"So far two HVAC service visits haven’t been able to diagnose the problem "

Could be a whole host of things! What model furnace?

Two visits by the same company/tech? Are they Burnham dealers?

Are you sure there are no sensors in there that could be shutting off the furnace?

Did the furnace exhibit the problems you described when the HVAC tech was present? If it did then you should be looking for a new HVAC service company. It sounds like a safety lock out problem. You shouldn't keep trying to force the furnace to operate if this is the problem. It is not safe to do this.

Hi - Thanks for the responses. The furnace model of Burnham is PVG/SCG. I had two different heating&cooling techs (different companies, not Burnham dealers as far as I can tell) ) come to take a look. For both times the furnace was working. In the second case, I asked what he would do if it had not been working, and with none of the thermostats able to turn it on. He didn't have an answer. The unit looks pretty basic (photos attached). Today it's cold here and the furnace has been cycling normally on and off all day. I've looked for a igniter but can't find one. I've already paid $350 for nothing so I'm reluctant to call in another tech until I can get more data.

It's got a spark igniter so flame sensing would be a part of that contraption.

It also has a pilot, that is a part of the ignition process.

If the main burners light? there is nothing more to guess at what might be wrong... unless they go out before the call ends.

The furnace is old, spark ignitors *like this* primarily belong in a museum these days.

They still occasionally use spark ignitors but they are attached to a board that will flash error codes to provide a hint what might be wrong.

Thanks for the information. It sounds like replacing this might be a place to start (if I can find one!)

I have a Burnham gas furnace (about 12 years old) that frequently stops running.

The Burnham PVG & SVG are boilers and not furnaces. The brochure talks about the safety switches.. I am not familiar with this type of boiler. I assume it is supplies heat to your forced air system through the heat exchanger. If it really 12 is years old then I would not give up repairing it. A new boiler is going to be very expensive. Maybe you should contact a Burnham dealer or the original installer. The key is to have it fail when the tech is present.

Sorry about my incorrect terminology. I have baseboard heating, not forced air. The only new information I can provide is that lately when I turn up the thermostat I hear the pump go on right away but it now can take 15-20 seconds for the boiler to ignite. It used to be immediate. Maybe this will help to localize the problem??

No need to apologize for incorrect terminology. One of the goals for this forum is for homeowners like you and I to share information and learn. A 12 year old boiler should still have another 20 years of life it is maintained properly and not abused. It can be fixed by someone skilled in boiler repair.

A good tech should be able to troubleshoot the problem you are describing. I encourage you to do a search on the problem you are seeing. You may find someone has posted a video with a similar problem. Intermittent problems are the hardest to find and resolve. Sometimes you have to do your own research in order to guide the tech doing the repair.

Do you know if you have the SCG or PVG model? I found the service manual for each. There are troubleshooting guides in the documentation. This would be useful to a tech not familiar with this boiler.

Thanks for the information. It sounds like replacing this might be a place to start (if I can find one!)

Nothing you couldn't have gotten by just analyzing the pictures you showed.

It's quite rare that the spark initiation contraption would fail, so replacing this *may* fix it --- may not. I'd say less than 30% chance... the other is that "sometimes it works or slow to work" doesn't say this 1 part has failed. Without the spark for ignition it won't work at all.

So the new tech, is going to charge you for your time and start replacing parts that might be bad but probably not? Doesn't sound like a good plan to me.

Because: The pictures you show scream lack of maintenance to me. Lack of maintenance often leads to equipment replacement. Due to the type of controls alone leads me to believe this is older than 12 years.

Supply a model # and serial number tag and we'll see if we can't find out how old it is.

"I have been able to get it running again by doing a restart (turn off gas, turn off power, wait 5 minutes, turn on gas, turn on power). There doesn’t seem to be a flame sensor in the furnace."

Some Burnham furnaces utilize a single electrode which functions as both ignitor and flame sensor. If flame is not detected, there is a built-in 5-minute delay before recycling/retrying burner ignition. A replacement pilot assembly (which includes the ignitor, gas pilot, and cables) runs around $80. Another potential problem, which is fairly common, is an issue with the differential pressure switch. It can get junk or condensate in either tube that results in a blower shutdown. You can disconnect and blow through the tubes to verify they are clear and do a visual inspection of the switch at the two connection points. A replacement switch runs around $42

Thanks for all the comments. I moved to my house 15 years ago and about three years in I switched from oil to gas. That said, the date on the sticker (with other boiler info) says 11/2007. That suggests the boiler was unused, but built a few years prior to when I installed it. The last maintenance was a couple of years ago when I had the expansion tank and a valve replaced. The HVAC tech didn't mention anything else that concerned him.

You have a gas-fired boiler-- not a furnace. There may be other control devices, e.g., a low-water cutoff, on the water circulating side that would shut down the boiler.

Was the original installer one of the HVAC companies you paid to diagnosis a fix? If not then that might be the next option to pursue. Whoever you hire next tell them to send a senior tech. You want an experienced person who installs and repairs boilers on a regular basis.

I completely agree - someone with a lot of experience is needed. I don't think the original installer is still in business (nothing came up in searches) so I tried two others.

It's not rocket science. The HVAC trade attracts some of the least technically capable folks of any employed in the trades, in my experience. And there's plenty of evidence here on houzz.com of just that. Unfortunately the OP paid for the inadequate knowledge and troubleshooting skills of two of them.