Strange behavior modulating furnace w/WR thermostat

[jeff-4]

Hello! I received extremely helpful advice form this forum when I had a Ruud modulating furnace in 2006 (the installer had no idea of what he was doing). They installed the just-then-introduced White-Rodgers (-)HC-TST401MDMS modulating touchscreen thermostat with the furnace.

I'll briefly describe the issue (and am happy to provide as many details as requested). This morning (-10 F outside temp, here in MN), after setting back the temperature for overnight, my house was at 65 F at 6:00 a.m. (It had been heating up for 2.5 hours before then from a minimum temperature of 58 at 3:30 am--yes, I checked it after waking up in the middle of the night.) At 6:00 am, the set point was switched to 68. The following is a history of time and temperature:
6:00 - 65
6:20 - 65
6:40 - 66
7:00 - 65
7:15 - 64
7:20 - 65
7:40 - 66
8:00 - 67
8:20 - 68

During the whole period (as far as I was able to determine), the thermostat indicated "Heat On +2" and the furnace showed "H" (and a constant flame). It seemed that the fan speed was going down and up, though.

Obviously, the period between 6:40 and 7:40 when the temperature varied between 64 and 66, was quite uncomfortable.

Any idea of what is happening? And, more importantly, how to fix this behavior? I would be happy to shell out money for a new thermostat, if it fixed this problem.

P.S. This is a recurring problem, most noticeable when the outside temperature drops below 0.
P.P.S. Once at 68, no problems. (I realize one solution is to keep the temp at 68, but I want to set back at night. I sleep better in a cool house.)

Thanks for all advice!

[beavercreekhc]

Could be too many different problems to diagnose over here. Best bet is to call an hvac company and have them come out and diagnose it and possibly fix it.

[George2]

I have seen furnaces work properly when temps. are moderate and fail to keep up when it gets (really) cold.

When I've seen this happen, it has been because (long story) of lack of airflow.

Have you ever had the static pressure checked?

Just one idea.

[dirtsprintfan]

I have the same furnace and stat. During the first heating season the inducer would hunt for proper speed. Pressure switches checked out OK, PVC is the right size nad amount of fittings. So I had the stat changed and that solved the problem. Since the stat does all the staging on the Ruud mod. Just my 2 cents

[sktn77a]

How big is your house? Where are you located? How big is your furnace? When did you last change the filter? Could be an undersized furnace or a problem with the furnace itself. Rheem/Ruud/WR had some problems with the TST401 thermostats. The TST411 was the replacement. This was then replaced with the TST412 in order to provide some additional humidity control.

Has the system worked OK for the last 5 years or is this a continuing problem? Have you ever had it maintained?

[ampulman]

I have a model 411 stat controlling my MOD. It's now in its 3rd heating season. My original (411) stat was changed due to having 'drooping' problems.

Using intelligent recovery, my unit was coming on at bout 5:30 AM for a 6:00 AM programmed time (it would take a bit longer to reach SP). Because I would frequently get up before the stat would call for heat, I would 'goose' the temperature a couple of degrees. I would also set the 'hold' time so that the furnace would go to program (ramp up) at the time it should be, to satisfy the stat at 6:00 AM.

The 2nd stat would behave similarly to the first, so I tried an experiment. First, getting up before the furnace ran, I killed power to it for 30 seconds. This forced the furnace to run a calibration cycle on the next run. Being that the house and its contents, ductwork and furnace (in basement) were at their coldest, I felt that this might positively affect (firing rates vs time) comfort.

I also discontinued turning up the stat shortly before program time, feeling that this would interfere with the 'intelligence' and just adds confusion (to the stat).

Well, for about a week, the furnace seems to be running perfectly with no droop. Since both 'fixes' were implemented at the same time, I don't know if either/both are responsible for the behavior. During the experiment, outdoor temperatures have varied from moderate to design. Will keep watching.

When you stated that your furnace was running at 3:00 AM, did you happen to note the indoor temperature at the stat? Could it have been running to maintain your setback temperature? Seems like a long time if starting up from scratch.

What is the size of your furnace and what type (size, efficiency) did it replace?

Amp

[jeff-4]

Thanks for the replies. I will get back with some more answers soon. First, some more information on this problem.

This morning was also cold (-12), and the system did it again, i.e., heated the house to 66, faltered back to 65, then a slow rise to 68.

However, this time I was able to observe that the thermostat status switched from "System On +2" to "System On," after reaching 66 and, amazingly, the flame actually shut off for a short period (with the furnace status light showing "F"). The flame did come back on (with the furnace status light showing "H"), but the temperature in the house dropped from 66 to 65.

After finally struggling to 68, I noticed that the "System On +2" indication was again showing on the thermostat.

This seems to be a clear indication of faulty operation of the thermostat in transitioning between the "System On +2" and "System On" states. I have read other posts on problems with the modulating WR thermostats. Is this a firmware bug or improper logic?

Would a newer thermostat solve this issue?

[jeff-4]

Thanks for the reply, ampulman! I hope some other Rheem/Ruud Modulating guys will also chime in.

The current furnace is the UGFD-07EMCK rated at 75,000 BTU/HR input (at 92% efficiency?). It replaced a larger unit rated at 80,000 BTU/HR, I think, with a claimed efficiency of 95%.

I was concerned about the smaller unit, so I used HVAC Calc and determined that its size was just adequate. In practice, this has proven true so far, since it can maintain a 68 interior temperature in -15 to -20 outdoor temps. But it does need to run a long time to make up the setback when it is that cold outside.

So, the observation at 3:30 am was an interior temp of 58. At 3:30 the set point was set to 62, followed by a set point of 65 at 5:00 am, and a set point of 68 at 6:00 am. I had programmed this ramp to try to avoid going from 58 to 68 in one step. I know it shouldn't matter, but the "droop" as you call it seemed far worse when a single jump in set point was programmed.

[jeff-4]

Could be too many different problems to diagnose over here. Best bet is to call an hvac company and have them come out and diagnose it and possibly fix it.

No one seems to have experience with the Rheem/Ruud modulating furnace. The Ruud dealers I have talked to are reluctant to do anything after I have described the problem with them.

[jeff-4]

I have the same furnace and stat. During the first heating season the inducer would hunt for proper speed. Pressure switches checked out OK, PVC is the right size nad amount of fittings. So I had the stat changed and that solved the problem. Since the stat does all the staging on the Ruud mod. Just my 2 cents

Thanks! I really think this issue is with the thermostat, too. It seems that it has had a history of issues, see http://hvac-talk.com/vbb/showthread.php?t=439932

[jeff-4]

How big is your house? Where are you located? How big is your furnace? When did you last change the filter? Could be an undersized furnace or a problem with the furnace itself. Rheem/Ruud/WR had some problems with the TST401 thermostats. The TST411 was the replacement. This was then replaced with the TST412 in order to provide some additional humidity control.

Has the system worked OK for the last 5 years or is this a continuing problem? Have you ever had it maintained?

See other post for more info on the furnace. My concern is not the long recovery time; I think the furnace is adequate but with little reserve power. My concern is the dip in temperature as the set point is approached, "drooping" as ampulman stated.

This issue has been going on since it was first installed (see http://hvac-talk.com/vbb/showthread.php?t=122652), but it only happens when the outside temperature falls to near 0 or below. Since the last several winters have been warmer than average, I didn't really notice. With several cold mornings this winter, the problem has returned.

I have not had any maintenance performed (other than myself replacing the air filter periodically).

Do you think my problem is a buggy thermostat, or should I be calling to ask for maintenance/repair?

[ampulman]

Jeff-4 stated: "My concern is the dip in temperature as the set point is approached, "drooping" as ampulman stated."

My drooping occured anywhere from 45 - 60 minutes after setpoint was reached.

Amp

[sktn77a]

This issue has been going on since it was first installed (see http://hvac-talk.com/vbb/showthread.php?t=122652), but it only happens when the outside temperature falls to near 0 or below. Since the last several winters have been warmer than average, I didn't really notice. With several cold mornings this winter, the problem has returned.

I have not had any maintenance performed (other than myself replacing the air filter periodically).

Do you think my problem is a buggy thermostat, or should I be calling to ask for maintenance/repair?

First of all, have the system serviced to make sure there's nothing obvious amiss there. Tell the service guy about the problems you've been having. An intermittent problem like this, only at very low tempertaures, doesn't sound like the thermostat problem - sounds more like a gas/vent/condensate issue.

I have two Rheem modulating furnaces. Only problems I've had with them is the condensate freezing in real cold weather and a vent problem when one of the furnaces was first installed.

[commerce48]

You didn't post the time that you set for full recovery. If that time was 8 or 8:20, your thermostat may be working fine. The temperature variance you posted before that time would be consistent with maintaining your setback temperature.

What is more interesting is that your furnace is apparently recovering on a higher stage. I would have thought that most smart stats would try to recover at a lower stage for a longer period of time to bring the thermal mass of the house to equilibrium, not just the air temperature. It also appears that relative to the very cold temperatures and your short recovery time, that your furnace is oversized.

Are you able to monitor what stage your furnace is in? That would be interesting to graph against your recovery.

[jeff-4]

You didn't post the time that you set for full recovery. If that time was 8 or 8:20, your thermostat may be working fine. The temperature variance you posted before that time would be consistent with maintaining your setback temperature.

What is more interesting is that your furnace is apparently recovering on a higher stage. I would have thought that most smart stats would try to recover at a lower stage for a longer period of time to bring the thermal mass of the house to equilibrium, not just the air temperature. It also appears that relative to the very cold temperatures and your short recovery time, that your furnace is oversized.

Are you able to monitor what stage your furnace is in? That would be interesting to graph against your recovery.

To avoid any vagaries, I turned off the intelligent recovery feature of the thermostat. So, at 6:00 am the furnace set point was changed from 65 to 68 (and was kept at 68 thereafter), with the before-described temperature profile.

I'd love to be able to monitor the furnace, but I do not have the tools or knowledge to do so. I am just an (overly) involved homeowner, not a JVAC tech.

[jeff-4]

First of all, have the system serviced to make sure there's nothing obvious amiss there. Tell the service guy about the problems you've been having. An intermittent problem like this, only at very low tempertaures, doesn't sound like the thermostat problem - sounds more like a gas/vent/condensate issue.

I have two Rheem modulating furnaces. Only problems I've had with them is the condensate freezing in real cold weather and a vent problem when one of the furnaces was first installed.

In my first conversation with a Ruud contractor, he also mentioned the possibility of venting issues.

But here's an experiment I did this morning (-6 F outside): At about 6:00 am, the temperature was 65. I then manually raised the setpoint to 70. The temperature in my home then went smoothly from 65 to 68 over the next hour, without dropping to a lower temperature in that hour.

Since I had bumped up the setpoint, the "System On +2" was always showing on the thermostat. Once the house was at 68, I hit the "Run Program" button and all was well. Kinda defeats the purpose of an automatic thermostat, though.

I am now pretty convinced that the thermostat is flaky, either via a software bug or something else. However, I will follow up with the contractor do do a system check, especially the vent, before I order a new thermostat (or have him put one in). Based on this post http://hvac-talk.com/vbb/showthread.php?t=439932 though, am I safe to do this or should I wait for a thermostat update?

[sktn77a]

My TST411 thermostats don't display "System On +2" in heat mode - only in cool mode (I have a 2 stage A/C). I don't know if the TST401 was different in this regard but double check it's wired correctly for modulating operation vs 2 stage operation.

I'm still inclined to think it's a gas/venting/condensate issue.

[sktn77a]

Another thought (generic recommendation for funky temp behavior) - ensure the hole behind the stat has been plugged so stat is not subject to drafts from inside the wall space.

[ampulman]

My TST411 thermostats don't display "System On +2" in heat mode - only in cool mode (I have a 2 stage A/C). I don't know if the TST401 was different in this regard but double check it's wired correctly for modulating operation vs 2 stage operation.


My 411 shows +2 when it starts up cold. I've seen it still registering +2 even as the unit runs at a much lower speed.

When it is maintaining temperature or cycling, it shows 'system on'.

Amp

[sktn77a]

Tried them both again just to confirm. Cranking up the t'stat manually by 10 degrees and it came on full bore but with only "system on" displayed (yes, it's wired correctly and the furnace dipswitch settings are correct). Both systems are working fine and ramp up and down as necessary. Kinda makes sense with 20 modulating heat settings - what would constitute "system on +2"?

[ampulman]

Another thought (generic recommendation for funky temp behavior) - ensure the hole behind the stat has been plugged so stat is not subject to drafts from inside the wall space.

Regarding the hole, I'm theorizing: would a draft more likely occur when the room temp (at stat) is near or at setpoint , as compared to the basement (equipment location) which is relatively cold (chimney effect)?

The droop, which I mentioned, seemed to occur after RT had stabilized at setpoint, and the unit ramped down to a very low level.

What would one use to plug that hole? I would think something that doesn't get hard, such as MoreTight aka rope caulk.

Amp

[sktn77a]

Most people just stuff the hole with fiberglass insulation.