Rheem touchscreen modulating t-stat is flawed?

[BB1]

I have a new Rheem modulating furnace that was recently installed along with the new touchscreen modulating t-stat (model HC-TST401MDMS). While I love the furnace, I am unhappy with the White-Rogers modified t-stat.

Yes, it looks cool and is easy to use. But the accuracy of the temperature readings of this unit is appalling.

While the furnace is in recovery mode, the t-stat temperature seems somewhat accurate as the temperature in the house rises. But once it starts getting near the setpoint, the temperature on the display will stop increasing and linger at the same value for a very long time.

While the t-stat temperature reading stays on the same value seemingly forever, every other thermometer nearby will continue increasing. Once the temperature starts to stabilize, the Rheem t-stat will consistently read 2 to 3 degrees cooler than all the other thermometers.

This appears to be a common problem with this unit. If you review each of these threads below, you'll see others that complain about the temperature reading "3 degrees too cold".

http://www.hvac-talk.com/vbb/showthread.php?threadid=118952
http://www.contractortalk.com/showthread.php?t=15861
http://www.hvac-talk.com/vbb/showthread.php?threadid=120940

In an attempt to remedy the problem, I went into the "installer menu" on the t-stat and set a +2 degF temperature offset. Doing this gave me a reasonably accurate temperature near setpoint.

HOWEVER, I discovered that placing a +2 degF or greater offset into the t-stat will effectively disable the modulating function entirely! I discovered that whenever a large offset is in place, the furnace fires at 100% continually!

It appears like this may be a firmware bug in the t-stat. It is as if the algorithm for determining how far to open the valve is not properly taking into account the temperature offset. i.e. the t-stat likely thinks it is ALWAYS at least 2 degrees or more away from setpoint, so it fires at 100%.

Interestingly, the heat-on & heat-off signaling in the t-stat does appear to apply the temperature offset correctly. It is as if the Rheem modifications for determing the valve setting do not work right with non-zero offsets, but the original White-Rogers logic for everything else does.

I have discussed this with my dealer, who has in turned discussed the issue with Rheem. Rheem says it shouldn't be behaving like this, so they are going to give me a new t-stat. But honestly, I am not confident this will fix the problems unless they actually send a t-stat with a new version number. (My current t-stat says "Rev A" on it.)

I know I am not alone in these problems based on the seemingly identical complaints I've read on various HVAC message boards. Anybody have any thoughts or additional info on this matter?

[stvc]

Did you read my post down about 10 threads now? I have the same thing and also had to adjust temp up 2 to get an accurate reading, it was really warm for awhile. I don't believe it is giong to 100% though, t-stat is not calling for 2nd stage, but does cycle pretty quick. Come to think of it the same as before the adjustment.

[BB1]

stvc,

So you are saying that with an offset of +2 degF you still get proper modulation? Are you sure?!?

If so, perhaps you can tell me the following...

1. What is the revision level of your t-stat? (If I pull my t-stat off its base, it has a sticker on the mounting bracket that shows it's revision "A").

2. How long would it normally take your furnace to recover 2 degF? I am wondering if your system is capable of heating your house up faster than mine in your current climate. If so, I would be curious to know what an offset of +4 degF causes your system to do.

I should mention that if I use an offset of +1 degF, my system does modulate but it tends to fire at a higher rate than if it had an offset of 0 degF. This seems to confirm my theory about what it's doing internally with the offset.

[stvc]

BB1,

1. I am 90% sure it is still modulating, it's kind of hard to tell, it is not going to 2nd stage, full fire, as it would if I raised the temp 2 deg.

2.The room temp never varies from the set point unless recovering from a setback period. I have never had the opportunity to time it but it is always what it should be when I look. Its about 10F here now.

I'll check the rev. etc. tommorow, past bedtime

[reznil]

Here is how to hook up a Rheem/Ruud modulating furnace (in my case a UGFD) to the TST401MDMS thermostat.
They are listed in this order: Furnace, thermostat, outdoor AC (TO FURNACE!)

W - W/E

V - V

YL - Y - YL (Y1) two stage cool only

YH - Y2 - Y (Y2 if two stage cooling)

G - G

C - C - C

R - RH

The AC connections were not used in my case, no air conditioning. If a heat pump is added, other connections on the thermostat would be used.

A jumper in the thermostat between RH and RC is a good idea even
without cooling, as this allows the thermostat to be lighted by the
24VAC power and saves the thermostat battery.

NO OTHER JUMPERS!

The thermostat should be programed for MS 2 (multi-stage, no heat
pump) and Gas before attempting the "special test mode for PWM (V)