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Thread: New furnace - old t-stat

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  1. #1
    DIYer, not in the trades LLigetfa's Avatar
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    Default New furnace - old t-stat

    My old furnace got condemned so I had a new Payne PG96VAT installed. The installer said my old Honeywell CT3300 would be just fine. I'm not so sure.

    The t-stat was set with screws A and B both in. I think these screws affect the duty cycle it will let the furnace run but it really is not explained in the manual. I know even with the old furnace that if I jacked up the setpoint, the furnace would not run continuously like it would with a mercury bulb t-stat.

    Anyway... with the new furnace being two stage, the t-stat is messing with how the second stage comes on now that it is getting a little colder. It will kick up to the second stage just about the time the t-stat decides the furnace ran long enough and it withdraws the call for heat despite the setpoint not having been reached.

    Page 7 of the Honeywell manual says for over 90% AFUE A should be out one turn so I changed it but now have to wait for a cold morning to see how it will react. Anyone care to hazard a guess what the outcome will be for turning screw A out?

    Me, personally would like to toss this old POS t-stat but for some reason the wife thinks it's a good one based on what the installer said. She distrusts all the newer stuff.

    Oh, I also read on page 2 in the Honeywell manual that this model is not compatible with a multi-stage furnace. Am I wasting my time with it?

  2. #2
    Plumber jimbo's Avatar
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    Quote Originally Posted by LLigetfa View Post
    Oh, I also read on page 2 in the Honeywell manual that this model is not compatible with a multi-stage furnace. Am I wasting my time with it?
    Why are we even having this discussion!!!!!!!!!!!!!!!!!

  3. #3
    DIYer, not in the trades LLigetfa's Avatar
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    Because the damn installer told the wife that the old t-stat is not only compatible but that it is better than any new fancy t-stat you can buy today.

    I'm so pissed at the installer for opening his big mouth I could spit.

    The Honeywell t-stat was installed in '98 and I think a lot has changed since then.

  4. #4
    DIYer, not in the trades LLigetfa's Avatar
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    Aside from the fact this stupid t-stat thinks it knows better and overrides the "call for heat" that a simple mercury bulb would provide without discrimination, I don't see anything that suggests it needs anything fancy. The manual makes very little mention of t-stat req's, basically just listing the connections:
    Thermostat Connections R, W/W1, W2 Y/Y2, Y1, G, Com 24V, DHUM

    Anyone know how the furnace decides when to kick the burner up to high? Is it based on a time factor where if the t-stat "call for heat" exceeds a specific time, it kicks it up?

    Does it look for a specific temp rise on the cold return within X amount of time, and if it doesn't see it, it kicks it up? If so, can the bit of hot air short circuited through the humidifier throw it off?

    Does it time how long since the last "call for heat" to determine the BTU it should put out?

  5. #5
    DIYer, not in the trades LLigetfa's Avatar
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    OK, woke up this frosty morning with the house at 68 and turned on the furnace. With the t-stat set at 72, it called for heat as usual but this time the furnace did eventually kick it up to high fire and ran on high for much longer than before. I was too busy to take note of what it was doing but it seemed to stay running on high until the setpoint was reached. The wife got up later and bumped it to 74. It short cycled twice on low before kicking into high but then dropped back down to low.

    The jury is is still out on this. Not sure if the t-stat is the culprit or the furnace can't make up its mind.

  6. #6
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by LLigetfa View Post
    Anyone know how the furnace decides when to kick the burner up to high?
    Looks like I stumped the panel of experts.

    I emailed Honeywell and they can't answer simple questions either. Got an email in to Payne as well. Will see what if anything they come back with.

  7. #7
    In the trades Dana's Avatar
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    Most 2- stage furnaces run on low-fire for a fixed amount of time, then f the T-stat is still calling for heat after the programmed low-fire period (typically ~10-15 minutes) it kicks up to high until the call for heat is satisfied. Simple as that.

    Continuously variable blowers usually have a more sophisticated algorithms for adjusting blower and burner rates to maintain a much more stable room temp rather than using a T-stat determined hysteresis around the setpoint. I'm not familiar with the Payne line-up, or if yours is one of those or one of the dumb time-out types.

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