[maggiesdad]

I have a Heil Model MULK125DK03 furnace. It works fine for AC and for heat as long as the outdoor temp is above 20 degrees. When it gets cold, especially when the temp is dropping the furnace intermittantly starts, gets to main flame and locks out with the ID fan running and nothing else. I come home to the stat calling for heat and the ID fan on.

It happens very infrequently. I tried cleaning the temp sensor but no help. When it locks out it doesn't appear to try again.

It fails when I really need it.

Roger

[hvac1000]

If you have a two pipe plastic PVC exhaust system try disconnect the intake air pipe. Now DO NOT disconnect the exhaust of you will have a CO poisoning problem.

The air coming into the furnace is about as cold as it is outside and can have a adverse effect on marginal electronic components. The units are designed to work with the combustion air coming from the outside but as parts get old they may loose the resistance to the cold temps and cause a problem. If this works it will get you out of the woods but not fix the problem.

Use your head as this is only a test procedure I have used for many years since the new 90%+ furnaces hit the market. This will get you by until you can contact a dealer in your area for the repair or you can read this post from one of my earlier posts on this subject.

.Your question is difficult to answer. It could be the small inducer motor or the capacitor for the inducer motor if so equiped the inducer might not want to spin at lower temps. Since it is temp related I would get a can of component freeze from Radio Shack. They use it on electronic circuits. You could spray it on some of the components when the unit is running and it might cause a shut down since you will be spraying a very cold product recreating cold conditions that the unit does not like to run in.

There are other items that can cause this problem but this may help out.
Hope this info helps.

[maggiesdad]

HVAC1000
Unfortunately I do not have a two PVC system. Intake air is from the room. My next step is the "shotgun" approach. I'll put a new temp sensor first, and then the electronic ignition module.

Thanks for the help.

If you have a two pipe plastic PVC exhaust system try disconnect the intake air pipe. Now DO NOT disconnect the exhaust of you will have a CO poisoning problem.

The air coming into the furnace is about as cold as it is outside and can have a adverse effect on marginal electronic components. The units are designed to work with the combustion air coming from the outside but as parts get old they may loose the resistance to the cold temps and cause a problem. If this works it will get you out of the woods but not fix the problem.

Use your head as this is only a test procedure I have used for many years since the new 90%+ furnaces hit the market. This will get you by until you can contact a dealer in your area for the repair or you can read this post from one of my earlier posts on this subject.

.Your question is difficult to answer. It could be the small inducer motor or the capacitor for the inducer motor if so equiped the inducer might not want to spin at lower temps. Since it is temp related I would get a can of component freeze from Radio Shack. They use it on electronic circuits. You could spray it on some of the components when the unit is running and it might cause a shut down since you will be spraying a very cold product recreating cold conditions that the unit does not like to run in.

There are other items that can cause this problem but this may help out.
Hope this info helps.

[hvac1000]

The freeze component spray works I have used it many times in the past to find problems. I could not locate your model number so I did not know how your system actually operated or was hooked up..

[maggiesdad]

I'll give it a try but the only thing that sees cold would be the gas line since the intake air comes from the basement. I guess that could cool the gas valve to the point of something sticking in it. I'll try cooling the gas valve.

[maggiesdad]

The freeze component spray works I have used it many times in the past to find problems. I could not locate your model number so I did not know how your system actually operated or was hooked up..

I'll try the cool spray. I mistyped the model number. It is a NULK125DK03 made by Heil. I'm going to order a flame sensor and try it. From other posts it seems like the most likely to be tempremental. I found one fairly cheap from Keith Specialty.
The temp locally went from 32 this morning to 24 when I got home from work to a house at 63 degrees, ID fan running, and thermostat calling for heat set at 68. It had worked fine all weekend with temps in the lower 30s. Thanks for your help.

[hvac1000]

Post back with results