Originally Posted by Dtodd
The gas valve & the electronic board have both been changed out.
When it fails the gas valve stays open less than a second and the burner(s) nearest the flame sensor don't light.
I am getting 23.7 volts to the gas valve & 24.2 v at the transformer
I'm wondering if one of the vacuum switches is acting up.
HVAC contractor said he checked them but since they have now been here 5 times now I don't have much faith in them.
I have tried removing the vent fan intake pipe letting the furnace draw basement air thinking that if the vent fan was weak it would help, but that makes no difference.
The vent intake & exhaust are 5' longer than max recommended but it ran for 15 years with no issue. Thought about up sizing from 2" to 3" but now sure if that would help.
Will let you know when the problem gets solved.
You said something there that i don't like. Your 24 volt is getting low. Some co. use transformers that just large enough to do the job. A 24 volt
transformer should start out about 27 volts. This voltage can change by the amount of line volt power you have. Is your furnace on its own circuit?
Check the incoming power to the furnace. It should be at least 118 volts.
If it is not then go to you main electrical and test your mains. You are paying for 120 volts and when the voltage drops the amps go up and you pay more for your power. Depending on the power use in your area the voltage can go up and down. So leave a tester in one of your outlets and check it at different times. If you don't have at least 118volts, call your power co. and have them bump it up . That only takes a few min. they can adjust it at you transformer on the pole.
The lower the line volt power the lower your 24 volt power.
If your line volt power is up then you might have to get a larger 120/24 volt transformer.
Transformers have a VA rating on them. Lets say you have a 30VA transformer. To find out how much amperage that will handle you divide the voltage into the VA, so 30Va divided by 24 volts is 1.25 amps.
Now if your control are drawing 1.75 amps then you 24 volt power will drop too low. Doing service work i have called the power co. many times.
If you can't get the voltage up or you are drawing more amps then your transformer can handle then you would have to go to a larger VA rated transformer. I have seen people run phone wire for a thermostat and that drops 24 volt power real fast.. Later Paul