Troubleshooting Gas Furnace
Our gas furnance stopped heating the other day. I fixed it. Here's my troubleshooting steps and solution. At 7am I knew absolutely nothing about furnace parts or furnace function. By 2pm I had resolved the issue with our unit.
Unit
Model: Goodman GMH8/GDH8
Installed: 2008
Preemptive Strike
First thing, I replaced the start/run capacitor (20mF, 370VAC) with a new one from Grainger ($15). I replaced a similar capacitor in my old bench grinder the other day. I understand what a a motor doesn't do right when a capacitor goes bad (start). Also, the service tech who was here last month said the cap in the furnace was reading 70%. Not sure I believed him, but it was a cheap part to replace either way.
Wall Thermostat Check
- Seems to be working fine. Batteries are new.
- The fan blower will run continuously when FAN switch is set to ON and the HEAT/COOL switch is set to OFF. Otherwise, the inducer will cycle 3 times ever hour or so, but no blower comes on and no heat comes out.
Furnace Unit Check
- No blinking light on the circuit control board. No error codes. Steady red LED. - 120V Power to unit confirmed. No breakers tripped.
- Fuse on circuit control board looks fine. - Gas flowing to other gas appliance confirmed. The gas range in kitchen works normally.
- Gas cock on pipe near furnace is open.
- Account with gas company in good standing (they have not turned the gas off).
Step-by-Step Run Test 1.) Set wall thermostat to a few degrees above ambient temperature.
2.) Inducer motor starts and runs for ~30 seconds. 3.) Igniter (hot service ignition, HSI) glows brightly and steadily for ~10 seconds.
4.) No ignition occurs, no flame. Problem appears to be at gas valve stage of operation cycle.
5.) Inducer and HSI cycle through 2 more times before automatic lock-out stops the cycling.
Suspected Problems
- Gas flows to valve, but not through valve. Valve stuck or valuve unit broken?
- Bad wiring connection at valve. Valve not getting any power or not enough power?
- Bad wiring connection between valve relay on circuit control board and valve unit?
- Faulty circuit control board (or relay component on board)?
Actions to Diagnose
Checked the voltage reading of slow ("M") and fast ("H") blower fan wiring terminals during the 5 second call to open the valve. I checked this at the white, plastic, push-in style connector. It has 3 wires labeled "H", "C", "M". The plastic plug has molded D-shaped receivers, so connector can only go in one way.
See this video on diagnosing gas valve: GrayFurnaceMan --> https://www.youtube.com/watch?v=j2W0P-u3wIs
I used my multimeter to test the readings on both H-to-C and C-to-M. "C" is the common. Reading of .....can't remember......... was returned during the 5 second call. I confirmed this was correct with the specs on a sticker stuck to the inside of the furnace unit. So the gas valve was energizing.
Removed both upper and lower doors on furnace unit. Held door switch on. Waited as inducer and igniter cycled up, then tapped on the gas valve with the handle of a screwdriver. I heard a click in the valve, the valve opened, the gas ignited, blower came on, and the house heated.
When house cooled off again, the heat/blower did not come back on. The inducer motor and HSI cycled through 3 times (original problem returns).
Solution
New gas valve needed. White-Rodgers #35G54 ($50-150). Found a new ones online for $55 and $140. Will update this post when new valve has been installed and checked for flow rate.
YouTube videos on replacing a gas valve:
RepairClinic.com --> https://www.youtube.com/watch?v=x_C--MHl3x0
SuperCoolSlideRule --> https://www.youtube.com/watch?v=EqODv-xqisI
GrayFurnaceMan --> https://www.youtube.com/watch?v=g8jMJIBVRGo
