Why is my furnace blowing cold air?

Question

My HEIL 7000 natural gas furnace has stopped working what are the most likely causes, and how to verify faulty parts?

Normal operation routine

• Thermostat sends "need heat" signal.
• Exhaust blower starts.
• Electric sparker lights gas.
• Once operating temperature is reached, circulation blower turns on.

Current operation routine

• Thermostat sends "need heat" signal.
• circulation blower turns on and blows cold air.

Things I have checked.

• House has gas (hot water tank is working).
• Followed reset procedure as described in owners manual.
• Looked for obvious signs of electrical short/arcing (black marks, melted wire sheathing, etc)

Answer 1

Turned out to be the High Limit switch. The switch had burnt out, which caused the burner not to fire. Turns out when the limit switch senses the heat box is too hot, it shuts down the gas and forces the blower to continue so it will clear the excess heat. When the switch failed it always told the system that the furnace was over heated, so the burner would not come on and the blower would run constantly to try and clear the excess heat.

To trouble shoot this issue I used an ohm meter to test each sensor, to make sure it was open/closed as it should be. Another way to test would have been to use jumpers to bypass the sensor all together, until the faulty sensor was located.

Answer 2

I am no expert on gas furnaces by any stretch, but have had similar problems with units over the years in rental properties. I often shadow my heating subcontractor because I am always curious to learn how these things work and pick up tips. I'm sure there is a control panel or control board that monitors gas flow in, ignition, fan functions etc. This logic control is designed to shut the supply of gas if ignition fails after a predetermined period of time. I can't tell you how to test the specific control board, but I would bet the problem is one of a few things. Bad igniter, faulty gas control valve, or control board failure. I know this is very vague and not really helpful. If you have spare parts, you could try the simple substitution routine. I'd start with the igniter, then the controller, last the gas feed. If you need heat now and fast, this might be a situation to call a tech, get it fixed fast and learn from the tech for the next time you have a problem.

Answer 3

I just had this problem when I came home from vacation. The heat had been off for almost 2 weeks, and when we turned it on, we got cold air. When I took the panel off the furnace I saw that the ignitor was not heating up. (Ours gets really hot and glows, instead of sparking like the OP's)

I "fixed" it by flipping the switch on the furnace control off and then on again. After that it started operating normally. So I don't really know what the root cause was, but resetting the controller may be a good place to start. ( Especially if it's 1 AM and your house is 60 degrees.)

Answer 4

Attempting to make this question the end all for "my furnace is blowing cold"

In my case, I had a furnace that just "blew cold" forever.

Turns out it didn't blow cold forever. Initially it would fire on, then shut off, fire on, shut off, 5x. Then blink 8 red blinks for an hour, while blowing cold air, then repeat.

Seems for me this meant the flame sensor was bad (for a york/evcon anyway). Cleaning it (even though it looked pretty clean already) seemed to fix it.

Never seen one just keep blowing after failing to sense flame like that...maybe it's newer than what I'm used to.

To test if this is the case for you, flip the switch that controls the heater off then on, then turn thermostat up and watch to see what happens at that point. Or just look for the 8 red blinks while it is blowing cold.

Tricky because this does cause the air to be "lukewarm" as it were. But there will be no flame going while it blows...ref: https://diy.stackexchange.com/a/177452/11265

Also check if fan is on AUTO: https://diy.stackexchange.com/a/25677/11265

If there is no flame ever, check the high temp limit.