When setting up your cage or rack to house your herps you will be faced with the decision of how to regulate the heat. The main choices are either a thermostat (on-off or proportional), or a dimmer or rheostat. I'll discuss the differences in them and my preferences here. To begin with let me point out that a rheostat is just a glorified dimmer switch, they function the same.
Using a Dimmer Switch
For years it's been common to use dimmer switches or rheostats to control cage temperatures. This common use began in the days before there were economical thermostats available and their use was widespread hence why they are still recommended today. Unfortunately in my opinion dimmers are not the ideal method to accomplish temperature regulation. I'll explain my reasons for this opinion and exactly how dimmers work.
A dimmer switch regulates the flow of electricity to the heating device at a reduced but constant rate. This decreases the heat output of the heater, but cannot actually control the temperature outside of it's preset flow of electricity. This means the temperature in the cage will rise and fall according to the temperature of the room because the heater is being supplied with electricity at a constant rate. For instance the overnight temperature in the house or herp room may be 70°F, but over the course of the day during the hotter parts of the year it may rise to 85 or even 90°F. This is especially true in a dedicated herp room which uses many heaters and/or basking lights during the day.
These devices all being used in the same room can raise the temperature in that room significantly higher than the rest of the house. While the room temperature rises, the heater controlled by the dimmer switch continues to receive the same amount of electricity regardless of the temperature inside the cage. This can result in overheating of the reptile's environment. The only way to prevent this from happening is to monitor the temperature inside the cage and adjust the dimmer as the room temperature rises.
This might be needed several times per day in some instances.In colder weather this problem is not normally as severe. The room temperature should usually remain below the optimum temperature needed by the reptile and the resulting temperature swings inside the cage will be decreased. Swings will still occur, but not usually to a degree that will affect the reptile unless the temperature at night falls low enough to cause the animal to go off feed.
Using a Thermostat
Thermostats operate by controlling the flow of electricity based on the temperature of the cage rather than controlling it to a preset point. There are basically two types of thermostats in use in the hobby today, the simple on/off type and the proportional variety such as Helix or the models manufactured by Big Apple Herp.
First the on/off type. These thermostats control the temperature by shutting off the flow of electricity to the heater when the temperature at the external probe has reached it's set point. These thermostats have a degree of swing in the temperature since the cage needs to cool below the set point for the heater to be turned back on. Usually the swing will be +/- 2 to 5°F. Not a dramatic swing, but not as tight as proportional models.
Proportional thermostats regulate the temperature by adjusting the flow of electricity based on the temperature at the probe rather than shutting it off completely. These type thermostats have a much tighter control of temperature, as little as +/- 0.5°F, but they are significantly more expensive that on/off type thermostats. If precision is required then they are worth the investment. However, that level of precision is rarely required in a cage habitat.
Dimmer switches come in at least three types, hardwired, inline, and plug in types.Hardwired devices require you to splice them in permanently and normally run $5 or so at the hardware store.Inline models come apart into two pieces and snap over the power line to the heater. Very easy to install, but cost around $20 at Lowe's. The plug in type simply plugs into the wall outlet and the heater plugs into the dimmer. Obviously the easiest to install, and they cost $15-20.
Thermostats come in many varieties and range in price from $25 to $135 depending on the type. My personal preference in on/off style thermostats are the Johnson Controls thermostats which run about $75 pre-wired. They are a very good quality thermostat with digital settings.
A cheaper on/off style is the Repti-Temp 500R from ZooMed. These can be purchased directly from ZooMed at herp shows, or online for $25-30 plus shipping. However, they are set with a dial and require the use of a seperate thermometer to read the temperature.
Overall, considering how long you will potentially be using the thermostat, the Johnson's are more than worth the higher initial investment. Proportional thermostats are significantly more expensive than the on/off type. Helix proportional thermostats run $150 or so but if precision is what you need then they are very good units. I have both in operation in my collection.
The primary competing brand for Johnson's in the reptile hobby is Ranco. They are also a very popular brand of thermostat and preferred by many keepers.
I have had quite a few Rancos in use myself over the years, but I no longer recommend them. I had three fail over the course of two winters. The failures were all after a power flicker which was common here at the time when it was windy.
They all had the same non resettable error. None of my Johnson's controllers have ever given any problems and I had them running along side the Rancos.
I'm not advising you on which brand to buy, just relating my experience. After losing three of them, I have switched entirely to Johnson's which are essentially the same price.
Another benefit of using some of these thermostats is you can easily control more than one cage with a single unit if the cages are of similar design and sitting fairly close together in the room. Ideally of course a cage would have a dedicated control unit regardless of the type, but this is simply not economical for collections of any size.
Years ago dimmer switches were basically the only option to control reptile cage heaters. They do the job to a point, and I'm not saying you are endangering your herps by using them, but they cannot create a stable environment in many situations and can result in unconfortably high or low temperatures for the reptile.Today we have several economical thermostats available and they do a much better job of keeping a constant temperature. I recommend you not use dimmers on your cages or racks unless the room temperature in which the cage resides in is very stable. This is rarely the case though. With the nominal difference in cost between the two types, it just makes sense to get the best control you can for the money.