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Airheatman
Airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 4567
Experience:  Furnace, A/C and heat pump specialist.
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I have a bryant gas furnace, the issue i have is the gas control

Customer Question

I have a bryant gas furnace, the issue i have is the gas control valve short cycles, could this be caused by the limit switch or could the gas control valve be faulty?
Submitted: 9 months ago.
Category: HVAC
Expert:  Airheatman replied 9 months ago.
Hello. My name is XXXXX XXXXX I'll do my best to help. Troubleshooting often requires meters and test equipment. Let's get started and see what we can do.
Can you tell me the model of the furnace? Then lets look and see if the furnace has a control board with an led light on it. If so, count the number of flashes on the light and that will tell us where to start.
Customer: replied 9 months ago.

I do not have the model number with me...It is an old unit most likely from the 1980s to early 90s ... There is no light on the board...What is occuring is the gas control valve shuts on and off, basically short cycling. My question is could a faulty gas control valve short cycle?

Customer: replied 9 months ago.

Its an old model, no light on the control board...I did test everything with a meter, as I mentioned the gas control valve short cycles, basically the power goes on and off....could a bad gas control valve coil short cycle or would something else cause the valve to short cycle?

Expert:  Airheatman replied 9 months ago.
Absolutely. However, it is usually a limit or something that is actually cutting it out. The way to test is to take a volt meter and place the leads on the gas valve terminals. On call for heat when burners ignite, we will get 24 volts. Hold the leads on the terminals and see if the 24 volts is dropping out when the burners go out or if it stays constant.If the 24 volts stays constant, issue is with the valve,If voltage is dropping out, we need to check limits and components to see what is cutting out.
Customer: replied 9 months ago.

I did check the volts on the gas valve and it does drop out, at times it drops out for a second or 2 and other moments it drops out for a longer period of time...Being that it is an old model it only has a one limit switch with a fusable link.

Expert:  Airheatman replied 9 months ago.
OK, you can do the same thing here. leave one lead on the valve and place other lead on the wire at each side of the limit. See if the 24 v is constant to the limit and dropping out at other side. If so, either limit is defective or we have an air flow issue. A dirty ac coil sitting on top of furnace will often cause this.
Customer: replied 9 months ago.

When I jumped out the limit switch the gas valve was still short cycling, I have already checked the ac coil and it is clean...is it possible that the gas control valve itself is the problem?

Expert:  Airheatman replied 9 months ago.
I would start by replacing the limit.The gas valve is NOT the problem if the voltage is dropping out.The valve is doing what it is supposed to. It has to have 24v constant to stay open. Please reconsider the rating and change to good or excellent. You have been provided excellent answers and troubleshooting and am willing to go step by step with you until we determine your problem.
If you come to JustAnswer/Pearl and ask questions.then leave poor ratings when you are being assisted with excellent help, other experts will be reluctant to try to assist you in future.
Note, leaving a poor rating does not keep you from paying for the answer. If after an expert has assisted if problem cannot be resolved, expert can request a refund of your deposit for you, but once you leave a rating, you are automatically billed.
If you would like further assistance on this I am here for you..
Airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 4567
Experience: Furnace, A/C and heat pump specialist.
Airheatman and 2 other HVAC Specialists are ready to help you
Customer: replied 9 months ago.

Still having the same problem...Here is the model number of the furnace 394gaw036075...I jumped out the limit switch and still the gas control valve continous to short cycle.

Expert:  Airheatman replied 9 months ago.
I cant seem to locate a manual on the wiring on the 394. I am going to open to other experts who may have readily available. We have to follow the control circuit and find out where the 24 volts is dropping out. Valve cannot stay open unless we 24 volts to it.
Expert:  Airheatman replied 9 months ago.
I found this video. There are some diagrams at the end. Maybe you can determine from the video and diagrams where we are losing the 24 volts.
http://www.youtube.com/watch?v=upYhuy0AMM8
Customer: replied 9 months ago.

I have seen that video before I messaged you on here the first time. However the video still doesnt explain to me why the gas valve short cycles so often, in most cases the gas valves will close and reopen so fast that the flame is still burning and at other times it will close and the flame goes completely out and then within a few more seconds it reignites. Basically the amount of time between short cycles are random, they can vary from 1 second to several seconds

Expert:  Airheatman replied 9 months ago.
Can you verify if we are losing the 24 volts at both the oick terminal and the hold terminal. It may be an issue with the pilot or may be ith the board. First we have to determine where we are losing the voltage,Then we can determine why.Still looking for a manual on the furnace.
Customer: replied 9 months ago.

The circuit board is new, I just changed it last week ...I did check the pick and hold terminals and its not losing from both at the same time.

Expert:  Airheatman replied 9 months ago.
Lets start with a probe on the common and a probe on the hold and verify 24 volts. Then follow the wire going to hold back to the next component and see exactly at what point the 24v is dropping out
Customer: replied 9 months ago.

I cant do that now I am not at the location where the unit is..but can you tell me what I need to check for to determine if I have a faulty gas control valve?

Expert:  Airheatman replied 9 months ago.
OK,if we were getting 24 volts constant at the gas valve and it was cutting on and off, we would have a bad gas valve. However. it is my understanding that your 24 volts is dropping out. A brand new gas valve would not stay open and burners lit if the 24 volts required to keep it open is dropping out.
What we have to do is with a volt meter, check each component in the circuit for 24 volts until we find which component is causing the 24 volts to drop out.

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