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.

[catmanacman]

im thinking bad gas valve

[ComfortablyNumb]

I'm still saying bad valve...

Or maybe a grounding issue.

Also just remembered - disconnect the power supply and check for voltage between neutral and ground. Shouldnt have more than 0.5vac if so you may have a neutral/grounding issue. It's put an end to me banging my head against the wall before... on a Trane 90% if I remember correctly. Read between 15-25v while also getting 95-105v hot to ground.

[gravity]

i installed a new system and went to fire off heat. i had a wire that went to safety switch on pump that broke 24v from board. when the unit went to energize gas valve it would drop out. I had 24v the entire time.

Come to find out, the wire i ran to the pump safety switch had a break in the wire. When the gas valve went to energize it would drop everything out.

make sure all your connections are good and no broken wires. check resistance thru your switches.

hope this helps

[Makinhole]

I too had a similar situation, found poor connection at the molex plug. Hard to find some times, try moving wires around and observing meters and listening while you pray. You will get it. Jump 24v to gas valve and make sure valve is good before investing time in searching for your problem. Good luck.

[cjpwalker]

Sounds like bad gas valve. Temporarily jump out the pressure switch and try a call for heat. Make sure you do it at the right time in the ignition sequence our you will fault on "swtich stuck closed." If no change, try unplugging the gas valve, make a call for heat and listen to the relays on the board. If the gas valve relay clicks and holds for the full four or five second trial for ignition it's most likely a short in the valve.

[Gerbs1983]

Follow up:

Went back, all indicators were bad gas valve. Removed old gas valve, before installing new valve I hooked up low voltage to it. It worked correctly, so I installed. Once installed, fired system and gas valve lost 24v in less than a second...still no error code.

Continued to troubleshoot, was preparing to try a different board when I suddenly got an open limit switch error. Traced to rollout switch...it was open, it hadn't been before. Replaced with new rollout, fired furnace...fired correctly.

Furnace ran for about 3 min, then tripped the new rollout. Checked temp in burner box, not sealed...upwards of 200 degrees...opened up coil...dime size holes in 3 cells.

The kicker was that the original rollout for some reason was allowing enough voltage to get by for the furnace to light, yet as soon as there was flame it would trip, but just fast enough to cause the board to remove voltage from gv, never giving a limit fault.

Very weird.

[ComfortablyNumb]

Funny how things end up working out.... but didn't you bypass/jump the limits first? Way to figure it out though.

So what about your guy that performed the PM????? He didn't see the dime sized holes? This problem just started the day after? Ouch.