Few things can frustrate a technician, and an anxious homeowner more than a furnace that should be running fine, but instead is putting out 90 degree air when it's below freezing outside. Last evening I had to go to a house out in Dansville because this owner's furnace wasn't keeping up. He did have a 1600 square foot house with vaulted ceilings and a ten foot high basement, which was uninsulated. The house itself was well insulated and the homeowner had said this problem was very recent, within the last few days. The furnace in question is an AUX060, so it's 60,000 B.T.U.s. Not blistering, but plenty adequate to heat a house of this size. The duct work and runs were in good order and well-balanced (the dampers were adjusted right). The filter was clean and so was the blower wheel, but if these were dirty, the heat output would be too high not too low. The furnace was started and the burners came alive with a "pop". I checked the heat rise and it about 25 degrees Fahrenheit, or about 14 Celsius. This was by checking the return air temperature against the discharge temperature from the supply side. The difference is the heat rise. I needed 30 to 65 degrees Fahrenheit, the former being the minimum and the latter being the maximum.
So, I checked the venting, inducer, and finally the manifold pressure. This was set to 10 inches of water column on low fire. I gasped in disbelief; apparently, the last tech who put the gas valve on didn't set the manifold for low fire correctly and could have damaged the heat exchanger. Since this is an LP furnace, I set the manifold to 6 inches for low fire. The high fire was set to ten, which was correct. Cycling the furnace revealed that although the gas came on, it didn't start the burners. The ignitor was at 15 ohm and I cleaned the flame sensor. Still no luck and the homeowner was getting anxious.
Without a word, I removed the manifold from the burner box, removed the ignitor and the burners. These were dirty and the runners too close together and corroded. I cleaned these out and spread the runners apart to the thickness of a dime. After that put them back in and started it up. The burners started smoothly and another unexpected result ensued, the furnace was warm again. Apparently, the dirt in the burners and runners was enough to cause the last tech to turn up the gas pressure on the valve to get these to light. Because this particular model has what I affectionately call a "three handed" burner design, the burners are a bit of a pain to get back together. Most techs avoid doing this as a result. I pull the whole burner box out on these ones to make it easier. All I do is turn this box upside down and put the burner in; no problem.