Symptom: The furnace turns on and runs normally for approximately 10 minutes at which time the gas valve begins to cycle off and on. The burners will go out but then immediately re-ignite then go out again. The furnace will continue to run but due to the cycling it will never heat the house to the target temperature and will just keep running. The unit is a Carrier 58DRC and is 30 years old. It has a pilot light with ignitor and two limit switches (overheat and flame roll-out with reset) it has no flame sensor probe. I replaced the filters and then tried a temporary bypass jumper of both limit switches but it does the same thing. One thing I have noticed is that when the gas valve turns on rather than making a clunk sound when the solenoid engages it makes a sort of chatter noise as if it's weak or undecided. It also makes this noise when it's doing the short cycle on/off symptom. So could be bad gas valve but not sure how to test that. Any tips or ideas? Thanks Paul

We’ve had cold weather below 30 and unit is blowing cold air. I’ve reset, rebooted, changed thermostat battery, put in clean air filter, checked connections. It seems to work periodically but there’s definitely something wrong. I have dual electric heat pump and oil system. I’m out of oil and since I thought it was for back up and only when I choose to switch to oil on my Ecobee thermostat, I didn’t think it would be a problem. It occurred to me I was likely wrong and because it’s been so cold the heat pump expected to switch to oil but couldn’t. Before I call professionals I’d appreciate input on whether I can fix myself or how to protect myself from unnecessary and expensive work. Background: I had what I thought was a completely new heat pump/air conditioning system put in because my oil furnace broke. Because of its age, replacement “boards” aren’t available for the oil anymore. However the tech who installed it decided he’d try to build a new board on his own time, which he successfully did. That’s why I thought I controlled which system it uses.