New furnace - old t-stat

Discussion in 'HVAC Heating & Cooling' started by LLigetfa, Oct 28, 2012.

  1. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,057
    Location:
    NW Ontario, Canada
    Installers were out today. Furnace fired up just fine. They found a straight section that went into a rolled 90 that was unsupported. It was run parallel to the plenum and by eye looked to be sloped back to the furnace but turned out the plenum was sloped and the vent pipe was perfectly level. The thinking is that condensate would run down to the rolled elbow and pile up there adding weight to the unsupported section. The longer the furnace ran the more condensate would build up causing it to sag more until there was enough there to trip the pressure sensor.

    Also, when they periscoped the end of the exhaust, rather than have the end shoot away from the house, they turned it down so it shot straight down to the pavers. The air intake right beside it was also turned down so the humid exhaust would get sucked back into the intake. They cut off the downturned elbow so that the exhaust now shoots away from the house.
  2. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,057
    Location:
    NW Ontario, Canada
    To this day I did not hear from them. I did call them a while back and was just told "no resource available" with no further explanation of what that meant or when a resource may become available. Anyway... my furnace was acting goofy on Christmas Day so I just turned it off for a while and put more wood on the stove.

    Boxing Day it again started acting up again so decided to go looking for deals on a new thermostat. I followed the recommendation of Honeywell and acquired a RTH8500 thermostat which I installed yesterday. The fire in the wood stove had burned down some and the house cooled enough while we were out shopping that I could give the new T-stat a good test. WELL... the furnace was still goofy.

    Nearest I can tell, the condensate drain needs an air gap close to the furnace. The old furnace needed the condensate line to form a trap so that exhaust gas didn't vent out it into the house. It was installed as a 20 foot length of 1/2" PEX with a direct connection and a hump put in it to form a trap. When they hooked it to the new furnace, they hooked it up directly as well.

    When I went down to the crawlspace the furnace exhaust blower was running and making a water splashing sound. There was a puddle of water on the floor and inside the furnace as well. When I pulled off the PEX line, water drained out of the furnace. I'm thinking that it must not like the long run with no air gap and with a hump forming a trap. For the time being, I put a heavy pail on the hump of the PEX to flatten it essentially getting rid of the trap. I'll keep an eye on it to see if soot forms in it like it did the old one before the hump was added.

    Anyway... I'm still not clear on whether or not I really needed to change the T-stat or not and how this whole "Heating cycle rate" thing is supposed to work. The new RTH8500 has Function 0240 with options 1, 3, 9, and 5 and they look suspiciously similar to what the A and B screws on the old unit did. The manual does nothing to explain what the net result is with each setting. I tried calling their tech support to get clarification. What I was told which was that none of the settings would limit the run-time of the furnace, that the "call for heat" would stay on continuously and the furnace would run (or not) based solely on how the furnace manufacturer designed it. I'm calling it BS.

    So... Honeywell points the finger at Payne. Payne refuses to give me details and says call the dealer. The dealer... well... who knows... I called them and asked them to get their facts straight.

    Back to Function 0240... my guess is that Option 1 will give me the longest continuous run-time and Option 9 the least. Option 3 is for greater than 90% efficiency and Option 5 is for less than 90% efficiency.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    The condensate drain must operate as a 'real' plumbing drain. It needs a trap, and the trap arm must exit lower than the inlet to the trap for gravity to work on the flow. If you don't have enough height to maintain the proper slope, then you may need to add a condensate pump. Then, the outlet would go to a trap, then to the condensate pump. The pump gives you more choices about where to drain things to as you can choose a pump with different head to get it nearly anywhere you need to. If you want to lengthen the run times, you can adjust the hysteresis, but this can affect your comfort level (this would let the house cool off more before it turns the heat back on). Overflowing the internal furnace's drain can quickly shorten the life of the whole thing...you must read, understand, and execute a proper drain system. Condensate is slightly acidic, and can really mess things up if it gets places it shouldn't. Sloshing it around with the blower motor is not good at all!
  4. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,057
    Location:
    NW Ontario, Canada
    The condensate doesn't go to the sewer. It goes to my sump pit that gets pumped to the surface outside. The sump pit also collects from the humidifier, the RO filter, the water softener, the iron filter, the HRV condensate, and the A/C condensate.

    The furnace has an integral trap. I verified it with the installer this afternoon. There is no need for a trap external to the furnace as there is no potential for either furnace exhaust nor sewer gas. The second trap caused a slug of air that prevented the furnace condensate from draining as it should have.

    The problem was the long run of 1/2" PEX strung along the floor with almost no slope to it, a deliberate hump in it, and and no air gap at the furnace end.

    As for the T-stat, I think you are missing my point WRT cycle time. The cycle time I speak of is an arbitrary run length limit that withdraws the "Call for heat" prior to reaching the set point, not the hysteresis. My dealer suggested that since the new T-stat supports two stage, that I replace the RGWY wire with one that also carries W2 and to jumper the furnace for the change. The furnace does support the use of single stage T-stats but it anticipates the need for high fire by tracking the runtime. It is when the T-stat imposes run length limits via the "Heating cycle rate" setting that the furnace improperly assumes low fire is all that is needed.

    One tidbit of fact I found out... If the furnace power is turned on while the T-stat is in "Call for heat", the furnace will flash an error code and blow cold air for a while.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    If the hump isn't higher than the trap, it should still drain through the line. If the house cools off more, it will need to run longer to reach the set point. If the furnace is oversized, there isn't a lot you can do to stretch run times. Make sure you have the furnace blower speed set properly.
  6. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,057
    Location:
    NW Ontario, Canada
    There is no trap per se in the line. The hump caused water to be trapped between it and the furnace.

    You seem to be missing my point. THe house cools off overnight, A) because the wood fire dies down, B) because outdoor temps fall overnight, and C) because the T-stat is programmed to setback the temp to 68 overnight. In the morning, the wife want it to be 73. The run limit on the T-stat did not let the furnace run long enough for it to sustain high fire to reach the 73 set point.

    The furnace is not oversized. The run time is not a matter of the furnace reaching set point too soon, it is a matter of the T-stat not letting the furnace run long enough. I must have made that point at least a dozen times.

    The furnace will run on low fire for the first 5 minutes. If after that it still has not reached the set point, it kicks it up to high fire unless the stupid T-stat prevents it.
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