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Roof A/C units while driving

June 21, 2011, 02:16:36 am

Returning to Houston from New Orleans our dash air went out. Running both roof A/C units would not keep the interior cool. We noticed when we stopped the air coming out was colder.  At slow speed and stationary the output was cold, as the motorhome accelerated, the output temperature rose. Is this a function of the design or do I have another problem? 

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Re: Roof A/C units while driving

Reply #1 – June 21, 2011, 07:42:04 am

You may have a vacuum draw on the units when in motion affecting their performance. If you are mixing outside air instead of subcooling the inside air this would cause this.
Also, if you have a pressure difference between the inside of the coach and the outside this can cause you to loose cooling.

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Re: Roof A/C units while driving

Reply #2 – June 21, 2011, 08:49:14 am

Be sure the dash HVAC is set on MAX/Recirculate.  If on any other setting, the forward motion of the coach, even if the fan is off will force huge amounts of hot, humid air into the interior.  No A/C can keep up with that.

You can also put a small fan on the floor just behind the driver's area blowing forward.  That helps a lot.

Brett

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Re: Roof A/C units while driving

Reply #3 – June 21, 2011, 09:05:18 am

Outside air can still come in through the dash if the controls are set for that even though the compressor and fan may be turned off.  Even if the dash air diverter doors are set to exclude outside air some can still get in if an overhead vent is open.  Hard to close off the kitchen exhaust fan, and maybe a good thing to have a little fresh air coming in.  The 3 dash air diverter doors will not operate properly  if the relay board is defective, I had that problem, got a new board from FT.  If the roof air units work correctly when stopped, there it must be an air flow problem when driving.  The Coleman polar units draw air in from  the back and out the top for the condenser, most other units pull air in from the sides and out the back.  Would the Polar type be less efficient when driving?  Could the heated air coming out the top recirculate through the condenser?  I have never had a cooling problem with my one old Coleman Super Mach and one Dometic Penguin when driving. 

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Re: Roof A/C units while driving

Reply #4 – June 21, 2011, 09:15:53 am

The dash air was selected to Recirc and we had a fan on the floor. The temperature in the front rose to nearly 90F. It did not appear that outside air was coming through the dash vents once we turned off the dash A/C. When I repaired the rear air a few months ago, I foound the seal that directs air over the condenser dislodged. I am going out today to remove the cover and check the seal on the front unit. 

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Re: Roof A/C units while driving

Reply #5 – June 21, 2011, 09:26:26 am

We are not aware of your problem.

At different speeds, check generator voltage to be sure it is not a genset problem. Is Kool O Matic cover in place to reduce air flow in or out? Check air flow from dash vents. Put a thermometer on roof air conditioner outputs to measure difference, as the roof air may be fine and warm air is coming in from somewhere else. Roof air filters clean?

Roof air conditioners have a condenser coil that have narrow openings between fins and a square length of foam on top of the coils to separate condenser in & out air flows. Everyone should lift off the large roof air cover once in a while (4 screws) to be sure the foam is not collapsed and the coils are clear. We found our coils VERY dirty and full of dust and our foam just about gone. Be careful to not bend fins and to protect motor when blowing out fins. Foam tape is about two inches square and available where Dometic parts are sold. Foam is very easy to replace. Clean top of coils with alcohol to help sticky tape find a clean surface.

We run with a 12" 120-volt (table) Vornado fan. We use it blow air to where we are, like forward to dash area when driving, to bedroom at night, on dash to clear windshield in high humidity or very cold weather, etc.

Barry & Cindy

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Re: Roof A/C units while driving

Reply #6 – June 21, 2011, 10:44:58 am

Definitely check that foam that seals off the intake and output sides of the coils. Mine started making "buffeting" sounds, and airflow was almost nil as as it was just recirculating within the unit itself. We were up in Helen, Ga, not close to anything when it happened. I took a foam mattress pad the we keep for the couch when making it into a bed, and cut some long thin strips and glued them to the tops of the coils. Problem solved. May not be your problem, however. Hope you find the answer.

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Re: Roof A/C units while driving

Reply #7 – July 04, 2011, 06:23:03 pm

I was finally able to take the time to work on the motorhome. When I removed the forward A/C cover it was obvious the seal was sufficiently flat enough to allow air to bypass the condenser when the cover was deformed by airflow at high speed. The seal on the front unit is attached to the condenser and baffles and gets flattened by the cover.  The seal on the rear unit is attached to the cover and consequently gravity tend to keep the seal in proper shape. I am wondering why there is no attaching hardware to keep the cover close to the seal to prevent leakage?

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