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

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.

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.

Yes nearly all heating over here is by means of hot water circulation, even in very large buildings.The boiler(furnace in your parlance) can be far more efficient, exceeding 100% in some cases.. Also water is far easier to mix and control than air. The hot water pipes can be much smaller diameter than air ducts & therefor can be better insulated. Domestic hot water can be produced from the same boiler, as and when required. (No need to store it hence reduced losses.) Things are expremely primitive in the USA compared with Europe.

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.

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.

MikeB

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

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.

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.

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

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

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

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

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

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.

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.

Digital thermostats can be very complicated to operate. Check on that before buying.

I'd yell at you too. Who needs 75? Set at 78 and be happy with 68. Wear a sweater.

Don't pull that cover off. There are electical forces there that will make your cold cream run.

In that case you could risk bending the spring or some other part that is holding the mercury capsule, but two major concerns. 1) Being a Mercury Capsule, it might head for earth orbit. 2) A good chance you'll bend it in the wrong direction (it's confusing) or bend it in the right direction but so much that the offset pointer won't be able to correct it. And you only get to bend it once or twice and soon it will break off. Plus you probably need TWO needlenosses to bend it.

You can try to tip the house while holding the thermostat in place.

Bad advice by me. I was actually closer with my last line. See below.

This advice was somewhat sarcastic, but it modified just a little, it was the best advice in my post. If the adjustment with the copper pointer is insufficient, you can rotate the entire thermostat. The amount you want to rotate it is the amount it takes to make the number for example 75 to show up in the same place that currently the number

As to buying a new thermostat, people here are often ready to buy new, to throw money at a problem.

As to accuracy, what does accuracy matter when people don't know if they want 74.4 or 74.7, or 75.3? And much moreso is it not important when they want something around 75 and their husband wants something around 68. Improved accuracy isn't going to help.

Temperature stabilty is adjustable iirc on the round Honeywell thermostats. It's called cycle delay or something. But one can't lower it too much because of the furnace, not because of the thermostat. It's not good for many furnaces to turn off and then turn on again when the temp drops 0.1 degrees. This is all the more true for ACs which need time for pressure to go down where it the refrigerant is compressed. The thermostat comes set for a reasonable value and it can be lessened if wanted with most furnaces. If it is also an AC thermostat, either there are two cycle delay adjustments, the cycle delay applies to the furnace only, or it can't be lowered more than is safe for the AC.