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Old 07-27-2021, 04:06 PM   #1
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Rooftop A/C efficiency when drivingÖ

Anyone else here notice a reduction in AC performance while the MH is in motion? I was parked for 4 days in 88-90 degree heat this past weekend with the 2 Dometic Penguin 13,500 btu AC units able to keep the inside of my 37í coach very comfortable. On the ride home to NJ from VA though, it seems like the AC units were just not able to keep up. Obviously this meant our windshield curtain was open since we were driving and Iím sure that contributed, but it seemed like even after the sun went down and the outside temps dropped we could not cool the coach down to a comfortable temp. I donít think they froze over as I could hear them still cycling. Why would they work fine for 4 days and then suddenly freeze over once we start driving? I have tinted my drivers window to try to reduce the sun baking me while I drive so I always assumed that it was the sun beating through the windshield, but Iím beginning to realize that it seems like the air coming from the registers just isnít as cold when we are in motion. I donít recall having this issue with my 35í gas class A.
Anyone else experience this?
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Old 07-27-2021, 04:19 PM   #2
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You’ve partly nailed it with the load of the windshield. The next thing is air infiltration - just as your AC unit at home might struggle when there are hard winds, or your coach AC might struggle on windy days (60 mph winds I might add).

You can test the units though - put a thermometer in the air inlet and outlet of the AC units and note the temperature difference as well as the supply air temp, both driving and not driving.
If that value remains fairly constant and supply air temp remains relative, then the problem is is heat load and filtration. If the differential narrows with the supply air temp going up, it could be the rooftop unit has a design issue that causes condenser air vortexes that limit heat exchanger on the unit while underway (think of what the empty beer can does in the back of a pickup going down the highway).
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Old 07-27-2021, 06:37 PM   #3
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When I get ready to travel I pull the thermostat down to about 60 to really get the AC unit cooling. That seems to keep the inside temperatures comfortable. If it gets to cold I have the DW raise it a couple of degrees, that would be cold to her.

When parked I can set it on whatever temp I want to maintain. Driving down the road you have the full windshield and wind hitting every opening. The ac units also have to contend with the 60+ mph winds.
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Old 07-28-2021, 04:12 AM   #4
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Thank you both. Iíll definitely start paying more attention now to the temp of the air coming out of the registers. I would think the rooftop units would be designed to work with the air rushing over them at 75 mph but it wouldnít be the first time an assumption like that would be wrong!
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Old 07-28-2021, 04:33 AM   #5
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Thank you both. Iíll definitely start paying more attention now to the temp of the air coming out of the registers. I would think the rooftop units would be designed to work with the air rushing over them at 75 mph but it wouldnít be the first time an assumption like that would be wrong!
If your using the ceiling ducts rather than the direct bypass, my experience on our current coach is that the ceiling ducting system both restricts the air flow out of the unit, and exposes the conditioned air in the duct system to a greater warmer surface area, thus warming it up.
Direct discharge gets more ďcoldĒ to the coach. Between that and a small oscillating fan will improve the cooling affect of the AC units running.
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Old 07-28-2021, 05:10 AM   #6
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Many owner's have found poor taping otherwise dividing incoming and discharge air under inside cover.. The natural suction driving at highway speed may make bypassing air much worse.
The foam doing similar under outside cover may be an issue as well.
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Old 07-28-2021, 05:34 AM   #7
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A couple of things as it looks like you have a gasser. There are many holes through the dash into the engine compartment. Might want to make sure these are all filled with foam. Go out at night and have some shine a flashlight under the dash as you look from the front as this will help find potential air leaks. Second, are you also running the dash AC? If NOT do you have it on recirculate? If not you should, as driving down the road a lot of hot air will enter there. The doghouse will create a nice warm spot too.
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Old 07-28-2021, 07:59 AM   #8
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Sounds like my old Safari. While camping the two 13.5's always kept the coach comfortable. Driving in hot weather not so much. Same for heat in very cold weather. I got busy looking for and finding air leaks. Biggest was a warn out bottom sweep under the large salon slide that was responsible for a lot of temp loss. I was able to keep the coach comfortable after I addressed the air leaks.
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Old 07-28-2021, 09:27 AM   #9
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Someone chime in if I'm incorrect but....the rooftop a/c units drawn air IN from the rear through the condenser coil and then blows it OUT the side vents.

I would suspect that while in-motion that air rushing past the EXHAUST vents on the side would inhibit the efficiency the air flow through the condenser and not allow for optimum cooling.

It would be great if that airflow could be reversed to let highway air blow inward toward the rear.
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Old 07-28-2021, 12:22 PM   #10
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Someone chime in if I'm incorrect but....the rooftop a/c units drawn air IN from the rear through the condenser coil and then blows it OUT the side vents.

I would suspect that while in-motion that air rushing past the EXHAUST vents on the side would inhibit the efficiency the air flow through the condenser and not allow for optimum cooling.

It would be great if that airflow could be reversed to let highway air blow inward toward the rear.
Don't know if I'm following correct but rooftop AC units are sealed from the outside. They only draw air in from inside the coach no outside air. The rooftop unit shouldn't care if you are stationary or moving.
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Old 07-28-2021, 12:24 PM   #11
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Yeah...no. Sealed for inside air across the evaporator coil of course. Not sealed as to air flow of condenser....see those big slats on the side of housing?? Air goes out those from fan to cool the condenser coil. Just like your home a/c etc. Big fan on top blows air UP and draws through the condenser coils.
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Old 07-29-2021, 08:43 PM   #12
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As others have mentioned the 30+ square foot magnifying glass you look through while driving creates a huge cooling load. The diesel engine sitting in the rear is also creating heat in the rear of the coach. Those two factors plus air leaks are most likely the culprits. Most of the highways I drive do nat have much shade so reflective heat from the roadway also impacts the ambient temperature around your coach. Are you able to use reflective screens in the windows and partially shade the windshield? Perhaps add extra insulation to the engine compartment or under the bed if it is over the engine. Best of luck.
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Old 07-30-2021, 04:25 AM   #13
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Thanks all for the replies. I donít feel an unusually high amount of engine heat in the bedroom. Air leaks are tough to verify as wife does not drive the MH. I will definitely try to get a temp reading on my next trip but Iím reasonably sure that the air coming out of the registers when driving is cool but not as cold as when parked. It seems like that coupled with the open windshield are the big culprits. As mentioned, I tinted the side windows already and that seemed to help. I contemplated tinting a strip at the top of the windshield a few inches lower that the factory tint but donít think Iíll be able to go far enough down to make a worthwhile difference without hindering vision.
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Old 07-30-2021, 04:46 AM   #14
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I don't see that anyone has mentioned the heat generated from the engine, drive train and exhaust. I walk around barefoot a lot and can tell you that the floor in the front half of the RV (gasser) gets noticeably warmer on a travel day. I nearly always run my generator when traveling, so also get some heat from that, though not as much as from the main engine and exhaust.
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