Roof Air

[john dolan]

Lanier...

I'm not an expert on this, just an observer...
Hope you get someone who knows for sure to help on your question, but
keep this factoid in mind: I have two identical home air conditioning
units out in the back yard. When the guy comes to service them and
top them off with freon, he always says, "Hmmm this one here on the
right has high head pressure... Well it's going to have to run thataway".
The only thing different about the two units is, his company and probably
he himself, replaced the start/run capacitor on the one on the right.
The original capacitor was a dual 10uf/15uf. The replacement is a single
20uf. Don't know if it is causally related, but thought I would mention
it.

I didn't like that mismatch and tried to get an exact replacement. They
don't make them anymore my supplier says. My air conditioner guy gets
his electric stuff there at the same supplier, I found out.

John

>
> What can be done to a roof air unit that has too much head pressure? Mine is
> the original 73 model, never been tapped, and in hot weather develops too
> much head pressure and kicks out the Onan - kills it dead and it won't
> restart. Also has blown bridge rectifier and Onan circuit breaker.
>

>
> What can be done to a roof air unit that has too much head pressure? Mine is
> the original 73 model, never been tapped, and in hot weather develops too
> much head pressure and kicks out the Onan - kills it dead and it won't
> restart. Also has blown bridge rectifier and Onan circuit breaker.
> Lanier

 

[mark grady]

Lanier, here are some ideas --

1). Make sure that the condenser fans are running and moving air. A dragging
bearing will reduce airflow. Low airflow means higher head pressure and
higher current demand. These Duotherm's are kinda' noisy anyway, so make
sure all the fans are running. Listen on the outside and put your hands over
the condenser to make sure you can feel the air flow.

2). Find a good commercial/home A/C service guy, or a really good appliance
repair guy that does window A/Cs. Get some "coil cleaner" from him. This
will remove the buildup of oil, road dirt and other crap that has gotten
stuck in the condenser core over the years.

CAUTION - Good coil cleaner is some wicked stuff. Be very cautious about
'run-off' as it may streak (or remove) the paint on the coach. This product
is an effective de-greaser. The oily/dirty film on the condenser reduces the
heat transfer efficiency.

As an alternative, take top cover off the unit, take the condenser fan
assembly loose and use a pressure washer (or car wash wand) and wash the
condenser out backwards. Try not to water the electrical stuff. It won't
grow anyway.

3). Make sure your Onan is producing 120V under load, and that you don't
have too much voltage drop in the circuit feeding the Duo-therm. Voltage
goes down, amperage goes up, Onan goes off-line. Measure the Onan output
voltage under load as well as with no-load. Keep in mind during this process
that life is full of compromises.

4). You didn't state, but if the Duo-therm kicks out the Onan only when it
tries to re-start, consider getting new compressor start (and run)
capacitors installed. A bad start capacitor can cause higher current draw
when the compressor tries to re-start. Again, a decent appliance repair or
A/C guy can help you out with this problem.

I don't have the PN for the capacitors Duotherm used, but they shouldn't be
hard to find as a replacement item. Just match up the value and physical
size. The voltage rating on the replacement can be higher.

We've ran both the front and rear together on our '77 before on the Onan, so
it's possible from a design perspective.

__/ __/ __/ __/ __/ __/ __/ __/ __/ __/

I make this last comment in a guarded way... so as not to annoy the Onan
gods of light --

Do you have the same problem when the Duotherm is plugged in to shore power?
(i.e. trips a circuit breaker on the shore power cord or GMC breaker panel?)
If you don't have a problem with the Duotherm on commercial power.... it
could be a problem with the Onan itself. I'm hoping that isn't the case.

We all know there is only solution to that problem. Go to the closest
Onan/Cummings service center with a drug-dealer sized roll of C-notes as a
token offering of peace. Repeat as necessary or until your bankroll is
eliminated. I'm not an expert on low output from the Onan. (Or much of
anything else, now that I think about it.)

You may have to wait on Duane's return for those questions.

Mark

> -----Original Message-----
> From: owner-gmcmotorhome
> [mailtowner-gmcmotorhome]On Behalf Of HLBF
> Sent: Saturday, May 01, 1999 8:14 PM
> To: GMCmotorhome
> Subject: GMC: Roof Air
>
>
> What can be done to a roof air unit that has too much head
> pressure? Mine is
> the original 73 model, never been tapped, and in hot weather develops too
> much head pressure and kicks out the Onan - kills it dead and it won't
> restart. Also has blown bridge rectifier and Onan circuit breaker.
> Lanier
>

 

[dave1]

>What can be done to a roof air unit that has too much head pressure?
Mine is
>the original 73 model, never been tapped, and in hot weather develops too
>much head pressure and kicks out .....

Check to be sure that there is good air flow through the condenser coil
(the hot one - outside). If its blocked with bugs, fuzz or other dirt, it
will not cool the freon properly and will cause higher head pressures.

Dave
73 Sequoia

 

[russ bethel]

Lainer,
If the air unit has never been tapped, how do you KNOW the head pressure
is too high? I would think you would have to put a set of gauges on the
unit to read the head pressure and that would require the high pressure side
to have a tap on it.

Russ Bethel
rbethel

- -----Original Message-----
From: Dave
To: gmcmotorhome
Date: Monday, May 03, 1999 10:43
Subject: Re: GMC: Roof Air

> >What can be done to a roof air unit that has too much head pressure?
>Mine is
> >the original 73 model, never been tapped, and in hot weather develops
too
> >much head pressure and kicks out .....