View Full Version : New furnace - old t-stat

10-28-2012, 03:12 PM
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?

10-28-2012, 04:23 PM
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!!!!!!!!!!!!!!!!!

10-28-2012, 04:38 PM
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.

10-28-2012, 06:01 PM
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?

10-29-2012, 04:56 AM
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.

11-02-2012, 07:11 AM
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.

11-02-2012, 08:36 AM
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.

11-02-2012, 12:23 PM
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.

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?

11-02-2012, 01:02 PM
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.

11-02-2012, 01:44 PM
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.

11-02-2012, 01:54 PM
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.

11-02-2012, 02:11 PM
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.

11-02-2012, 02:13 PM
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.

11-02-2012, 02:23 PM
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.

11-03-2012, 06:13 AM
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.

11-03-2012, 07:48 AM
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!!

11-03-2012, 05:37 PM
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.

11-05-2012, 10:36 AM
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.

11-05-2012, 04:32 PM
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.

11-05-2012, 05:11 PM
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.

11-06-2012, 04:31 PM
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.

12-27-2012, 12:45 PM
I signed up for the peaksaver PLUS™ program with the PoCo and will be getting a new t-stat.

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.

12-27-2012, 07:14 PM
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!

12-27-2012, 09:39 PM
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.

12-28-2012, 12:45 PM
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.

12-28-2012, 01:27 PM
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.

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.

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.