Gas and Oil Home Heating Furnaces - Gas furance burner question

Hi,

I have a old Heil furance that has run fine for years. Recently I noticed it sometimes will NOT start the burner. When the thermostat activates I can hear the usual click from the furance (an electrical relay I assume) but no 'woosh' from the burner. If I turn the thermostat down and back up I will hear the electrical click again, then the burner 'woosh'. Sometimes I will have to repeat the thermostat trick a couple of times to get the burner to iginite. Othertimes the furance will run for days with no problems.

I have checked and redone the wiring to be sure I have good connections.

At first I thought it was the Honeywell Gas Valve, but the valve always closes to stop the burner. I'm guessing if the valve was 'sticky' the valve might stay open and the furance would keep running. Unless the valves have some safety feature to be sure they close?

Next thought is that one of the relays is bad. I made myself a schematic. There is one relay for the fan and temperature range for ON/OFF. Then there is another relay (not very well labeled) that has 2 input lines and 2 output lines. The only thing I found on this was 24V - 5VA. I'm guessing its a relay/transformer?

The best solution is a new furance, and I'm working on that, but for now I need to keep the old one running. Any help with my problem would be greatly appreciated, and thanks in advance.

What kind of ignition system does it have? Is it a standing pilot, or a spark generated to first lite the pilot? Next time the gas valve won't open, check the terminals on the valve to see if you have 24 volts. If the furnace has an ignition module, post the brand and model #.

Standing pilot. I will check the voltage the next chance I get. I spent part of yesterday searching the DIY archive. I came across one that looks similar the conclusion was: 'The gas valve wont open if the pilot light is not detected' Is this a reasonable possibility in my case? If so is there something I can read about how to clean/adjust this? BTW, I have NOT had any problems with the pilot light.

The pilot never goes out? That means the thermocouple and the pilot burnner are ok. Check the voltage at the gas valve terminals the next time it won't lite. Don't reset it or manipulate the thermostat, just check for 24 volts at the gas valve terminals. If you have the voltage but the gas valve won't open, replace the gas valve.

Well, I tried. When I got home yesterday the thermostat was on for 70, the house was at 63. I got my multi-meter and set it for DC, range 200V (for the range scale mine goes from 20 then 200). As soon as I touched the leads to the two posts from the relays (NOT the thermo coupler) the burner ignited. I repeated the sequence again later that night with the same results.
I could NOT get a voltage reading. I'm assuming I measure potential across the two relay wires, not ONE terminal to ground, correct? Any thoughts about what it means if this causes the valve to open? Again this past weekend I removed each wire (one at a time) cut the end off and strip back the insulation back and firmly reattached the wire to the terminal. So I do not believe the connections are loose. The underside of the terminal screw could have oxidation on it, I did not check that, usually re-tighten a screw takes care of that. As always any thoughts or ideas are greatly appreciated.

The voltage at the gas valve terminals will be Alternating Current. Set your meter for AC volts not DC.

0 Volts A/C. Across the pilot/thermocoupler 0 AC, .286 V DC.
At the transformer (that the thermostat connects to) the red and white wire (going to the relay and Fan control) there was 27VAC. At the input to the relay and the Fan control there was 26.8 VAC. Because I was standing there I gave the relay a 'flick' with my index finger and the burner ignited. Thus I think it may be the relay. There is very little writting on it: Honeywell R8225B 10071, is this really a 'relay'? I'm moving slowing but I think I'm narrowing down what is good and possibly bad, again thanks for the help so far.

Yes, that's a 24 volt relay and it is probably the relay for the blower motor. However, it does not control the gas valve. The heating control circuit should go from the transformer to the thermostat, from the thermostat back to the furnace, then to the high temperature limit control and then to the gas valve. I think the limit control was open preventing power from getting to the gas valve terminals and tapping on the relay may have affected it. Starting at the gas valve, trace the wires back. They should go to a limit control mounted on the bulkhead. It is probably a combination fan/limit control and, if it's a Honeywell, it will have a number like L4064_ _ _ _ _. Next time the furnace won't lite, check for power in and out of the limit control. IOW- from both terminals on the limit switch side to common on the transformer. Common is the side of the transformer that DOES NOT have a wire going to terminal R on the thermostat. I would be very surprised if you write back and say the wires from the gas valve go to the relay, but anything is possible. I looked up the relay at Honeywell's website- it's a spdt blower motor relay.