Trane furnace start up issue

PuppyRage

Hi, I have a 10 year old Trane Model TDH2B060A9V3VAC that has a constant issue at call for heat:

Thermostat calls, an audible relay click and the inducer starts up. 2 seconds later another click and the glow plug lights up, but 5 second later the inducer motor will drop to lower speed followed 2 seconds later by another click and the unit shuts down. It doesn't get far enough into the cycle to open the gas valve.

It will then immediately retry the process which 99% of the time works perfectly with a click after 10 seconds or so of inducer start to gas valve/main flame on. Once it does this, it never fails again during any part of the cycle.

The one odd time it will fail like the first paragraph describes a second time before being successful a third try.

I've pulled the inducer motor out which turns freely, no corrosion, no water build up in housing. The condensate drains freely. Both the combustion and flue gas piping are clear to outdoors. The air proving switch clicks easily with a gentle blow into tubing.

Any ideas on the cause?

Thanks

JUGHNE

I had a 2 speed inducer problem, (different brand of furnace), it would start on high speed, ignition and then when dropping to low speed it would flame out and start the sequence again.
It turned out that the low speed failed in the inducer motor. It all happened so fast that watching pressures was impossible.

Finally, an amp clamp on the Neutral going to the motor showed no current flow on low speed, you have to monitor the neutral not the hots as this happened quickly.

Ironman

Something’s definitely out of sequence there: the igniter shouldn’t light after two seconds, there should be at least a 30-60 seconds pre-purge before lighting.

May be a bad board, but you need a competent tech on site to diagnose it.

PuppyRage

Ironman said:

Something’s definitely out of sequence there: the igniter shouldn’t light after two seconds, there should be at least a 30-60 seconds pre-purge before lighting.

May be a bad board, but you need a competent tech on site to diagnose it.

That was my first hunch was the board, but Trane is very difficult to find parts for and a board is almost double of most other brands replacements. So trying that is a big risk if it ends up being something simpler.

When it works properly the pre purge ends up being around 15 seconds. In my case the glow plug is on for far longer than I would expect without the furnace opening the gas valve.

mattmia2

Might want to look at the voltage on the gas valve. Are you sure that it isn't trying to pen the valve but the valve is sticking closed?

Also look at the sequence of operations in the installation manual and compare to what you're seeing.

EBEBRATT-Ed

@PuppyRage

Either you or your technician need an electrical meter and to do some troubleshooting. Guessing seldom works and costs $$$ unless you get lucky

HVACNUT

Check for 24v at the gas valve. That's you're starting point since the HSI is on.
If there's 24v, check gas pressure. 
If there isn't 24v, put a manometer on the inducer to see if it drops out before ignition. 
Is it a 2 speed inducer and 2 pressure switches?

JUGHNE

In the situation I had, the manometer was not quick enough to show the drop as the low speed did not start, it shut off the burner and then went back to pre purge.

Amp clamp on the inducer neutral would have saved me a lot of time.
Furnace was only weeks old, assumptions made.......

PuppyRage

HVACNUT said:

Check for 24v at the gas valve. That's you're starting point since the HSI is on.
If there's 24v, check gas pressure. 
If there isn't 24v, put a manometer on the inducer to see if it drops out before ignition. 
Is it a 2 speed inducer and 2 pressure switches?

It doesnt get to energizing the valve as the failure happens at about 8 seconds in. This furnace has quite loud relays so you can clearly hear when it energizes the glow plug and when it opens the gas valve on the second successful try.

The failure timing is so consistent that i've ruled out inducer/pressure switches being the problem. If it was a mechanical issue it should be less consistent and happen during other parts of the heating cycle to. Its gotta be in the main board....was hoping someone came across this and theres something on the board itself that was changable. If not I might be stuck with an expensive change.

PuppyRage

mattmia2 said:

Might want to look at the voltage on the gas valve. Are you sure that it isn't trying to pen the valve but the valve is sticking closed?

Also look at the sequence of operations in the installation manual and compare to what you're seeing.

The relays are quite loud so you can clearly hear it energize the gas valve at the 15 second mark during the second attempt.

Just found a manual, on call for heat inducer start followed after airflow proving satisified and the glow plug starts, so that part is normal. Gas valve should be opened at 15 seconds which is what it does on the second try. Its the 8 second mark it gives up every time and restarts the process. No relay clicks can be heard when it does this. Just drops the inducer to a different speed. It sure seems like an air pressure problem but it never does it again once it starts and its at the exact same moment every time as well.

PuppyRage

Ironman said:

Something’s definitely out of sequence there: the igniter shouldn’t light after two seconds, there should be at least a 30-60 seconds pre-purge before lighting.

May be a bad board, but you need a competent tech on site to diagnose it.

Found the manual for sequence. Glow plug is supposed to come on almost immediately after inducer start. 15 seconds is the sequence timing for opening the gas valve....which is spot on for what it does for the successful second try.

mattmia2

Check the pressure switch with a voltmeter or ohmmeter. Sounds like it isn't proving draft and is shutting down.

EBEBRATT-Ed

@PuppyRage

Tubing on a pressure switch is a common problem. Doesn't take much to plug them