How to Use Thermostats

This is a guide to how thermostats should be used, to ensure they function properly and your dragons are protected from overheating etc. If you have any suggestions about things to add, please Email Us.

1. Basic Principles

A thermostat ("stat") is used to control the temperature of a cage. They work by adjusting the power to your heating device to keep the temperature by your probe (or the box itself if you have a probeless one) to the temperature you set the thermostat to.

2. Types

"Mat Stat" / on-off: This turns the power off when the desired temperature is hit, then turn the power back on when the temperature falls a certain amount below what it's set to (the "on threshold" value). Depending on the stat, this can be a few degrees. If you set the stat to 85f, and it had a 3f sensitivity, the temperature would go: 85f -> 82f -> 85f and so on. On-off stats are commonly used to regulate central heating in rooms in your house.

"Pulse Propoertional": This sends power to the device in pulses, rather than being simple on, then off, then on, then off etc. They are usually used with ceramics - if used with a light bulb, they will cause the bulb to flash.

"Dimming": These reduce the power to the heat source (but don't turn it off completely) when the desired temperature is approached. They are commonly used with lightbulbs, and can prolong the life of a bulb by not switching it on and off, and causes less confusion to your pet, by keeping the light levels fairly constant.

3. How to use

For an animal that requires a temperature gradient, a stat is a safety feature, NOT a control device. For an animal that needs a single ambient temperature it's slightly different, but for an animal wanting a gradient, they should not be used to control the warm end temperatures.

A stat is fitted to ensure that there is always a cool end to retreat to. There's a reason why most only go up to 92f; they weren't intended to be controlling 110f bearded dragon basking spots!

If a beardie (or any other reptile that has a basking spot) gets too hot, it will seek a cool area to cool off in. If you have your stat right under the basking light, set to 92f and it's a hot day, the rest of the viv may get to 92f, as there will be very little heat loss, due to there only being a small thermal gradient to the rest of the room. If your beardie has basked and got itself up to 100f (desired temperature is 93-96f, depending on the individual) and tries to cool off, it will take a very long time to lose heat with only an 8f difference, even in the cool end. Staying at too high a temperature for a prolonged period is not good for them - when they want to cool off, they have to be able to do it quickly! A cool end of 80-85f gives a 15-20f gradient for heat loss; this will allow much faster cooling (read up on thermodynamics if you want to know why...)

The stat / probe needs to be in the cool end, to ensure that, even on the hottest day, there is somewhere to cool off.

Once you have the cool end under control, you can then think about setting the basking spot:

4. The Basking Spot

Once you have a nice, stable cool end (which may take an hour or two to settle down), you can start to think about creating a basking spot. The stat controls the total wattage of energy going into the cage; the basking spot is controlled by the intensity of the light on the basking spot - think of it in terms of energy per square inch of basking spot. A lot of energy means a hot basking spot; a little energy means a cooler one.

Using a narrower beamed light can help - the wattage is the same, but the light is more focussed onto the basking spot, so the intensity is higher. Changing the distance between the light and the basking spot also helps - it increases the intensity of energy hitting the basking spot. Often this will mean suspending the light from the top of the vivarium, to avoid having to build amazingly tall and complex basking spots.

5. Common Problems

There are a number of commonly-encountered problems / faux-pas associated with the use of stats:

a) They only control the temperature at the probe. A stat can't control the rest of the cage. You have to set the cage, with the stat, under normal operating conditions, then tinker around with the rest of the temperatures.

b) Heat rises. Having a stat placed 18" up the backwall in a cage for a 1" tall baby beardie doesn't make much sense - the beardie doesn't care how warm it is 18" above its head; it cares how warm it is on the ground! Although this is a "good" kind of error, it leads to inefficiency in heating - if the temperature is controlled at 85f 18" up, it may only be 75-80f on the ground. Whilst this is good in that there's a decent cool spot, it will make setting the basking spot harder, and can lead to premature dimming of the bulb on hot days - the ground level where the beardie is may still only be 75-80f, but the stat will sense 85f, and will turn the basking light (and hence the basking spot) right down. A prolonged period without a basking spot isn't ideal for a beardie.

c) A 100W bulb dimmed to 40W is much less bright than a 60W dimmed to 40W. If your bulb is dim all the time, switch down to a lower wattage. You want the basking light to be as bright as possible, whilst keeping the correct temperatures. I use 60W bulbs in my 4' x 2' x 18" vivariums. The number of people using 100W bulbs in tiny cages that give off almost no light is pretty high!

d) Ventilation is needed to allow heat loss. If heat can't escape from the cage, the cool end will get to 85f, the light will dim...and will stay dimmed. The whole cage is now at 85f, and you've just lost your thermal gradient and basking spot. A couple of 75mm vents in the cool end allow heat to be lost at that end. This makes sure that as heat is transferred from the warm end, it is lost and doesn't just build up in the cool end. Heat loss is vital for keeping a good thermal gradient.

6. Advanced Ideas

There are a lot more things that can be done with thermostats than what is described above. I'd really recommend that if you've never kept reptiles that need basking spots / cool ends, and aren't familiar with the concept of thermostats, you stick to what's described above. It's simple, it's safe and it's functional.

However...

If you are using large, ventilated cages, you could replace the dimming stat with an on-off, and have it as a true safety feature. Set it to 85f, place it in the cool end as usual, but set up the rest of the cage (ventilation, size of cage etc) so that, even on the warmest day, the cool end remains below 85f. You are likely to need a large cage to do this.

This means that your bulb is now "unregulated", with only a safety cut out in place (incase something goes wrong / we get an unexpected heat wave). What this means is that your cage will now track room temperature, and you'll get a bit more temperature variation, as you would in the wild. If you use this in an unregulated / only slightly heated room, the cage temperature will now track even more closely to outside temperature, giving true temperature fluctuations, depending on the weather. For animals that can sense air pressure (and hence weather) changes, such as beardies, it may be less confusing to them if they sense that there will be rain, and the cage gets a little colder than usual, than if they sense rain and the cage retains it's 110f basking spot...

Note: the on-off safety cutout described above is the only way to "stat" a Mercury Vapor Bulb (MVB) - they cannot be used with dimming / pulse stats.

Also on the market are time-variable stats, that adjust the temperature depending on the time of day. They are expensive, but could be used for some wonderful setups. If you are using a large cage, how about having 2-3 of them, each linked to their own lightbulb (with their own basking spot). Put the 2-3 probes together to create one permanant cool end. Set one stat to come on first thing, set to 85f. Set the other(s) to 70f first thing. This will give you one basking spot for the morning. Set the first stat to switch down to 70f at lunchtime, and the second stat to switch up to 85f. Your basking spot will move, representing the movement of the sun across the sky (in nature, reptiles have to move during the day to track the sun and the best basking spots). If you have a 3rd stat, do the same, but with it coming on in the late afternoon. It may be a solution to all of the lazy bearded dragons that are only too happy to sit around by their light and not move for an entire day...

Hopefully this has cleared up some misconceptions, and possibly given people some ideas about how thermostats should (and potentially could) be used. My intention with this guide is to improve safety in reptile cages, by greatly reducing the risk of overheating, especially with summer on its way. People who only got into reptiles in the last 18 months may never have experienced a "proper" summer (as the last one scarcely counts in terms of temperatures) with their reptiles, and may not be aware of what an exceptionally hot cage can do to temperatures and gradients.

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