Millivolt system loses pilot after burn.

Unknown

Both a hot tip and a cold base. The base could be heating up.

It doesn't sound like a high resistance or a short, but those are possibilities, too.

500 mv running, on pilot, is pretty normal.

I bet it drops as the whole pilot generator heats up.

Noel

Rudy

Millivolt system loses pilot after burn.

I ran across a puzzler the other day.

It's a sealed combustion natural gas log fireplace, millivolt system. Gas valve is Robertshaw-Grayson 7000MVRB-5-LC.

The pilot looks good, good 1/4" to 3/8" contact with the powerpile.

With no load: One powerpile lead removed and pilot held open manually, the powerpile generates 618 mV.

Reconnected, and powering the pilot only, the voltage at the normally connected powerpile leads drops to 485 to 495 mV.

There are two on/off switches to turn on the main valve: a wall switch or a switch at the gas valve. When either swich is used, the voltage at the normally connected powerpile leads drops to about 220 mV. No voltage drop across the switch.

The burners light smoothly, look fine and operate well.

The problem is that after letting the fireplace burn for a while, like an evening in front of the TV, the next morning the pilot will be out. But there is no problem while it's operating, they can enjoy an evening of normal fireplace operation.

I've never seen this happen, but this is the customer's complaint. While I was there at cold startup, everything checked out as I described above. The pilot stayed lit after I left, too.

When the main gas was shut off at using either switch, the flames went out smoothly with no rolling burner flames that could have extinguished the pilot.

The fireplace operated fine for years, so I doubt that it's a draft problem, although I didn't do an actual visual check of the vent piping. Wiring connections are all sound.

At this point, I'm tempted to tell the customer to run the fireplace for a couple hours before I get there to see if something has changed when the fireplace gets up to full heat. But then it would be too hot to work on. I'm thinking that perhaps there's something going on with the powerpile operation once the fireplace gets up to full heat. Or an intermittent problem, but the pilot outage after a long burn seems happen consistently.

Any one ever come across a problem like this? What was your solution?

Thanks.

Unknown

Duncan - give me a call

401-437-0557. I do not have time to give an answer here for you but if you call me around 7 PM tonight I will be in and can give you some help.

Dale

Just the powerpile?

A few things to think about. First alot of the new sealed fireplaces have a valve that powers both a special non standard thermocouple and a powerpile, the reason is to add safety by shutting the gas off in less time than a standard powerpile so the sealed glasss cover stays in place. That thermocouple controls the pilot gas and the main gas. Next, the wall switch should be the special type for powerpiles, determine where the main coil is on the valve and see how much voltage is on it with the unit running, should be 130mv minimum, then measure the millivolt drop accross the wall switch, should be less than 10 mv. If there are 2 switches I would guess one should be 120 volts for the add on blower. Next, is there a single wire running up to a klix-on usually near the upper right of the unit behind the glass? This is a funny normally open safety that I usually see as a manual reset, if it is an auto reset it may be opening because of poor venting and then resetting. Gas fireplaces are alot tougher to troubleshoot that most people would suspect. Good luck

Unknown

Q335 Quick Drop out

thermocouple that Dale is talking about is very sensitive, it will drop out in less than 30 seconds with any kind of residual combustion products.

With your open circuit reading of 618 millivolts the closed circuit should be around 200-220 millivolts as you have very litle resistance with a gas fireplace. The reading through the pilot stat should be 460 millivolts (you had 485 to 495) that may indicate the power unit on the gas valve is starting to go. What Dale gave you will cover the rest.

I would also watch out for excessive ambient temperatures around the gas valve (150 degrees F) and pilot itself, as someone already posted if the cold junction on the powerpile generator is getting over heated it will drop out.

I will be around in the AM tomorrow if you want to give me a call.

[Deleted User]

3 ohms

Probably some resistance in the control circuit. But first check the voltage produced by they thermopile, should be 400+ millivolts with heater off. When gas valve opens it will drop by 50%, (200 volts. Then check total resistance in control circuit. Should be a total of no more than 3 ohms, (be sure to zero your meter). If you get more than 3 ohms, start checking switches, and all connections, anything else in that series circuit, one at a time until you find the offending item. I have seen spade connectors crack. They look fine, but then they heat up and the crack widens, more resistance. An earlier post mentioned special switches, no such thing, any switch with no resistance will work. If you find a switch or thermostat with more than 3 ohms, turn it off an on several times, or run it up and down, that might knock some crud off the contacts.

Rudy

Really great answers, Thank you.

I appreciate your help, and thanks for the good info!

Noel, Thanks - I never knew (or forgot?) it needed a cold base. Since this problem occurs after a long burn, this could very well be the problem. I think I’ll check out voltages after a long burn.

Dale you said “…so the sealed glass cover stays in place” Cracked me up! You’ve got a way with words. No thermocouple, powerpile only. You know, I said there was no voltage drop across the switch, but looking over my notes, I don’t see that specifically written down. Having seen thermostats with a resistance anticipator used on millivolt systems, I know what you’re getting at. Any little voltage drop could steal enough voltage to drop out the valve. I will re-check the switch. Both SPST switches I mentioned turn on the burner.

Hey, there IS a SINGLE WIRE coming off the gas valve, I wondered what the heck that was! So if it gets over temperature, the N/O switch closes and sinks all the current to ground, robbing the gas valve. But the main burner burns all night long until it’s turned off. The problem is that the next morning after a burn, the pilot is out. Are you thinking the switch is trying to close or partially closing?

Tim I was hoping you’d see my post, you da man! It’s reassuring that the 618 mV and 220 mV seem OK, but I wonder about the 485-495 mV vs. 460 mV with the pilot only (what’s 25 mv between friends?). So what’s happening there? When the main valve opens, the pilot and main valve are now in series, sharing the total voltage?

I guess I’m wondering now how to conclusively check the power unit on the valve and know if it’s good or bad one way or the other? If it’s the pilot part of the valve starting to go, I guess it could be causing the problem. And if it's flaky and intermittent, I suppose there's no way to check it. I just don't wanna be a parts jockey.

Sorry I didn’t have a chance to call this AM, but I’ll get in touch.

Mike, thanks for your help. At 485 mV with switch off, and 220 mV with main burner on, it seems like I’m in the ballpark. But I will definitely isolate and check out the control circuit resistance more closely. It’s just that I had two main burner on/off switches in parallel, so I figured BOTH couldn’t be bad, and both on at once should have offered no resistance. One was a little rocker switch near the gas valve, the other was a remote wall switch, regular 120V on/off with a large rocker switchpad. But I will check it out more thoroughly. Good call. Like I told Dale, I’ve seen errratic operation of millivolt furnaces with a (wrong) resistance-type anticipator in the circuit. If I see resistance, I’ll start wiggling wires.

Once again, thanks for all the help and ideas gentlemen, good info and much appreciated. Thanks for helping me get on track with this.