Troubleshooting Limit Circuit Fault

Do you own this house or are you a tenant?

Can you ask the previous people who were living there last winter if they had the same problem.

My first go to would be the high limit behind the gas valve. You could put meter leads directly on the 2 terminals of the limit. Leave all the wires connected to the limits.

With the furnace running normally you should have no voltage showing across those two leads.

If the limit opens then you will see 24 volts. This test works on any simple 2 wire limit switch.

Gotta check static pressure and temperature rise. Gas pressure measurements and combustion analysis need to be performed.

Its probably oversized equipment installed on undersized ductwork.

If one of the rollout switches opened then chances are good that your heat exchanger is shot or the vent is blocked

Probably not. it is much more likely that the safety is opening because the condition it protects against is occurring than the switch is bad.

Because the blower puts the hx under positive pressure and listed CO detectors don't alarm until there are huge concentrations of CO it is unlikely that the amount of CO that would leak out of a bad HX would set off a CO detector. A better at least quick check for the integrity of the HX is to watch the flame when the blower starts. There should be no change to the flame when the blower starts, it shouldn't move or change shape or color. If the blower starting changes the flame that means that it is pushing air in through some sort of a hole in the HX.

The meter test will tell you which limit opens. It is not necessarily a bad switch.

You are simply looking for which limit shuts down the burner. The roll out switches are on either side of the burner and are manual reset type. That is you have to push the small red button to reset them back to being closed. The other limit is on the inducer fan housing, it may be or not be manual reset. If you have never had to reset any of those 3 limits then the most likely limit is the main limit above the burners behind the gas valve.

Do you know if you have a variable speed blower (ECM) or an old school 3-4 speed blower motor?

With the old type you could put the tstat fan switch to on and it might run on high speed (AC speed).

That would provide more air flow thru the furnace and perhaps keep it from overheating.

I had a problem with a replacement furnace that the owner insisted to oversize. I did so and was still smaller than the original unit. The main limit eventually failed open and had to be replaced. It was cycling the burners by that limit and cool down and then relight. This could have been the case with the original furnace also.

I downfired the gas pressure and had to go to the cheap air filters to keep the furnace from overheating. You can measure the temp rise across the furnace and compare it to the label on the furnace to see if you are overheating.

The 25" circumference you speak of, is that round steel pipe or insulated flex? And how many runs are there?

Not to be a tool but have you changed your furnaces air filter ? When confronted w a furnace that is tripping the limit i usually first things first is check the condition of the air filter either at a filter grill or a seperate media filter or worse the cheap cleanable which is in the blower section of the furnace . If you have recently replaced your filter w a higher merv filter try removing and reset the unit and see if it operates . You can also remove the blower compartment door and tap the safety switch so the unit will operate and have your thermostat call for heat . If the unit operates then your have return or blower issues . As for a blocked flue you would disconnect the vent piping and check the chimney for obstructions . I would gather that the inducer blower is running before it goes into lock out or is it firing up and the blower starts running and then locking out if so then it is air flow issue . Before going to nuts are all your supply air vents open and return open also ?

peace and good luck clammy

There you go @neilc, off topic again talking about the birds and the bees!