20kw Generac generator will not run Rheem heat pump unless stove is on---help!

I posted on HVAC-talk.com but the moderator would only give me a hint of the solution since that is not a DIY website.
We have a Generac whole house standby 20kw propane powered generator which while running during an outage will attempt to start the compressor and fan in our 5 ton Rheem condenser unit (heat pump), but after about 2 seconds the heat pump goes into time delay and shuts the heat pump down. If one bypasses the condenser circuit board the generator easily starts and runs the condenser unit---so we replaced the circuit board but same problem. Generac authorized our dealer to replace the generator engine but that does not help and we have plenty of propane flow. ODDLY, when he asked me to turn on the kitchen stove and then see if the generator would start and run the condenser THAT worked! So if there is a resistive load on the generator to start with, the generator starts our condenser unit. We tried a hard start kit early in the process and that did not work--nor did shutting down all house circuits except for HVAC. The air handler and zone systems are not affected--only the condenser. In past Rheem and Ruud had issues with condenser boards but this board is new. A moderator at HVAC talk.com insists there is an issue in our air handler but since they are not DIY will not give me more information on that. My HVAC serviceman does not believe that. What would be an easily solvable issue in our air handler which causes the problem I have presented?

 

Okay, let's think about this. First, a generator does not attempt to start anything. Once started it will deliver
20 kw to wherever it is connected. That's it!

So, we need to go from there, your generator is on and delivering 20kw of power to the panelboard it is connected to. What else is connected to the emergency panelboard? Does your generator disconnect the main panelboard before engaging the emergency circuit's panelboard?

Given that you are only generating 83 amps (+/-) of amperage, is that enough to drive all the connected circuits you have on the emergency panelboard?

 

The generator size was chosen to cover all house circuits except the oven and there is no load shedding. When I have ALL breakers at the inside panel off except the AC and Heat, the gen behavior is still the same--- condenser shuts off within a couple of seconds unless the circuit board is bypassed in which case there is no problem with startup of compressor. IF I turn our kitchen stove on, that load on the generator somehow allows the gen to start and run the condenser compressor and fan normally. So the gen is plenty enough to accommodate the condenser load (line voltage). What goes on with triggering time delay on the condenser board is uncertain but does not occur when another load is added to the gen.

 

Okay, now I am totally confused. To begin with at max, a 20KW generator will only yield 83 amps (+/-). Simple math.

I just re-read your original post. If you have an HVAC man, he needs to be doing this. It is not an issue with the generator. The generator just delivers the electrical supply to the panelboard.

I cannot even begin to understand why starting the stove makes any difference at all. The stove should be on a dedicated branch circuit.

 

The stove is on a separate circuit from the panel box but that box is powered by the generator via the ATS which is rated at 200 amps. The only item NOT on the generator is the oven. For sure having the stove drawing power from the gen allows something to change which powers up the condenser. Generac is sending a soft start kit for my gen guy to install--he also has HVAC experience as well. If that does not work he knows a veteran HVAC guy who teaches HVAC at the local technical school who will come out to troubleshoot. And if that does not work we still have AIG-Rheem able to provide a warranty visit by a large local HVAC company. Others with this problem past years switched out the condenser circuit board for another brand but had to change the reverser valve around since with Rheem it operates through the heat side and not the cool side as other brands do. That may void my warranty though.

 

I am also an Electrical Instructor at a technical college here in the Tidewater area of Virginia. This problem is very intriguing!

Is your generator an Generac 7039? If so, can I get the serial number so I can pull the manuals and review them, please. Also, any information you can get me on the auto cutoff switch and your service panelboard that the generator is supplying would help me.

Given that I have no accurate information on your specific installation:

Regardless of the rating of the main breaker in your service panelboard (200 amp) the generator can supply up to 83 amps. No more. So if you are trying to service your entire home with 83 amps you may not have a sufficient supply.

The way the generator's system should work is that the automatic switch should sense the loss of incoming power to the main service panelboard. At that point, the automatic switch should break contact with the incoming power feed (to prevent back-feeding on to the electrical power lines), start the generator and supply power to either the main service panelboard or the emergency service panelboard.

As long as the demand for power is within the supply limitations of the generator, it will not care what is connected to the panelboard it is using. Theoretically speaking, anyway.

Again, (forgive the repetitive questions please) The output of the generator flows through the switch and connects to either the main service panelboard or a separate emergency panelboard, which one is it?
The stove is connected to which panelboard or both panelboards?
The HVAC is connected to what?

When and if you get a technical resolution to this can you supply the answer to this thread, please.

 

FYI - I just sent a note to one of my fellows from the HVAC field to see if he would have a clue as to what is going on here. Don

 

My gen is a Generac model 0062440---20kw. Serial # is 8466956. The whole house inside service panel is Eaton brand with Eaton and Siemens breakers. I know nothing about an auto cutoff switch unless you mean the automatic transfer switch which is rated at 200 amps. According to Generac, our 20kw can easily supply all of our circuits with one minor limitation---cannot run clothes dryer without turning off something else such as AC or well pump. In past we have had no issues--this is a new problem. When the ATS senses the loss of utility power it transfers to gen which supplies our main inside panel (no emergency panel) to which all circuits are connected. No breaker in that panel or anywhere else trips during use of the generator when this problem occurs--condenser board sends that unit into time delay repeatedly unless the condenser board is bypassed or the stove or similar load is on . Generac is sending our generator man a soft start kit to see if that helps---otherwise he plans to switch out the gen circuit board since it regulates electrical power to loads. Propane supply is verified normal. Should these not correct the issue, we will consult with another experienced HVAC man who now teaches HVAC at the local technical school here in Panama City FL. and who is a close friend of our generator man.

 

Hi Coolman51,

What is the amp draw on the condensing unit outside and the amp draw on the air handler? Also what is the surge watts on the generator?-
Any ac electric motor is going to roughly draw 2 x the running current on start up, This 2 x draw will last will for a second or so.
You have around 90 amps from the generator if the 20kw is running and not surge watts.
Also you do loss a little wattage on the generator running on propane.

 

Hi Mr. Modify. You are correct on the running amps for the generator---83. The surge is very short as you state and it is my understanding that even a lower kw generator can handle the surge for a very short period of time. I do not know the exact numbers on amp draw for the condenser unit or air handler though my generator and HVAC servicemen measured those. The propane causes little drop c/w natural gas so I read. This generator for past 4 yrs has started and run the condenser without problem during outages--but not now unless another load is active such as the stove OR the condenser board bypassed so it does not trip into time delay. Wish I could give u more info.

 

I think your problem is with the generator or the auto transfer switch gear.

I don't get why the resistive load would let the condenser start, seems that would make it worse. I did not understand that it use to run everything.

Has anything changed from when it worked to when it no longer worked?

Have you taken any voltage or amp measurements with the generator running and or surge loaded?

If you have a known large amp draw say the stove for instance, take your measurements to see what the gen is doing. you could possibly have a imbalance on the windings in the generator could be as simple as dirt, moisture.

Also the engine has to maintain very close to either 1800 or 3600 RPM to keep the 60Htz cycle. Running on LPG,NG, or gasoline RPM's should be 3600.

But come to think of his may get you a direction.

 

The servicemen have taken measurements but not when the stove load was applied---which might tell us what the difference is with the generator supply to the condenser. Good suggestion. The generator engine was just replaced in hopes that would solve the problem but did not (not the windings). I do know that 60Hz was maintained at all times though there was a shortterm V drop during applied load---generator guy said that was expected. As I say, nothing changed between last yr and this yr with either unit except a new condenser coil had to be installed in the condenser unit due to refrigerant leak. Someone at HVAC-talk.com indicated there may be a problem in the air handler but that functions just fine along with the zone system (which is on a separate 40 VA transformer)---so not sure what he means. That is not a DIY site so he would not give me further info.

 

The inrush current can be as much as 5 to 6 times the running current. This is basically a instantaneous spike caused because the motor has not built its counter EMF.

If this were the problem, then the breaker on the generator (100 amp) would trip, but is does not trip, so I doubt the spike is causing the problem.

 

Also, think about this, please. The power drop to the home from the utility is dead, the transfer switch activates to remove the utilities lines from the circuit and then the power from the generator (up to a max of 83.3 amps) flows to the panelboard.

At this point in time, the only difference between the generator's supplied power and the utilities supplied power is the amperage.

Neither power source care what is on the end of the line for them. They are just the supply. Would you call the utility company to resolve this issue if it manifested itself when they are feeding the panelboard?

Also, when you turn on the stove, you are not increasing the resistive load for the generator. As far as the generator is concerned nothing has changed, it is still supplying up to 83.3 amps.

I suspect that the issue is downstream of the main panelboard or in the way the HVAC is wired to the panelboard. But I can be wrong, it certainly would not be the first time in my life. Coolman51, when you get this resolved, please post the fix.

 

Coolman51- My HVAC partner in crime wants to know if you leave the stove off, but turn the dryer on will the compressor work?

 

The dean of the electrical department said he has seen this before and it is caused by improper grounding at the generator - transfer switch. Bad Neutral terminations at the generator, transfer switch and panel board.

One of the HVAC instructors said this is an issue with the connections on the Transfer switch and main panel board.