York P9M Gas Fired Modulating Furnace (2-questions)


  #1  
Old 01-25-11, 06:22 AM
H
Member
Thread Starter
Join Date: Oct 2006
Posts: 191
Received 0 Upvotes on 0 Posts
York P9M Gas Fired Modulating Furnace (2-questions)

Furnace is about 4 years old and worked fine until this fall.

System will start at low fire rate and run fine, then when it goes to a high-fire rate, the burner goes out, the blower keeps running for 5-10 minutes, dumping cold air throughout the house. Keep getting the 3-blink, "Pressure Switch Stuck Open" error. I have blown this out manually several times and the furnace would then work OK, but this is a real temp fix. I could run fine for a few days, or only a few hours.

1. From all I have read so far, it sounds like the issue is the pressure switch. I have ordered a new one, and it should be here in a day or two.

2. Is there anyway to force the unit to run at a low-fire rate all the time?

I can have the stat set for 70, at 68, it fires the furnace up, I have seen the temp get to 69 on low fire. Then the furnace goes to high fire, burner goes out and after 5-10 minutes of dumping cold air, the temp drops to 66.

Today I am going to take the old switch off and really try and clean out the orifice and associated tubing.
 
  #2  
Old 01-25-11, 08:57 AM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 5,491
Upvotes: 0
Received 4 Upvotes on 4 Posts
Pressure switch is almost always never the problem. The way to properly check your highly sophisticated equipment out is with a manometer. What size flue is used on your system? What is the length of the flue run and number of elbows and degree of elbows?

also check to see if the intake is at all blocked.

With a modulating furnace it is best to call a PRO. There are so many different things that could be causing a problem. It may very well be a Press Switch. However, I don't recommend throwing parts at furnace to try and repair it. Best to have it diagnosed and then replace only the part/s needed.
 
  #3  
Old 01-25-11, 09:14 AM
lawrosa's Avatar
Super Moderator
Join Date: Dec 2010
Location: Galivants Ferry SC USA
Posts: 15,993
Received 85 Upvotes on 77 Posts
Sometimes the condensate does not drain properly and blocks the pressure switch. Check that everthing is draining.

Mike NJ
 
  #4  
Old 01-25-11, 11:33 AM
H
Member
Thread Starter
Join Date: Oct 2006
Posts: 191
Received 0 Upvotes on 0 Posts
The reason I lean towards the Switch is that York posted a memo to all distributors and field service tech back in mid-late '09 identifying an issue with factory mounted switches mfg'd prior to 3/09 that were faulty. Mine was mfg'd in 2006.

The flue is 2" PVC, has 2 90's in the house, 1 outside. Totol pipe run is about 16' (including vertical and horiz).

Condensate appears to be draining properly, I can hear it, but perhaps I need to recheck.

Again, it works fine for low-fire, but craps out on high fire.
 

Last edited by HerContractor; 01-25-11 at 11:35 AM. Reason: subscribed
  #5  
Old 01-25-11, 04:53 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 5,491
Upvotes: 0
Received 4 Upvotes on 4 Posts
The length of the run sounds fine, but wouldn't know for sure with out your installers guide. One thing to check on your flue is to ensure the exhaust is sloped all the way back to the furnace, at least 1/4" per foot. Ensure there are no sags in the exhaust where condensate could get trapped and could cause the unit to trip on pressure. When the unit shuts down when going to high fire, have you checked to see if the switch has truly stuck open by checking voltage across the switch contacts with the wires still attached. It is also possible that the inducer has not ramped up when going to high fire to provide the correct pressure. Again, to truly determine if the switch is faulty you need a manometer. One thing you can do is to TEMPORARILY remove the intake at the furnace and see if it will go into high fire. If it does, this is an indicator of insufficient combustion air or a partial blockage of the intake.
 
  #6  
Old 01-26-11, 05:42 AM
H
Member
Thread Starter
Join Date: Oct 2006
Posts: 191
Received 0 Upvotes on 0 Posts
The combustion air for the furnace comes straight from the basement. It is unoccupied and is roughly 2000 sq. ft. The intake is a simple 2" pvc pipe 6" long with a 90 deg el.

The flue pipe is pitched sufficiently.

The inducer seems to ramp up properly for high fire.

Now 2 new things occurred yesterday and last night

1. When the Tstat was calling for heat with only a 1 degree differential, the furnace went straight to high fire, the inducer did not ramp up properly. The unit then ran about 2 minutes, shut down, did it again, shut down, then tried the low fire route.

2. Last night (this morning actually at 1:20 am), the furnace just finished a cycle, satisfied the Tstat, shut down. About 30 seconds later, it did the same thing in no. 1. Sadly, all the nice warm heat the house just got was replaced by cold air.

I have a new switch arriving tomorrow, but today I intend to look closer at the condensate. The fact that I have most issues late at night when the demand is greater, is making me think that it may be a condensate issue. That being said, I can watch the furnace fail, then disconnect the pressure tube from the upper chamber, blow in it a few times, reconnect and the furnace fires up...
 
  #7  
Old 01-29-11, 02:34 PM
H
Member
Thread Starter
Join Date: Oct 2006
Posts: 191
Received 0 Upvotes on 0 Posts
A new development:

I disconnected and cleaned out the Condensate trap. When I put it all back together, I noticed that the inducer drain line to the trap actually was pitched towards the inducer. Mind you that this was all installed at the factory.....I trimmed about 1/4" off the drain, reconnected so it pitched towards the trap. Furnace has run flawlessly since.....

Oh, the new switch arrived today with a date code of 03/10, so at least I have a good one, if the existing switch ever truly does fail.

Oh what fun it has been!
 

Last edited by HerContractor; 01-29-11 at 02:35 PM. Reason: stuff
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: