Factory Gen Set will not handle both AC units

[dgb]

2017 Georgetown 329DS. Just stopped in Las Vegas (it was VERY warm outside). When trying to run both AC units at the same time using the generator, both air conditioners started and ran for a little while. Then, the auto transfer relay switch started making a buzzing sound when the 2nd (rear) air conditioner was cycling on. The front air conditioner then shut down completely.

The front AC unit is a Dometic 15K. The rear AC is a Dometic 13.5K. The generator is a Onan RV OG 5500. The Transfer Relay is a Progressive Dynamics Model PD52.

Seems like the generator should be large enough to handle both AC units running at the same time. The buzzing noise sounds like an overload situation. Could the Transfer Relay be the problem?

Has anyone experienced this?

[chrisbailey13]

One of the easiest work arounds I have found is to turn off the electric Water Heater. If I run out of hot water, I simply use the propane water heater elements.
Also, stagger when you turn on the AC Units. I typically turn on the larger unit first and let it run until I hear the compressor kick on, then I turn on the second smaller unit. what you don't want is both units kicking on the compressor at the same time, that surge will choke your GENSET.

[Trempert]

No problem at all for the generator. Sounds like a transfer switch problem.

[Donamac]

I have had the same problem with my 2014, 328TS. It happens when we're tailgating in RV lot on a 90+ degree day and sun shining on the back storage compartment where the transfer switch is located. It has helped to open the storage doors on both sides to get ventilation and cool down the switch.

[AquaMan]

When stopped in very warm (hot) conditions, I open the door to the generator and/or transfer switch to allow air to move around and not get trapped. I've had my generator vapor lock on very hot days (even while going down the road). The heat from the engine combined with the generator heat caused the problem. Hasn't been an issue since that one time. It was 103-105 degrees in SD last summer.

[Dijoeboss]

I've been fighting this exact same problem since 2013, 18K miles later..... NO FIX !!!! I've added hard start capacitors to the A/C units, I've changed the transfer switch TWICE!! I've verified the oil level is good in the generator, I've had the A/C units amp draws checked by myself and an RV dealer, no problem there, I've had a Cummins dealer check the HZ level of the generator, all ok. Good luck with this, I would still love to hear any NEW ideas, but after 4 years, I'm not very hopeful.

[dgb]

I tried it again when I got home (CA). Ran the front AC for a while, then turned on the rear one. They both ran without shutting off. The generator was running with a definite load. I checked the Transfer Switch in the back compartment. The switch box was definitely hot. Had a slight burning smell. I will open the box later today when it has cooled off and check for burnt wires. I failed to note that when I was running the system yesterday (Vegas), it was 120 degrees. When I ran the units at home it was only 95. The only other thing that draws power is the 120v fridge. I would think that the system was designed to handle the loads that the factory installed. I will report on what I find in the transfer switch.

[Bluepill]

1. Dometic calls for 5KW genny capacity for dual 15K BTU A/C units. That is just for the A/C with no other loads like reefer, water heater, converter/charger, etc.

2. Generators derate at high temps. Onan Manual states "1% power derate for every 10 degrees above 77". At 120 degrees you were down about 250 watts. They also derate for altitude.

3. You might consider a split system with a small portable genny to run loads other than the A/C units.

[WayneLee]

IMO, it was the heat that caused your problem. First of all, your 5500 generator would put out approximately 45 amps in a normal operating environment. As was stated, the efficiency of generators decreases rapidly with temperature rises.

In my experience, the 13.5 and 15 K AC both use around 18 to 20 amps each while running. Under startup, the amps jump up really high. So running both AC units was using somewhere around 36 to 40 amps, leaving you with very few amps to handle the other loads. That, combined with the drop in generator efficiency, IMO is the source of your problem.

My Class A's had Onan's that handled both AC units with power to spare BUT they all had EMS units to control the usage. The really big diesels have up to 12K generators.

[AquaMan]

Originally Posted by dgb

I tried it again when I got home (CA). Ran the front AC for a while, then turned on the rear one. They both ran without shutting off. The generator was running with a definite load. I checked the Transfer Switch in the back compartment. The switch box was definitely hot. Had a slight burning smell. I will open the box later today when it has cooled off and check for burnt wires. I failed to note that when I was running the system yesterday (Vegas), it was 120 degrees. When I ran the units at home it was only 95. The only other thing that draws power is the 120v fridge. I would think that the system was designed to handle the loads that the factory installed. I will report on what I find in the transfer switch.

The heat is what is causing your problems. Like I had mentioned before, open your access doors to genny and transfer switch. I would check inside the transfer switch and make sure the lugs are tight also. I'm pretty sure I would melt at 120 degrees!

[Bayouownbeer]

Originally Posted by AquaMan

The heat is what is causing your problems. Like I had mentioned before, open your access doors to genny and transfer switch. I would check inside the transfer switch and make sure the lugs are tight also. I'm pretty sure I would melt at 120 degrees!

I had exc same issue. I turn off my water heaters and took the metal cover off the transfer switch. Yes the wires are exposed but only I should be digging in there. Seems to have helped

[B and B]

Every TV RF Truck I used had the this same Onan Genset. We added large 10 inch fans in the bay doors to get the air moving.