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Help! My Thermostat Isn’t Working Properly

by B.Annie on January 19, 2018

When it’s working a thermostat is a remarkable device, allowing you to easily control the temperature in your home. But when your thermostat isn’t working properly, it can leave you in a potentially dangerous situation. Not being able to turn on the air conditioner may make your uncomfortable but winter cold and an inability to turn on the heater is a frightening combination.

First, a warning –  Fixing a malfunctioning thermostat can be complicated and is usually best left to a professional technician.

However, there is one easy fix that the homeowner can try is to replace the batteries in a thermostat.  If the heater is working, but there are no digits registering on the LCD display faceplate, the battery could be the problem.  Simply pop off the faceplate on the thermometer and replace the two batteries.

If you push the faceplate back into place and everything turns on again, it was a battery problem. You are a lucky guy! Now grab a cup of coffee and settle back into your easy chair for a night of TV viewing.

Testing equipment

If new batteries did not fix the problem, your HVAC person should be called to the scene. Most likely he will arrive with a multimeter for testing different wiring problems.

But don’t panic, it might be an easy fix. A close visual inspection might reveal that whoever installed the device might have nicked one of the wires when he was stripping the ends for connection into the terminals. This means that a small spot on the wire that should be covered with insulation is bare. If the bare wire is touching another live wire or a metal bracket, it could cause a short and the thermostat wouldn’t function properly.

Another trouble spot is when two of the wires are touching each other and causing a short. Often the place where the short is happening is not visible. The technician doesn’t need Superman’s x-ray vision to diagnose this situation.  Instead he uses a multimeter. He disconnects the two wires from the thermostat and uses the multimeter’s alligator clips to clip onto each wire. The device will indicate a reading in ohms. A low reading is good, meaning the current is flowing easily through the wires.  A reading of 2.0 or more is bad. That resistance, plus resistance in the other wires, is probably enough to blow the 3.0-ohm fuse.

Portable fuse

The use of the multimeter is helpful for the technician to be able to isolate which wires are the trouble spots and which wires can be left alone. Some technicians also will bring with them a portable fuse “Lil Popper.” This device is useful after a fuse has blown.  By connecting the Lil Popper to the fuse plug he can quickly determine which wires are overloading the fuse and concentrate his efforts there.

Here are some of the ground rules the technician will use during his examination to see why the thermostat is not working properly. Usually, the red wire coming into the device carries 24 volts. What is being required of the thermostat will determine which of the other signal wires carries the current out.

Another simple test would be to wire the red “hot” wire with the white wire. The technician might simply secure those with a screw-on cap.  If the furnace begins running, then that circuit is working, and not the problem.

The technician can check out the other wires in a similar fashion to see if any of them are part of the problem.

If your thermostat flunks these tests and a new one must be installed, run an extra line through the system. For example, if the system needs four lines, run a fifth line. That way if there is a problem in the future, the technician can simply disconnect the troublesome wire and connect the back-up line.

The homeowner who finds his thermostat is not working properly needs to contact a reliable HVAC company.  The company technician can carefully test the wiring of the device to see if it needs to be replaced. Be sure that the company you select uses experienced, trained technicians. The company’s reputation for reliability is important to ensure that your thermostat is properly tested and fixed or replaced.

If the red wire connects or touches the blue wire it would probably short out the fuse which protects the transformer from suffering damage.

One way to make future fixes easy when installing a thermostat is to run an extra line through the system. For example, if the system needs four lines, run a fifth line. That way if there is a problem in the future, the technician can simply disconnect the troublesome wire and connect to the back-up line.