York PV9 Furnace Overheating


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Old 02-09-14, 11:37 AM
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York PV9 Furnace Overheating

Hey all,

I have a York PV9B12N080UP11D upflow two stage furnace installed in the basement. Its 7 years old.

Docs for this model located here:
Heating - Furnaces - Old Models - York - PV9-UP

The furnace appears to be overheating and I'm looking for assistance troubleshooting. I have tried to be as specific as possible below to describe what's going on but if more info is needed, I'm happy to provide it as I'm running out of ideas.

We had a company come in and do the annual inspection in November. The tech mentioned that high fire was disabled and he didn't know why so he re-enabled it.

On Christmas day, the furnace quits. It was reset by cycling the master power switch off for 2 minutes. It seemed to come back to life and operate normally, but since then, it does the same thing - usually when the house temp needs to be increased from the overnight setback.

Originally, the problem only presented when the outside air temp went below -10C/10F and the furnace otherwise seemed to perform normally. The LED shows 4 red flashes which translates to Open Limit Switch. This will occur 3 times and then the furnace locks out for an hour. The problem only occurs when the furnace is operating on stage 2 heat in high fire about 5 minutes after stage 2 starts.

The induction motor recently quit at the end of January and was replaced which further confirms to me this furnace is overheating. The HVAC techs want to throw more parts at it (with 1000% mark-up) so I'm looking for a better approach.

I have done my own testing and there are 3 limit switches in the wire diagram that could be tripping:

1) Primary Limit
2) Roll-out
3) Induction Motor Limit Switch.

The Induction Motor Limit Switch is the one tripping open (0V on one post to ground, 24V on the other post to ground when tripped). After 2 minutes, voltage is restore across the switch and the furnace fires up again. This makes sense if this limit switch is tripping (rated at 160F -20), cools 20 degrees and the switch closes again.

The return air filter was replaced. I was using one of those high quality allergen filters and bought the cheapest one I could find to troubleshoot - the problem continues.

I replaced the induction motor limit switch with correct part number identical switch - the problem continues.

I have removed and cleaned the fins on the Condensing Coil - it was dirty but there are nolonger blockages there - the problem continues.

I have counted green LED flashes and the CFM is reading 1400. The blower does ramp up in second stage.

Until the problem is resolved second stage heat has been disabled and the unit operates "normally" with the exception that it takes a lot longer to bring the house up to temperature.

Any ideas are much appreciated as to where to look next. The HVAC techs are from the company that installed the unit 7 years ago and told me the unit is not overfired but seem confused as to what to do next.

I am going outside today to pull a brush through the exhaust vent and intake pipes as this is the last thing I can think of to eliminate blockages here as an issue.
 
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Old 02-09-14, 11:41 AM
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A previous poster had a problem with an oversized furnace overheating. Maybe you have a similar problem?

Tech sez reason I'm always hitting a limit is the furnace is too big

Read more: http://www.doityourself.com/forum/ga...#ixzz2sqrsV5GK
 
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Old 02-09-14, 11:47 AM
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Welcome to the forums.

It does appear to be overheating. When they installed that furnace they may not have done a heat loss calculation and just stuck that one in.

It would appear that now the duct work may be too small to handle the heat that is produced when the furnace is in high fire mode which is probably why it was set to low fire only.

Have they tried increasing the blower speed ?
 
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Old 02-11-14, 06:15 PM
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I have removed and cleaned the fins on the Condensing Coil - it was dirty but there are nolonger blockages there - the problem continues.
Do you have a/c? Was the indoor coil which sits above the furnace checked for dust buildup? (presumably you were referring to the secondary heat exchanger)

Are all your vents open?

How many vents are there? (what sizes?)

Post supply and return air duct sizes.

--------------------------------------------


Measure the temperature rise (temperature difference between supply and return) on high fire and compare it to the range printed on the rating plate.

If it's above the range, you either have an airflow issue or the furnace is overfired.

The blower motor in your furnace is designed to deliver constant airflow as operating conditions change (for example, your filter getting dirty), so for the limit to trip, there would have to be a very major airflow restriction or grossly undersized ductwork.

Throwing parts at the problem definitely won't fix it; only measurements* can tell you what's going on.

*Temperature rise, gas pressure, static pressure, gas input (google "clock gas meter"
->Bolded things are DIY friendly
 
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Old 02-16-14, 05:51 PM
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Thanks everyone for help. I am only able to look at this on weekends due to work schedule so there may be lag in my replies.

Have they tried increasing the blower speed ?
-No. Is there a simple way to do that? I think its already set to max CFM by the jumpers on the circuit board.


Do you have a/c? Was the indoor coil which sits above the furnace checked for dust buildup? (presumably you were referring to the secondary heat exchanger)
-We do have AC. I was only able to visually check from the top and it seemed clean. I will try to look from the bottom next at the end of the week but the data below tells me that the rise is likely ok.


Are all your vents open?
-Yes, all vents are open.

How many vents are there? (what sizes?)
-There are 12 standard floor vents connected with round ducting and there are 2 more connected by thin rectangular pipes to wall outlets.

-There are 3 return air vents:
2 side wall type 24 x 6.5"
1 ceiling type 6 x 10"

Post supply and return air duct sizes.
-If this is the size of the tin attached to the furnace itself, then:
Supply = 18.5 x 21.5"
Return = 10 x 18"



Measure the temperature rise (temperature difference between supply and return) on high fire and compare it to the range printed on the rating plate.
-I have tested this and it seems to be in range.

The Air Specs state the following:
Air Temperature Rise High Fire is 45-75F (25-41.7C)
Air Temperature Rise Low Fire is 45-75F (25-41.7C)
Max Outlet Air Temperature 175F (79.4C)
Max External Static Pressure 0.5 IN.W.C. (0.12 KPA)

I didn't measure intake temp but lets says its close to room temp of 21C. The outlet air temp about 6 feet from the top of the upper condenser is 55C - a rise of 34C. This occured over a time of 17m12s at which time the limit switch tripped and the test was stopped.

If it's above the range, you either have an airflow issue or the furnace is overfired.

The blower motor in your furnace is designed to deliver constant airflow as operating conditions change (for example, your filter getting dirty), so for the limit to trip, there would have to be a very major airflow restriction or grossly undersized ductwork.

Throwing parts at the problem definitely won't fix it; only measurements* can tell you what's going on.

*Temperature rise, gas pressure, static pressure, gas input (google "clock gas meter"
->Bolded things are DIY friendly
Ok so I tried this and got strange numbers, not certain if I did it right. The York manual states that I should use the following formula:

For Natural Gas multiply the Heat content of the gas MJ/m3 (or Default 39.2), times 0.0566 m3 of gas measured at the gas meter, times a barometric pressure and temperature correction factor of 0.960; times 3600, then divided by the time it took to measure 2 cubic ft. (0.0566 m3) of gas from the gas meter.

MJ/m3 x m3 x 0.960 x 3600 = MJ/H x 0.2777 = kW x 3412.14 = BTU/H
Seconds it took to measure 2 cu.ft. of gas

My gas meter has a dial marked 0.01m3 and another marked 0.05m3. I'm assuming the 1 full revolution is equal to these numbers. So I measured 1 full rotation of the 0.05 plus another increment to get as close to 0.0566 as possible.


I used:
39.2 x 0.1 x 0.960 x 3600
95 seconds

=142.6055 x 0.2777 = 39.60154 x 3412.14 = 135,126 BTU/H

This seems really really high seeing as the high fire input from the data plate should be no greater then 80,000 BTU/H.

Bad math or something really wrong? The manual states:

If the actual input is significantly higher than the furnace input specified on the rating plate then replace the gas orifice spuds with the gas orifice spuds of the proper size for the type of gas you are using.

Is this likely?
 
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Old 02-16-14, 07:49 PM
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Check the temperature rise as close to the furnace as possible.

Measure the return air temp; don't assume that the return air temp is what the t-stat is reading. (often, it's not)

One other thing you can try:

Physically remove the limit, put foil tape over the hole, and stick a thermometer through the tape. The measured temperature should be well below the limit switch temp rating.

Something else in the circuit could be dropping out on high fire only - verify it's the high limit by checking resistance across when it trips. (or voltage drop)

---------------------
As for gas meter clocking: Make sure your water heater isn't on at the same time and subtract at least 1000 btu/hr for each standing pilot. (better yet, shut the pilots)

I would ignore the service manual pressure info (decent gas meters compensate anyhow) and just calculate the input.

So for example, if one cubic meter of gas contains 36 000 btu (it varies - could be a little lower in some areas) and it takes 20 seconds for the 0.01 dial to turn...


BTU input = (3600 seconds in hour / 20) * 0.01 m3 * 36 000 btu per m3
= 64 000 BTU/hr
 
 

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