How to adjust honeywell round thermostat (mercury switch) to the right temperature

[Glenda Copeland]

How do I adjust a thermostat so that the heat reaches the set temperature?

My Honeywell round thermostat has a bulb with mercury in it and a coil of metal that seems to control when it makes electrical contact.

When I set the Honeywell bulb thermostat to 68°F, the heat only gets to about 58°F. Likewise, when I set the thermostat to 85°F, the heat shuts off when it reaches about 75°F.

This works fine except my husband keeps yelling at me for setting the thermostat to 85°.

I know the simple answer is to change him but why do thermostats have numbers on them if they're not even close to those numbers?

Is there a way to adjust the temperature? When I pull the round cover off, I see a copper pointer on the bottom with etch marks on the plate below but that pointer is already all the way to the left and moving it to the right just seems to make the offset error worse.

What's the procedure to adjust a Honewell round thermostat to reach the set point before turning off?

[hrhofmann]

You have to turn the mounting of the coil spring that the mercury bulb is connected to. I don't have that type of thermostat any more, so I can't give you detailed instructions. But, I'm sure others here will, give them a few hours.

[RBM]

"Glenda Copeland" wrote in message news: [e-mail]...

It's probably easier to just replace the mercury thermostat with a digital. It'll be more accurate and have less temperature fluctuation. The little pointy thing is supposed to be aiming at whatever current amount the device it's controlling is drawing. It may be a relay or a zone valve, so you'd have to know this, then find the thing, then find the label on it where it give it's current draw. You could just point the thing far to one side, then see how it affects the temperature, then far to the other side, and make whatever adjustments you need. Keep in mind also, that Honeywell T-87 yo-yo thermostats must be level as well. There are marks on it, that you hold a plumb bob up to ,to properly level it.

[Marina]

Glenda Copeland wrote in news: [e-mail]:

You can change it to digital therm. on your own. I did it so anyone can do it. Just don't allow the wires to fall back into the wall. Seeing that winter is half over, I'd wait until you don't need the heat. Then if you mess up, it won't matter so much.

Tell your husband that just because it's pointed to 85, it doesn't mean the air temp is 85. Get a room thermometer and set it near the thermostat. Then show him the thermometer's reading of the air temp. And tell him to mind his own business.... he he.

[Bob F]

I don't have that particular unit, but on mine, it looks like the is a nut behind the metal thermo spring that could be turned a bit to calibrate it. What you need is to turn it in the direction that makes the mercury switch less level - just a little. If such an adjustment is not evident, bending the metal where the switch attaches to the thermospring could do it.

The copper pointer at the bottom probably adjusts the difference between turn on and turn off.

[Lp1331 1p1331]

The adjustment you are talkig about s called a heat anticipator. RBM is correct in that is adjusted to the amperage draw of whatever the thermostat is controlling. The others are also correct in that digitals are much more accurate, and now a lot of them actually cost less than mercury bulb mechanical types. Larry

[BQ340]

1st check that it is level (across the top 2 posts that hold the trim ring on).

MikeB

[ransley]

The heat anticipator is not whats off with that issue, the anticipator is correctly set to what it controls and controls how wide a swing you get before the thermostat comes on and goes off, the issue is its 10f off which happens with those dial units if someone messed with it not knowing how to calibrate it to the thermometers "temp" or forced it over to one side. Honywell used to supply with every round thermostat a little wrench that slips over the nut so you can turn the dial to get it set to run as the thermometer temp states, its a real real thin wrench but maybe Ace or a heating supply house has one. Often they wernt packed in the box so you had to ask and maybe pay 35c for one. One of mine is off now and I cant find my wrench, its off 6f, but it works fine with a 2f swing, so get used to it or get a wrench. Now if temp swings to much from on to off, then look at the anticipator. Those units are foolproof in design, and would outlast digital, they are not made anymore because of the Mercury bulb and disposal concerns. They still make the round style, but took out the mercury.

[HeyBub]

Tell him the house has to warm up before you can.

[Tony Hwang]

Hi, First thing first. Did you have this problem all the time or it is acting up lately? Sounds like good cleaning and calibration is needed.

[bud--]

I agree that checking if it is level is a starting point. If it is level, you could shift it off level in the correct direction as an alternative to finding the real thin wrench that is used to change the calibration. To change the leveling you may have to unscrew 2 or 3 recessed screws to remove the thermostat from the baseplate, and loosen the screws holding the baseplate to the wall. The baseplate has level marks.

[Lp1331 1p1331]

This doesn't sound like the OP's problem, but I just thought I'd throw this out as a FYI. About 12 years or so ago, I ran into a round Honeywell T87 that I had installed the year before, and something had happened to the mercury. Instead of it being a shiny silver, it looked like molten lead, and acted about the same. Normally,as soon as the bulb gets the least bit off level either way, the mercury will roll in the lower direction and turn the unit off or on. On this, it wouldn't move until the bulb was slanted all the way, and it moved like a glob instead of a fluid. For all the temperature control it did, it might as well have been a toggle switch. I replaced it under warranty. But the really strange thing was, over a relatively short time, I ran into two others, both also T87's that were exactly the same way, and haven't seen any more since. They always say things like that come in threes. FWIW, the first was on a gas wall furnace, one of the others was on a gas central, though the call was for the a/c, and the other an electric central, and I don't remember if it was an a/c or heat call. Larry

[Glenda Copeland]

It's our first winter in this house and it's all the time so far.

[Stormin Mormon]

First, make sure the thermostat is level. If it's tilted, it won't work right.

[trader4]

Agree with you and Ransley. First thing is to make sure it's level. Then if necessary you need a thin wrench that you can hold the nut with so the mechanism doesn't move while you rotate the dial.

All in all, considering the conveniences of a programmable digital one, I'd just replace it. Wouldn't it be nice to set the heat back at night and have it come back up shortly before you wake up?

[Bob F]

Worst case (Or maybe simplest), you could just tilt the unit ( Twist it on the wall) to adjust it.

[Glenda Copeland]

The word "Honeywell" in the center is perfectly level.

[ransley]

I have nothing against technology, I have all sorts of stuff, the round honywell is is even in the museum of modern art. Its a classic, I can adjust it, calibrate it, set anything with ease, now my digital if I dont have my manual I cant adjust temp swing or calibrate the thermometer and I dont know where my manual is anymore. My round Honywell has setback, I set it back, it has advance, I advance it. It will outlast new stuff.

[harry]

The problem you have is called hysteresis. When the temperature rises the 'stat switches off & the temp. falls but the stat does not close at the same temperature as when it was rising. The hysteresis should only be a couple of degrees but now it's obviously a lot more. This is due to ancient technology and probably wear and tear and lack of lubrication. The mercury bulb is indestructable but the rest of the 'stat wears. Get a new themostat. It will pay for itself in a year.

[harry]

I don't know if such things are available in America but in Europe you can get an "intelligent thermostat." There is an additional outside sensor and the heating is switched on earlier in cold weather and later in warmer weather. The exact amounts depend on the insulation of your house (nonexistant I expect in America). The thermostat determines this for itself after the first run. Dependant on your lifestyle, between 10 and 20% can be knocked off your heating bill. Some of these devices also compensate for wind . A digital thermostat can save 10%. It's important that the thermostat is correctly sited. In Europe every room is independently controlled in new buildings.