Lennox Gas Furnace--Voltage/Gas Valve - http://www.hvacmechanic.com/ Forums

Original message

boydch

72.146.27.121

"Lennox Gas Furnace--Voltage/Gas Valve" , posted Sat 16 Feb 12:16

I have a Lennox natural gas packaged unit that is not igniting. I have checked the voltage to the gas valve and it was reading about 30V. I replaced the gas valve and the furnace worked for a couple days. It then stopped igniting again.

I tested the voltage to the gas valve again and it fluctuated from 4V to 20, usually it was around 15-16V. I also jumpered the switches and it did not make a difference.

What could be causing the variations in voltage? Does the gas valve need at least 24V to open? Do I need to replace the ignition control board? It was last replaced in 2002, when apparently there was an electrical fire.

Thanks.

Replies:

marchvac

24.57.149.86

"Re(1):Lennox Gas Furnace--Voltage/Gas Valve" , posted Sat 16 Feb 12:39

Yes you need 24v for the valve to open. What ignition module do you have and what is the model and serial#s of the unit.

Be careful sometimes you don't get a second chance

boydch

72.146.27.121

"Re(2):Lennox Gas Furnace--Voltage/Gas Valve" , posted Sat 16 Feb 13:47

The ignition control board is the one in the link below.

http://www.cozyparts.com/Lennox-Parts/ProductInfo.aspx?productid=60M32

The model is a Lennox 10GCS030-75-IPA. Thanks.

marchvac

24.57.149.86

"Re(3):Lennox Gas Furnace--Voltage/Gas Valve" , posted Sat 16 Feb 14:17

It probably is the module. But there are other safteys that have to be checked. Here is a manual. If it is 20 years old you
might want to think of replacing it soon.
http://pirl.lennox.com/PDFs/503968.pdf

Be careful sometimes you don't get a second chance

dwcaveney

69.104.143.117

"Re(4):Lennox Gas Furnace--Voltage/Gas Valve" , posted Sat 16 Feb 14:34

The 120vac-24vac transformer? My two stage draws 0.3 amps low, 0.43 high fire. (different manufacturer)

boydch

72.146.27.121

"Re(5):Lennox Gas Furnace--Voltage/Gas Valve" , posted Sat 16 Feb 14:38

How do you know whether the transformer is causing the problem or the control board is causing the problem? A technician measured 240V at the board. Thanks.

marchvac

24.57.149.86

"Re(6):Lennox Gas Furnace--Voltage/Gas Valve" , posted Sat 16 Feb 14:57

If you have 24v across the W and C terminal and the limit switches are closed then the ventor motor will start and send 24v back to the module when the pressure switch closes. Then you will get spark followed by the gas valve opening and ignition. Last thing that happens is the flame sensor senses flame and then the main blower comes on. What is the diagnostic light on the module doing (flash codes).

Be careful sometimes you don't get a second chance

dwcaveney

69.104.143.117

"found this on wiki" , posted Sun 17 Feb 13:07

Hi,
24 volt control transformers come in all kinds of sizes and voltage options.
If you have a all electric system then chances are you have 230 volt A.C. primary to 24 V.A.C. secondary.
If you have a gas system i'm assuming 120 volt A.C. primary to 24 V.A.C. secondary output transformer.
The above two are in no case all there are out there but the most common in the H.V.A.C. field.
These small control transformers are typically rated in VA ratings.
Make sure your transformer is showing its full voltage potential reading under full load on the output 24volt A.C. side. If it is not , then you either have a transformer that is to small from some addition to your system or wrong transformer replacement in past. You can check the amperage output with a amproble if necessary on secondary side. It should not go above the name plate rating. If it does, check to see if your equipments literature or supplier & manufacture can tell you what original equipment was designed for this system.
Sometimes the transformer will hum when under heavy load. Whatever part of the system that started drawing power when this occurse is usually your culprit to your problem.
Transformers can just age and go bad. (OR just go bad period) if it hums alot then it has weakly wrapped windings which can cause insulation failure within the coil.
But please do remember most transformers get louder the closer they come to their nominal designed conditions.
Sometimes if your Amprobe will not pick up the small signal you can wrap a wire around the wire you are checking 10 times and connect at ends and it will magnify your reading x10 so you can see it on your amprobe.
Transformers usually really stink when they go down. They have a real sharp burned smell that stands out quiet strongly. So Use your nose.
Basically this is what you will see in most home situations:
central heat only=20 va rating 24 volt transformer
central heat with cooling = 40 va rating 24 volt transformer
Heat Pumps and dual fuel systems typically use =50va output ratings or sometimes 60 va. because of all the extra devices they have to operate in the more complicated systems.

Remember any transformer should be seen and not heard if all is well.

Hope this helps: Jimiwane