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Home thermostats can wear out

QuestionDo home heating thermostats wear out? I am talking about a simple mercury type with a glass tube that rocks back and forth on top of a coil. When the thermostat calls for heat, I usually hear the burner light and then, about 90 seconds later, the fan comes on and runs until the house warms up. On many occasions though, during the 90-second warm-up period, the mercury-filled glass tube rocks back to the "no call for heat" mode, and the burner goes out.

The furnace vent runs through the wall about a foot away from the thermostat. I wonder if this generates enough heat to cause the thermostat to behave like this.

I only notice this problem in the spring and fall, when there is a 10- to 15-degree difference between the outside and inside temperatures. Is it possible for the house to heat up enough on its own so that the thermostat no longer calls for heat? In the middle of winter when it is very cold, the thermostat does not behave like this.

The thermostat on my house is the original thermostat, so it is about 30 years old (Honeywell Chronotherm T882A1047). Also, it is for heat only as I do not have air conditioning. The clock portion of this thermostat has never worked and I only use the upper control to set the heat. The lower control is always set at the lowest setting. Any info you can give me will be greatly appreciated.

AnswerThanks for your detailed question. The simple answer is that thermostats can wear out. The main reason a thermostat wears out or doesn't work is because it may not be level, e.g., it may have been removed when the wall was painted and was not reinstalled in a level position. It's also possible that drops of paint have fallen on the coil, the unit is full of dust and dirt, or it has picked up grease from the kitchen. In these cases the thermostat should be replaced. And yes, it is possible that you are not getting enough good air circulation at an average temperature around the thermostat. Drafts, hot or cold air from water pipes or ducts, or even radiant heat from the sun or an appliance can affect how the thermostat reacts.

I contacted Honeywell (800-468-1502, 8:00 to 4:30 Central time) and they sent me a copy of the original owner's manual.

Your thermostat has a bimetal coil made of two strips of metal with different coefficients of expansion bonded together. As the temperature changes, the different metals expand at different rates causing the coil to expand and contract. At the top end of the coil is a glass tube that holds a mercury ball and two electrical contacts. When the coil turns far enough to tip the tube, the mercury ball rolls onto the two contacts, closing the circuit and switching on the furnace automatically.

With time the metal coil could experience fatigue as it changes its range and that could cause the thermostat to function abnormally (normally, though, dust, dirt, and grease cause the coil to be offset). Your Honeywell Chronotherm thermostat has a combined temperature control and synchronous electric clock to automatically lower the temperature to a lower setpoint for a specific period of the time every 24 hours. The timer was adjusted at the factory to lower the temperature at 10:30 p.m. and to raise the temperature at 6 a.m. The minimum period for the HI setting is 5 1/2 hours; the minimum period for the LO setting is 4 1/2 hours.

It's possible that the HI and/or LO pointers are adjusted too close together or the nonworking clock affects the characteristics of the thermostat (I suspect your nonworking clock has also affected the mechanics of the thermostat.) On the other hand, the two levers on the right-hand side on the top of the thermostat are the LO (left) and HI (right) that separately control the lower and higher temperature settings.

Perhaps the levers are too far apart—to maintain the temperature at a constant level, the temperature levers should be placed together at the desired setting.

It is also possible that the heat anticipator needs to be adjusted to match the current draw of the primary gas burner valve. The pointer on the scale (near the coil) should correspond with the rating found on the gas valve. A thermostat voltage meter would be used to check the required setting. It sounds like the heat anticipator is adjusted too low which gives a shorter burner-on time. It is also possible that your thermostat needs recalibration.

The manual provides detailed trouble-shooting for your thermostat including replacing the clock motor. It is also possible that the clock doesn't work because a wire is loose at the clock transformer or inside the thermostat, contacts are corroded, one or both wires are broken somewhere along the wire's path, or the transformer is dead.

I would be more than happy to forward a copy of the owner's manual for the reading but I believe it would be best if you contacted a heating tech to service or replace the thermostat that has served so faithfully over 30 years.

Copyright © 2005, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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