in the fins near the hot gas inlet will do.  Even
better is a thermocouple stuck to the hot gas tube headed from the
compressor to condenser.  A small round dial-type thermometer as
used to check food temperature will work in a pinch.

If your condensing temperature is much above 120 degrees, there are
several remedies, all aimed toward keeping heat off the condenser.

1) make sure the fan is running at full speed.  As the fan motor
bearings wear out and the rotor gets out of center in the stator, it
will slow until it finally stalls.  You pretty much need some sort
of optical tach to determine this.  Measuring the fan current draw
and comparing to nameplate will give some indication.

2) clean the condenser.  Not only will more air flow but the natural
finish of the aluminum will reflect off heat better than the dark,
dirty surface.

3) Shield the condenser from direct sunlight.  The Maxxair
replacement cover does a fairly decent job of this by interposing a
white plastic grill in front of the condenser.  Some sort of
improvised shielding is also possible.

4) since someone installed a service valve, the unit could be
overcharged.  There's an easy way to check.  During normal
operation, there should be liquid freon in the last row or two of
condenser tubing at the bottom but no higher.  Liquid freon conducts
away heat much faster than vapor.  Therefore to figure out where the
liquid level is, simply heat the "U" turns on the condenser for a
few seconds with a propane torch and then feel the tube.  A tube
containing liquid will not have heated.  A tube with gas in it will
be hot for a few seconds before the flow cools it again.

I kinda doubt that it is overcharged if it's working OK otherwise. 
I suspect that it is simply overheating.

John