Over the summer I replaced my older non-programmable thermostat with a new Honeywell RTH6300B thermostat. It worked well all winter long. It continues to function fine, but the temperature it shows is not accurate. It is registering about 4-5 degrees higher than the actual room temperature(based on multiple thermometers).

This is an older home, about 75 years old, and so the forced air heating system was retrofitted to the house (before we bought it). In putting the system in the installers created a chase, running from our basement up to the attic (3 floors) where they ran the ductwork, also in the chase is the house's chimney (used to vent the gas furnace and a gas water heater). The thermostat is actually mounted on the wall of this chase. Is it possible that that is causing the reading to be inaccurate?

So, does it seem like the thermostat is just broken? Do thermostats need to be calibrated? Could the location of the thermostat be the problem? What do you suggest? Do I move the thermostat? If Honeywell thermostats are able to be calibrated, how do I do it? The manual doesn't give instructions for this.

Any help would be appreciated!

If you checked the wall with other thermometers and they read the same as the thermostat, then it could be the heat that is coming from the chase that makes the thermostat read a little high.

The fan stops and starts but the burners are functioning

What is the quick remedy

Cure for the problem.

Make a spacer for behind the thermostat. This will leave a air space between the thermostat and the chase allowing the thermostat to only read to room air temperature. Just use small pieces of wood or as we always did short pieces of 1/2 inch plastic electric tubing or plastic water pipe (plastic or wood is used to prevent and short circuits from forming like might happen with metal pipe if the back of the thermostat has any of the printed circuits on it or other wiring. Then we just used longer screws to pass through the thermostat then the spacer then into the wall. Just tighten enough to hold thermostat on wall you do not want to bow or deform the plastic part of the thermostat.

Next make sure that the hole in the wall the thermostat cable passes through is caulked tight around the wire so no air from the chase can interfere with the thermostat temperature reading or activation of the thermostat.

This has been done before and it works well to cure this type of problem. You will probably find out after completing the above cure your thermostat will not need any calibration at all.

The thermostat reads lower than the one in the kitchen

The thermostat has a gap and the hole is sealed and the thermostat show lower than the one in the kitchen

Yes thermostats need to be calibrated, its called cycles per hour(on new digital thermostats), same thing as a heat anticipator on old mercuray thermostats. The instructions should indeed tell how to set this. Most likely, if you have a 80 % furnace, it will need to be set at 5 cycle per hour(usually factory setting), if you have a 90+ efficiency furnace, you will most likey have to change the cycle rate to 3 cycles per hour.

Read all instructions very carefully, you are looking for cycles.

He is talking about the temperature that the thermostat shows like in a thermometer not how long the unit is running I believe.

Calibration or anticipator settings are used when the furnace runs to long or to short overshooting or undershooting the set point of the thermostat. Anticipator settings will not change the base point of the thermometer reading for room temp. Now some thermostats are have the base line temp changeable but that is a different set of instrutions and that will be addressed differently in the thermostat manual. This feature is not universal and not all thermostats offer this setting adjustmant.

The thermostat is directly attached to a to hot or to warm chase causing the thermostat to read incorrectly. The wire can be caulked but without the spacers it still will not work correctly if the wall it is mounted to is to cold or hot as compared to the surrounding temperature. Been there done that but many times no one wants to fix it correctly since it takes some skill or additional labor to mount it to the chase with the spacers.

This tip was given to me 40 years ago by Honeywell at one of there trade seminars and it will solve these kind of problems.

Thanks everyone. Indeed, it maintains the set temperature VERY well, unfortunately, as HVAC indicated, it is misreading the temperature.

The hole the wire runs through is actually quite large, perhaps 2" square, so I am sure a fair amount of warm air is hitting the thermostat.

HVAC, your suggestion to use spacers sounds perfect. I will use the spacer to make sure I cover over the hole left by the installer. How thick of a spacer should I use? 1"? More? Less?

Also, would it be better to either mount the thermostat on a different wall(possible but a bit of work) or get a wireless thermostat(expensive)?

Thanks!

One inch spacers will work.

A different wall will also work BUT it needs to be a inside wall not affected by temperature as you have now but also being located where furnace heated or cooled air is not blowing directly on it.

Wireless is OK but you still have to worry about the mounting of the temperature sensor and they are expensive.

HVAC-

Thanks a ton. I think I am going to try the spacers. There is an alternate wall I can mount on, interior wall, not affected by temperature and not in a direct line with a vent, but I'd have to fish a line over there, and the spacer idea seams easier. Is it just as effective? The wireless would be a last resort given cost.

Am I right to assume that it is important for me to close up that hole? What if I cut a piece of wood, 1"DX3"Wx3"H, drilled a small hole in the middle to feed the tsat wire, affixed the tstat to the wood and the wood to the wall. Would that work? Would that all for sufficient airflow? Or am I missing the point? Would the heat from the chase pass through to the wood and cause the same trouble?

A different wall will work and eliminate the problem completely but you still have to close the hole where the wires pass through. The idea behind the spacers is to allow room temp air to circulate behind the thermostat there by allowing it to read room air without the warm chase making a difference. The use of wood might work but I would be afraid that eventually the wood might absorbe the additional heat of the chase wall and in turn continue to cause the problem by passing that heat by direct contact to the thermostat.

We are dealing with a imperfect science here. LOL

Great information. I will just move the t-stat to a different wall. Closing up the hole shouldn't be too much trouble, fortunately I have plenty of experience with that!

Thanks!

Slow and easy on the wire fishing and you will be OK. Have a nice holiday next week.

One more quick question...

I am not convinced that the current wire will be long enough, maybe, but most likely not. Is it hard to wire it at the actual furnace/ac unit? You wouldn't suggest splicing on some addition wire to the current wire, right? Any suggestions?

Just make sure you write down and mark each wire as to where it goes on the thermostat and heating unit. Naturally it is always better to run a new on piece wire but you can splice.

And wiring it up at the HVAC unit isn't too difficult?

Simple wire to wire. Just write it all down as to where they came so you can reattach.

Great, thanks.