AC Condenser not starting on Generator

[Npstewart]

Hopefully this post doesent sound like a rambling brain dump. It’s been a long week running entirely on generator after being hit by a hurricane. This has always been extremely puzzling to me any and info I find online has lots of answers, but no conclusion. Most forums aren’t like this one and questions go unanswered.

I have a 12kW Dual Fuel LP/Gas, Duromax XP12000. I love the generator and runs like a tank. Under most circumstances, the generator starts my AC without issue, with or without load. However, many times, when I turn the thermostat down, the AC won’t start and it doesent even TRY to start. Basically when I turn the AC down, the contactor never pulls in. Even when I manually push in the contactor, it only starts the compressor, but never the fan. The only time it consistently starts is when I start the AC for the first time on generator. After that, it sometimes takes 45 mins to start after a call for cooking, and sometimes it won’t start at all and the only way I can get it to start is by turning off the AC completely for an hour (probably after compressor cools off). Last, I’ve noticed that sometimes if I turn something else on in the house , it will start the generator, but not consistently.

I have power monitoring on my panel and my resting usage for the whole home is normally 1.2 kW, and when my AC condenser is on, my load goes up to about 4.7 kW. The generator usually gives me 123v and 125v, respectively (L-N).

Yesterday, I added a hard start kit, changed the capacitor, and changed the contactor. The hard start hit helped the starting amps but didn’t do anything for the original starting issue.

Condenser: Rheem RP1548AJINA (4 ton, RLA = 18.5, Fan FLA = 2.8A, LRA = 128A, HP = 1/3rd, MCA =26).


Some possibilities I found on other forums include:

* The generator is producing a square sine wave and/or dirty power which isn’t working well with 24v control wiring. (This makes sense, but not sure why the fan doesent start so makes me think something else is stopping the fan. )

* Add a 240v - 240v isolation transformer

* The Ecobee thermostat (smart thermostat) doesent like the type of power it’s getting.

*Use a soft start instead of a hard start hit

* Turn on a resistive load before starting (not a solution but this may be helping already per the above).

* The defrost control and/or time delay on the unit is preventing the condenser from starting on generator due to the power quality.

A lot of the solutions seem to be with power quality, but how do I fix the power quality in a practical way?


I know this isn’t an HVAC forum but it’s filled with people 100x smarter so hoping for any practical input or advice from experience. Trying to turn this into a learning experience and if I’m lucky it will help someone else in the future stuck in a hurricane hell.

Thank You all!


Sent from my iPhone using Tapatalk

 

[synchro]

Some possibilities I found on other forums include:

* The generator is producing a square sine wave and/or dirty power which isn’t working well with 24v control wiring. (This makes sense, but not sure why the fan doesent start so makes me think something else is stopping the fan. )
...
* The Ecobee thermostat (smart thermostat) doesent like the type of power it’s getting.

I suggest measuring the voltage between the R (hot) and Y (cooling) terminals, and between the R and G (fan) terminals of the thermostat when you turn the thermostat down. The voltage across those pairs of terminals should drop to a low value if the thermostat is working correctly. Optionally, you could jumper across those terminals to activate the compressor and fan.

 

[mtnelect]

Your generator is too small for the starting current. Also, remove the hard start and replace the combination capacitor with one made in the United States of America, rated at 440 volts.

 

[curt swartz]

Your generator is too small for the starting current. Also, remove the hard start and replace the combination capacitor with one made in the United States of America, rated at 440 volts.

If the contactor is not closing there is no starting current.

 

[qcroanoke]

Optionally, you could jumper across those terminals to activate the compressor and fan.

That's exactly what I would do. Take the thermostat loose and jump out r and y in the unit and r and g in the air handler and try to run it

 

[drcampbell]

... when I turn the thermostat down, the AC won’t start and it doesn't even TRY to start. Basically when I turn the AC down, the contactor never pulls in. Even when I manually push in the contactor, it only starts the compressor, but never the fan. The only time it consistently starts is when I start the AC for the first time on generator. After that, it sometimes takes 45 mins to start after a call for [cooling], and sometimes it won’t start at all and the only way I can get it to start is by turning off the AC completely for an hour (probably after compressor cools off). Last, I’ve noticed that sometimes if I turn something else on in the house , it will start the generator, but not consistently.

I have power monitoring on my panel and my resting usage for the whole home is normally 1.2 kW, and when my AC condenser is on, my load goes up to about 4.7 kW. The generator usually gives me 123v and 125v, respectively (L-N).

Yesterday, I added a hard start kit, changed the capacitor, and changed the contactor. The hard start hit helped the starting amps but didn’t do anything for the original starting issue.

Condenser: Rheem RP1548AJINA (4 ton, RLA = 18.5, Fan FLA = 2.8A, LRA = 128A, HP = 1/3rd, MCA =26).


Some possibilities I found on other forums include:

* The generator is producing a square sine wave and/or dirty power which isn’t working well with 24v control wiring. (This makes sense, but not sure why the fan doesn't start so makes me think something else is stopping the fan. )

* Add a 240v - 240v isolation transformer

* The Ecobee thermostat (smart thermostat) doesn't like the type of power it’s getting.

*Use a soft start instead of a hard start hit

* Turn on a resistive load before starting (not a solution but this may be helping already per the above).

* The defrost control and/or time delay on the unit is preventing the condenser from starting on generator due to the power quality. ...

"Power quality" is often the answer when someone has no idea what they're talking about.
I'm not saying it isn't the problem, but don't be quick to condemn it until you've actually measured a power-quality problem.

Failure to start unless it's been off for a long time is often a failure to equalize refrigerant pressures. Compressor temperature usually isn't an issue because the refrigerant vapor passes directly over the motor windings and through the compressor body. (unless the refrigerant temperature is too high or the refrigerant flowrate is too low) Many controls in larger equipment will refuse to start a compressor until it's been off for 3, 5, or 10 minutes.
BUT ... if the contactor isn't pulling in, you have a controls problem, not a hard-start problem.

Turning on a large [resistive] load before starting might be helpful if the generator responds poorly to load changes.

Have you measured voltages (both 240 & 24v) while on utility power and while on the generator?

Fancy-schmancy electronic thermostats are often not-quite-compatible with existing equipment. It may be necessary to interpose old-school electromagnetic relays between the two to provide the condensing unit with hard dry contacts.

 

[hbiss]

Basically, when I turn the AC down, the contactor never pulls in. Even when I manually push in the contactor, it only starts the compressor, but never the fan.

This in a nutshell describes the problem.

The first thing I would do, with the thermostat calling, check the voltage on the contactor coil. If there isn't 24VAC then you have a problem with the thermostat or whatever controls it. If by manually pushing the contactor in the compressor starts consistently, you have a control problem.

As for the fan, check for voltage at the leads, check the capacitor. That said, if you have a more recent unit, it may have an ECM condenser fan motor. That doesn't have a cap. It will have at least four leads, two are hot all the time and the other two are 24VAC control that turns the motor on. Check that.

Likely also that you have a fussy control board that controls everything and that may not like the power from the generator. Good luck with that.

-Hal

 

[qcroanoke]

Turning on a large [resistive] load before starting might be helpful if the generator responds poorly to load changes.

I've seen that help on generators that were powering UPS's. They would surge and hunt for the right frequency and as soon as a resistive load was added they smoothed out and ran well.