Why does the gas valve on my furnace sometimes clunk on and off? ANNOYING.

Factual Questions
1

I live in butt-ass cold Minnesota. Therfore I have a foreced-air gas furnace, which has been acting up on me lately.

I have an electronic thermostat, which automatically changes the desired temp from 63 to 68 degrees in the morning. Once every few days, as the temperature is making its slow steady climb, the valve on the gas will turn off and on and off with the electronic ignition trying to relight it as this happens. This has an erratic cycle, sometimes with the valve closing and opening every few seconds, or sometimes a couple cycles within a second.

The valve happens to be the loudest part of the furnace.

This is a high-efficiency unit and the exhaust discharge fan continues to run while the valve cycles, as does the main blower. The valve does this for several minutes, then the furnace gives up and shuts off even though the temp is still below set point. A few minutes later the furnace comes back to life, and usually brings the temp up to 68 without further problem.

So I’ve done a little auto-diagnosis, but didn’t get very far. I have tried the following (some of which are based on my hypothesis that the furnaces senses overheating but has no offset):

  1. Replaced the Honeywell controller (auto ignition unit). No effect.

  2. Replaced filter and opened all vents in the house to get maximum air flow – possible reduction in how often the cycling happens (or maybe it’s just wishful thinking).

  3. Partially closed the gas line to reduce the heat load on the exchanger to ~ 1/2. Again, possibly reduced frequency. But not by much.

I would just call the furnace repair guy, but I’m sure it won’t do it for him, so I’ll have taken vacation and shelled out $100 and not fixed the problem.

I would take a multimeter to the controler and valve and other assored wired parts, but I can’t find instructions on what voltages/currents I should find.

Any suggestions on other troubleshooting?

2

glilly - I have not worked on this system, so my suggestions are guesses :

1> Change the electronic thermostat. I believe its the thermostat malfunctioning. I don’t know how advanced the thermostat is - but it may be of the PID type (Proportional Integral Differential). And higher value of the Proportional or Differntial Component will give the instability you are talking about. Call up the thermostat company, and they may help you with tuning the thermostat.

2> Can you describe the Honeywell Controller ?