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Old 12-17-2013, 06:58 PM   #16
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2008 York Modulation Furnace PM9C Issues


I agree, check the venting if it is a 2 pipe system. the venting/pipes have to be in the same pressure zone and installed EXACTLY to the specs/pictures in the install manual on some units or they recirculate exhaust into the intake or trip pressure switches. also the effective length of venting/size of pipe, length and number of elbows and slope matters too.

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Old 12-17-2013, 08:19 PM   #17
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2008 York Modulation Furnace PM9C Issues


So, we left the house at 7:30 am with it at 70 deg and the Tstat now in setback to 67. I came home at 4:30 and the house was down to 58, furnace blowing cold air, full blast, 3-red flashes.... This time, I shut it off, disconnected the cond. drain line from the inducer fan outlet pipe and gently blew into it, pushing condensate out the trap into the drain. I reconnected the drain line, turned the furnace back on and it ran flawlessly for 2 hrs while it brought the temp back up to 70.

Attached is a pic of the cond. trap as well as the circuit board. Tomorrow I have a tech coming to look at it. I am also going to look closer at the trap and look for cracks.

As for the venting etc. This unit is vented outside with 2" pvc, 2 90's and about maybe 10' of pipe, all pitched 1/4" per foot back to the unit. The intake is via ambient air, though I did sway from the installation manual. I added about 24" of 2" pvc for the inlet as with no inlet pipe, the furnace sounded like a rocket.

The Tstat is a LUX 1500 series programmable. set to 3 cph I believe.
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Old 12-17-2013, 10:32 PM   #18
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2008 York Modulation Furnace PM9C Issues


that hose from the bottom of the trap to the floor drain looks kinda kinked. these MOD furnaces are finicky and if they don't drain the water fast enough from the secondary heat exchanger they will trip pressure switches when they change the firing rate. at higher fire it produces a lot more water and if the secondary heat exchanger has water backed up in it then that is enough to trip the switch. Lennox had a few g71s like that but quickly re-engineered the heat exchanger mounts with more slope and problem solved plus we slope the furnace 1/2" at install. they test them in a lab etc but in the REAL world sometimes things are different and have to be made to work. where the hoses attach to the trap at the top they must be real tight or air can get in/air locking. I use black electrical tie straps on some and cinch em up real tight.
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Old 12-17-2013, 10:49 PM   #19
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2008 York Modulation Furnace PM9C Issues


York modulating furnace needs to be set to 6 cph
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Old 12-18-2013, 04:18 AM   #20
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2008 York Modulation Furnace PM9C Issues


Quote:
Originally Posted by hvactech126 View Post
York modulating furnace needs to be set to 6 cph
The reason for them saying to set the CPH to 6, is it keeps the Mod from going into extended run time until it gets colder outside. So setting the CPH to 3 only allows it to go into extended run time sooner. No other effect then that thought.

javan might want to set it for a 10 higher temp rise also. Makes a bit less condensate that way, also quieter at higher modulations.

Also, being piped as a single pipe, wind conditions can effect the pressure switch. Its best to have it as a 2 pipe, so that the air pressure on both pipes is the same.
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Old 12-18-2013, 10:28 AM   #21
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2008 York Modulation Furnace PM9C Issues


Last night when I got home, I gently blew into the drain tubing from the combustion fan outlet and pushed some condensate out. Later I changed the jumper setting to the +10 deg rise.

The furnace ran perfect all night. Nice and quiet, no 3-red flashes, etc.

This am I repiped the gas inlet, fixed the trap outlet and added some extra slope to the unit. So far so good! fingers crossed. tech comes at 2 for a look.
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Old 12-18-2013, 02:09 PM   #22
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2008 York Modulation Furnace PM9C Issues


Tech just left.

No great words of wisdom other than suggesting I upsize the vent from 2" to 3". Manual states that I can use 2" if under 30ft of pipe. With pipe and elbows, I am 31'. He also recommended I take my combustion air from the outside. No real advice on the potential condensate issues, as he could not see anything that would warrant concern.

Now I am wondering what would happen (if anything) if I upsize the vent to 3" but leave the combustion air at room ambient, instead of piping outside?
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Old 12-18-2013, 03:00 PM   #23
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2008 York Modulation Furnace PM9C Issues


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Originally Posted by javan View Post
Tech just left.

No great words of wisdom other than suggesting I upsize the vent from 2" to 3". Manual states that I can use 2" if under 30ft of pipe. With pipe and elbows, I am 31'. He also recommended I take my combustion air from the outside. No real advice on the potential condensate issues, as he could not see anything that would warrant concern.

Now I am wondering what would happen (if anything) if I upsize the vent to 3" but leave the combustion air at room ambient, instead of piping outside?
When you say including elbows you are at 31' is that adding 5' for every elbow? If so you're borderline and just barely over which could explain the unit working in low fire but not when it needs to move more air for high fire. If you didn't add 5' for each elbow you're likely way over & need to upsize.

In either case I'd go to 3" on the exhaust & see if that fixes the problem, if you're still having issues add the outdoor fresh air intake. But like I said being marginally undersized would explain what your unit is doing.

All 2 stage gas furnaces I've seen have 2 pressure switches, one for high fire one for low. If your flue was barely undersized I would imagine it would work in low fire, but when your inducer speeds up to go into high fire it may not have enough volume to make the 2nd pressure switch.
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Old 12-18-2013, 03:19 PM   #24
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2008 York Modulation Furnace PM9C Issues


Yes, current vent has 16' of 2" pipe with 2 elbows mid stream and one at end. This gives me 31'.
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Old 12-18-2013, 03:22 PM   #25
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2008 York Modulation Furnace PM9C Issues


You know that using inside air for combustion. Creates more condensation then using outdoor air, right.
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Old 12-18-2013, 05:02 PM   #26
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2008 York Modulation Furnace PM9C Issues


Inside air is higher humidity. Cannot see why anyone would not want to use outdoor air unless it is impossible to have the 2 pipes due to some obstructions. Saves $$ as you are not sucking cold air in around doors and windows to replace it. Like I said they design these furnaces in a lab and not real world conditions and that is not their fault. If you have a negative or positive pressure in the house (cannot simulate that in a lab) it can affect the combustion air and your basic 1 or 2 stage furnace is not nearly as finicky as a Mod. the pressure switch(s) on them have to be safety rated for all the different firing rates instead of 1 or 2 so that is a difficult compromise. Probably will work better with outside air as it will be more stable load on the ventor fan.
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Old 12-18-2013, 06:43 PM   #27
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2008 York Modulation Furnace PM9C Issues


Yes I am aware of the higher humidity from the house ambient air. The decision, was based on 2 things. 1 being limited locations to route the combustion air pipe through the exterior wall, and 2. being the size. In hind-site, we should have routed through the wall instead of using ambient. Again, this does not explain why the 1st year of operation was flawless.

Note that the house is 55 years old and sits on a basement and 4 crawlspaces. House SF is about 2000, with about 2/3 of that on basement. One crawlspace that is about 600 sf and has an screened exterior access that is about 2 SF. So, this said, I am not worried about pulling cold air through windows and doors, as in reality it is being pulled in from the outdoors via the basement.

Going forward, I will upsize the vent and add the 2nd pipe and hopefully be done with this issue.

I still dislike the way this furnace keeps trying to cycle even without a flame.
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Old 12-19-2013, 07:28 AM   #28
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2008 York Modulation Furnace PM9C Issues


Furnace ran great yesterday into last night, then around 12:30 am, it quit.

I had told the wife that she will know when it has an issue as the sound will be nice and gentle and will suddenly ramp up to full speed immediately.

At 12:30 I awoke when I heard a bang (ductwork). I layed in bed and listened. Then the wife came in from the bathroom and reminded me of what I told her and I told that I heard it too and was waiting. Sure enough a couple minutes later same thing. I ran to the basement and reset the furnace.

Here is what happened. Furnace starts up, inducer fan starts. Flame never ignites, distribution fan immediately ramps up to full, then stops (where the bang occurs in ductwork. funny that I have never heard that section of d/w flex and bang like that before). Furnace tries to start again, same thing happens. I shut it off and go get some water to drink. Come back restart furnace. Same thing happens, but this time after the distribution fan shuts off the furnace tries again and it ignites properly and all well until I left for work (hoping it is still good).

One more thing on this, when the unit was trying to start, I never saw the igniter start to glow. The inducer fan came on and ran for about 1 minute, then the dist fan just ramped to max very fast and all shut down, then tried again.... This is troubling as it is the first I have seen this issue. I wish I had let that furnace code the 1st time....

Last edited by javan; 12-19-2013 at 08:47 AM. Reason: just thought of this....
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Old 12-19-2013, 03:22 PM   #29
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2008 York Modulation Furnace PM9C Issues


It remembers the last 5 error codes.

Sounds like the pressure switch didn't close.
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Old 12-19-2013, 07:01 PM   #30
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2008 York Modulation Furnace PM9C Issues


So, I was hoping (in a sick sort of way) that the furnace would have tripped during the day, but it was on and the house nice and comfy when I got home.

Around 7:30 pm, the furnace is running nice and quiet, then suddenly it ramps up and I am waiting for it the shutdown, but it never comes. It is blowing cold air and has the 3-red flashes. Yet another stuck pressure switch error. Saturday I will be upsizing the vent in the hopes that it will help alleviate this.

This is driving me crazy. Now the wife is talking to co-workers. replace this, check that, consider this, etc.... Wife wants me to go after York..... I really have no problem with York other than the fact that the furnace does not shut off with a stuck pressure switch error.

In my simple mind, the furnace should try 3 times in a row, then shutdown. But do not keep running the fan with no flame.

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