We had the annual furnace inspection a few days ago. I’ve inspected furnaces before, ones with Standing Pilots, Standard Efficiency, Standard Flues. Ours uses an ignitor, high efficency, forced draft.
I’m not real happy with the degree he inspected it. But it was done thoroughly last year, the Secondary Heat Exchanger has a plastic box cover that was cracked in places and was replaced, as they diagnosed that, they tested the heck out of everything.
There’s an Induced Draft Motor that comes on first. It activates a Pressure Switch. The computer checks to see if the Forced Draft is established and sufficient, lights the igniter, turns on the gas, I’m not sure to what degree, maybe it lights, is proved lit, then opens the valve. There’s a Flame Sensor, it’s immersed in the flame, and in the absence of a flame, shuts off the gas valve.
There’s a high limit switch on the primary combustion compartment, if it gets too hot, it closes the gas valve until the switch closes with a drop in temperature, then opens the gas valve. The homeowner usually doesn’t know this is happening, but the furnace filter may be dirty, the furnace may be oversized, or the heat registers closed or closed too far, or the cold air returns may be blocked, an A-Coil for the AC atop the furnace may be partially plugged, or a combination of items.
Gas furnaces usually have a blower door switch that keeps the furnace from running since it sucks in air from the cold air return, but with the cover off, sucks in air from the basement, and may pull flue gases in with it, sending them into the house. Electric models might have them too, sucking air from the basement can pull air back through the water heater flue, pulling carbon monoxide into the home.
And there are Roll Out Switches that open if flames are backing out of the combustion chamber into the gas valve, and wiring area, outside the heat exchanger.
Furnaces used to have a Fan Limit switch, it waits until the heat exchanger warms, turns on the blower, preventing cold air from initially being pushed into the home. It also waits until the furnace has cooled to a certain temperature once the thermostat is satisfied to prevent lost heat from sitting unused in the heat exchanger. The limit cycles the gas valve on and off, while leaving the blower run, to cool the heat exchanger. Now many furnaces use a time delay before the blower comes on, and holds it running for a delay before it shuts off the blower, but of course, the High Temperature Limit is still there.
You’re really supposed to test these things, he didn’t, except the switch that proves the forced draft is present.
But, of interest to me, he had this Meter, featured image with this post was his actual meter:
UEi Test Instruments DL379 Digital Clamp-On Meter
Click Here for more about this meter. DL379B (Described at the link) is the current model on Amazon, watch prices, they vary wildly on this item.
• Capacitance 2000µF
• AC current: 2000µA (.002 Amps, or 2 MilliAmps, a very low value DA)
• AC current (Clamp): 400A
• Temperature range (K-Type): -22˚ to 752˚F (-30˚ to 400˚C) (
• Hi/Lo Non-Contact Voltage 24-600V AC
• Volts: 750V AC/1000V DC
• Frequency/ Duty cycle
• Resistance: 40MΩ
• Diode test
But the meter does a plethora of things, all in one. I’ve used a ton of meters in my life, never one that incorporates so many things. I can’t vouch for it, I’ve not used it, but I wouldn’t mind doing so if the chance arose. The furnace inspector last year had the same model.
Did yinz (Pittsburgh version of You’ All) change your smoke detector batteries, do you have a smoke detector and CO Detector (or combination) in your basement and near your bedroom? It’s important that you do.