Rupp Furnace Ignition

I have a older 1991/93 Ruud furnace model UGVC09NAAGR installed in my attic.
have had several repair techs look at and recently had a new generic igniter installed. Previously went from a manual to programable thermostat to solve problem. no avail.

Problem: When the furnace is set to go down into the 60's during the night it will not come back on to the 72 daytime temperature. I can override the condition by manually turning on the fan for a few minutes until warm air starts to blow then every thing works untill the next night. Seams to be more of a problem when out side temperature dips into the 30's if the out side only goes to the 50's sometimes the fan will come on.

present Repair man (who installed the generic igniter) seams to think the problem can be resolved with a tailored rather than generic igniter which he hasn't been able to get from RUUD. I'm wondering if there is a fan relay circuit that isn't working. any Ideas??????

 

I think the trouble-maker is your fan-limit controller.

Please confirm the following:
a) you have a hot surface igniter (glow coil)
b) you have a Honeywell S89G electronic ignition module
c) you have a Honeywell L4064T fan-limit controller (somewhat similar to the one in the pic below)



From what you describe, this is what is happening (pls confirm):
1- thermostat calls for heat
2- inducer/combustion blower kicks in
3- igniter glows
4- gas valve opens and burners fire-up
5- the main (house) blower does not kick ON, rather, after 30-50 seconds or so the burner flame ceases unless you first manually turn the main blower ON from the thermostat (switching from AUTO to ON).

The L4064T has a tiny internal heater element, with its two end wires connected in parallel with the terminals of the gas valve.
When the gas valve is energized, the heater is too...this heater warms up an internal bimetal which in turn will, after a delay, turn ON the main blower.
If this heater element is defective, or not getting sufficient voltage (say, b/c of a weak transformer), it may just take a bit too long for it to turn the main blower ON, and if so, the limit switch in the furnace turns OFF power to the gas valve and the burners stop firing. The situation gets worst the colder it gets in the attic...b/c the heater will have a tougher time (and take longer at) warming up the bimetal as it gets colder around it.

I'd recommend you test these two wires for continuity (by first disconnecting from the gas valve), and if it passes that test, check the voltage across the gas valve (if lower than 25-26V you have a weak transformer). In all likelihood you may have to replace the L4064T.

The L4064T is old technology. If that was my furnace, I would adapt a Fan-ON/Fan-OFF delay relay to switch the blower on a more reliable time-delay operation. Talk to your tech about it.

I do not think the generic ignitor is the root of the problem. If it glows and lights up the burners it is doing its job.


p.s.: your furnace is really a Ruud, not Rupp...I believe.

 

Correct it is a Ruud. It has a glow coil but I'm not sure about the two Honeywell units. It is installed in the attic and I'm an amputee and can't get up to it.

What you described sounds like my problem and I'll get with my Tech

Thanks.

 

Good luck!
let us know later how you make out.