Hello,

I have a mid 90-s gas furnace that has worked last spring, but will not start now. I took the time to learn some basic troubleshooting and I believe I have isolated the problem to the control board.

• The limiter switch seems to work, shows zero ohms when furnace is cold
• The pressure switch shows indefinite resistance, which apparently also is correct until negative pressure is present
• The flame rollout-switch also shows zero ohms
• The blower fan works when set to On at the thermostat
• The low voltage side gets power, it's roughly 29VAC

When I call for heat, I hear a click in one of the relays of the control board. However, the terminal for the inducer motor does not get power. I bypassed it just to make sure the motor spins, which is does.

My conclusion so far is the control board is defective. But I'm wondering if there could be any other prerequisite that prevents the whole startup sequence from starting. The only thing that comes to my mind is the gas valve which has its own switch and control circuitry. I just don't want to isolate the problem incorrectly to the wrong component. Looks like the control board is rougly $85, but I'd rather know that I need it. Looking at the schematic, the inducer motor should pretty much start right away without any dependencies on other circuits, but it also doesn't show any of the internal logic that powers the terminal for it.

I did notice there was a clicky-clacky noise last spring when the furnace would start (when the flame was lit, more specifically). It basically hesitated a little bit before it "realized" that the gas was burning normally. In my opinion it was a relay sound, but a furnace guy said it was the solenoid in the gas valve that was acting up. But he wasn't able to reproduce it so his opinion was just a guess. If it was the relay, maybe it just died over the summer. I know the furnace is old, but if I could give it another winter with an inexpensive fix, I'd rather do that than a replacement.

If there are any further ideas for testing, I'd be thankful for that. Is there a way to bypass the gas valve just to see if the inducer motor starts? I do have a pretty decent understanding of electronics; I program microcontrollers and do electronics prototyping for my own enjoyment.

The furnace in question is an Armstrong Ultra SX 80, model: GUJ075D10-2B

 

Your diagnostic methods are good.

Double check that you have 24VAC or thereabouts to the W terminal when the thermostat is calling for heat, which you probably do.

IF you do, the circuit board should supply the 120 VAC to the inducer motor to operate it. If it doesn't, the circuit board is bad and should be replaced.

 

Thanks for the tip. I did check the W terminal and I do get between 33-35VAC on it when the thermostat calls for heat, and 0VAC when it isn't calling for heat - logically. The terminal for the inducer motor reads 0VAC. So I guess it's the relay for the inducer motor that probably has seen its best days. I wonder if the coil voltage is a tad high since the relay is labeled as 22VAC coil voltage.

I'll get a new control board. Thanks again for your help.

 

Have you tested for 120V on the terminals at the control board where the inducer motor leads connect?

 

33-35 vac on the control circuit is a bit high. You wrote in your first post it was 29 vac. That's a little high but ok.

I would check the output right at the transformer.

 

Yes, the inducer motor terminal reads 0VAC when the thermostat calls for heat. Everything else appears to be as it should.

 

PJmax, I did a direct measurement from the transformer. It starts below 30VAC but keeps rising all the way to 38VAC over the next two minutes, then hovers between 36 and 38VAC. I wonder if that's a problem. Usually power supplies show a higher voltage when they aren't under a load, so I don't know if the same thing is happening with the furnace transformer.

29VAC was when I just made a quick measurement yesterday, didn't think of keeping measuring. I guess if the transformer is bad, then its higher-than-normal secondary voltage could have destroyed the control board too.

 

Yes, 36-38VAC is too high. Are you sure your meter is accurate? I have seen a couple of instances where the voltage coming into a house was too high like 255-258V and caused the output on the transformer to be too high. The power company had to be called to come out and adjust the voltage. You should get 120 VAC to turn on the inducer motor when you have 24 VAC applied to the W terminal. If you don't it points to replacing the control board.

 

firedawgsatx, thanks for making me check. My meter had a low battery so I changed it, and now everything is looking better. The transformer now shows a ~27VAC voltage. I took my Kill-a-watt to see what I'm getting at the outlet, then compared and my meter was way off. I guess this is what you get with a cheap meter. Anyway, all the previous diagnostics still apply to my case and the control board seems like the culprit at this time.

 

Glad to hear your meter is working. Yes, low batteries will cause erratic readings. It is always a good practice to check your meter using a wall outlet to see if it reads properly. Yes, it does look like your control board.

 

I got a new board today. Replaced the old one and everything is working again. The only thing that still bothers me is that once the gas is ignited, the gas valve (I believe) makes one or more clicks and there's some kind of hesitation that I mentioned in my first post. It does that during the first ignition and there's a bit of a roll-out but once it's going, the clickiness goes away too. This whole thing takes no more than 4s. The gas valve is a Honeywell SV9500H 2724. Does anyone happen to know if it's a common problem with these valves?

Thanks again for the help with diagnosing the problem!

 

As I noted earlier, the burners shouldn't fire if the inducer motor hasn't come up to speed.

Replacing the circuit board may cause the inducer motor to switch on, but you need to check to see if that safety switch is working properly.

Disconnect the wire to the inducer motor so it wont start. Then turn up the thermostat to see if the burner fire up. If it does, you likely have a defective pressure switch which needs to be checked out and perhaps replaced.