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Thread: New furnace - old t-stat

  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?

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    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.

  8. #8
    DIYer, not in the trades LLigetfa's Avatar
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    My guess as well is that it is nothing more than a simple timer but as I mentioned, the run-limit on the t-stat messes with it. I found the following thread that describes pretty much what is happening to me but I have not actually timed neither the t-stat nor the furnace. In that thread the OP claims the t-stat lets his furnace run for 25 minutes and that his furnace goes to high-fire after 10 minutes. I'm pretty sure my times are much shorter. I'll have to get up early on the weekend and time it.
    http://archives.hvacmechanic.com/archives/22523

    Turning screw A out did lengthen the on-time and the furnace does now spend more time on high-fire than before. Honeywell won't tell me what the actual time limit is on the t-stat and Payne won't tell me how their furnace decides to go to high-fire. Both say to contact my installer... the same installer that didn't have the smarts to adjust the A and B screws after I told him the t-stat has a short run-limit.

    So, I'm prepared to purchase a new t-stat and I asked Honeywell to recommend one that has a longer run-limit but I insisted that they tell me what the run limit is first. Their answer is to contact my dealer. Maybe I need to look to a different brand, but how can I be sure that whatever I buy doesn't have the same stupid limitation?

  9. #9
    In the trades Dana's Avatar
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    There are a gazillion T-stats out there with no run-limit, simply running a (sometimes programmable) hysteresis on room temp, some with a small (also sometimes programmable) anticipation feature. Those screw adjustments are likely adjustments to both hysteresis and anticipation, but they don't tell you squat about it in the documentation, eh?

    If 1 turn was good, 1.5 turns is probably better, which may be enough to "fix" the operational problem for now but it'll probably overshoot the setpoint. Most thermostats assume bang-bang one speed control and oversized equipment, so if the design assumes no recovery ramp from setbacks will ever take longer than 25 minutes they've pretty much incompatible with multi-stage equipment.

  10. #10
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Dana View Post
    If 1 turn was good, 1.5 turns is probably better...
    From what I discerned via google search results, the screws do nothing more than an on/off function. If the OP in that other thread really got 25 minutes, I'm thinking he used a different combination of settings. I could probably settle for that, if after 10 minutes, the furnace spent another 15 minutes on high-fire. At least for him, the ratio of high-fire is higher than low-fire. For me it seldom goes to high-fire. It has not gotten all that cold yet so not much of an issue now. I set the program to start raising the temp early enough that the house is nearly up to temp when I get out of bed. The wife then bumps it up some more when she gets up.

    There are 3 screws and a fuel switch so 2^4, or 16 different combinations are not beyond experimentation. Still, it burns me that it is not only not documented in the manual, but that they will not divulge it on a direct inquiry either.

    Anyway... been reading about the Filtrete WiFi t-stat and wondering if it had an undocumented run-limit or not. If I could find one locally or just across the border, I'd be tempted to try it out but I'd have to order one online from the US. They don't appear to be available in Canada.

  11. #11
    DIYer, not in the trades LLigetfa's Avatar
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    As I mentioned in my OP, my old furnace was condemned. What I didn't mention was it was because of a cracked heat exchanger. Normally one might expect something like that particularly on a furnace installed in '99 but then I don't use the furnace all that much. I supplement my heat with wood so once it gets cold enough, the furnace may only run for a short bit in the morning before I get the wood fire going. I wonder now if this programmable t-stat didn't hasten its demise with all the short cycling.

  12. #12
    Retired Defense Industry Engineer jadnashua's Avatar
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    Many of the Honeywell thermostats have a learning function that determines when it will turn on from a setback. Instead of you setting it to turn on at 0530 so the house is warm at 0600, depending on how fast the house cooled off and how fast it recovered the last time, it may start at 0545 or 0500 if it is either a mild night or a really cold one.

    If the thermostat wiring has both a low and a high wire, and the furnace is capable of accepting both, with the right thermostat, wired with both stages for control, it may go immediately to high if it sees the current temp is larger than a certain point, or only go to low, if it thinks that's all that's needed.

    Many two stage furnaces, when only wired with a single stage thermostat, rely on time, always starting out on low. The only way to get it to start conditionally on high is if the furnace can handle it along with having a thermostat wired and capable of it...this may require another wire if your cable doesn't have an extra one.
    Jim DeBruycker
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    In the trades Dana's Avatar
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    If it was only getting a couple or three burns per hour on the ramp, maybe, 12 burns per day, it probably isn't related. The wear and tear on ignition and on the air handler motor from startup surges from short cycling would be more typical. I'd hazard that HX cracking would usually partly related to corrosion, more so than a dozen or so thermal cycles/day for 12-13 years. There are many furnaces out there with 25 years of similar use with intact HX, and you'd normally expect at least 20 years out of the HX, even if other components started to fail, pitted motor relay contacts, flaky gas valves, intermittent flame sensors, etc.

  14. #14
    DIYer, not in the trades LLigetfa's Avatar
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    My furnace guy told me that he had to replace every furnace or at least the HX on every one of that model from that time frame. He was surprised that mine lasted as long as it did. The manufacturer has a lifetime (parts only) warranty on the HX but they wanted $300 for shipping and another $500 to install. I figured that the blowers were the next to go so decided to forklift it.

    Jim, my t-stat is not that smart and even if it was, I would confuse the hell out of it with my supplemental wood heat.

  15. #15
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by LLigetfa View Post
    In that thread the OP claims the t-stat lets his furnace run for 25 minutes and that his furnace goes to high-fire after 10 minutes. I'm pretty sure my times are much shorter. I'll have to get up early on the weekend and time it.
    OK, timed it this morning. I had turned off the furnace last night so woke up to 68. Set it to 72 at 7:17 and it ran on low-fire for 5 minutes. Oddly, the furnace stops (7:22) to switch to high-fire. About a minute later (7:23) the blower comes on high speed and runs that way until 7:39 where the setpoint of 72 was reached. The t-stat withdrew the call for heat and the furnace blower drops to low speed for the cool down.

    So, it looks like the t-stat kept up the call for heat for 22 minutes and probably would have kept it up for 3 more minutes but it will take a colder morning than what I had today to find out. I think with the A screw in it was running for a little more or less than 5 minutes. Just long enough that the furnace might just kick up to high-fire but then get thwarted when the t-stat withdrew the call for heat.

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