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Furnace Pressure Switch Hose Connections? - Three Hoses

BigRobBigRob Member Posts: 269
edited October 2017 in THE MAIN WALL
Hello all, I recently picked up a vacation home and am getting everything sorted out, including the heating system. The home is pretty remote and there isn't access to professional HVAC service providers. The previous owner might have also been Micky Mouse. For example, the installed filter was literally half the proper size - amazing it didn't fly into the fan.

It's operating well now, but pressure switch two (differential switch) is not connected. Pressure switch one (not differential; single hose) is installed and monitoring the inducer fan (assumed negative pressure, but there are a couple taps on the inducer housing), and proves correctly, allowing operation. There is proper gas pressure at the input and output of the gas valve, and a nice blue flame. I bore scoped the primary and secondary heat exchangers and they are amazingly clean. There is a lot of air volume coming out of the exhaust PVC snorkel (actually pretty loud; might need a muffler at some point). I checked for CO in the conditioned space air stream and it's clean.

I found pressure switch two (differential) is not connected electrically, but is connected pneumatically across the secondary heat exchanger. There is a hose connection at the entrance and exit of the secondary heat exchanger. At first, I was thinking the differential pressure switch had it's two connections connected at the inlet and outlet of the secondary pressure switch. Connected like that, it will not prove, I suspect because there is not a large enough pressure drop. I can hear the switch when I blow into the tube (confirmed by multimeter).

Pressure switch one and two are supposed to be electrically in series based on the circuit board cover diagram, and they both need to close on pressure to activate the furnace.

After this long winded explanation, my question is this: Is there another configuration of the three pneumatic hoses that is industry standard for these furnaces?

The connections I have are:
Secondary HX inlet
Secondary HX outlet
Inducer fan

The furnace is a York P1HDD20N11501, and I cannot find the service manual online. Does anyone have access to the service manual or can you confirm the pressure switch connections for me?


Here are some pictures of the inducer. Pressure switch one is connected to the smaller nipple that appears to be on the inside of the fan rotor, so assuming negative pressure:


  • ratioratio Member Posts: 2,341
    The pressure switch across the HX is to prove the proper airflow to safely fire. If it's not making, either the switch is bad (out of cal), or you aren't moving enough air. You can check the switch with a Magnahelic and a draft simulator (Fieldpiece has a combined unit), or just replace the switch. If the switch is good, then you've got a leak in the HX (or hoses possibly).

    I actually came across just this situation last week. Combustion test was good (<30 PPM CO), HX test was good , pressure switch was good, 0.1 in WC too low to make the switch. There's actually about 0.15 in WC difference cold to hot HX, another clue. We priced a new HX and a new variable speed multi stage unit to the homeowner, as the system was zoned & I know that 120k btuh unit ran hot when 50% of it's airflow went away!

  • BigRobBigRob Member Posts: 269
    Hi ratio, thx. I'm at 7000 feet, so I bet it's right at the margin or the switch is bad. I confirmed the hose connections at the secondary HX and switches are good. The previous owner reported the furnace operated "intermittently," but I did find a couple other issues, so not sure. I ordered an adjustable differential pressure switch.
  • ratioratio Member Posts: 2,341
    You might need to involve tech support in this. 7000' is pretty far outside my experience, but I'd still be nervous about changing the pressure switch without someone telling me to. Have you been able to find a high-altitude kit for that unit? It might specify a new pressure switch.

    You have to keep in mind the lawyers & insurance companies—if anything untoward happened you'd be personally liable once it came out that you'd replaced the pressure switch.

  • BigRobBigRob Member Posts: 269
    I have an email out to York- pretty much zero info on this model online. Maybe I’m using the wrong keywords. I can see the pressure switch was changed before I bought the home. From a reasonable safety point of view, I feel ok since other 90% models seem to only use one switch at the inducer, but I do want to get it operating as indented by the manufacturer. What I really need is that service manual.

    Anybody have access to York documentation?
  • ratioratio Member Posts: 2,341
    Is that really a '95 model? What's the serial number?

    I'll ask around, but I expect recommendations to replace at that age. How's the combustion? Do you have a digital combustion analyzer?

  • BigRobBigRob Member Posts: 269
    It took me four calls to get somebody that would send me the install guide, but I have it. It turns out the differential switch is connected to the inducer fan nipple and then to the inlet of the secondary HX. The regular pressure switch is connected to the outlet of the secondary HX. I'm very confident this is the problem. The primary and secondary HX's are in good shape (verified by snake camera). The documents say the secondary HX has stainless tubes- pretty good construction. Thanks for your suggestions.
  • BigRobBigRob Member Posts: 269
    edited October 2017
    Thanks again for the help.
  • ratioratio Member Posts: 2,341
    That would certainly cause problems, congratulations on getting the paperworks out of them!
  • Gundo_EricGundo_Eric Member Posts: 1
    Hi all - I have been having issues with my York furnace not running and have been getting both the 3 & 4 red flash faults indicating potential pressure switch related issue... It wouldn't turn on last night but did turn on by mid day today and has been working since.

    I have never messed with the furnace aside from replacing the air filters, but I did noticed today that there was a hose from what I assume is one of the HX (sorry i didn't get that in the pic) that wasn't connected to anything, and that the lower pressure switch does has two ports for hoses (see pic). To be honest it does not look like that hose was ever installed (no visible stretch marks near end) so I am not sure if that is the issue. The lower hose is connected to the front of the inductor fan and the upper hose is connected on the far side of the fan exhaust tube. I took the hoses off, and using my mouth sucked/blew gently to hear the switches click. I didn't have a toothpick to check, but it could be possible there might be blockage on the inductor fan front hose point as it looked like it was slightly closed off (could not tell if that was just due to a shield or something being there or not)

    I have not performed any other troubleshooting - Any chance it could be as simple as that hose not being attached to the lower pressure switch? My guess is no since the heater started working again.

    I did not reattach it yet because i was not sure if it should be and did not want to damage the furnace. Wish york put out hose diagrams!
  • BigRobBigRob Member Posts: 269
    Hard to know without the docs. I would also start a new thread if you are still having issues. Usually one switch proves the inducer is running and the other looks for blockages in the exhaust system. The two switches are usually wired/programmed in series so both have to prove for the gas valve to open.
  • BigRobBigRob Member Posts: 269
    In my case, turns out somebody with no technical ability was messing with the system, probably to fix an intermittent issue, and crossed up the pressure switch hoses to get it working. Upon further inspection, LP static & dynamic gas pressure to the regulator was at or below the gas valve regulation range depending on what other appliances were on. The heater is up in the attic about 50 to 60ft above the LP tanks (house is on a grade) - wish they had used 3/4" pipe instead of the 1/2" to the furnace. The orifices were also too big for 7000ft. The LP tank company bumped up the tank pressure a bit and the furnace has been running like a champ ever since. It was for sure running rich since installed. Oops. Also, the gas dryer was never converted to LP. Double oops. Pretty sure that dropped the LP gas line pressure a a ton when running, probably causing the furnace to drop out. Good thing they build a ton of safety into gas appliances!

    The funny thing: there are HVAC company stickers on each from installation- won't be calling them!
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