Thread: HVAC Help?

Im hoping that someone here is either an HVAC tech or has had a similar problem and can give me a few things to check before I get bent by an HVAC company.

I have a gas furnace that is probably around 5-7 years old, made by American Standard. There is a digital thermostat that controls it that I installed 3 years ago and seems to work fine. This furnace has no pilot light that I can find.

When the thermostat kicks the furnace on, sometimes only tha fan will come on. The burner never lights or even tries to from what I can tell. The fan just kicks on and blows recycled air through the vents. If I cycle the power of the furnace with the breaker and toggle the thermostat switch from heat to off and let everything sit for a few minutes. And then turn everything back on the burner will kick on as it should and continue to run until it reaches the prescribed temp on the thermostat. Then the burner and fan will turn off. Once the temp drops and the thermostat turns the furnace back on the burner wont light and I get just fan again.

I have changed the filters so theyre brand new. There is no problem with gas flow because the hot water heater is running perfectly. The thermostat has been running flawlessly for three years.

A coworker suggested that there is a sensor in there and that it gets dirty and causes this type of symptom. He said to clean it with some fine steel wool and re-install it. He says the sensor is relatively easy to find and can usually be accessed by removing a few screws. He's not an HVAC guy.

Does this sound like a possible solution? Anything else I should try?

Thanks for any and all suggestions

Given that you said you can't find a pilot light, it sounds like your electronic starter is dying. It will start inconsistently before finally going completely toes up. Don't know if you can replace it yourself (I didn't) - might be hard to find the right model.

Yes.

I would never condone lying to the utility company but I have "heard" that if you say someone smelled gas near your heater you can often get them out there to test things out and oftentimes they will have the part on hand to fix it and you can save yourself the service call money.

Also, if your furnace has a pilot light, there is a part that is connected to the gas valve that works essentially as a mini thermostat and will not allow gas to come out if it thinks your pilot light is out (for obvious reasons). This $10 part can fail sometimes, too. It's called a thermocouple, and in the above scenario that I mentioned this was the issue. Total cost with utility company guy fix: $35.

If your furnace has an electronic controller (on the furnace under the main cover) then it probably has a diagnostic light that flashes X times to indicate a fault. There's usually a list on the controller to decode the flashes. That's the starting point. It'll be the ignitor (like PB said) or a safety interlock (e.g. a vane switch in the duct so the ignitor only fires if the fan is running and the filters aren't clogged). In either case it'll take a service call. Have the model number handy so the furnace guy brings the right parts. Sometimes it's the controller that's failed. The price will make you cry but at least you're stimulating the economy!

Sounds like it works fine as long as you reset it (Breaker off...wait, restart)

So I would think it's your ignition control box (Brains of the unit).
$75 at Johnstone for a universal replacement, easy-ish to rewire (read the directions)

I got the furnace up and running yesterday. I checked the ignition control box and the blinker indicated that everything was fine. That was a relief. So I grabbed a bit of fine steel wool and started cleaning everything that resembled a sensor. There were two that were extremely dirty. I also cleaned what appeared to be a thermostat (made by T.I. and inserted into the heat box area). I then reset everything and it has been working well since.

thanks to you all for your suggestions!