Lennox Zone Sensor (17A30) Reads Low While Air Handler Running

JustinS

Good morning all

Before I begin, a little summary of my system

Past 10 years since construction, house has had separate heating/cooling systems. It used a gas boiler with HW baseboard for heating and a single-stage AC system for cooling. The issue did not occur with that system at any point.

----

I've got quite the conundrum with zone sensor; once the air handler is turned, the sensor reading will slowly (over ~5-10 min) drop by as much as 2-3F (generally 2F but I have seen 3F). Once the air handler is no longer running, the reading will slowly increase back to "normal" over a similar period as when it first starts running.

This happens whether the system is heating or the air handler is just on (circulate / test).

If the system is heating, this results in a corresponding sensor overshoot once heating is complete. By this, i mean if it's at 70F setpoint and normal heat call occurs, it goes 70F => 69F => 68F while in fact still being 70F. As it heats and the thermostat reads 68F => 69F => 70F, the room temperature is actually 70F => 71F => 72F.

To me, this implies some sort of convective effect from the moving air; however, I have hung party streamers around the thermostat and they appear to be unaffected by any sort of draft while the system is running. I would think that any airflow sufficient enough to cause a 2F false drop would be noticeable via the party streamers?

Installer first said that this meant there was insufficient CFM to the room. This struck me as a very odd position since the drop occurs quite rapidly (much more so than one caused by heat loss) and isn't affected by the outside temp.

I then happened to notice that the drop only occurs when the door to the zone is closed. This seems to reinforce the air flow theory but the party streamers remain unaffected whether the door is open/closed.

Their current position is that the placement of the thermostat is the cause and that it should be moved elsewhere out of the apparent draft. I am reluctant to do this as I am not convinced that it is due to some air flow over the thermostat (unaffected streamers).

Furthermore, the location of the thermostat is unchanged from my old system (gas boiler HW baseboard + AC single stage with much higher air flows) and the issue didn't exist with it. Their response is that the previous system only used the ducts when cooling. While I recognize the difference, it seems to me that an air flow issue would have manifested when cooling in some similar fashion, no?

Hopefully, I was able to convey this strange situation adequately.

Any thoughts?

Mosherd1

Is it possible, since you say it only happens when the door to that zone is closed, that that area is being depressureizrd and is pulling cool air from inside the wall, through the back of the thermostat, causing a false reading. If a new thermostat was installed, possibly they unsealed the tstat wire coming through the Sheetrock. An easy thing to try, pull the cover off the thermostat and put some putty, or caulk, or stuff some tissue in there to seal and drafts that could affect the thermostat.  This is a very common issue.

JustinS

Mosherd1 said:

Is it possible, since you say it only happens when the door to that zone is closed, that that area is being depressureizrd and is pulling cool air from inside the wall, through the back of the thermostat, causing a false reading. If a new thermostat was installed, possibly they unsealed the tstat wire coming through the Sheetrock. An easy thing to try, pull the cover off the thermostat and put some putty, or caulk, or stuff some tissue in there to seal and drafts that could affect the thermostat.  This is a very common issue.

I believe that this is the case

I quickly tested it the other day using painter's tape and it had a noticeable effect. For the most part, I don't think that I am seeing a false drop when the door is closed, although I believe that the heating cycle still takes longer. I am interpreting this as the tape not being 100% effective and a reduced draft still happens.

A new thermostat was installed, along with new wiring as it requires shielded wire. As you can see, they banged up the wall pretty good.

JustinS

Hi all

Spoke to the installer again and their opinion is that there's insufficient return for the room. They've proposed increasing the return register (currently 13" square).

I measured the supply registers (11" square x2, 9" square x2). CFM to that area is currently 220 CFM

Thoughts?