View Full Version : OT- Heat Pump Trouble

02-14-2015, 06:12 AM
I'm not getting any electric backup heat coming on. It's a Heil unit, 11 years old. The refrigerant mode heat works as it should, but when it goes into defrost the electric coils dont come one to compensate. And, if I switch to emergency heat when the overnight temps go below 20 or so, the electric coils never come on.

The stat is calling for electric, I have checked the voltage of the control wiring and there is voltage to the W1 terminal which initiates the electric heat. During this time, I hav clamped and amprobe around the primary leads and it's only pulling 2.5 amps, which is the fan motor running.

We are calling for single digits tonight and for a week, so I sure would like to fix this.

The equipment literature is clear, a signal via low voltage to W1 will initiate the first coil, and the unit will sequence in the second coil. Or both W1 and W2 can be jumped together if so desired. Either way I'm not getting anything.

I'm not sure if the circuit board does the sequencing or if it is a seperate item, but something has to tell the coils to come on.

At this time, I'm sure I could nevr get an parts until Monday, but I would feel better if I knew exactly what to get.


02-14-2015, 07:40 AM
W1 and W2 are stage one and two heat (emergency heat), originating from the thermostat. In the unit, these signals pull in relays to turn the ac on to the heating coils themselves Given the troubleshooting you have done already, the most likey cause is burned out heat elements. The electric heat elements are optional when the unit is new and field installed. The module mounts into the unit with just 3-4 screws, a plug in wire connector, and the heavy ac current wires.

I would shut power to the unit down and pull the heat strip module out and have a look. The elements are wire and if broken are easy to see. Look it over. If bad, they are pretty easy to replace.

FYI 5kw heat strips usually only use W1 , 10KW heat strips and higher have 2 elements and can be setup in 2 stages, using W1 and W2, althernately W1 and W2 can be tied together making it a single stage element (most common). The circuit board in the unit is a defrost control board, handling that function, nothing to do with the backup electric heat strips.

BE SURE THE POWER TO THE UNIT IS TOTALLY OFF, sometimes the electric heat strips are on a seperate circuit from the heat pump, can be setup either way.

Hope this helps,

02-14-2015, 07:46 AM
Heating coils themselves sometimes fry and go open circuit, I would check for voltage across them, or power down the unit and check for resistance across them.

Other then that, id suggest an electric space heater or two in the mean time.

Jon Heron
02-14-2015, 09:16 AM
Further to what sparky said above, usually sequencers are used for electric heating, they are also prone to go bad so be sure to check them too.
They look like this.
You can check the heater element with an ohm meter, if it reads open its cooked for sure.
Good luck!

02-14-2015, 10:48 AM
Thanks guys. Going to look at these now. How do you test a sequencer though? A relay is easy to check, so is continuity in the coils.

02-14-2015, 11:43 AM
A few years back I had relays/sequencers like that fail. If I remember correctly, they failed open. The control side showed 240V, the side connected to the heat coils showed 0V. The unit had two sets of them, the second set failed just a year after the first went.

Jon Heron
02-14-2015, 12:14 PM
A sequencer is basically a time delay relay, it has a heater in it that eventually bends a bi-metallic strip to close the contact.


02-14-2015, 12:41 PM
I pulled the coil pac out. Two coils, two relays. These must be electronic relays instead of the soleniod "pull in" type. There is a sticker that says "use only DC relays, or circuit board damage will result". The wiring diagram and wiring isn't hard to follow. Each coil is 120 volt, each pulling off one leg on the main breaker. The control wiring goes through the limiter, which is made.

Put it back in, turned it all back on and bumped up the emps for Aux heat, had 24 volts on W1 as I should. I could not get a voltage reading at the relays on the control wires, set at AC. Turned it to DC and got 11 volts. I'm not convinced this is right. I did get 120 voltson the coil terminals, but the amprobe was only showing 2.5 amps as before, and there was not a difference in supply air temp over heating on refrigerant. I haven't done it yet, but I will switch to emergency heat which locks out the compressor, and I bet I still have no coil heat.

The coil control wiring plugs right into the circuit board that has the fan control and where all the control wiring terminals are. If I could prove the coil relays are OK, then it would have to be the board, I think......There just isn't much to the coil pac other than the relays.

J Tiers
02-14-2015, 01:01 PM
The relays control heaters....

Is there heater voltage actually available? Check that. If not, there is your issue.

Also check the heater units for continuity. Shut off power and ohmmeter check each one. They are typically a slide-in frame setup, with the screws for connection on one side. Should be easy enough to see the wiring.

If you have to, you can jump bad relays out to get heat, or do other haywire rigging to keep something working. You'd be in total manual control, but it is do-able. If the relays don't function, then jump them. If the board does not give the control signal, you may be able to find the control voltage and jump it to the coils.

Check the fuse on the board.... it is likely powered by 24VAC, and there will be some sort of fuse on it, and most likely a spare in a clip right there. At least our clients who make that stuff provide one on their units.

I'd do something like that before I let the house freeze, for sure.

02-14-2015, 02:01 PM
Don't worry, I burn wood so nothing will freeze. I may end up just using the fan to circulate the air, to get it all over. JT, I did pull the coils and checked them, they are fine.

Jon Heron
02-14-2015, 02:53 PM
I am not sure I am following you. Do you mean the contactors are pulling in providing power to the heaters but your not getting any heat?
Or, do you mean you tested the contactors, they work, but they are not getting closed by the control thus giving you heat?
Are there sequencers or just contactors controlling the power to the heaters?

02-14-2015, 03:56 PM
Jon, the contactors are not like ones I am used to seeing, where they are solenoid activated, pulling the contacts closed with a definite click. These are a sealed unit per se. Then, the sticker says they are DC, not AC and not to use AC contactors. So I first turned the heat up to make the Aux kick in, 24 volts at W1. I tried to get a voltage reading with my meter set at AC, nothing. I turned it to DC, and got 11 volts. Turned it back to AC and could get a 120 volt reading on the coil terminal. However, there is no amp draw. The amprobe shows 2.5 amps with the fan running, and with W1 energized. It reads 2.5 amps with just the fan running alone.

I don't understand why the contactors would be DC and everything else is AC. And I don't understand why I get 11 volts DC and not 24 volts. And I don't understand why there is voltage to the coil terminals but no increase in amps.

I turned the unit on to emergencey heat, no compressor running but W1 energized, and the supply temp is room temperature, doing nothing but circulating air, no heat.

02-14-2015, 04:22 PM
Just a thought but make sure you are waiting a while when making measurements. As mentioned, the old mechanical sequencers were heaters warming up bimetallic switches and took a minute or two to make. This was not by accident. They don't want the full load kicking in all at once so the purpose of the sequencers was to stage the load. Stage two did not start getting power to the sequencer until stage 1 was up and running. Clearly yours is all electronic but it would not surprise me if the electronics simulated the same delay in powering up. I am not suggesting that there is no problem, only to be sure the real problem is where it is and not where a quick measurement might indicate.


02-14-2015, 10:09 PM
I'm having a little trouble following your explanations:

So I first turned the heat up to make the Aux kick in, 24 volts at W1. I tried to get a voltage reading with my meter set at AC, nothing. I turned it to DC, and got 11 volts.
It sounds like you are saying W1 should be 24 volts but you measured 11 volts. My question would be referenced to what? You need to reference the reading
to the 24 volt return.

Turned it back to AC and could get a 120 volt reading on the coil terminal.
I thought the coil was low voltage DC. Did you mean one side of the switched contacts?

I don't understand why the contactors would be DC and everything else is AC.
Because the circuitry on the control board runs on DC so it would make sense to use contactors with low voltage DC coils.

02-15-2015, 12:31 AM
Contactors can be DC because its 100x easier for a small control board to switch 12v DC to a contactor then switch 24v AC or any voltage AC for that matter.

If its getting 12v DC across the coil but no output, its 99% most likely the contactor has failed

Try placing a jumper clip across its 120v AC terminals and see if your heater turns on (or monitor current through the jumper clip). Also check to see if the 12v DC disappears when the thermostat is set to not heat the place.

If so, Relay failed
If no, Heating coil failed.

Don Young
02-15-2015, 10:15 PM
If you are getting 120V at the (heating) coil terminal but no current and the heater is good, you must also have 120V at the other coil terminal, thus no voltage ACROSS the coil. You did say "there is voltage at the coil terminal"s", which sounds like the same reading to the neutral at both terminals. Somehow, I suspect a fuse or breaker problem and that the heating coil is 240V. If it is 120V, then there must be an open contactor or connection in the neutral lead. Of course, I may have mis-interpeted your measurements.

If you measure voltages from a common neutral point, you need to measure both sides of any load to determine the voltage DIFFERENCE.

02-16-2015, 05:37 AM
I pulled the coil package out again yesterday to check things. The relays are indeed DC volt controled. They are rated at 240ACV load and 22VDC coil, N.O. I didn't have a means of rigging 22 VDC to test them. The heat coils are continuous so they should be fine. Once re-installed, I again checked the voltage to the relay control terminals. The board is only feeding the relays with 11 VDC to pull them in. I'm not sure if this is enough or not.

So this morning, I'm going to try to buy a replacement control board which I think is the most important, and if I can I'll get two new relays. It may be I have to get an entire coil assembly, don't know. I cannot imagine what else could be the problem. The stat is sending the right signals.

02-16-2015, 07:33 AM
I've never seen one of these particular heat pumps but i do know electrical/electronics/etc.

You say that your only getting 11 volts at the relay - well it's possible that the relay has gone partially short-circuit and that is accounting for why your only seeing 11v, the control board can only supply a finite amount of current so it's possible the relay is dragging the volts down as a result.

I had my gas boiler doing the same kind of thing about a month ago; the gas solenoid was partially short-circuited and on the face of it made the control board look as though it was not supplying enough drive to open the valve. Local plumbers merchant said it would be the control board as that's the common item. Once i'd fitted the board there was no returning it as i'm not "trade".

So i rigged up a standard 100w lightbulb across the control output (to give a bit of loading) and measured the voltage - it looked a bit on the high-side (about 28v, solenoid was rated at 24v) but i expected that.

Back to the local plumbers merchant for a new gas valve ("it'll never be that" says the guy behind the counter plus one of his trade mates leaning on the counter). Went home and connected that up - was rewarded with a nice thump as the control board opened/closed the new valve. Installed valve and the boiler sprung to life.....

new control board was ~150, new gas valve/solenoid was just under 40. I'm glad i did a bit of diagnostics work first... :-)


02-16-2015, 10:24 AM
Got my parts, put the new board in and two new relays. I have heat!!!!!!!!!!!!

02-16-2015, 12:41 PM

do your coils really come on when defrosting? mine dont.

02-16-2015, 02:56 PM
It's only 13 here so no point in running the compressor. If the evaporator unit calls for it, it should. Crossing fingers here.

It's been running on emergency heat since I put the parts in, the house is toasty!

What I want to do, like I should keep messing with it, is put a temp sensor in the supply duct that will display on the stat. The stat (honeywell) has contacts for an "outside" sensor to display. It would be nice to have the sensor in the duct, and at a glance I could see both supply temp plus room temp. Since the stat is right above the return air grill, that would be return air temp and it would be easy to tell if I'm getting the proper temperature exchange across the coil.