[S_Jeff]

I have a Carrier (Model 80U-19A) furnace from the mid-1970s that is not working. When the thermostat turns on, normally the flames ignite and a short time later the blower kicks in. However, the flames are not igniting, now. The blower still kicks in after the usual wait period. It is a simple two wire heat-only thermostat so I do not think the thermostat is the problem, since it is turning the blower on. The pilot is lit and the furnace valve knob is set to on.

There are no circuit boards/electronics. This thing is pretty basic. A simple panel, with wires linking 5 internal parts in various ways. They are: 1) a transformer-looking thing, 2) a fan and limit control (like this), 3) the gas valve, 4) the thermocouple, and 5) the blower motor.

Based on a test of an identical unit I know that the wires to the gas valve are supposed to send 24 volts when the main burner kicks on. However, on mine, this does not happen.

What is wrong with my furnace? I have a multimeter and can check things if you need more info.

I will reply on Monday.

Thanks!

 

[how]

Trace your wiring from the transformer to the gas valve, testing with a meter at each junction for where the 24V has been interrupted.
The main fan limit or a yet unfound aux temp limit will be the most common suspect.

 

[user_12345a]

Yah, check for 24vac at the gas valve with the stat calling for heat, then work your way backwards.

There's probably a vent safety switch on the draft hood - check it for continuity.

Check the transformer itself for voltage.

----------
DO have the heat exchanger of this unit thoroughly inspected if you want to keep it.

A new condensing furnace would save 20-30% on gas compared to what you have now. (but won't last 40 years)

 

[jogr]

Your pilot light thermocouple is probably dirty or bad. It's usually pretty easy and inexpensive to replace.

 

[how]

@jogr

The OP already says the pilot is lit. How do you see the thermocoupler as an issue?

 

[jogr]

In layman's terms the thermocouple tells the controller that the pilot light is burning and it's safe to release gas. If it doesn't sense the pilot light burning then it won't let the valve open because that wouldn't be safe.

So if it fails (or is too dirty to conduct the heat of the pilot light well) then the gas valve won't open. Blower blows but no heat.

 

[ben's plumbing]

thermocupler produces about 250 milli volts or 1/4 of 1 volt...it allowes for the pilot on a gas valve to be in the ready position for when 24 volts is applied to the gas valve.if pilot is lit we have to assume thermocuppler is working ...

 

[how]

@jogr

In laymens terms...
The thermocoupler produces the dc volts to keep the NC safety solenoid in the gas valve open. When there is not enough voltage to keep that solinoid open then it shuts off all the gas. A pilot working on in it's own is the proof that the thermocouple is actually working.

The exception could be a faulty gas valve which a new thermocouple will not fix, or if that gas valve was ten years older than the OP stated it was.

 

[jogr]

Thanks Ben and How. I didn't realize that the thermocouple would shut off gas flow to the pilot light too but it makes sense.

 

[readar]

I have a furnace from the same era so I am familiar with the operation and the only reason the blower starts (when in auto) is the heat in the exchanger triggers the limit switch. The op states that the blower "still kicks in after the usual wait period". Op also said that there are no circuit boards/electronics and also only a 2 wire thermostat (means no fan control), so it seems very unusual that the blower would turn on for really no reason. What would be triggering the blower?

 

[how]

@readar
The photo that the OP presented was of a fan/limit control with preheater controls that will turn on the fan with just the powering of the 24V stat circuit. These controls are often used with counterflow furnaces to activate the blower sooner than straight bimetal controls normally could.
It is a good thing to know before sticking ones hand into a blower cage, even when there is not even a pilot burning..

 

[readar]

Ah yes, I see the description of that now. The one thing my furnace does not have. You're right then, sounds like the 24v circuit wiring is broken somewhere, should be easy to find, there's not much that can go wrong on these old beasts. The 24v circuit will be AC, make sure the meter is set for that.