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Something that’s often neglected, even by proactive folks who actually check on the heat before the first frost, is the humble thermostat. That tiny appliance on your wall—whether it’s an old-timey mechanical thermostat, a modest battery-powered programmable version, or one of those smart thermostats stealing all your personal datais crucial during the cold weather. If your thermostat stops working, your heat won’t turn on.

That’s no big deal if it happens during decent weather—a quick run to the hardware store or a call to a handyman or HVAC contractor will get a new thermostat in place in no time. But if your thermostat breaks while a blizzard is raging or in the middle of the night during sub-zero temperatures, you’re going to have an uncomfortable experience at best, and a dangerous one at worst.

Fortunately, a thermostat is a pretty simple device, and you can bypass it without many tools or fancy knowledge. Here’s how you can get your heat on in an emergency even if your thermostat gives up the ghost.

Thermostat wiring 101

First, a quick review of how your thermostat is wired up. Note that there will be some variations to this—specific HVAC systems will sometimes have a slightly different configuration to the wiring, and depending on your exact setup you might not have all of the wires listed here, or you might have some extra ones. But these are the basics that you need to know for the ugly hack you might commit.

To start, take the thermostat off the wall. This might involve some screws, or you might be able to just snap it off a clip that’s attached to your wall. What you’ll see in there is a small circuit board with wires emerging from the wall, screwed down into terminals. Each of these wires is color-coded, and each terminal most probably has a letter code:

  • Red: The red wire is the wire bringing power to the thermostat, and connects to the terminal marked “R.” If you have both heating and cooling you might see two separate terminals marked Rc (or cooling) and Rh (for heating).
  • Green: The green wire connects to the “G” terminal and runs the fan or blower on your system. Not all systems have a fan, though, so you might not see this one.
  • Black or blue: These wires connect to the “C” terminal and are the neutral or common wire.
  • Yellow: The yellow wire connects to your cooling system and uses the “Y” terminal.
  • White: The white wire runs your heat, and connects to the “W” terminal.

Again, your home’s HVAC system may be more or less complicated than this. For example, if your thermostat only controls a furnace and no air conditioning, you might only have a red and white wire. And if the person who wired up your thermostat was lazy or inexperienced, they may have ignored color coding conventions, so check that the color matches the terminal. In other words, if a green wire is plugged into the R terminal, that’s likely the power wire—but be extra careful because the job wasn’t done correctly.

Assuming your thermostat was wired correctly, the good news is that for the purposes of getting your heat going on a cold evening, you only need to worry about three wires: red, white, and green.

Hotwiring a thermostat

If your heat has stopped working in the house but your power is still on, the first thing you’ll want to do is to check if the problem is your thermostat, furnace, or boiler. Check if your thermostat uses batteries, and change them if it does. Most battery-powered thermostats will warn you when the battery gets low, but this is a crucial first step if you want to avoid feeling pretty dumb.

If the battery situation is fine, your next step is to check to make sure there’s power coming to the thermostat, if you can. For this you’ll need a voltage tester, which is a generally handy thing to have if you’re the type to consider opening up stuff like thermostats and touching wires coming out of your walls. Touch the leads of the tester to both the R and C terminals of your thermostat. You should get a reading between 24 and 30 volts. If you get less than 20 (or nothing at all), you’ve got bigger problems and bypassing your thermostat won’t help. You’ll likely need to contact an HVAC professional or electrician.

If you’re getting good voltage, your thermostat may be broken, so bypassing it might work. Assuming you’ve removed the thermostat from the wall and exposed the wires, turn off the power at your furnace to avoid getting a nasty shock. If you’re not sure how to turn it off at the furnace, you can turn it off at the breaker. However you do it, make sure you’re not handling live wires.

Once that’s done, there are three approaches you can take:

  1. Literal hotwire. If you’ve seen an older movie depicting someone crossing wires in a stolen car to get it started, it’s the same concept: Unscrew and remove the red (power), white (heat), and green (fan—if there is one) wires from their respective terminals and twist them together. When you turn the power back on, your heat should come on full force. If there’s no green wire, just twist the red and white together. Pro tip: If it looks like the wires might slip back inside the wall, slide a pencil or other implement behind them to prevent it.
  2. Jumpers. Using either alligator-style jumper wires or magnetic jumper wires, connect the red, white, and (if present) green terminals. Again, once in place turning on the power should get the heat going immediately.
  3. MacGyver it. Don’t have specialized electrical tools lying around and don’t want to twist wires together? You can jump the red and white wires with another piece of wire—just peel back the insulation on both ends and connect the exposed ends to the R and W terminals, screwing them in place. In a pinch you can even use a paperclip—all you need to do is create a connection between the power and the heat.

Note that you can also get your air conditioning on this way if it’s a heat wave emergency and your thermostat dies: instead of the white wire, connect the red and green to the yellow wire connected to the Y terminal. Also note that under no circumstances should you connect the red and black or blue wires connected to the C terminal. This will burn out your furnace’s circuit board and you will have a mess on your hands.

Finally, keep in mind that this is for emergencies only. This will cause your heat (or air conditioning) to run constantly, and these systems aren’t designed for that. Over time it will damage your HVAC systems, so make sure you replace your thermostat as soon as you possibly can, and/or disconnect the power to give your furnace a break.



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