Honeywell T87F accuracy of thermometer

xpsd300

Hi,

I'm replacing a Honeywell T87F thermostat that got damaged during our remodel with a direct replacement.

I have two NOS units and noticed that, laying flat in their boxes, the thermometer on one matches the room temperature, but the other is off by +6 degrees.

Does it sound like the second unit is defective, or does it just need to be recalibrated? The thermometer needle does move when brought from a hot to a cold space.

Does the T87F need to be installed on a wall to get an accurate room temperature reading?

Thanks.

bmwpowere36m3

Is that the basic round model from honeywell (what used to be mercury-switch)? Mine holds the temp in the house correctly (setting), but it reads 4* low on actual temp (lower needle).

Meaning when set to 68*, the house stays at 68* according to two digital thermometers. However it indicates the to is 64*...

I've read a few reviews, with similar responses... the displayed/actual temp is way off. For me it's just a temp thermostat, till I find a nice hardwired, programmable one.

JUGHNE

I think there was a procedure for recalibrating the thermometer, it may have involved a special wrench. Most people never worried about it and just set temp to be comfortable. (Before digital thermometers it was close enough)

xpsd300

The spare T87 thermostats I have are mercury switches. They were manufactured in 2002.

Last night, I found 4 old thermostats (circa 1980) tucked behind my oil burner and I did a test overnight.

This morning, the old ones and one of the spares reported a room temperature of 68 degrees, which I confirmed to be accurate with a hand-held digital thermometer. But the other spare was still off by 6 degrees.

Regarding recalibration, the instruction sheet with the 2002 units only mention using a 104994 Calibration Wrench in passing. I did find more detailed instructions online, but recalibration only appears to fix problems related to the mercury switch, not the room temperature thermometer.

JUGHNE

Yes, I remembered that wrongly. Most never worried about the thermometer accuracy. Just get it level as in the instructions.
If you wanted to use that T-stat, I would just buy some type of wall mount digital T-stat, maybe with the outdoor remote sensor and then you have it all in one place. Just ignore the HW T-meter, it has no function in operation. FIWM

More important is the setting of the heat anticipator....shorter or longer cycles....do you have steam?

xpsd300

So I figured out how to recalibrate the room temp thermometer after completely disassembling one of the old ones I had on hand.

There is a screw located on the top of the dial beneath the "Honeywell" decorative button (see attached).

To get access to it, carefully remove the plastic dome by inserting a small screwdriver into the notch on the face of the dial. Then, pry off the decorative button. Use a flathead screwdriver to rotate the room temp needle as needed. Finally, reinstall the decorative button and the plastic dome (the dome will crack if too much pressure is applied).

Regarding the heat anticipator, I have a hot water oil burner with Taco 571-2 zone valves (0.9A rating). Setting the heat anticipator to 0.9 causes the heat to kick in when the room temp dips 1/2-1 degree below the set temp, which I find to be comfortable.

I've tried a few mercury free thermostats in the past, but I've found it difficult to replicate the 0.9 setting.

JUGHNE

Good work, the key word is "comfortable". I would recommend putting the digital thermometer in a drawer out of sight.

ChrisJ

1/2 to 1 degree?
I find it hard to believe this cannot be achieved with a Visionpro 800 series set to 2 or 3 CPH?

The original T87 is a fine unit and I'm glad you shared how to calibrate the thermometer but I just find it hard to believe a quality digital thermostat can't replicate this performance if not better.

Canucker

I haven't been able to equal the control my old t87 had for the basement zone. 2 cycles overshoots/undershoots by at least 1 degree celsius. I tried 3 and 4 cph but it ended up short cycling my boiler to a degree I wasn't comfortable with. The t87 did dip under the set point that I noticed but it never overshot, ever. I watched that with a few different thermometers

Techman

The "room indicator" is adjustable. The heat anticipator,heats up the "spring" and turns the heat off slightly before the room gets up to temp by anticipating that the "residual" heat in the system will finish the job and bring the room temp up to room temp.
I re-calibrate those "old" T'stats with that special wrench.

ChrisJ

Canucker said:

I haven't been able to equal the control my old t87 had for the basement zone. 2 cycles overshoots/undershoots by at least 1 degree celsius. I tried 3 and 4 cph but it ended up short cycling my boiler to a degree I wasn't comfortable with. The t87 did dip under the set point that I noticed but it never overshot, ever. I watched that with a few different thermometers

How long did you run 3 or 4 CPH?
The VP8000 slowly learns and tweaks it self but it days a while.

JUGHNE

Yep, agree with all that. But for the OP....no batteries..no concern about a "common" wire.....and they are "free" to him.
Plus that word "comfortable" comes into play.

nicholas bonham-carter

The benefit of a good digital is in the ability to raise the temp just one degree, if someone complains. That's harder to do with the old style.--NBC

Gordy

Key word comfortable. That's a feeling not anobservation

Canucker

ChrisJ said:

Canucker said:

I haven't been able to equal the control my old t87 had for the basement zone. 2 cycles overshoots/undershoots by at least 1 degree celsius. I tried 3 and 4 cph but it ended up short cycling my boiler to a degree I wasn't comfortable with. The t87 did dip under the set point that I noticed but it never overshot, ever. I watched that with a few different thermometers

How long did you run 3 or 4 CPH?
The VP8000 slowly learns and tweaks it self but it days a while.

I only ran it for a couple of days. It's not a high end stat, do all the honeywells have a learning curve? I ask because the other 2 zones use a honeywell model that adjusts when it recovers from set back so you're at temp when you requested instead of starting at that time. The basement one doesn't and i've never adjusted their cph. (They overshoot too, especially when it's warmer out)

ChrisJ

Canucker said:

ChrisJ said:

Canucker said:

I haven't been able to equal the control my old t87 had for the basement zone. 2 cycles overshoots/undershoots by at least 1 degree celsius. I tried 3 and 4 cph but it ended up short cycling my boiler to a degree I wasn't comfortable with. The t87 did dip under the set point that I noticed but it never overshot, ever. I watched that with a few different thermometers

How long did you run 3 or 4 CPH?
The VP8000 slowly learns and tweaks it self but it days a while.

I only ran it for a couple of days. It's not a high end stat, do all the honeywells have a learning curve? I ask because the other 2 zones use a honeywell model that adjusts when it recovers from set back so you're at temp when you requested instead of starting at that time. The basement one doesn't and i've never adjusted their cph. (They overshoot too, especially when it's warmer out)

Not sure.
I know the VP8000 does but Honeywell doesn't say much about how their thermostats work which is annoying.

For example, I still have no idea what the difference between a normal VP8000 is, and the commercial version that was a lot more money.

Canucker

I noticed that about Honeywell, hard to find out what their features actually do. I checked my tstats and set them all to 4 cph from the 3 they were set at. I guess I'll watch them for a week and see what happens.

fulljionslly

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STEVEusaPA

@fulljionslly...booooooooo.
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