I have a Day & Night home forced air heater apx 30 years old. Model U-19A. Recently, when the unit is turned on, the pilot light ignites the flames, but they only burn for a few seconds, then shut off. Then they light up again, then shut off. Sometimes they stay on for a period, but most times just on and off, on and off. The fan continues to run no matter if flames are burning or not during the cycle.

Any ideas is appreciated.

Is this a spark ignition furnace? What is the brand name on the gas valve and model number ?

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Originally Posted by hvac1000

Is this a spark ignition furnace? What is the brand name on the gas valve and model number ?

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The information is on back side of valve, and very hard to read even with mirror reading backwards. It is pilot light lite (which works fine), and the model # is "1/2 A 643" (I think). I could not make out brand name

At this point with out accurate information I have no other recourse except to say you might call a HVAC person to have a look at it. There is definitely something wrong but without a furnace model and gas valve name and model number I cannot reference any of my repair information.

Do you have a volt meter? If so, try checking for 24 volts on the gas valve coil when the gas goes off. If you still have voltage, the valve is bad. It the voltage goes off, you need to follow both wires back and see where the break in the circuit is.

To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

This furnace has a limit switch that will turn the fire off if it gets to a point that is set on the limit switch. There are several reasons for the switch to not work properly, one, and the most common is to have a dirty filter. Anything that restricts the air going through the heat exchanger of the furnace will make the limit cut out. This would also include a dirty evaporator coil, this is a very common thing if you have not always kept the filter clean. Anything blocking the return air system, like something stored in the return air space. One other thing could be that you have some of the room registers shut, if so this will retard the air, slowing it down and over heating the limit. And one last thing could be a worn out limit switch. Good Luck. If I help you let me know.

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Originally Posted by hvac1000

At this point with out accurate information I have no other recourse except to say you might call a HVAC person to have a look at it. There is definitely something wrong but without a furnace model and gas valve name and model number I cannot reference any of my repair information.

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Thank you for your time. I am Director, Engineering, at SeaWorld San Diego, have own tools, and will continue to check.

I have removed the same 1/2 A643 gas valve from my Bryant boiler and looking around for a replacement. I can give details of the entire unit (without having to read it backwards!) if you like. Please let me know if you found a reasonable replacement unit (so far the only this a "A643" gas valve search provides is a $250 RobertShaw unit from AmericanHVAC.