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BillyHvac, Journeyman HVAC Tech
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I have a YORK modulating furnace installed 8/07. The first

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I have a YORK modulating furnace installed 8/07. The first heating season was fine. In early November 08 it would shut off. My installer came and based on the diagnostic lights replaced the pressure switch. Furnace worked fine for 2 months. On January 9 this year;failed again. Same problem; pressure switch. Replaced again. Furnace worked fine again until this morning. Same diagnostic code. None have been a immediate hard fail. Shutting power off and restarting would resulting in furnace working but would fail later in day. Also furnace always starts but shuts off about 5 minutes later. My technician has checked out other causes and seems very competent. Other than this problem, I love this furnace.

Do you have a 2 pipe system?


one for exhaust and one for combustion air?



Customer: replied 8 years ago.
I have a 2 pipe system. Looks like 4 inch PVC. Every thing is within the mfg recomendation. My installer is a very reputable company. Been in business over 30 years and have a very good reputation.

Hello again,


It is good to hear that you have a great company servicing the unit.


I can really get into this more via any technical releases if you can get me the model number of the furnace with serial number.


This will enable me to examine engineering data as well as any technical updates or issues with the unit.


I have seen evrything from the pitch of the furnace off an inch, to condensation jelling and not draining properly resulting in sporadic nuisance lock outs.


If you can get me that info I can supply you with an analyisis of the unit.



Customer: replied 8 years ago.
Its a PM9 Affinity upfloe 100K BTU.

Model #PC36C3XN1A

Serial # XXXXX

Just to restate...this furnace ran fine for one complete heating season......the pressure sw has been replaced twice and the problem went away twice.

One other thing. The initial fail always occurs at night after the furnace has been turn down for the night. This morning it did come start on time, but when off shortly.

Also, my understanding is that this furnace fires and after about 5 minutes will increase the firing rate if more heat is required. Not positive, but I believe that at this is when the furnace shuts down. The fail code is aways 3 blinking red.



here we go. You have a common denominator which is pressure related.


You have already changed pressure switches which rule a defective switch out of the mix. That means something either due to installation or something introduced into the system is responsible.


I feel if the piping for vent and intake were inadequate, too may elbows etc you would have experienced it from the beginning. So I discharge that option.


Likewise on draft fan, It would be a regular occurance if not daily if we lost rpm or volume.


What else can affect the switch? The condensate drain. Obstruction in vent or intake pipe. Now we are down to 2 fair possibilities. But the first thing techs check when a unit faults on pressure is the intake. Which brings my rambling to a interrmittant issue with the condensate removal.


When condensate builds up in the unit it can fault the switch....meanwhile it still slowly drains and then is not evident. I have found units that needed a water sterilizer tab put in the collector box to keep the water "thin". Hot air with moisture made a nice breeding ground to thicken the condensate which then would not drain or drain very slowly.


I have also found units slightly out of "pitch". The drains in these units drain back to front. When the unit is not pitched right it collects water instead of free flowing.

The unit is humming along on low speed when all of a sudden the draft fan ramps up and jerks that water a bit. All it takes is a microsecond to drop the pressure switch and start all over.


I know it sounds like I have already held court and decided what is wrong...but through process of elimination and what I have seen and experienced in the field, my bet is on the drain.


Check the trap, pitch, extra water in inducer housing, proper drain assembly for unit and the viscosity of the condensate itself.


I am available for follow up.



Customer: replied 8 years ago.
What you say makes since, and my local technician has checked the drain. The unit appears to be level. Some of the other things I am not qualified to check.

It still bothers me that this ran a complete heating season before the first fail. And it ran from January 9 until yesterday during the coldest part winter flawlessly.The first switch appeared to be the problem. It failed multiple times over several days and appeared to be getting worst. The second time I called my technician after two fails given it was a Friday on a cold winter day in northern New England. Since it failed yesterday, It has not re-occured. Furnace has come on about 10 times.

Do you have a theory why the initial fail is always at night after the temp has been turned down for the evening.

One other thing. I have an AC unit also on this furnace. Is it possible for AC water to get down into the furnace.

I think you have been quite thourough. Get back with be on the above and I will accept the charges.

Hello again,


Keep in mind this items I discuss are currently "possibilities". The best find I have had in my career involved a 90% furnace that had 3 contractors that between them had basically rebuilt the furnace. New exchanger, draft motor, pressure switch, reroute of pvc etc. This furnace had literally every part except the outer cabinet replaced.


I went out and found the code of pressure switch (which the unit had done for 2 years).

I pulled the drains and even the collector box to examine the unit and noticed the water moved slow. I then, through research and taking a sample found that bacteria, spores or other organic matter was being drawn into the intake pipe, through the heating process and took up comfortable residence in the warm, wet environment inside the furnace and started to "jell" the condensate.


I then found that the company that made the furnace was aware of this, but had not received the right amount of complaints to qualify for a "technical release".

I installed a sanitary pan strip and at the request of the manufacturer a pice of copper tube in the collector box inside the furnace.


The copper creates a reaction that prohibits the growth of the micro-organisms.


The good news is the furnace never faulted again and the customer was relieved to find out he did not need to replace his furnace.


I am making this assumption based on your information of the quality and credibility of your contractor,and feel comfortable that they have addressed design, piping, mechanical or installation flaws. We already what is faulting....the issue now is to find why.


My theory of why it faults after sitting dormant for a while is the thickening of the condensate while sitting inactive.


I certainly hope you rectify your issue and I am available for any follow up questions you may have.



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