Troubleshooting a Thermostat

Make your thermostat troubleshooting easier with these steps

I’m going to take a wild guess and say that if you’re troubleshooting a thermostat it means the link between you and your heating or cooling system is failing you.

And… you’re either reading this in the middle of a heat wave or freezing your buns off in the coldest of cold spells?

I know the feeling all too well. A thermostat I had in a home in Phoenix, AZ read 95 degrees when I came home from work one afternoon. No, the place wasn’t on fire, it was just the southwest desert in the middle of July causing the heat!

I expected the house to be roasting while I was away (and it was!), so my first thought was to do some thermostat troubleshooting. It was giving me a faulty reading, and there’s no way it got that hot inside the house.

Besides, I can hear the system fan whirring away in the attic. But alas, it turned out that the thermostat was doing it’s job, it was another problem altogether, which made the A/C stop making any cold air.

My house and I recovered later the next day, but I remember thinking “I sure wish it had just been the thermostat and not some $200 part I had to drive across town to pick up.

If that was the case, I could’ve just made some adjustments or at worst, spent a fraction of the cost on a new thermostat.

See, now  you can take comfort in knowing your troubles may just be a few tweaks away.

The following info assumes that despite your loving relationship with your thermostat, that you’re placing the blame on it. 

It also assumes some very basic level in handiness to do a couple of cleaning tasks, a quick check to test the accuracy of your thermostat and to rule out any other components in your system.

It’ll be so fun you’ll forget all about freezing (or steaming!) away in your house!

Let’s get cookin’…

Is it the Thermostat’s Fault?

Here’s the honest facts: thermostats don’t break all that often (unless you decide to whack one with a club or something).

But for the sake of this topic, let’s look at the facts in more detail.

Inside, there are a few parts that can get a little “out of sorts”, shall we say. Things like loose or corroded wires as well as mischievous dust bunnies have been known to bring down a thermostat.

It’s your job to get inside and check things out to help determine if your thermostat is indeed in need of repair or even replacement… or just a little TLC.

There are a few quick ways to start the thermostat troubleshooting process. We’ll start with basic visual stuff, then move on to a couple other tests.

You’ll graduate to”Dr. Thermostat” in no time!

But first…

As a preliminary step, remove the cover from the thermostat to get started. Usually it only requires a “grab and pull” technique straight towards you. You may need to gently pry or push a button or clip to let it snap free.

Also, it goes without saying that for line-voltage thermostats, ex. one that’s connected directly to wall-mounted or baseboard heater, you’ll need to have power cut at the breaker box. Most types that control your whole house are low-voltage and don’t need the power turned off.

1. Basic Thermostat Troubleshooting:

Feel for loose or disconnected wire connections

Do any wires feel loose or look like they”re not connected any more? Tighten those babies up! Use a screwdriver to check that screws are securely in place. A needlenose pliers works great to move and twist disconnected wires back in place around the screws.

Clean the thermostat’s key parts (see below). Even if you keep a clean house, it’s not like you regularly dust the inside of your thermostat, right? (*But you should once a year!)

2. Advanced Thermostat Troubleshooting (but things you can do!):

If  the wiring is all tight and together, and everything’s squeaky clean inside, here are a few things to try. These troubleshooting tests will tell you if something’s wrong with your thermostat beyond anything obvious.

Test for a malfunction

To see if your thermostat truly is faulty (gone off the deep end!), you can bridge two terminals together and check an expected result. By the way, “terminals” = screws.

Inside, you’ll see these with colored wires wrapped around them. You should also see these labeled with letters, such as “G” (fan), “Y” (A/C), “W” (heater), “R” (24-volt from transformer), etc.

Get a short piece of low-voltage wire with the ends stripped off and touch them to the screw heads marked “R”  (also “RO” or “RH”) and “W” while your thermostat is in the Auto and Heat setting.

Nothing should happen…

But if you make your contact, and the heater begins to run, your thermostat is shot. All you can do now, is replace it.

Test the transformer:

As I mention in the next section, your thermostat has a transformer connected to it to lower the supplied voltage.

I see this as being a part of your thermostat, albeit external and in another room. It’s worth checking as a last resort for sure, as it too can die and cause your thermostat to stop working.

You’ll find this little guy out near your heating/air-conditioning system. It may also be just inside your furnace access panel. To test, you’ll use a multimeter (volt meter).

Set it to the 50-volt AC (VAC) setting. Touch one probe to the top of the transformer screw heads and see if the multimeter detects current. If it doesn’t, it’s time to replace the transformer.

Cleaning Your Thermostat

Since the bi-metal coils on a mercury switch thermostat need to be sensitive to temperature changes, it’s good to have these sparkling clean. You should check coils or strips for dust and grime.  

Gently clean with a small, soft paint brush or cloth.

Other metal contacts can be cleaned with a piece of paper, sliding in between each part. 

Another option for cleaning parts is a Q-tip and rubbing alcohol. Unlike water, alcohol evaporates quickly and picks up dirt really well.

Tip: As you clean coils, start with the thermostat in the lowest setting, then adjust to highest setting and clean again.

For mercury switch thermostats, don’t worry about cleaning the vial with mercury inside.

That main thing is it stay intact. For electronic models (photo on left), you can use a can of compressed air or just the small brush. Get all those dusties out.

Tuning after Thermostat Troubleshooting Is Complete

Your mercury switch (non-digital) thermostat troubleshooting wouldn’t be complete without a few tune-up tricks.

Like a car, a thermostat can do its job better after a tune-up. Don’t worry, you won’t have to change the oil.

Check and adjust accuracy

Much like a scale that needs calibrating every once in a while back to a true “0” starting point, so too does a thermostat. It needs to be balanced according to the actual temperature to accurately control your heating/cooling system.

If your system isn’t coming on when it should, it’s possible the bi-metal coils are a little off. I’ve had to adjust thermostats that were off by as much as 3 degrees.

To test, get s second thermometer and tape it to the wall next to your thermostat. Give it a few minutes and then compare it.

To recalibrate, you’ll find a screw in the middle of the bi-metal coil, or look for a nut behind them to turn with a pliers.

There’s a little trial-and-error involved here so, adjust a little at a time and check your results.

Next, check and see if your model has what’s called an anticipator.

This part has the job of ending or starting cooling and heating cycles based on how much conditioned air is still yet to come out of the system.

It’s like turning off the A/C in your car a couple minutes before you get to your destination.

There is still some cold air to come. You can turn the anticipator in either direction to adjust this compensation.

Finally, mercury switches need to be level in order to work properly.

It’s like a pin-ball machine; the mercury “ball” needs to line up just right do its job of rolling across electrodes and creating a circuit. Use a level  to check  for level.

Reposition and/or tighten screws on the mounting plate. Use drywall and anchor screws for better holding power. Even an inadvertent shoulder bump can knock things askew.

FAQs

How Do You Know a Thermostat Is Bad? (Common Signs)

Here are some tell-tale signs that your thermostat is malfunctioning and needs to be fixed or replaced:

• The HVAC system keeps turning on and off on its own.
• The system gives you consistently wrong temperature readings.
• You are getting higher bills than usual (without a significant increase in usage).
• Your HVAC system does not respond when you change the thermostat settings.
• Your thermostat is short-cycling.

What Happens If You Wire a Thermostat Wrong?

Experts often recommend fixing or installing a thermostat on your own for this reason alone.

Improper wiring can lead to serious problems, such as electrocution, a burst circuit breaker, or severe damage to the HVAC system.

Most likely though, things will just not work, i.e. your heater won’t turn on. The above are worst case scenarios.

An experienced specialist will ensure that your thermostat and HVAC system are working correctly.

How Do I Reset My Thermostat?

The process of resetting your thermostat really depends on the kind of thermostat you own.

Here’s how you can reset a mechanical thermostat, which is the most common type:

1. First, turn your thermostat off.
2. Turn off the breaker to which your HVAC system is connected.
3. Wait for one minute before turning the breaker back on.
4. Turn on your thermostat to to complete the resetting process.

As you can see, there is a bit that goes into thermostat troubleshooting. Your eyeballs will be your greatest tool at first.

Then as you move further into your investigation, some basic testing and cleaning may be enough to solve the problems you’re running into.

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