York P2MPD20L08001A "no heat, all fan" problem

[blue11]

I have an older York system, Model # P2MPD20L08001A, installed in 1994.

It has an intermittent issue as - after it reaches the end of the heating cycle and it's time to shut down, the flame cuts out and the blower fan should stop after about 50 seconds or a little longer, but the blower fan will continue to run until I unplug the unit. If I don’t unplug the unit, the blower fan will run all day and the thermostat will no longer control the system.

After I unplug the unit I can plug it back in after a few moments and the blower fan will stay off until it's ready for the next heating cycle. Once I've "rebooted" it by unplugging it, it will work fine for the next 10-50 cycles of heat, then the problem returns. It happened 3 or 4 times last winter, but this year it's happening almost every day, so I need to fix it.

As I said, the problem is intermittent, and seems to happen more overnight (when it's cold in the basement) than during the day. I'm in Los Angeles, so "cold" is a relative term. Around 40F.

My first reaction is that some relay is getting stuck somewhere, and powering down the rig resets it

I am an experienced electronics geek with good troubleshooting skills, and have the proper tools to do measurements, replace parts, etc., but this is my first furnace problem, and so I would appreciate any ideas anyone may have on what the solution might be. Thanks.

[4ftBanger]

Thinking through this one -

If the fan relay were just sticking closed causing the fan to run continuously then the ignition controls would not "know" that the fan was on because the call for fan would have interrupted on schedule at, say 50 (90-120) seconds and the board would assume that the fan had stopped. Then when the next call for heat came from the thermostat the ignition should just start the sequence as if the fan weren't running. I've seen stuck fan relays before and typically the furnace still makes heat on demand from the thermostat while the blower runs continuously.

Your situation is different in that once the fan sticks on the furnace stops responding to any call from the thermostat, which would suggest a safety lockout of some type. Based on the knowledge that it tends to happen during colder weather when the furnace would be running longer cycles, I'd be looking at limit switches. I'm not sure if York uses this logic, but some manufacturers have a limit trip limit - If the primary limit opens more than twice during a cycle it will lock out on multiple limit trip error. Resetting power clears the lockout and error from the board.

[heatseeker]

yes has to be a safety switch or malfunctioning circuit board.

[blue11]

Thanks for the replies.

Can you be more specific about what types of limit switches and/or safety lockouts are typically used in forced air *

After several days, incidentally, the problem definitely occurs during the night and early morning a LOT more than during the day, where it never happens more than once, even if the furnace is cycling all day long.

Can you speak to your experience with the "Universal, we work on most systems" replacement control boards I see on web sites and *

Thanks.

[heatseeker]

yes, don't buy stuff of the internet and if you want to know the name of the components on your heater just look at your schematic.

[Freon]

Post the make/model of the control board on your system.

To be clear, you are saying that once the heat cycle stops and the fan enters into its extended, non-stop, mode of running, your thermostat has not control over the * Did you try calling for heat to see if the thermostat and furnace worked together despite the fan's continuous *

I would suspect your control board especially if you have a simple mechanical thermostat.

[4ftBanger]

The safety I'd suspect is called the "primary limit" or just "high limit" - It's just a simple bi-metal temperature activated switch and they lose sensitivity, accuracy and wear out over time. A limit switch that opened at 175F when it was new might open at 145F or 150F when its 10 years old...

There should be lights on the control board that would indicate an error associated with the lockout. If you can identify what the error is, it can co a long way to pointing to a cause for the lockouts.

[blue11]

4ftBanger -
> There should be lights on the control board that would indicate an error associated
> with the lockout. If you can identify what the error is, it can co a long way to
> pointing to a cause for the lockouts.

There is a single LED as far as I can see, and the next time it happens, I'll look and see what its state is.

Freon -
> Post the make/model of the control board on your system.

The control module, which appears to be a solid block of off-white plastic with the circuit board sort of embedded in it, has this silk screened on -

WHITE RODGERS 50A50-241
York 031-01266-000

Looks exactly like ebay item 190605592249 (I wanted to post the actual link, but wasn't allowed to do so)

> To be clear, you are saying that once the heat cycle stops and the fan enters into
> its extended, non-stop, mode of running, your thermostat has not control over the
> * Did you try calling for heat to see if the thermostat and furnace worked
> together despite the fan's continuous *

That is - once the heat cycles off and the fan goes into its extended non-stop mode, the thermostat loses all control over the furnace, including the ability to turn the fan off or calling for heat. If I do nothing, the fan will run forever as the temperature in the house drops below the thermostat setting. Once I cycle power, it goes back to being fine.

> I would suspect your control board especially if you have a simple mechanical thermostat.

I have a "digital" thermostat, a Honeywell Chromotherm III.

[heatseeker]

Don't buy off internet. It sounds like stuck relay on circuit board or limit switch possibly. If you have not found the problem yet start throwing parts at it eventually you will get lucky of course after the money you have spent you could have bought a new furnace but since you don't want to listen do it your way. Whatever you do don't call a professional they just want to rip you off anyways.

[blue11]

Sorry I haven't posted sooner - my dad is in the hospital and that has taken priority. I have temporarily worked around my problem by using an X-10 wireless controller and appliance module, and turning the furnace off when it locks up, then on a few minutes later.

So I managed to be under the house and actually looking at the furnace's innards while it was running this evening, and when it failed, I got 8 blinks on the diagnostic LED.

According to the -

"This fault is indicated if the flame is lost 5 times (4 recycles) during the heating cycle. This could be caused by low gas pressure, dirty or faulty flame sensor or faulty gas valve. The furnace will lock out for one hour and then restart."

In fact, I watched the flame go out and restart several times during the cycle. I also heard intermittent clicking noises from (I think) the gas valve, like a relay trying to pull in but not quite making it.

The only symptom I don't have is the restart after an hour, which perhaps my controller doesn't support.

While it all seems to point to the gas valve, it doesn't seem to explain why the fan keeps running forever.

Any opinions based on this additional *

> Whatever you do don't call a professional they just want to rip you off anyways.

I would hire a professional if I could, but this has not been a stellar year, and faced with a choice of mortgage payment or heat this month, I have to go with the mortgage payment. Which is why I'm trying to fix this 18 year old system myself if I possibly can.

But I'll tell you - professionally, I'm a visual effects supervisor in the movie/TV industry, and if you have some questions about that, or about higher end home theater, I'll be glad to answer them.