I’ve a central HVAC and recently, compressor started to not stop with thermostat indicating a temp lower than setting. Thermostat was also showing that compressor was stopped. At the beginning, it was happening once per 3-4 days. Remedy was to cycle the compressor breaker off / on and everything was OK.
I thought condensor contactor was the culpitt. Till I received the new contactor, problem became more and more frequent. I changed the contactor and it was not the culpitt. Problem is still there and now compressor never stops.
I looked at the electronic thermostat (Robertshaw 300-225) which seems OK but to be sure I disconnected the 24 V supply and you know what … as soon as I power the compressor it starts even without 24 volts.
I imagine someone has already seen a similar problem. What’s happening ?
Sounds like the thermostat contacts are welded together - you have cut off the 24v supply so there is no way that any sensor can be responsible - so a short across the compressor contactor control relay seems to be the most likely. If the control relay contacts are welded, you can often free them with a sharp tap with a hard object - such as a screwdriver handle - this may break the contact weld, however they will certainly stick together again very soon.
You could try to find out if the outputs are calling for the compressor to run, that way you can decide if it is the thermostat or if the problem lies elsewhere.
The circuit diagram shows that terminals Y1 and Y2 are responsible for bringing in either 1st or 2nd stage compressors online.
Suggest that you knock al power off, disconnect the Y1 & Y2 lines (tape up the loose ends) and try it.
If the compressor does not run, that would indicate you problem is in the thermostat control relay, if it still runs then it points to the compressor contactor being a problem.
It does not look as if it is intended for service repair, I personally would strip it and decide if its worth replacing the part - but you may not know where to obtain such an item.
You other option is to buy another, but if it were me, I would want to consider how to prevent reoccurrence, and that means either modifying to take a more heavy duty relay - with perhaps a bit of work to fit it inside, or look for another more reliable make.
I removed completly the thermostat (that’s a plug in model) to make some tests on the wiring from where the thermostat is. First, without thermostat and 24 volts on, I measured the Y wire : 0 volts. Red wire as 26 as it should be (so no short between Red and Y). That’s good news. Compressor started when powered.
Then I disconnected the 24 volts transfo again without thermostat. I had 0 volts everywhere as it should be and compressor started when powered.
So without 24v, without thermostat, compressor starts. To me, problem is certainly in the outside unit.
As I said, I replaced the relay/contactor with a heavy duty one rated to 50 amps. Could a new relay contactors be welded ??? Should I try knocking an the contactor ? To be honest, I didn’t check the relay before installing it but it I will.
I asked question before going there as my unit is on the roof and not easily accessible.
IF the problem is NOT the new contactor, and it pulls in without the thermostat calling for cooling, my guess is you have a relay between the stat and the contactor and the relay is what is stuck in the closed (made) position.
The contactor control circuit must be getting power from somewhere. Are you sure there is not an override somewhere?
These must be some other way power is getting to it, you will probably need to get the cover off the outside unit, and start working out where the feed to the contactor is, and work your way through - something is either shorted or is bypassing the controller.
You might get lucky and find a diagram in the outside unit - if not then look up the manufacturer online and find out if they have a set of installation wiring diagrams, there will probably be several different options, and you challenge will be to work out which one applies to yours.
If you do find such a diagram, perhaps you can link to it.
I just climbed the roof … I now know that the problem is in the outside unit.
I tested the 24v relay/contactor operation. It operates perfectly, telling me that thermostat, wiring and 24v relay are in good shape.
For the experts my outside unit is a Samsung US24A2RC. This is a unit for a split system. They converted it in adding a 24 volt relay. It’s too bad that I cannot join a drawing. It seems special in the way that contactor cuts only one side of the 240v line. See this drawing with the following link on top of p. 27 https://www.quietside.com/archives/downloads/samsung/AS18A0RCD/Service%20AS18A0RCD%20AS24A2RC.pdf
It has been working for 8 years, so I imagine a part is now defective and it started as an intermittent problem.
I now have to study the electric schematic a bit … to try to find where could be the problem. Just a question ; could a compressor work with only 120v ???
Is the part you changed what they call PR on the schematic? The “power relay”?
What do you mean by “they converted it in adding a 24 volt relay?”
As to your question about the compressor running on 120 volts, it most certainly will not.
Some contactors only switch one side of the 240 line. It will work that way. My unit was originally that way, but when the contactor failed, the replacement switched both lines.
Yes, I replaced the Power Relay. If you look in the top middle of the diagram you will see that the initial relay winding was connected directly to the L1 line. Power relay could be energized from inside unit through terminal 1. So it was either a 120v or 240v relay.
That schematic is for a split unit. As mine is connected to a central HVAC they had to change the relay for a 24 volts. I should have said “change for” not “add” a 24 volts PR. Inside the condenser unit, they tagged the wires feeding the winding " 24 volts" certainly to indicate the change.
Now, I suspect wiring is not as in the schematic. As power relay is doing it’s job, as line L2 only connection is through the power relay, the compressor could not run. So it has probably been modified more heavily than just 24v PR.
I could solve the problem in switching both lines, but what bugs me is why was that perfectly working for 8 years before it started to deteriorate and finaly completly failed. Something seems to have failed and it may or not affect performance. I’d like to know.
I didn’t see anything but, according to you, could they add something prone to failure ? Also, in another schematic I see an “O.L.P.” between the relay contact and C compressor. Is this an Over Load Protector ?
I will realize a real wiring diagram and I may find a final answer.
Thanks again for your help. As an electrical engineer, I can understand circuits but I’ve zero knowledge in HVAC.
Hi, I’ve been out for the evening. Just looking at this again.
I’m a bit confused. You have the outdoor unit Samsung US24A2RC but you do NOT have the indoor unit that it normally connects to?
So you have a regular A-coil either over a furnace or just alone in an air handler? You describe it as “central HVAC” so it is NOT a stand alone wall unit with all the electronics that is shown on the indoor unit schematic??
If that is the case, then the Y1 wire from your thermostat should power the contactor for the compressor (and the condensor fan too) on a call for cooling. The outdoor unit schematic shows the compressor is powered on by terminal 1 and the fan hi/low on terminals 2 and 3 respectively from Samsung’s indoor unit. You have a normal 24 volt stat so I assume then that that part of the schematic is NOT as wired and you do NOT have Samsung’s indoor unit.
Since this worked for 8 years, it is probably something simple that has failed. What is the coil voltage on the Power Relay (what I call the contactor) that you replaced? I still suspect that there is an added relay between the stat and Power Relay that is stuck closed.
(This is just a theory, but if the coil voltage of the Power Relay is 120 or 240, then there must be another relay switching the PR on and off that has a coil voltage of 24. If so, I suspect it is stuck closed.)
I found it in doing the real wiring diagram. The installer feeded the original power relay (240v coil) with another 24v relay controlled by the thermostat ! Great engineering plan ! When I knew it was a relay, I knock on it and it started working as it should. I will fix the problem in removing this relay and wire the compressor through the 24 volts relay as it should have been done initially.
I got mixed by the fact that the 240v power relay is not at all as a regular power relay, it’s exactly as a terminao block.
Thanks for the good hint : knocking on the relay and searching for another relay.
And I appreciate you coming back to fill us in. A lot of times things are just left hanging here on the internet.
I’m a retired HVAC tech and I couldn’t think of anything else it could be. Especially when I saw that the original relay had a line voltage coil but your stat puts out a nominal 24v.
By the way, there is nothing wrong with the way the installer added the 24 volt relay. It’s a simple, cheap way to power the line voltage coil of the contactor. I would have done the same thing. And many (large) units are wired this way.
Your problem was, you didn’t have a correct as-wired schematic. You seem to be drawing your own, so you will now.
A last question : I discovered a device that is not on the original wiring diagram you have seen. So I imagine that the original inside wall unit was doing this funtion.
It’s a 3 wire connection device with a small wheel with degrees on it. It also has a small copper tube going on the outside of the unit (certainly to sense external temperature degree). One wire is from power relay switched L2, second wire is connected to TB2 - 2 (brown fan wire) and third wire is connected to TB2 - 3 (yellow fan wire).
As all 3 wires are related to condenser fan control, could this device be controlling fan to avoid too much cooling when it’s cooler outside ? I’m living in a place having very cold winter.