Thermostats: Buying, Replacing, Repairing

You've probably seen or used a thermostat a thousand times.

This devic­e controls the heating and air-conditioning systems in your house -- the two pieces of equipment that use the most energy. In these days of rising energy prices, you might ­be interested to see how your thermostat works. It is surprisingly simple and contains some pretty neat technology.
In this article, we'll take apart a household thermostat and learn how it works. We'll also learn a little about digital thermostats, talking thermostats, telephone thermostats and system zoning. Let's start by taking a look at the parts of a basic thermostat.
The top layer houses the mercury switch and the thermometer coil . The bottom layer houses the circuit card , the mode switch and the fan switch.
The mercury switch is a glass vial with a small amount of mercury in it. Mercury is a liquid metal -- it conducts electricity and flows like water. Inside the glass vial are three wires. One wire goes all the way across the bottom of the vial, so the mercury is always in contact with it. One wire ends on the left side of the vial, so when the vial tilts to the left, the mercury contacts it -- making contact between this wire and the one on the bottom of the vial. The third wire ends on the right side of the vial, so when the vial tilts to the right, the mercury makes contact between this wire and the bottom wire.

Bulb thermometers are good for measuring temperature accurately, but they are harder to use when the goal is to control the temperature. The bimetallic strip thermometer, because it is made of metal, is good at controlling things.
The principle behind a bimetallic strip thermometer relies on the fact that different metals expand at different rates as they warm up . By bonding two different metals together, you can make a simple electric controller that can withstand fairly high temperatures. This sort of controller is often found in ovens. Here is the general layout:

A thermostat that operates both heating and cooling units has two contacts at each end of the vial. When the vial tilts one direction, the mercury flows to that end and completes a circuit that calls for heat. When the system is switched to the cooling cycle, the mercury flows to the other end of the vial to turn on the cooling.
Disposal of the small amount of mercury from this type has become an issue in recent years. Pilot programs are being tested in some states for recycling old mercury-containing thermostats by mail; Honeywell has been a very active player in these initiatives. Some electromechanical thermostats, such as those made by GE, work similarly, but complete the circuit with a magnetic snap reed switch.
Electronic thermostats utilize an electronic heat-sensing element and circuitry to sense temperature changes and turn on heating or cooling equipment. Like a small computer, they are programmable; you can schedule the settings to change with household activities during the day or week. This means you can closely align room temperatures with your needs, ensuring comfort without wasting energy.

The case for electronic thermostats

Electronic thermostats offer considerably more control than electromechanical models, and their electronic sensing element is said to be more responsive to temperature changes than bi-metal sensors.
A programmable electronic model allows you to set room temperature so that, in the winter, your home's temperature can dip down to a fairly low temperature-say, 60 degrees-after you've gone to bed and then rise to a comfortable 70 degrees when you get up in the morning. Or, if no one is home during the day, you can set it so the house cools down when you leave for work and returns to a comfortable temperature before you arrive home. Then, on the weekend, it can change to an entirely different schedule.
Using a programmable thermostat, you can eliminate considerable wasted energy. As a rule of thumb, every degree you turn down a thermostat will save 3% of energy costs over a 24-hour period. In other words, if you turn down the temperature from 70 to 61 degrees for an 8-hour period every night, you can save about 9%. Reducing daytime usage similarly can double your savings.

Suiting your equipment

Both electromechanical and electronic thermostats work with most types of gas, oil, electric and even hydronic heating systems as well as air conditioners. Most have some type of adjustment to suit the system--wires that are jumpered, a small switch on the back or the like. This adjustment is necessary because some heating systems are much slower to heat up than others and cycle differently. And some types of heating require system fans and others don't.

If you have a system that provides multiple stages of heating and cooling, such as a dual-speed air conditioner, a furnace with two sets of burners, or a heat pump, you need a thermostat that is designed to handle this complication. Such a thermostat may be either electromechanical or electronic and, in fact, even some conventional thermostats have a jumper or switch that can be set to modify the thermostat for such equipment. Even a zoned heating system, with multiple heating or cooling zones, can be operated with electromechanical thermostats. But you can really fine-tune your home's areas for comfort with a programmable electronic thermostat.
Most heat pumps have an auxiliary electric heating element that kicks on when room temperature is about 2 degrees lower than the thermostat's setting ("set point"). If the room temperature is allowed to drop to 60 degrees at night and is then turned up to 70 degrees in the morning, the auxiliary heat will come on until the room temperature reaches about 68 degrees. In most parts of the country, using this much electric heat is quite expensive.

A sophisticated electronic heat-pump thermostat, on the other hand, automatically calculates when the heat must come on to bring room temperature up to your set point by the time you've set. It then tells the heat pump to bring it up from 60 to 61 degrees, then from 61 to 62 degrees, and so forth. That way, the electric auxiliary heat is "fooled" into staying off.

Making a choice

Unless you have a multiple-stage system that requires a particular type of thermostat, choice usually boils down to personal preference, which is generally a dynamic of features and price. People who are techno-phobic often prefer the simplicity of a conventional electromechanical model. Those who are interested in more control and potential energy savings opt for an electronic type.

Most electronic thermostats cost more than electromechanical types. For example, a professionally installed, Honeywell T-87 Round(r) thermostat, a very familiar model, may run about $40. A high-end electronic model can cost up to about $200 installed. You can, however, buy an electronic thermostat for as little as $40.

Though all electronic thermostats may look alike, they're not. If one is considerably cheaper than another, there is probably a reason for the price difference. Some low-priced ones may not be able to maintain a narrow "comfort window." And some may not be properly programmed to anticipate at what point the furnace must turn off to avoid wasting residual heat in the system or overshooting the desired room temperature. Also, ease of programmability is important; some types are more difficult to use than others.

Today, most manufacturers are focusing on features. For example, in the pursuit of the ultimate user-friendly electronic thermostat, Carrier and Bryant have each developed a line of thermostats based on input from consumer focus groups. The resulting products are aesthetically pleasing, have large "positive action" pushbuttons made from soft rubber, and give you useful feedback and system alerts such as a "clean filter" indicator that lets you know when its time to change the filter. The units automatically change over from heating to cooling. In the event of a power outage, they hold their programming up to 72 hours--without backup batteries. These are the types of features to look for if you want a state-of-the-art thermostat that will offer you comfortable, efficient control for years to come.

How to Check Your Thermostat

If your heating equipment isn't working properly, maybe the thermostat is defective. Here's how to see if it's any good:
1) With the power to the furnace off, remove the thermostat or thermostat cover to expose the wires.
2) Unscrew and remove the two wires from their terminals. Do not let them fall back in through the hole in the wall (you can wrap them around a pencil if necessary). If there are more than two wires, pick Red and White--normally, these are standard colors for power and heat. For a problem with cooling, you would choose Red and Green.
3) Wrap the two wires together.
4) Turn the power on and watch the furnace. If the blower goes on and the furnace burner ignites, the thermostat is defective. Replace it. If the burner does not ignite, check the continuity of the wires from the thermostat to the furnace. (If you're checking the air conditioning, only the blower will go on.)
5) Check and tighten terminal screws at all wire connections.
Replacing a Thermostat
Turn off the power to the furnace, remove the old thermostat's cover, unscrew the low-voltage wires from the terminals and label them with tape according to their terminal designations. Remove the base plate from the wall.
Following the new thermostat's package directions, push the wires through the baseplate and screw it to the wall. Connect the terminals, then mount the cover.

Repairing a Thermostat

Many heating and cooling problems first become evident when you adjust the thermostat --and the thermostat is often the cause of the malfunction. Fortunately, some thermostat problems are easy to fix. At the very worst, you can replace a thermostat yourself for under $50.

Thermostat misreads temperature

When your thermostat says one temperature and you know the room is another, it may be dirty, tilted on the wall, or located where it can't get a proper reading.
1) Turn the power off to the heating system.
2) Remove the thermostat's cover.
3) Use a soft brush or vacuum with brush attachment to gently and carefully remove dust and dirt. If the thermostat has two parallel metal strips, wipe them off with a soft cloth.
4) Look for corrosion. You can try to remove corrosion with electronic contact cleaner. If it's an outdated thermostat, consider replacing it with a newer model.
5) If you have a mechanical (not electronic/programmable) thermostat, be sure it is mounted level on the wall. Check it with a small bubble level.
6) A thermostat should be mounted about 5 feet from the floor, located where it can easily sense an air sample that is consistent with room temperatures. This means it should not be put in a corner, behind a door, in a closet, near a window or door, or near a heat source. If it is mounted in one of these spots, consider relocating it. This involves re-routing wires so, unless you're pretty handy with this sort of thing, it's probably better to hire an electrical contractor or heating technician to do the work.

How to Adjust the Heat Anticipator

If your gas- or oil-burning furnace cycles on and off too frequently or there are major swings in room temperature before the furnace goes on, the thermostat may just need a simple adjustment.

To adjust the thermostat:

1) Remove the thermostat's cover. For a mechanical thermostat with a mercury switch (a small vial filled with mercury), first use a small level to make sure the thermostat is mounted level on the wall. If it isn't level, it won't measure temperatures properly.
2) On many thermostats, you'll see a small lever that moves along a calibrated scale (not the heat temperature lever) and may be marked "longer." This is the heat-anticipator adjustment.
3) Adjust the heat-anticipator lever one calibration mark closer to the "longer" setting if the furnace goes off and on too frequently. If the furnace allows room temperature to drop to low or raise too high before the furnace goes on or off, move the lever one mark away from the "longer" setting.
4) Wait several hours for the thermostat to stabilize at this new setting.
5) Repeat the adjustment if necessary.
6) If making these adjustments doesn't solve the problem, consider replacing your thermostat.