Part of the Washington State SOP is removal of the access covers for furnaces. It seems like a bit of common sense, but not every inspector will do it, especially if the furnace is relatively new or electric.
Yesterday's inspection gave me a great example of why the cover should be removed. The induced-draft furnace, a Carrier wasn't that old. It was a bit of a problem to get to it since the builder put in the crawlspace.
Before I clambered into the under-floor area, I set the thermostat up by ten degrees so I could put a heat demand on the system. When I reached the scuttle, I could hear the inducer spinning up and the furnace ignite. All good.
I let it run while I did the crawlspace inspection (also required under Washington's SOP.) After taking pictures of the issues there, I turned my attention to the furnace.
It was side-mounted with the burners to my left and the blower compartment to my right. To get the cover off for the burners, both covers needed to be removed. This shut down the whole furnace, as it should, when the blower door safety switch released.
I fiddled with the blower door and remounted it with it to restart the furnace which obligingly did so. Saw the glow of the hot surface igniter and heard the woosh of the gas burning. And, from the corner of my eye, an oddity. The flame on start-up didn't look right. A few seconds later, the furnace shut down on its own.
Most modern furnaces has a sight glass to the electronic control boards. On these boards, there is an LED (light emitting diode) or two that will signal an error code. In this case, it was three short lights followed by three long. The legend indicated a flame roll out or limit switch failure.
I used the serviceman's switch (a switch located within sight of the piece of equipment that turns power off to it) and reset everything. The furnace started, ignition, and ran without incident.
Have I mentioned I hate intermittent problems?
So I reset everything, again.
And the furnace failed, again. This time I got the (not very good) picture of the flame roll out. If you look closely to the left side of the picture, a tongue of flame dodges the normal flame path and appears to be hitting a wire.
If I had taken the easy way, to start the system and count it as working by just the first start-up, I would have missed a critical detail of the operating status of the furnace. By pulling covers and testing both safety switches, I gathered better and more accurate information. Better still, I found a condition that might lead to a house fire. Protecting the people in that home is the most important part of my job.
The cost of repair probably isn't much, in this case. I say probably because, while the issue may be as simple as adjusting the gas and air mixtures, a cracked heat exchanger can present this as well. In that case, replacement of the whole furnace may be necessary. In between, the venting of the furnace may be a cause.
As a home inspector, determining the cause is beyond my purview. Finding it, though - that's solidly in the inspector wheelhouse.