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Thread: New furnace - old t-stat

  1. #16
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by LLigetfa View Post
    Oddly, the furnace stops ... to switch to high-fire.
    Is it normal for the furnace to stop when switching to high-fire? It is annoying and may be causing me to falsely accuse the t-stat at times. I think at times the t-stat may be confusing the furnace but at other times perhaps the furnace is confusing the t-stat. I think the two are confusing me. ARGH!!

  2. #17
    Moderator & Master Plumber hj's Avatar
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    I would go back to your original statement that the MANUFACTURER, who should know, says it is NOT COMPATIBLE WITH TWO STAGE SYSTEMS, and buy a new thermostat. I am surprised one did not come with the unit, unless he took it to sell to someone who had a thermostat like yours and wanted to upgrade to a modern one. Very few "good technicians" would NOT try to sell you a modern thermostat with a new installation. But if they did recommend reusing the old one, they would have set it up properly, if it was compatible.
    Last edited by hj; 11-03-2012 at 05:39 PM.
    Licensed residential and commercial plumber

  3. #18
    DIYer, not in the trades LLigetfa's Avatar
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    Ja, well... easy to take that one line and say it applies to all two stage systems but that could depend on the specific system. So far in my back-and-forth with Honeywell, they have not outright said it is not compatible. They haven't said it is either. When I pushed them to recommend another Honeywell model and to reassure me that it doesn't have the same run-limit, they sidestep the question.

    I already rendered my opinion on the installer so no way I will be asking for his advice. So far nobody here has suggested another model either.

    Between chores over the weekend, I would keep an ear out for the furnace. Anyway, at one point I was outside near the exhaust and heard the sound of water gurgling. I also noticed a lot of water condensing on the paving stones and dripping from the exhaust. I went inside and followed the 2 inch PVC all the way back to the furnace to see if they left a sag in it. I didn't find any sag but they didn't strap it well enough and looked like the weight of the periscoped end was creating a belly so I tie wrapped it to the support and levelled it out. The furnace was done its cycle and the wood heat was on preventing a "call for heat". Anyway... I noticed a few odd cycles afterwards but did not think to go listen at the exhaust, blaming the t-stat instead.

    This morning the wife calls me at work saying the furnace won't work. I go over all the settings of the t-stat. It was indicating a "call for heat" but we know that doesn't actually mean the furnace is getting the call. I have her cycle the t-stat as well as the furnace power off/on. Still no heat. I get her to go outside to where the exhaust is and she hears a water gurgling noise and cold air blowing out the exhaust. I had her turn everything off and call the installer. My guess is that they probably did not properly configure the condensate drain for the furnace to be laying down on its side.

  4. #19
    Retired Defense Industry Engineer jadnashua's Avatar
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    It's critical on a condensing burner to get the vents properly run. On those I've read about, they must slope back to the unit, not out, as if they do, things can freeze up on a cold day and shut things down.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #20
    DIYer, not in the trades LLigetfa's Avatar
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    It is all properly sloped back to the furnace. It has to be because outside, the vent is periscoped up to get the req'd height.

    The wife called the installer but he did a no-show. Good thing we have wood heat to fall back on.

    As for the t-stat, I finally called Honeywell since I could not get an answer from them via email. They were not interested in trying to get such an old model tweaked and invoked the page 2 clause.

    I signed up for the peaksaver PLUS™ program with the PoCo and will be getting a new t-stat.
    http://www.hydroone.com/MyHome/SaveE...Peaksaver.aspx

  6. #21
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by jadnashua View Post
    It's critical on a condensing burner to get the vents properly run.
    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.

  7. #22
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by LLigetfa View Post
    I signed up for the peaksaver PLUS™ program with the PoCo and will be getting a new t-stat.
    http://www.hydroone.com/MyHome/SaveE...Peaksaver.aspx
    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.

  8. #23
    Retired Defense Industry Engineer jadnashua's Avatar
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    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!
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #24
    DIYer, not in the trades LLigetfa's Avatar
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    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.

  10. #25
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #26
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by jadnashua View Post
    If the hump isn't higher than the trap, it should still drain through the line...
    There is no trap per se in the line. The hump caused water to be trapped between it and the furnace.

    Quote Originally Posted by jadnashua View Post
    If the house cools off more, it will need to run longer to reach the set point...
    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.

    Quote Originally Posted by jadnashua View Post
    If the furnace is oversized, there isn't a lot you can do to stretch run times...
    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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