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Question DetailsAsked on 12/12/2017

Furnace is blowing mild air, pilot is lit and thermostat working.

The burners will ignite temporarily and then it goes out. The pilot remains lit. I have a new filter, thermostat is set to heat and has new batteries. Old unit so sensors are not there from what I can tell. It does make a clicking noise like it is going to ignite and then nothing happens, but the pilot remains lit. Unsure of the issue. Thermocouple maybe??

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1 Answer


Because of the clicking (trying to fire up), sounds like thermostat is good.

Because you said no safety devices you can see, I presume that also means no LED indicator lights to show either a color pattern or a single LED blink pattern to indicate an error code. (If so, see owner's manual for troubleshooting code meanings.)

Could be thermocouple though a thermocouple failure usually shuts all the gas flow off - including the pilot gas. But voltage output can drop enough that pilot stays on but will not allow main burner to come on. That is good starting point, and for many people a DIY thing to change out.

Other common possibilities, assuming this does unit does not have the safety sensors modern ones have, include (especially for older units):

1) relay failure in the control unit - so you hear the relay clicking into engaged position to send power to the gas control valve to turn on the gas to the burner, but the contacts are so worn and burnt from use that the juice is not getting to the gas control valve to turn it on. Some repair techs will change out a dead relay - most just replace the entire control box for commonly $300-600 total cost but occasionally higher with some models (though usually only with newer electronic units)

2) failed gas control valve - one of the internal springs or the electric solenoid (or similar device) is not engaging the internal gas valve to open it - in which case it would have to be replaced, for commonly $300-500 total cost

3) access panel safety switch has failed - so it thinks the front (or side on some units) pnel is open so that is interrupting the power to the gas control valve - checked by checking power is making it past the switch when it is depressed, and that it is properly mounted (not loose) and engaging the panel correctly when it closes.

4) blower is not kicking on - rarer, but some models require that the blower kick on immediately BEFORE the main burner ignites - if blower is not on then gas control valve will not turn on. There are different ways used to determine if that is happening - depending on how far back you go in time, I have seen centrifugal switches, governor-like centrifugal contact sliders, magnetic impulse switches, even an air blade switch where the moving air moves a reed switch attached to a paddle blade pushed by the moving air.

5) very rarely but happens - transformer (in the furnace) has an internal issue that means it is not putting out the correct low voltage, so may not be enough to operate the relays correctly, or may be too high and burned out part of the system

6) A check that there is gas flow at the valve (shutting off the gas shutoff valve at the appliance, disconnecting the gas line, then VERY briefly opening the shutoff valve to see there is good gas flow would be a check against a blockage in the line letting just enough minor gsleakage to light the pilot, but not the main burner. This can also happen if enough water has built up in the gas line so it is largely blocking the gas flow, but enough gas diffuses through the water to fire the pilot. Rare, but happens - draining the drip leg and checking for gas flow there would eliminate or confirm that. Pressure should be checked at the same time i there is any doubt - rare, and would presumably not be the case if other gas appliances in the house are working OK (like range, clothes dryer, water heater for instance), but on occasion the main gas regulator (outdoors) fails and starts just letting a very minor gas flow through - that has happened twice on my house with different brand regulators, so not all that uncommon. Commonly lets just a trickle of gas through, but not enough to fire an appliance.

7) failed control board, if electronic controlled unit

8) rare, but a with some quite old units which use the frame/housing as neutral return path and as ground, a bad control box or transformer connection to the frame can cause circuit failure.

A check with a volt-ohm meter to see if power is getting to the contacts where the wire to the gas control valve comes off the relay,when the unit kicks on, would tell if the relay is the problem. Gas control valve would be tested by being sure power is getting into the contact tabs on it when furnace is trying to kick on. These may sound like the same check, but on older units the minor vibrations and moving the wires around during maintenance commonly breaks one of the wires (especially if solid wire as they usually are) - almost always at one end at the connection, so while the wires may look intact may be broken and separated just inside the insulation, which is holding the broken end in place so it looks OK. Corrosion of the wires at the connection also occurs in some cases - like in damp basements and especially with crawlspace units, so you need to check not only that the wires have juice but also that the connector screws or tabs actually have power. Very rarely a connection will be bad because of a loose tab connector or connection screw too.

A tech tracing the power flow (and voltages) could determine whether power is getting where it should be - if power of right voltage is getting to the gas control valve from the control box and also from the thermocouple and the main burner still is not kicking on, then the gas control valve would be suspect.

Obviously, if the above did not allow you to solve it, then a Heating and A/C tech (your Search the List category) iswhat you need.

Here are links to a bunch of very similar questions with answers FYI too - some of them also have additional links to other (and some duplicate) questions with answers :

Answered 2 years ago by LCD

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