A/C HEAT PUMP MYSTERY AND QUESTION

davanz

I apologize for the length of my first post. I have been an electrical contractor in East Texas for 46 years and have learned to reach out for help when I am stumped. I have recently installed a Generac system at a residence and am looking to a collection of HVAC experience on this forum for help.

First, the equipment. I have a Heil 3-1/2 ton heat pump system with a scroll compressor, about 5 years old. The wiring diagram shows it to have a dual capacitor, (H-C-F), a separate start capacitor, start relay and thermistor. The reversing valve in this system is energized in cooling mode. The generator is a Guardian 24 KW, about 6 months old, with a 200 amp Service Rated ATS.

Here is the mystery, and I have confirmed the below information with multiple tests, to the point that I am sure of the consistency. And before each test, I let the compressor sit for about 10-12 minutes to let the gas pressure stabilize.

In heat mode, I put the thermostat set point one point above room temperature to turn on the compressor. I use this method because with just one point difference, this Honeywell unit does not turn on the heat strips in the furnace. The compressor starts and runs normally with just a slightly noticeable, brief bog of the generator. The compressor runs at about 14 amps. In other words, in heating mode, all is well.

When I try to run the system in cooling mode, (again, with time between tests for gas pressure to stabilize), the compressor will try to start, bog the generator, and disconnect after about 2 seconds. Then it will attempt another start in about 6-8 seconds, same action, and repeat multiple times before I abort the test. I assume the compressor is tripping out by means of a thermal overload or current sensing device.

I know this information may lead to thoughts of soft start devices or other such solutions. But the mystery to me is why the difference. Specifically why does the compressor run fine in heat pump mode, but will not even start in cooling mode? Does it have anything to do with the timing of the reversing valve actuation?

Please give me any information or experiences to help me to solve this.

davanz

I am still thinking and have another question.

The thermostat is a Honeywell RTHL3550. Does this thermostat keep the reversing valve energized anytime it is set in cooling, or does it send 24 volts on the orange R/V wire and yellow compressor wire at the same time when temperature rise calls for cooling.

Also, according to the wiring diagram for the heat pump compressor, both the orange and yellow land on a control board, and the control board switches the R/V and compressor contactor separately. Is there a time delay there to delay compresssor start until after the R/V is energized?

JUGHNE

How long between the heating run test and the cooling run test?

davanz

I have usually waited about 10-12 minutes between tests.


I am still thinking and have another question.

The thermostat is a Honeywell RTHL3550. Does this thermostat keep the reversing valve energized anytime it is set in cooling, or does it send 24 volts on the orange R/V wire and yellow compressor wire at the same time when temperature rise calls for cooling.

Also, according to the wiring diagram for the heat pump compressor, both the orange and yellow land on a control board, and the control board switches the R/V and compressor contactor separately. Is there a time delay there to delay compresssor start until after the R/V is energized?

ratio

You might be on to something re the reversing valve. I think I'd try jumping it into cooling & then repeating your tests. Do you have a clamp meter that will capture inrush current? That may be insightful as well. The other thing I'd look at right off the bat is the hard start kit. There's several different styles, IIRC a thermistor is one method, a start relay is another—check carefully which one you have, & consider switching to an aftermarket one.

JUGHNE

What happens on city power if you wait only 10-12 minutes between change over?

pedmec

are you sure that the system pressures are equalizing. did you put gauges on the system. and what do you by "disconnect". more info is needed by im going to suggest that you have a plugged bypass and your tripping out the clicks-on or high pressure switch due to high head. but like i said more info is needed.

Ironman

David,
Check your email.

EBEBRATT-Ed

Maybe the reversing valve is hung up

Bill_Kitsch69

ya, need more info. you need a tech there. You could call Heil. we could do this by phone troubleshoot. I didn't hear you say you tried equalizing by disconnecting power (throwing the reversing valve) then starting cooling, test. Haven't seen any pressure tests, capacitor tests.

Look for (rule out) blockage in the cooling deck. Some of this is a stretch: blocked bypass drier in the cooling mode (it's bypassing in heat.) Blockage at indoor expansion device; or a "non-bleed" expansion valve anywhere, failed can lock closed.Not likely outdoor expansion device is non-bleed. Also, screens in play. Checks that won't allow bypass. Your description almost certainly sounds mechanical (lock up) not electrical. You can ruin your unit learning on it.

One day after enough trip outs, that internal compressor OL is not going to reset itself.. ruht roh...

Would be awfully nice to have all the wiring diagrams.

knkreb

Without knowing the model directly, I would say 5 years ago, that the heating side used a piston at the liquid line connection at the outdoor unit. That would equalize quickly. Cooling with a TXV, not so much. Did you try dropping power completely to indoor and outdoor unit to simulate a power failure? That should cause the system to default in the heating mode for the reversing valve and equalize pressure. See if that test changes your results.

Harvey Ramer

The compressor will overload if you have to much refrigerant volume or liquid at the suction port. Does the condenser have a compensator? You should probably wait an hour between heating/cooling tests. The TXV's need time to adjust. 

dmax73

Does the contactor drop out or is it just the compressor shutting down?

Geosman

It's good that you're asking for assistance. This time see if it behaves the same on line power and not the genset. If it still wants to act hard starting or faults on AC start-up you really need to have the customer contact their HVAC specialist to diagnose what is going on. Without a meter and gauges on the system you are only grasping at straws for a solution and I highly suspect the solution will require an experienced tech.

wmgeorge

If it works ok on Shore power and not on Genset there's your problem. Nothing wrong with the unit as everyone is troubleshooting here??? You need more time delay on the Genset because it can not start the unit do to limits in your wiring or the generator its self can not handle the starting load. Yes a proper hard start kit would help if all the rest has been sorted out for starting on the Genset which may be too small. FYI if its running on NG you will not get the full rated kW from the Genset and if the AC is trying to start when other heavy loads are online that might make a big difference.
Snip from the Spec's notice the derating on NG and its only got a 100 amp breaker so instead of a 200 amp service you really only have a 100.

MrScott

A generator is of course powered by a gas engine.  Adding a big load to a gas engine at idle tends to stall it, just like abruptly letting the clutch out on a stopped car would -- you want to give it some gas first.  I've learned to put on a bit of load, 100..200W, to get the generator engine up above idle before suddenly putting a heavy load on.  

Lance

While I was a Generac tech for residential and commercial as well as an HVAC master I am unsure I have enough to diagnose this cause. Generator size and demand load are ok, unless there is a higher load seen by the generator. What is the amp draw at the generator feed? I would check for voltage drop at the outdoor contactor and or review the refrigerant pressures during this cycling. It may come down to adding a time delay relay on the Heil low voltage circuit to prevent a short cycle startup issues related to pressure or temp. But I think the problem is elsewhere. When you say compressor 14 amps, is it just the compressor or the entire outdoor unit feed between the disconnect and the unit? FYI, A scroll compressor on shutdown can reverse direction during pressure equalization. Starting a scroll in reverse does not work too well. Without more info I cannot offer anymore. In general I review before and after an event, cycling frequency, the usual measured changes in electric and pressure, also factor in heat causes, bearings, all connections. If I can divide I can conquer and narrow the problem to a smaller part of the system. GAs pressure drop to the generator? Reduced combustion power can be a factor also.
Good luck.

Lance

PS, Don't count on controlling the electric heat by the tstat alone. In a defrost cycle, the electric strip is energized also. In controls anything can be designed and anything can go wrong.

wmgeorge

MrScott said:

A generator is of course powered by a gas engine.  Adding a big load to a gas engine at idle tends to stall it, just like abruptly letting the clutch out on a stopped car would -- you want to give it some gas first.  I've learned to put on a bit of load, 100..200W, to get the generator engine up above idle before suddenly putting a heavy load on.  

Powered by gas, except with connected to Natural Gas (please read the chart I posted) like most of the whole house ones, and I have never seen standard AC ones idle down they run at either 1725 or 3450 rpm to maintain the correct Hz or Cycles, DC engine powered welders idle down and newer Inverter AC gensets but the OP did not state it was.

tanklessman01

davanz said:

I am still thinking and have another question.

The thermostat is a Honeywell RTHL3550. Does this thermostat keep the reversing valve energized anytime it is set in cooling, or does it send 24 volts on the orange R/V wire and yellow compressor wire at the same time when temperature rise calls for cooling.

Also, according to the wiring diagram for the heat pump compressor, both the orange and yellow land on a control board, and the control board switches the R/V and compressor contactor separately. Is there a time delay there to delay compresssor start until after the R/V is energized?

Looks like I may be late for the party here; but let me start by saying; I agree with the most responded to your inquiry that, this should be dealt with qualified HVAC tech. as so many little details need to be axplored

However, since your situation is somewhat electricity related, I will answer your question none the less;

IN SHORT;
while the unit outside is running in heating mode put a jumper between O and Y terminals at the outdoor unit.
That will energize the reversing valve on the fly.

You will hear big swoosh sound and compressor will make loud grinding noises at first then smooth out and continue working.

If that happens, you know the problem is not line voltage,...your job is done,...you are outta there..

follow the steps below to make sure nothing is out of place..


If the RV(reversing valve) or SOV(switch over valve) is energized in cooling(most manufacturers do),
"O" Terminal from the thermostat energizes it when you switch to "COOL" mode.

There are many ways you can test it but in your specific situation there is one test you can do to eliminate the question about the RV working or not.

You said "The heat pump starts and works fine in heating mode"

That means it has no problem starting and operating without RV being engaged,...so far we know that to be the fact.

Here is how I would proceed if I was trouble shooting it; I'll try covering every little detail as possible..

1- Follow thermostat wire into the OD(outdoor) unit's control compartment
2- Make sure they are color coded like, Y=yellow, O= orange R=red etc. these 3 are the most important to your situation
3- Remove wire nuts from orange and yellow but make sure the wires under the wire nuts are not separated, another words, connection to control board is not interrupted,...
yellow goes to Y orange goes to O terminal on the board.
4- Start heat pump in HEAT mode from the thermostat and do what I said IN SHORT.

Good Luck, if not this one,...on your next adventure.

JUGHNE

Did you ever try the 10 minute wait from HP to AC on utility power??

I think you are changing over too quickly, needs more time to equalize.

Yes, the reversing valve does change over for defrost but the compressor is already running.

hvac01453

some of the heat pumps have a TDR on the SOV so they start without reversing, then after about 5 seconds it energizes the SOV. This may help your situation...but without test instruments its just a WAG.

JK_Brown

Have you watched the voltage drop on start up for heating and then cooling to see if there is a difference?

And if so, you might watch the voltage drop at the contactor 24v control wiring in each mode. One difference between heating and cooling is that the control wiring is powering both the contactor and the reversing valve coils so a voltage drop would impact it more than in heating when it's just the contactor.

That being said, is it the contractor dropping out or is it just the compressor?

wmgeorge

So it works fine on standard house supplied power, but not when on the engine driven AC power ? So where would you start troubleshooting....? You need to get some recording amp meters and voltmeters. I have some Fluke meters I could loan you, both the voltage meter and the ampere meter record peaks.

JUGHNE

The OP seems to be MIA........

jpm659er

knkreb said:

Without knowing the model directly, I would say 5 years ago, that the heating side used a piston at the liquid line connection at the outdoor unit. That would equalize quickly. Cooling with a TXV, not so much. Did you try dropping power completely to indoor and outdoor unit to simulate a power failure? That should cause the system to default in the heating mode for the reversing valve and equalize pressure. See if that test changes your results.

I think you have the answer. Piston metering device for heat. TXV metering device for cooling (non- equaling). This would require more current (and more time) to get compressor started.

davanz

First, I appreciate all the input and suggestions and I had planned on doing further diagnostics.

But yes, I have been MIA for a while. My mother had just moved into an assisted living facility and has fallen twice, (trips to ER etc.). So, I have had to take some time off work to finish with her furniture moving and set up, as well as just being with her. But I have visited with a local AC contractor that I have worked with in the past, and he has "taken over" the challenge. I will probably be back in the saddle tomorrow and get caught up on the situation.

After that I will report back, and thanks again.

knkreb

davanz said:

First, I appreciate all the input and suggestions and I had planned on doing further diagnostics.

But yes, I have been MIA for a while. My mother had just moved into an assisted living facility and has fallen twice, (trips to ER etc.). So, I have had to take some time off work to finish with her furniture moving and set up, as well as just being with her. But I have visited with a local AC contractor that I have worked with in the past, and he has "taken over" the challenge. I will probably be back in the saddle tomorrow and get caught up on the situation.

After that I will report back, and thanks again.

Sorry you're having such a challenging time... If you pass him the link to this discussion, it may speed up troubleshooting. Take care!

wmgeorge

Sorry to hear about your mother, went through all that with my wife's Mom. But she was in denial and refused to use her walker and finally broke a hip.

I think your HVAC tech will solve your problem especially even when on house power it has some issues.

davanz

As a follow up and, I hope my last concern with this project. The A/C guy I was consulting suggested that I forget the question of why the compressor was starting with no problem in heating mode but would not start in cooling, (both on generator power). He urged me to put a soft start kit on it.

I put the kit on yesterday, and all is well. The compressor now comes on in heat or cool with no problem on the 24KW unit.

Thanks for all the input, I may be back when I have my next A/C system challenge.

ChrisJ

wmgeorge said:

If it works ok on Shore power and not on Genset there's your problem. Nothing wrong with the unit as everyone is troubleshooting here??? You need more time delay on the Genset because it can not start the unit do to limits in your wiring or the generator its self can not handle the starting load. Yes a proper hard start kit would help if all the rest has been sorted out for starting on the Genset which may be too small. FYI if its running on NG you will not get the full rated kW from the Genset and if the AC is trying to start when other heavy loads are online that might make a big difference.
Snip from the Spec's notice the derating on NG and its only got a 100 amp breaker so instead of a 200 amp service you really only have a 100.

I just wanted to try and clarify something, that may actually help things make more sense.

On natural gas that generator says it's good for 87.5A. So, 87.5 @ 240VAC.
That seems like plenty for a 14A load, but that compressor isn't a 14A load while starting.

Off the top of my head, I think it's perfectly reasonable for an air conditioner compressor to pull 60-80A when starting for a short time. I think it's also perfectly reasonable to assume a house likely needs 20A or so under normal conditions.

So, it could be borderline depending on what else is going on in the house and how the system behaves in cool.


With all of that said.
My parent's 16KW Generac starts their 3 ton cool only system without any issues repeatedly.

And my 9KW home built portable generator starts my 3 ton cool only system but I cheat. My system is 2 stage and the compressor starts in low stage and the generator really doesn't appreciate it. I wouldn't let the thermostat control it, it's more like a "let it drop 5 degrees and shut it off manually" situation.

@davanz I'm sure the generator and the compressor appreciate that soft start. I think it was a wise move.

wmgeorge

General rule is a AC motor takes 3 to 4 times the FLA to start. We used to call those kits hard start kits when you had a single phase motor on low voltage or was short cycled, the newer ones were just a start Cap and a solid state relay and they jus plugged in across the run Cap. Cost was about $70 or so, I have no idea what is in the soft start kit but the $350 cost makes me wonder? All that is required is to put the Start winding out of phase with the Run winding. Scroll compressors since they self balance so quickly start with out issues with just the normal Run cap only.

ChrisJ

wmgeorge said:

General rule is a AC motor takes 3 to 4 times the FLA to start. We used to call those kits hard start kits when you had a single phase motor on low voltage or was short cycled, the newer ones were just a start Cap and a solid state relay and they jus plugged in across the run Cap. Cost was about $70 or so, I have no idea what is in the soft start kit but the $350 cost makes me wonder? All that is required is to put the Start winding out of phase with the Run winding. Scroll compressors since they self balance so quickly start with out issues with just the normal Run cap only.

I actually have no idea but I'm going to assume the soft  starts being sold are like some kind of VFD that ramps up.

From what I've seen they're very different from a hard start

ratio

ChrisJ said:

wmgeorge said: ...We used to call those kits hard start kits ...

I actually have no idea but I'm going to assume the soft  starts being sold are like some kind of VFD that ramps up.

From what I've seen they're very different from a hard start

Agree. I'd like to see a pic of the soft-start kit myself, or a model number or something so I can look it up.

wmgeorge

Only VFDs are used on three phase, I have worked with them a lot. Like to see the soft start kit with the cover off? Wiring diagram would help if someone has installed one. Found a hook up diagram, so they must be altering both the Common and Start side voltages and phase shifting to accomplish the starting. Lots different from the old hard start kits.

ChrisJ

wmgeorge said:

Only VFDs are used on three phase, I have worked with them a lot. Like to see the soft start kit with the cover off? Wiring diagram would help if someone has installed one. Found a hook up diagram, so they must be altering both the Common and Start side voltages and phase shifting to accomplish the starting. Lots different from the old hard start kits.

While I don't know how a soft start works there's most certainly single phase VFDs.

wmgeorge

ChrisJ said:

wmgeorge said:

Only VFDs are used on three phase, I have worked with them a lot. Like to see the soft start kit with the cover off? Wiring diagram would help if someone has installed one. Found a hook up diagram, so they must be altering both the Common and Start side voltages and phase shifting to accomplish the starting. Lots different from the old hard start kits.

While I don't know how a soft start works there's most certainly single phase VFDs.



Yes, single phase input VFD feeding a three phase motor. I have installed several. Try putting a VFD on a regular single phase motor and let me know how it turns out.

JK_Brown

A soft starter seems like good advice for any system being run by a generator. It slowly brings up the voltage, and thus limits the in-rush current, to reduce the voltage drop on the line. Once the magnetic field in the windings is producing the impedance to limit the current and the motor starts, they switch to direct connection of the motor to the line voltage.

It occurs to me that your heating/cooling difference could be something as simple as when the indoor blower started up in the different modes.

jpm659er

https://www.micro-air.com/products_easystart_368_softstarter_microair.cfm
Found this on the web. Designed to reduce LRA during generator use.
Emerson also supplies an electronic soft start for Lennox for their noisy start issues.