Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

incorrect condensate piping!

Del Member Posts: 52
Got a few questions regarding pressure switches.  Had a no heat job where a 90% armstong air was horizontal left mounted in a dropped ceiling.  First time there, I drained about a gallon of water out of the exhaust line due to the condensate drain being plugged.  Now, this is a building that was recently converted into condos and whoever did the HVAC work was really terrible.  This was the first summer they were occupied, and we found out that refrigerant lines, thermostat wires and even power supplies were mismatched between apartments.  They didn;t even put wire nuts on the 24V to the condensers which resulted in shorting out some of the boards on the furnaces. 

   Anyway, on the unit in the ceiling, the condensate lines from the furnace all run into the wall out of view.  There is one from the horizontal exhaust pipe which uses 3/8'' clear plastic tubing to tee into the 3/4 A/C condensate line, and then one from the bottom of the heat ex. which ties right into the A/C condensate drain which is not trapped (I know, dangerous)  Seeing how that is not trapped, I doubt that there is a trap hidden in the wall where the other lines tie in.  On this unit, there are 2 pressure switches.  One is the blocked vent/ intake switch which connects to the burner box and inducer, and the secondary is for blocked condensate line which is connected via a tee at the condensate drain for the secondary heat exchanger. This line was not plugged.  Once I cleared out the drain for the exhaust pipe, the unit fired up and will run for about 4-5 minutes until the secondary pressure switch begins to open/ close rapidly (held my tester on it during the cycle) and eventually opens and the burner shuts  off.

   Based on the other furnaces I have installed, I know that all the condensate from the furnace needs to go into the same trap which is usually supplied with the unit.  It would seem that unless that secondary pressure switch is sensing a drain with a trap, the positive pressure would mostly instead go down the drain and not into the pressure switch.  I jumped it out and let the unit run for 1/2 hour, then checked the drains. All clear.  Talked to the building super and he said they have had the same problems with a lot of the units, so he had his guys jump out the 2nd pressure switch! Am I correct in thinking that un-trapped condensate lines will problems with the secondary pressure switches?

   Also, why do some pressure switches tee into the gas valve?  Is it some sort of air vent in case pressure builds up in the gas valve?   Thanks, and sorry to ramble. I hate cleaning up other people's messes! If it was up to me, I would have walked from this job on first glance.  Too much potential for call backs with all the bad installation. 




  • Tim McElwain
    Tim McElwain Member Posts: 4,518
    What is the Model

    number of the Armstrong unit? I need that so I can see how they are using the pressure switches, many times there are two pressure switches because it is a two stage system and one switch is for low fire and the other is for hi fire.

    The connections from the regulator on the gas valve through the pressure switches to the combustion chamber is so that the regulator on the gas valve is sensing the same atmosphere as the combustion chamber. A normal gas valve the vent on the regulator is in the same room as the air for combustion on an atmospheric burner so they sense the same pressure. When the chamber is sealed and gets it air from outdoors the regulator has to sense that atmosphere from within the combustion chamber.

    Most applications for condensing furnaces do not allow the condensate from the secondary heat exchanger to be mixed with the condensate line form the AC..
  • Del
    Del Member Posts: 52
    not two stage

    Hi Tim thanks for the reply.  Good to know about the vent on the gas valve.  This unit is definitely not a two stage, I don't remember the model # off hand.  I ended up setting it up with a goodman condensate trap because that is what I had on the truck, and then I dumped that into a tee in the 3/4" a/c drain line because that was all there is up in the ceiling for drainage options. Seems to be working, it got pretty cold last night and there was no call today!  I'm still curious as to why the installers didn't use the trap that came with the unit. 

        The second switch is in series with the primary switch, and it only has one hose connected to the outlet of the secondary heat ex. When I jumped it out, the furnace did not give the "pressure switch closed on startup" error.    Thanks again.
This discussion has been closed.