Suburban Furnace

Turbojimmy has a great point- you get a voltage drop across the thermostat when the contacts are oxidized. When you first turn the thermostat up for heat, there is more pressure on the contacts -> less voltage drop. You also typically get a voltage drop across the sail switch (a.k.a. air prover) and high limit switch.
The board will usually work down to around 10.5 volts, but it's nothing to loose 3 volts across those items.
If you have the original thermostat, I would change it, no question. Cheap, easy, and really needs to be done.

That said, I still recommend an inspection and cleaning.

Hemi RT wrote:

turbojimmy wrote:
I would check the thermostat before you dig into the furnace. I had an intermittent no-start problem with mine (1984 vintage) and it was the thermostat. I replaced it with a new Suburban unit and the furnace has been fine.

Not the thermostat, gas valve doesn't always open & when it does it ignites but won't stay lite, the electrodes are new & gapped properly

Mine would light pretty much every time when you first turn it on. It would reach the set temp and shut off. When the temp dropped again, the fan would fire up but the burner would only light about half the time. Sometimes it would work fine all night, other times I'd wake up freezing cold and have to shut the Tstat off and on several times to snap it back into reality. Eventually even that stopped working. New Tstat fixed it. YMMV.

turbojimmy wrote:
I would check the thermostat before you dig into the furnace. I had an intermittent no-start problem with mine (1984 vintage) and it was the thermostat. I replaced it with a new Suburban unit and the furnace has been fine.

Not the thermostat, gas valve doesn't always open & when it does it ignites but won't stay lite, the electrodes are new & gapped properly

I would check the thermostat before you dig into the furnace. I had an intermittent no-start problem with mine (1984 vintage) and it was the thermostat. I replaced it with a new Suburban unit and the furnace has been fine.

The standard Dinosaur UIB-L or -S would work- the L would be a direct replacement- no new holes, the S would work fine, drilling one new mounting hole.
You could use the fan control board, but that would require some rewiring, and your model uses the non time delay relay along with a fan switch, which really works pretty well.

Do be aware that furnace is OLD though- you really need to carefully examine the heat chamber, make sure the cast iron burner is clean (but don't enlarge the slots at all), and replace all the gaskets. Suburban offered parts for a while to change the burner to a new Stainless Steel burner, but they stopped.

That said- being old is not an automatic condemnation of a furnace. Old ones were often built heavier.