Could be a loose balancing damper. If it has a hex nut or wing nut, try tightening it. Dan Martyn

I agree with daft punk. A furnace does not produce heat for delivery to the registers in only a few seconds of firing. Modern furnaces may not be exactly as I describe, but this is the way that many work. The wall thermostat clicks on, "calling" for heat. The wall thermostat controls only the burner, nothing else. The burner starts and begins to heat the combustion chamber and the heat exchanger. When the heat exchanger reaches the preset "ON" temperature (120 degs. for example) for the fan control, that device sends power to the fan. When the room thermostat is "satisfied", it shuts off the burner. The fan continues to run, extracting some of the residual heat from the heat exchanger. This boosts furnace efficiency. The fan control also has an "OFF" setting (85 deg. for example) and when the temperature of heat exchanger drops to that level, the switch opens and the fan stops. After the fan stops, the residual heat from the combustion chamber (the hottest part of the furnace, if it is an oil furnace) rises and the fan switch senses the rise in temperature and the fan runs briefly again. The fan control, if adjustable, can be adjusted to stop the extra brief fan operation. The differential between the ON and OFF can be increased to that end. But that does not help with efficiency, except to decrease it somewhat, perhaps slightly. My trade is not HVAC, but I had a case once where I was the electrician and plumber for a home addition and was asked to extend the ductwork to the addition. I have my late father's sheet metal tools, but only some of his skills. The extended ducts did not adequately serve the addition and starting over was not in the budget. So I used booster fans in the ductwork near the registers. Good results for temperature, bad for noise. I told them that for their budget, the choice was the noise of the fans or the noise of chattering teeth. So you now need to make a choice about your system. My two typing fingers are tired now.

"Oh its not the t-stat now is it?" I don't know Jeff. ".. sounds like a lockout to me." A lockout needs a hard reset. Once a primary trips - it won't reset simply by cycling the breaker. You have to re-arm the primary manually. "or a number of things that can't be checked over the internet" True, Jeff but diagnosing a lockout over the internet is no more sure than diagnosing a t-stat over the internet. Jeff - no slams here; just diagnosing what we've been told. Cheese- All the stat does is make and break a low-voltage connection. Essentially it's a part of a relay circuit. It opens and closes what is usually a 24v circuit, that's all it does. The stat gets it's working power from the batteries. If the batts are weak; the stat won't reliably close the connection. IOW it isn't always telling the furnace "it's cold, make heat." Jeff wasn't wrong-I just don't believe that a diagnosing a bad t-stat is over your head, 24 volts will not hurt you like 120v or 240v. If you lose power in your display - it's not the 24v from the furnace. It's the stats internal battery power failing - or some other element of the stat failing ie. bad component on the board, or bad contact on the board etc. Are your batteries new? -br

I can't blame a layman or old timer for confusing the terms when in fact years ago there were hot water furnaces. And in fact still are outdoor open system wood boilers/furnaces. You see the term furnace also depicted an open hot water heating vessel. The proper term boiler became prevalent when describing a closed system vessel. So going back to the 50's and before, when risers and open tanks were used in attics, sometimes with feeders, in fact they were "hot water furnaces" judiec, Any fix yet? Being related to an appliance with a solenoid that uses water, I'm betting on a singing PRV on the boiler or it's the tank water heater, or piping.