Yesterday my son called, said his furnace was not working. Would turn on, run for a bit then shut down, and then in a few minutes try again.
I had one WETT Inspection to perform, and so got that finished and headed to my sons, he had taken the day off for his oldest son to go into pre-op exam so my wife went along and we stayed with his other two sons while he took the oldest to doctor.
Back to the furnace. It was a furnace made by Goodman, sold under the Amana name. Nothing wrong with that, a common furnace in our area. A high efficiency, two stage model.
I turned the furnace off, removed the flame sensor and cleaned it. While this was not the problem, it is something that should be checked as it will oxidize and cause the flame to shut down if not clean and able to detect flame. The flame sensor works by passing a small electrical current through the flame, and detecting that voltage. Flame is actually not a very good conductor, so the amount of voltage is very small, and the slightest amount of build-up on the sensor can be a problem.
I then turned the furnace on to see what exactly was happening. Now the furnace had been off for several minutes so the main board computer would have reset during the cleaning of the sensor. The furnace went through proper start up cycle and ignited. I waited to see if it would shutdown, as my son said that after a few minutes it would turn off. It was providing some heat, but only managing to keep the home up to perhaps 55 or 60F. So we waited, an hour and half later it turned off and the house was up to 72F.
Well, perhaps that fixed it but let’s wait and see.
After a few minutes, we turned thermostat up a couple of degrees to let it start again. This time it began fine with the draft inducer blower running starting as expected, this creates the draft to vent the exhaust outside. Then the igniter would begin to glow, looking good so far. But then the ignitor would dim, glow bright again and then dim. After doing this perhaps three or four times, it shut down.
Several seconds later it attempted to start again, and went through same cycle. Then the LED on main board started flashing 3 times indicating low pressure switch not closing.
High and low pressure switches are placed into the furnaces to detect proper operation of the draft, ensuring the flue gases are exhausted outside properly. Blocked vents can cause problems, as can water in lines blocking pressure variations to the sensors. It has just been running fine, but we checked the vent anyway as well as pulling the lines off and checking for blockage or water building. No problems found. I then took a multi-meter, placed on contacts of the low-pressure switch and measured an open contact. Placing a small vacuum pressure on the hose, the switch operated and the meter indicated a closed contact. The switch was operating; however, the pressure change is very low during furnace performance, and one actually needs a manometer to accurately verify the air pressure change.
The pressure switches work by having a round diaphragm in a closed disc shaped enclosure. The hose is connected to one side and positive pressure will press the diaphragm away, while negative pressure (or vacuum) will draw the diaphragm towards the source. There is a small micro switch on the diaphragm, which opens and closes with the diaphragm movement. During operation, the low-pressure switch should be closed indicating to the computer that the minimum exhaust pressure is present, and the gases will be properly vented. These diaphragms are constructed out of a silicon, however the exhaust gases are corrosive and will eventually eat small holes or even tears in the diaphragm material. A small hole, if present, will cause the switch to not close or be intermittent as some of the vacuum is leaking past the diaphragm. This aligns perfectly with the ignitor glowing and dimming when attempting to start, as soon as the intermittent contact would open the ignitor would dim.
I pulled out a new low pressure switch, and replaced it. Always use an OEM switch, and one that if factory calibrated to the proper setting. We replaced both high and low pressure, I told him both had been in use the same amount of time, exposed to the gases, and it is always good practice to simply replace them both. Both switches and bracket are less than $100.
Once replaced, we tried again, this time the ignitor remained on constantly and the furnace ignited properly. However, it had done that once before on our very first attempt so I wanted to be sure. We stayed a couple more hours through a few cycles, and all appeared fine. Checking late last night, and again early this morning everything appears good.
A inexpensive and easy fix. However, if you do not know what you are doing, leave this to an experienced HVAC technician.
Oh and about the WETT Inspection, as many of you do not likely know what that is, I will cover in another blog.
Have a good day everybody, I have a home inspection tomorrow, but am working on kitchen renovation at my home today.