My friend’s furnace is like 20 years old. When he calls for heat, the pilot light turns on, but no hot air. The air conditioner works fine, as we used it last week. Any idea what’s wrong? He’s calling a guy today, but I want to be able to converse intelligently with him about it. Oh, after the pilot light kicks on, it makes this clicking noise, like the stove makes before it ignites. My friend let it run for 30 minutes and the furnace did not turn on. It’s currently off (via the off switch on the side). Thanks for the replies.
My guess would be a bad flame sensor or thermocouple. The gas to the burner won’t come on if it doesn’t detect a pilot light flame.
Do you actually see the pilot flame get lit? I had a problem in my 20-year-old furnace which sounds similar. What would happen is this:
- Pilot gas value was opened
- Unit sends spark to the pilot gas
- Pilot gas lights up
- Unit continues to spark the pilot gas
- Blower motor starts up (blowing cold air through the house)
- Unit stops sparking the pilot gas
- Unit closes gas valve
- Unit never releases furnace gas
The problem in my system was the flame sensor wasn’t working. So the control unit didn’t think the pilot flame had been lit. The clicks I was hearing was the unit continuing to spark the gas. I had to replace the control unit with a new one. The control unit was a plastic box screwed to the side of the furnace with a lot of wires coming out of it.
Does your friend have two identical furnaces perhaps? If so, try swapping one box for the other and see if that fixes the problem. Just remember to mark where the wires go.
What brand of furnace is this?
It’s a carrier furnace. Nothing blows. The only thing that turns on is the pilot. Thanks for the replies. I spoke with some guy at my office as he was leaving and said it might be the gas valve. But, if the gas pilot light is on isn’t there gas going in? My gf’s dad said to look for the sensor, but I don’t think this furnace has one.
Are there 3 wires that run to the pilot light? (yellow,green and white, iirc…)
Carrier and Bryant used a “3 wire pilot assembly” to light the pilot and flame sensing.
It sounds like the pilot assembly needs to be replaced.
…
Take a look at these links… (the pictures can be enlarged…)
http://universalsupplyonline.com/index.php?main_page=product_info&cPath=22&products_id=4
And…
http://www.myhvacparts.com/Catalogue/Gas%20Furnace%20Controls/Carrier%20Pilot%20Burner.htm
Does it look like what you have?
Yes my friend confirmed that that’s the one. Like I said, it lights. Does it also control the burners for the furnace?
Yes your pilot is lit, but it doesn’t know it. The sensor (part of the pilot assembly) is not functioning properly.
Your furnace has precautionary measures to prevent the gas to the burners (separate from the pilot) from flowing unless it knows the pilot is lit.
Hmmm…that doesn’t look too difficult to install. I sense a DIY project. How hard can this be? I love DIY where it isn’t my house. 
I had a hunch you had a Carrier.
Carrier was the only one that used this type of pilot assembly. The general approach is to replace the whole assembly. However…some people have taken the assembly out and used some sand paper to clean the metal strip that the flame ‘strikes’ or comes into contact with.
This is not to be confused with other flame sensors from other manufacturers -----many/most of which can be simply cleaned with sand paper and reinstalled.
This is a “bi-metal” device that “warps” when the flame is applied to it----warped enough to close a set of contacts and allow the gas valve to open.
Because the bi-metal surface can’t tell that there is actually a flame present it’s doing 2 things: 1) it keeps clicking, trying to establish a flame/pilot, and; 2) it’s preventing the gas valve from opening. (because if there was no flame raw unburned gas would be flowing into your home)
Basically, your pilot assembly doesn’t know that you actually do have a pilot flame.
If money is tight, you may elect to take out the assembly and try the sand paper thing. (and if you PM me I can give you my phone number and walk you through it) As I said, this type of flame sensor is different than most, the bulk of which can be fixed with sand paper. Yours is bi-metal and works on a different principle and therefore it is a dicier proposition as to whether it will work.
The other option is to replace the “3 wire pilot assembly.”
I don’t know how handy you are, but with a little direction it is doable as DIY.
Explained in greater detail in post #12, but yes it does control the gas valve/ burners.
Does anyone?
Holy crap raindog, you’re a furnace genius! Well, he was supposed to have a guy come out yesterday but that dude never showed. Which sucked because we were supposed to have Monday Night Football at his place. I’ll see if I can talk him into a DIY and let me learn from his issue. Thanks again for all the replies.
You’re welcome!
If you try it as a DIY, feel free to PM me and I’ll give you my phone number in case you have any questions/problems along the way.
Good luck.
If you lived next to your friend, in a duplex, it’s entirely possible that the original builder would have put 2 identical units into the structure. One in each unit.
Or if he owns a duplex, but converted it to a single residence, both could be “his.”
</wise ass reply> 
With regards to my comment about two furnaces… I did have two identical furnaces–one for upstairs and one for downstairs. I was able to switch the control boxes from one to the other to diagnose my problem.