That's exactly right. Its like chasing a problem with a motor when the problem is intermittent. Probably didn't try this, but if you pulled the fuse to the furnace and reinserted it, did it fire then?

Steve

 

No, I didn't try that. But the furnace was "operating" the whole time. It would come on, make a burner ignited noise for a few seconds and then cut the burner noise off. On my home furnace I would say that the thermocouple didn't get the signal it wanted so the board shut down the gas. The it would start all over again.

 

So if it always answers the stat, but doesn't maintain flame, here is what you know.

You know you have a complete circuit through all the components. If you did not, you would have no ignition at all. That means your voltage is good because the blower motor has to spool up and run fast enough to close the sail switch, which in turn passes power through the high limit control back to the board.

If voltage is too low, the motor will not run fast enough to close the sail switch. You know the high limit is closed or the board could not go to ignition.

There is a delay built in order to insure the blower runs long enough to clear the combustion chamber of any residual propane before the board goes to ignition. (there never is any build-up, but it is a safety feature).

When the board get power back from the high limit, it simultaneously opens the gas valve and powers the ignitor which sparks. There is no thermocouple. Once the gas valve opens, the board has to find a ground through the flame in order to complete the ignition circuit. The flame provides the pathway. Since you have a flame, you should have a ground, but a board which has failed can not find the ground.

If the board does not find a ground. The blower will continue to run and try two more ignition cycles, then go to what is called "lock-out'. Once that happens, you have to turn the thermostat off and back on to recycle the board.

Steve

 

That sounds like what my system is doing. I've heard that older boards will keep retrying until the battery is depleted. I think mine kept trying. It was not in a wait state as near as I could tell. The new board does stop trying according to the description on their web site.

 

Steve, I checked the propane system this weekend. At the stovetop's burner the measurement was slightly less than 10.5" of water, probably closer to 10.25". Does that warrant adjusting the regulator?

 

For sure no. Leave it alone. That is easily withing a good working range.

Steve

 

Did you check with both tanks?

 

Tank selection would make a difference is if the OP completely ran one tank empty while the other was shut off when the problem occurred and then shut off the empty tank and turned on the opposite tank to do his testing. So far he knows the regulator is good. You really can get a very good estimation of pressure by simply observing the cooktop flame.

If there is still a concern about pressure the next check would be at the valve on the furnace, which is actually best done by disassembly and measuring pressure through the burner orifice. I believe that will prove to be a dead end.

In actuality, although the manufacturer gives the ideal pressure range for which the furnace is operational, in fact in the field you will find that furnaces successfully fire with pressures much below what the manufacturers specifies.

Steve

 

No, I didn't. The other tank is empty and I have the regulator switched to the non-empty tank. Weather was in the high 50's and the tank was probably 80% full.

 

what about the flexible hose pinching off.... under moving fuel...

or like car brake hoses... a bit of inside hose becomes a check valve... closing off the hose while "Working".

I am no expert with propane.. just putting out ideas.
I have seen odd stuff on gasoline and brake systems...

its like a running engine..

Air, fuel, spark.... it starts.. then dies... something is reducing the main parts to maintain "Fire"... not enough Air or fuel.

RV Tech, pointed out to me.. last year.. when my furnace closed the fuel after 5 minutes .. then 2 minutes it re-fired .... for 5 minutes... to much dust/dirt in the squirrel cage blower (over temperature I think)

Thank You..

 

Here's something worth bearing mind. Whatever I got furnace calls from folks doing their own work, they were almost always convinced it was the furnace not getting enough fuel, which is either it is or it isn't. There is no in between! If you can light all the burners on your cooktop, the propane system is capable of supplying your furnace.

And again sounding like a broken record, there is a test for every single component in this scenario.

Steve

 

It looks like fuel isn't the problem based on my test of the pressure in the lines. I have a standby circuit board I can put in should the system act up again. It would be nice to be able to replicate the issue but I'm not going to hang out overnight at the storage lot waiting for it to fail.

 

Is the flame sensor a separate component? Can you test it?

 

The flame sensor is on the board. Essentially the board goes to ignition once the delay on the board to insure the combustion chamber is passed That means the furnace gas valve opens and there is spark at the ignitor for just a few seconds and during that time the board has to be able to establish a ground through the flame.

If there is not flame, the board times out and loses its ground. Then there is a delay and on all but the very oldest boards, it will go to ignition two more times. After that if ignition has still failed it goes to "lock out" and will not retry until the thermistat is turned down and back up to again call for heat.

This cycle, by the way, is very similar to how the furnace works in your home if you have a new model furnace.

So the sensor on the board can not be directly tested unless you have a board tester. However, one of our dealers here will normally test a board at not charge as they hope to sell a new board. That means the first thing to do if you think you have a bad board is to have it tested.

Here is the caveat and I think it is what is happening to the OP. Often when electrical components start to failure, at first the operation is erratic and more often occurs under harsher operating conditions. That means it is very difficult to diagnosis the problem because it is intermittent just like an intermittent problem problem with a motor is more difficult to diagnose. Once it fails diagnose becomes very simple.

Steve

 

Ok . I guess you just replace the board. I had to remove the furnace in my trailer to work on it.