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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: The End Approacheth
Date: Tue, 05 Aug 2008 17:25:15 -0400
Message-ID: <>

On Tue, 05 Aug 2008 11:41:17 -0500, Bob Giddings <> wrote:

>If you use a half gallon per hour on your genny - just guessing,
>so fill me in here - that equates to maybe a half mile per gallon
>penalty on your mileage.
>That's doable if it really works.  I noticed when we started out
>like that in the Lazy that the room air wasn't really penetrating
>down into the forward cab (this was when the dash air wasn't
>So maybe a strategically placed fan to force the room air into
>the cab would be necessary.
>Maybe this is a workable solution.  This thing has a GOOD rooftop
>AC.  It's not like you'd do it every day, unless you like to
>cruise the southland in the good ole summertime.
>Whattaya think?  Anybody got any relative figures on using the
>roof air while traveling?

I run my roof air a lot when traveling for two reasons.  One, when I get tired
or hungry and want to nap or snack, I don't want to have wait for it to cool
off back there.  In the case of napping, I don't want my bed to be heat-soaked
which makes me sweat.  Second reason is that I like the cool breeze on the
back of my neck.  One of my several generator control panels is right over my
head so I just reach up and start the genny a half hour or so before I'm ready
to stop.  Or when I want to give my neck a blow job :-)

I don't know what my Generac's fuel consumption rate is but I'm about to
measure it as part of my battery powered AC project.  According to the book,
from 3/8 to a half a gallon an hour with the load involved.  Mine's an
inverter generator which really shines at low load.

Here's a little trick that will GREATLY improve the roof air operation while
underway and help a little while stopped.  Replace the condenser fan blade
with one of the opposite pitch.  This causes the fan to suck from the side
vents and blow out the back of the unit.  A couple of air scoops, one on
either side, grabs the air flow and rams it in, probably supplying more air
than the fan itself can.

What got me to this place was that at certain low speeds, in the 35-40 mph
range, my roof air would gradually start drawing more current (as indicated by
the generator speeding up) until it tripped the breaker.  I'd pull over, reset
the breaker and everything was fine.

It took me a long time and a lot of work to find out the cause.  Originally,
the fan sucked air from the back of the unit through the condenser and out the
side vents.  At a certain vehicle speed, a low pressure area would build
behind the unit and coupled with the little bit of ram air effect the side
louvers provided, would prevent the fan from moving any air through the
condenser.  The condenser would heat up, the freon pressure would rise and the
compressor amp draw would increase correspondingly until the breaker tripped
or occasionally the klixon in the compressor would trip from over-temp.

Changing the fan pitch so that the condenser fan sucks through the side
louvers and exhausts into the low pressure area behind the unit not only
solved the problem but made the unit cool a LOT better.

I simply took the fan down to my friendly local electric motor shop and told
my friend that I wanted a fan just like that one but with a reversed pitch.
About $20.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Air conditioner woes.
Date: Thu, 18 Sep 2008 00:58:54 -0400
Message-ID: <>

On Tue, 16 Sep 2008 13:40:44 -0400, John H. <> wrote:

>>Was the AC oriented so that the sun was shining on the fins?
>That I don't remember. Didn't notice.
>>Was any wind blowing?  If so, from what direction relative to the RV?
>Very little. Lots of trees.
>>Do you know what the incoming voltage from the park was?

OK, here are my thoughts.  It may be low park voltage but not likely.

I think that the compressor tripped on high condensing pressure.  Here's my

Refrigerant 22, what is used in these ACs has a fairly low critical
temperature (153 deg if my memory holds.)  That is, above that temperature the
refrigerant cannot become a liquid regardless of the pressure.  The R22
replacements have even lower critical temperatures.  When that temperature is
approached, the pressure and thus the load on the compressor skyrocket.

A unit with a small condenser such as an RV AC runs a fairly high condensing
temperature, usually 20 to 30 degrees above ambient.  That means that on a 95
degree day (probably 100+ on the roof), the condensing temperature is already
at from 120 to 130 deg.

If the RV is arranged so that direct sunlight hits the condenser fins, it can
raise the temperature close to the critical temperature.  Even if the critical
temperature is not reached, the pressure rises rapidly as the critical
temperature is approached.  The compressor's current draw is roughly
proportional to pressure and so would increase.  Perhaps enough to trip the
breaker if the breaker is properly sized.

Another effect has to do with the wind.  Normally, the AC draws air in from
the rear and discharges it out through the side vents.  If the wind speed and
direction is just right (wrong?) it can slow or stop the condenser air flow.
The temperature and pressure rises until the breaker trips.

I've experienced the sunlight effect several times, to the point that I pay
attention to the sun's path as I choose where the park.  I have also
experienced the wind effect.  I don't think that the wind effect applies in
your case, though, as it doesn't sound like the wind was blowing hard enough.

There is another possibility, given that there were lots of trees in the area.
If the tree limbs were down close to the top of the RV, the leaves could have
held in much of the hot air discharge from the condenser, causing the hot air
to recirculate.

I've encountered this problem many times with home AC units where the customer
has planted shrubs tightly around the unit to hide it.

In any event, I don't think that there is anything wrong with your unit.  I
think that pathological outside conditions caused it overheat and trip.

I'll mention one more effect that can happen to a MH when the AC is running
while underway.  This hit me hard in Lubbock TX one miserably hot summer day.

When moving, a low pressure area builds up behind the AC unit while the air
velocity passing by the sides raises the pressure.  At a certain speed range,
the pressures become such that the fan can no longer suck air from the low
pressure area and discharge it out the sides where the high pressure flowing
air resides.

In my instance, it was over 100 deg, the dash air wasn't doing nearly enough
and I REALLY needed the roof air's assistance.  It would run for awhile and
then the generator would load down and almost stall.  Wait awhile, restart and
repeat.  No problems when driving at interstate speeds or when stopped.

After scratching my head, instrumenting the AC with a differential pressure
gauge and driving around, I found the problem.  My solution was to fit a fan
blade of the same size but opposite pitch.  Now the AC sucks air in from the
side vents and discharges out the back.  Forward motion actually improves the
air flow and AC operation.  Especially since I fabricated a couple of little
sheet metal air scoops to the side vents.

If this happens again, reading the condenser temperature will give you a good
clue, as will noting the orientation of the sun.


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