Controlling Temperature: Thermostat vs. Rheostat


Temperature control with thermostats and rheostats works differently, but each has its uses.

Both thermostats and rheostats can help you control the temperature in your box turtles’ habitat

If you want to set up an easy system for controlling the temperature in your box turtle’s habitat, attaching a thermostat or rheostat to your heat source can help.

Using one of these devices can make temperature control more hands-off. So you don’t need to be constantly checking and adjusting.

But you may not be sure which you need. Do they do the same thing? If not, what’s the difference? Is one better than the other?

The short answer is: They do similar things in different ways. A thermostat maintains a specific temperature (more or less). A rheostat controls how much heat a lamp or heat mat puts out, keeping that constant without regard for the actual temperature. Neither one can make your heat device any warmer than it would be if plugged directly into the wall.

And the one you need depends on what you are trying to do.

How Does a Rheostat Work?

A rheostat works like a light dimmer. Adjusting the dial controls how much electricity flows to your heating device. And that controls how much heat it puts out.

Once you set it, it stays on that setting, sending the same amount of electricity into your heating device until you change it. That means it puts the same amount of heat into the habitat no matter what the temperature is in the room. Which means the temperature in the habitat will rise and fall the same as the room temperature.

The absolute temperature in the habitat and room will be different, but the change will be the same. So for example, let’s say your room is 70° F and you have adjusted the rheostat so that it puts out enough heat that the habitat is at 76° F. Now say the room temperature falls to 65° F. The habitat temperature will also fall, to about 71° F. If you want the temperature to go back up to 76° F, you need to manually adjust the rheostat.

How does a Thermostat Work?

When you use a thermostat, you set it to a specific temperature and put the temperature sensor in the habitat. It will then keep the temperature in that spot within a small range around the set temperature. The temperature swings vary by device, but can be as much as 5 degrees above or below the set temperature.

There are two basic kinds of thermostats:

  • The simplest are just on/off devices. They turn your heater/lamp on when the temperature drops too much and off when it gets too warm. These are the kinds that have the biggest temperature swings.
  • Proportional thermostats are a “smarter” kind of thermostat. Instead of turning on or off completely, they can increase or decrease the amount of electricity going to the device. So they are kind of a cross between a rheostat and an on/off thermostat. They let you vary the amount of electricity going to your heater like a rheostat. But they can change the amount by themselves, in response to changes in temperature.

The advantage of proportional thermostats is the temperature rises and falls more slowly and the temperature swings are smaller. This gives you better temperature control.

Thermostats can also be analog (with a dial) or digital.

You may have seen some (like the Zoo Med ReptiTemp Thermostat) that don’t have a way for you to set a specific temperature. The dial only allows you to adjust the temperature higher or lower. Sometimes people mistake these for rheostats, because the dial is so similar.

But these are thermostats that do measure the temperature and turn on and off in order to maintain the temperature you have set. They are a bit more irritating to set since you have to use a separate thermometer to see the temperature and guess at how much to adjust the dial. But if you’re on a tight budget, these thermostats tend to cost much less than many others.

Which Should I Use to Control a Heat Lamp?

The short answer: It depends.

If you’re using a heat lamp that also produces white light, the simple on/off thermostats will basically just make the light flash on and off all day and night. This can be disturbing for both you and your pet. Nighttime red bulbs will also flash, although it may be less disturbing since these generally give off only a faint light. But the constant flashing can weaken the bulb’s filament and cause the bulb to burn out quicker.

A proportional thermostat will solve the flashing problem, because the light will just change intensity, not completely turn on and off.

And, of course a rheostat will keep the light and heat output at a constant.

 But …
Read the package on your bulb first! Some bulbs, like the Zoo Med PowerSun UV bulbs can’t be used with thermostats or rheostats. Many fluorescent bulbs also don’t work with dimmers.

If you’re using something like a ceramic heat emitter, which doesn’t give off light, you can use any controller you want, including an on/off type thermostat.

No matter what kind of heat lamp you’re using, if the temperature in the room fluctuates a lot, you’re probably better off with a thermostat, since it will maintain a stable temperature without you having to reset it constantly. But if you keep your room temperature stable, a rheostat (or even just a regular lamp dimmer) will work just fine and generally cost less.

How Should I Control the Heat in a Heated Rock?

Although we’re not to fond of these things, because reptiles have been known to get burned on ones that got too hot, attaching them to a rheostat can help prevent this and make them safer for your pet.

The rheostat would “dim” the heat output of the rock so it can’t overheat. You can adjust it so that it’s warm enough to bask on but not so hot it will burn your pet.

Keep in mind that you will need to use a thermometer to measure the temperature of the rock’s surface (not the air around the rock) as you adjust your rheostat. And you will want to check the temperature regularly to be sure the rheostat is still working.

You would not want to use an on/off thermostat to control this, because the temperature fluctuates too much, and that could allow the rock to get too hot.

A Few Final Thoughts …

Thermostats and rheostats each have their uses. It’s important to understand what each can and cannot do. And it’s also important to read the information manufacturers provide about their products. Certain bulbs need to be plugged directly into the wall. And some controllers are not suitable for certain heating devices. Often this information will be on the packaging or in the manual.

Also, each thermostat and rheostat has a maximum wattage it supports. Plugging too much into it could cause it to fail prematurely. This is usually only an issue if you’re plugging two devices into one controller.

Box turtle eating strawberrySo, both kinds of controllers are very useful for managing the temperature in your box turtle habitat. You just need to decide what you’re trying to do and which one will serve your needs best before you buy one.


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