This call has me confused!

[Gerbs1983]

One of our guys performed pm on a two system house, got a call a day later that the second floor system wasn't working/keeping up. He worked on it for a while, I was told to go see if I could figure it out. Here are the details:

Call for heat, inducer kicks on, pulls 1.97" wc, ps needs 1.35 to close. Ps closes, ignitor heats up. Gv energized, opens for less than a second and closes. Not open long enough for the flame to flow acrossed all four burners, literally less than a second and the gv is de-energized by the board.

Call for heat does not drop out.
Pressure switch does not drop out.
Inducer does not drop out, continues to pull 1.87 to 2" wc throughout the cycle.
No limits are opening to cause interruption.

Burners are clear all the way acrossed, flame sensor is clean (not on long enough for flame sensor to come into play at this point)

Here is the kicker, twice while trouble shooting the furnace lit and stayed lit. Once I shut it down and attempted to refire, problem reoccurred.

Have a new board installed at this time, same issue.

Furnace is a 94 Trane 90% upflow.

Any ideas/thoughts are greatly appreciated!

[mark beiser]

Does it use the old radiant flame sensor system, or does it have a normal flame sensor rod?

[billg]

Some times things happen faster than your meter can respond. Did you jump out pressure switch or thermostat? Also check wiring and gas pressure when valve is powered. At least it happens enough for you to see it.

[Helioson]

Check your ground. A simple test is to make a "test" 12g ground wire from the board to a "good" known ground. It doesn't take long, is cheap and if it doesn't work you can at least cross it off your list. Good luck

[suBcools]

The few times I've had the gas valve being mysteriously de-energized it turned out to be the pressure switch dropping out faster than my meter would respond. For a split second.

[ComfortablyNumb]

What piece of equipment are you working on?

Did you try to jumper the pressure switch? Do you continue to have 24v signal to the valve?

I had similar occurrence with an older Rheem 80%. Valve would open for a split second then close, occasionally staying open.

I'm calling a failing solenoid in your gas valve.

In past experience mentioned, I would measure ~20-22 volts when the valve would open for a split second, sometimes not open and measure ~15v, sometimes 24v and would hold and valve would stay open, but rarely and very intermittently.

Removed leads from solenoid and test, would measure 24v and hold consistently.

Replaced gas valve and batta bing.

And btw although those funky voltage readings did obviously tell me of a problem, they may jot be exactly entirely accurate, as like someone said above, can happen so fast your meter barely detects fluctuations.

[timebuilder]

The ground jumper should generally be used between the burner closest to the flame sensor, and to the board ground. Aside from boards in modern equipment that sense the presences of the equipment grounding conductor that comes from the service panel, the ground between the burners and the board are all that the board cares about for flame sensing.

Yes, you can jump out the safeties one at a time, but I'd consider if another gas appliance is running when this one tries to fire, causing low inlet pressure. Or, a defective gas valve.

[hvacrmedic]

You have a poor connection somewhere, track it down. Could be literally anywhere in the active circuits, both low and high voltage, including the premise wiring, breakers, etc.,. It's a process of elimination. Test jumpers should help. If you're not losing 24 volts to the valve, then it's the valve.

[Snapperhead]

snug every wire nut , and every screw with a wire

spade connectors , take off , gently pinch tighter with pliers , put spades back on

Now keep your meter on the gas valve and fire it up ... see if you lose power to valve when it drops out

[uniservice]

This is one of the times that an old analog meter will be better at checking the valve input than a digital.

[Gerbs1983]

In response to all of the above advice:

Normal flame sensor rod.

I have jumped out the stat, but not the pressure switch - I will try that.
I understand that these things can happen faster than a meter may be able to respond, yet did not think to jumper out pressure switch. I did monitor inducer pressure, did not go low enough to affect ps, yet could be that my meter response time is too slow.

I am losing 24 volts to gas valve.

I will make sure that no other gas appliances are fired. Incoming gas pressure: 6.5. Unable to measure manifold gas pressure, drops out too quickly.

I will check all connections, ensure good ground, and jumper out limits one by one when I go back. Thanks for the advice so far!

[ch4man]

do you get a fault code?

im thinking cold solder joint on the board or possibly in the gas valve internals.

sometimes the best tool for this is you... start wiggling wires. hw smartvalves have been known for poor internal connections a grabbing the harness and moving it around works on thoses.. can happen on the IFC at the point where the gas valve wires connect.

good luck

[Cosmicmuffin]

I made up a "Test" 24 volt transformer. plug into 120volt.,two jumper wires with clips! works great to test gas valves etc.