Saturday, November 5, 2011

Understanding 78% Efficient Furnaces.

As of this writing and to the best of this writer's knowledge, the minimum A.F.U.E. (annual fuel utilization efficiency) on a gas furnace is 78%. This means that 78% of your energy dollar heats your home and the other 22% keeps the chimney warm. When natural gas was cheap, this wasn't a big deal and the savings on the first cost might be justified to someone who didn't want to take the plunge for a base 80%. The only real difference that this writer can see between the two is the 78 has a standing pilot while the 80 has a spark ignitor on early models and a hot surface ignitor on later models. As the price of natural gas seems to go up every year, even the 80% furnace seems to be going the way of the standing pilot light.

There are more than a few of these 78% furnaces out there and for the most part they do their job reliably. Just change the filter and service them once a year and they'll give you years of trouble free heating. To the service tech however, these can be counter intuitive. The idea of having a standing pilot and an inducer motor seems a contradiction, but these do work. The inducer does cause the pilot flame to flicker and can blow it out after a time. This is usually after the inducer has been running for an extended amount of time after the main burner has failed to light and not a problem with the pilot flame itself. Make sure the burner is clean and the flame surrounds the thermocouple though. As with any standing pilot, the thermocouple or gas valve may need replacement if there are problems with flame staying on as well. If the flame does stay on when you light it and release the plunger on the gas valve, it should stay on long enough for the main burners to light. Just adjusting the pilot flame will not correct an underlying problem. A call back will be the result.

First and foremost, check to make sure the main burners come on within a few seconds of the inducer coming on. You should be able to hear the pressure switch at the inducer click and the main valve at the gas valve open, followed by main burners. If not, you need to check the pressure switch hose for cracks, damage, etc. The spud on the inducer motor gets really hot on these and the hose will tend to crack after 10 or 20 years on hot metal. If the hose is alright, put a tee somewhere on it and check it with a manometer against the rating on the pressure switch. Do not condemn the pressure switch without checking with a manometer and an ohmmeter. You'll have a call back at best and a prison sentence, or wrongful death lawsuit if you change the value of the switch. These seldom fail anyway. If the reading you're getting is wrong the vent could be plugged, the inducer motor could be breached, or the heat exchanger could be cracked or blown out. You, the technician need to determine the actual cause and not just "cut and run" which is running the risk of the furnace not operating. Take your time and do the whole job. Your customers will love you. Maranatha!

No comments: