I have Brynt (16 yr old) heat pump system. It was running fine few days ago, and all the sudden outside unit quit responding to the thermostat. When put on heat, the inside fan will run, but the outside unit (fan and compressor) does not turn on. It seemed like it is not getting any power, so I took the cover off, and checked if it (unit) is getting the power. The contactor is getting the power, and the unit runs, when contactor is manually pressed. As soon as I release the button, it stops running. I have checked the thermostat wiring to best of my abilities and they seem to be right (plus unit was working fine with the current wiring few days ago). Apparently contactor is not being pulled when put on heat or on cold; it (outside unit) does not respond to any setting on thermostat. I had my system checked by tech few days ago, because it was frosting, he manually defrosted the unit (test pins), and it ran fine; he told me that I should change the board, and I did. The feron level is fine as he tested few days ago, and everythign was running well. All the sudden outside unit does not respond.

I am completely new to the forum, and the heat pump system. I need to replace the unit (pretty old, and want to increase the capacity), so I figure it is good time for me to learn the system (even if I mess up, it needs to be changed, and if I can fix I can hold on to buying new unit for few more months). If any one knows the solution, please provide step by step instructions (I am completely new to the system). Thank you so much in advance.

Did it quit running right after you replaced the defros control board? If you get the 24v IN and OUT reversed on the board the time delay logic will not work.

You are getting 24v on the contactor when the thermostat is calling for heat? If so then you have a bad contactor. Their's no reason that the unit should not come on if the contactor coil is receiving 24v unless it has gone bad. Let me know if this solves your problem.

Oh! I almost forgot... make sure that the dual run capacitor is good. Sometimes you can tell they have gone bad when they look swollen or are leaking fluid. The capacitor looks like a big battery with wires attached to the top. This is very dangerous for someone who doesn't know anything about electricity to change. Get a H.V.A.C. technician to replace these parts for you. Some units have two capacitors. One for the fan the other for the compressor.

Sorry, never mind this answer. I forgot you said it comes on when you push the contactor. That rules out the capacitor theory.

No, it quit running before, and I thought my board went bad, so I replaced the board. (Technician told me that my board is going bad, because the defrost cycle did not kick in, and he suggested I should change the board; but the board was working fine, so I figure no need to change, then all the sudden one day my ourside unit stopped working (the problem described above); I thought may be my board had gone bad, and I changed the board as advised by the tech); The respons eis same with the old or new board, no difference what so ever. Thanks for help.

From what I tested, no I am not getting any voltage on the contactor coil. This could be because I am not checking the voltage properly or at the right wires. Could you please tell me, which wires and how to check the voltage on that with volt meter? Sorry, completely new to this stuff. I am getting voltage from the source (with ground 124V each and between them 248V; black and white wires at the bottom of contactor); I got two wires going from the left side of contactor Red (124V; measured to ground), and Black (124 V; measured to ground); At the top of contactor I have four wires two black and two yellow (all of them have 124V, measured to ground); now the right side, which I think is the coil side of contatcor (not sure), has three wires Brown, yellow, and yellow (none of them have any voltage; again I am measuring with ground); My ground being the metal case. My capicotrs look in good shape.

Just for reference here are pics of My contactor, and defrost board wiring.

http://postimage.org/image/1ynn4a9pg/
http://postimage.org/image/1yolukk2s/

It's the contactor

It might be the contactor to check to see if you are getting 24 volts check contacts on the side ot the contactor not the high voltage if not see it you have continuity between your control wires tie both wires that connect to the circuit they are different from the t stat the t stat will have 5 or 6 wire the other will have two so tie the two together and check continuity between both wires of you get an OL then you have a break in your wire find break and fix it

Sorry I've been real busy lately. I saw the picture of the contactor. On your last reply you said that you checked the contactor on what you thought was the low voltage side (24volt). From the picture it looks like the brown wire on the right side and the two yellow wires are the low voltage side. To test if they are receiving power make sure to set your thermostat to cooling and drop the temperature as low as it will go. Then go to the contactor to check and see if you are receiving 24 volts. The two yellow wires on the right side are the 24 volts + and the brown is the common. The other thing you can check is in the middle of the contactor. There you will see a little spring. If the coil is receiving 24 volts and that button is not pulled in then you have a bad contactor. If their isn't any 24 volts between the brown and yellow wires on the right side then you need to trace those wires coming from the furnace to the outdoor unit. Make sure it's intact. You can also do what the other tech suggested. A continuity test. Let me know if this solves your problem or if its already been taken care of. Sorry for the late response.