Fridge & Freezer Thermostats
On older fridge and freezer units as well as many low-tech or older units you will find that they operate by way of a pretty simple device that is known as a thermostat. This is much like most thermostats in that it will generally, in any refrigerator or freezer at least, offer only two operating positions as it is, in effect, a simple mains switch, on and off.
This article does not cover fridges and freezers that have electronic controls and use thermistors although some of the general principles of operation will apply.
General Principles Of A Fridge Freezer
Where most people struggle with refrigeration, including some engineers, is that the temperatures work in reverse and it is important to remember that the idea of a mechanical thermostat is simply to supply power to a lamp and/or a defrost heater in some cases and, it's most important function, to switch on or off the compressor.
The compressor gets it's name from the fact that it "compresses" the gas so that the pressures required for refrigeration are reached. For the sake of allowing you to simply understand how this particular component works with the thermostat we will explain a little further.
The compressor is often called the "black bottle" like thing or motor. In fact, motor is more accurate as that is what it truly is inside the outer casing. Inside there is a motor, normally suspended on springs and sat in a pool of oil to help with cooling and sound insulation. It is a very simple electric motor really as shown below.
The thermostat simply switches this motor on or off and gas is compressed and sent around the refrigeration system.
How A Fridge Or Freezer Thermostat Normally Operates
This is really very simple indeed. Fridge and freezer thermostats will, like those for cookers and other appliances, operate on a "mean temperature". That is to say that they will operate cyclically in a temperature range that is typically 10% either side of the set temperature.
This means that, for example, a fridge thermostat set to a 3˚C temperature would typically cycle and switch on 10% higher or off 10% lower than that temperature. In reality most thermostats don't prove to be that accurate and would usually cycle between 1-2˚C to switch the compressor off and 4-5˚C to switch it on, remembering of course that the compressor will be switch on to lower the temperature inside the fridge or freezer.
Thermostats used in commercial units are more accurate as are many more upmarket domestic fridges and freezers as, quite simply, the more accurate the thermostat is the more expensive they are. Low cost products will usually use low cost components which are almost invariably of poorer quality and accuracy.
It is worth remembering this when if you ever get offered thermostats that seem too cheap as they are often wrong or just rubbish. Of course there are so-called "universal thermostats" from Ranco, Danfoss and a plethora of other far less well known component manufacturers which are cheap simply because of the sheer volumes that they are produced in. However, when it comes to these types of thermostat we'd strongly recommend that, unless there is no alternative, you ONLY fit Ranco or Danfoss ones as, to be frank, the rest are usually pretty poor. Put it this way, it's what we engineers use as they fit, work and don't give any hassle.
Why Fridge & Freezer Temperatures are a Safety Issue
The idea behind this is that the food kept in a fridge or freezer is stored at the optimum mean temperature. In days gone by for a fridge this would have been about +3-5˚C and -18˚C in a freezer. These storage temperatures represent the best possible storage for your food in general terms in a domestic setting.
At these temperatures bacteriological growth and degradation of the food is kept to a minimum but, go outside these temperatures significantly unless appropriate and your food may not be stored safely. This can lead to problems of food spoiling more quickly than it should or allow bacteria, such as salmonella or suchlike to culture within the food. You don't want that and that's the point of maintaining a suitable environment in which to store food.
These days however, although the optimum of -18˚C remains for frozen food, refrigerated food storage has changed considerably and with the introduction of far more accurate and cost effective electronics it is now much easier for manufacturers to offer refrigerators with multiple temperature zones. Of course any foods can still be kept in a "normal" fridge very easily but many will keep longer and stay fresher in a tighter controlled temperature zone.
Largely this is due to the shift in people's buying habits for food and the types of food being stored, not the refrigeration as it's only followed the trend of what people want. But it is now commonplace to see units with multiple temperature zones and, a few, use traditional thermostats. Where these are used it is usually for a "Cool Zone" or an area to keep meats and deli products or even pre-packed foods. These areas have a much more accurate thermostat in the and the temperature is maintained at just above freezing point, typically between +1˚C and +2˚C. You cannot in any way use a universal thermostat for this sort of application and, if you were to do so, it is highly likely that food would be being stored at the incorrect temperature.
The point is however that the whole point of having a fridge is to ensure that your food is as fresh as possible and, for as long as possible so as to maximise safety as well as minimise any food you may have to throw away prematurely. Having the fridge or freezer be able to operate in these normal ranges is imperative to ensuring that this happens and, the more accurate the thermostat is, the better.
A normal generic fridge freezer thermostat and what the parts are.
When you buy a new thermostat or remove the old one, one of the first things that you will see is the phial which is also commonly known as the capillary.
This is the long, usually silver, very fine metallic tube that will normally attach to either the back wall or to the fridge or freezer evaporator.
Where the end of the phial goes is EXTREMELY important and it MUST be sited where you found it! This is because on many fridges and freezers that the phial will only operate as it should, cutting in and out at the correct times and temperatures only when correctly positioned. This is especially the case where the phial is attached to the plastic inner wall of a fridge, it can be vital.
Also you must not cut or kink the thermostat phial, you can roll it up or coil it, but you MUST NOT kink or cut it, doing so will render the thermostat utterly useless and unusable ever again. This is because the phial contains a minute quantity of gas that expands and contracts operating the simple bellows inside the main body of the thermostat which, in turn, operates the switch. Once the gas is out or blocked from operating correctly, the thermostat is useless.
The Main Body
The main body of the thermostat contains the switching and also the bellows that operates the switch itself as described above.
The bodies of most thermostats are pretty robust and can be handled easily without any fear of breaking it. Problem is of course, you have to watch you don't kink the phial getting them into some housings.
We'll go into these in more detail in another article but, generally, you will have spade terminals to make the electrical connections required and, normally, two, three and occasionally four terminal posts on the thermostat.
The tags are virtually always numbered, take a note of them and what goes where!
It should be a case of fitting them like for like, old and new thermostat using the numbers.
The Thermostat Knob Shaft
Obviously this is the shaft which the control knob that you would be able to regulate the setting of the fridge or freezer will attach to.
There will almost invariably be a threaded section at the bottom of this shaft on the main body. This is often used to fix the thermostat into the housing of the fridge or freezer by way of a nut (which is usually included with a replacement) but you will often see this fixing even in thermostats that do not require it, that's perfectly normal.
Fridge & Freezer Thermostat Types
There are, literally, thousands of different thermostats out there. Some are special one-off items and some are generic. Many, many can be replaced using these standard thermostat types that we discuss here where it's a standard fridge, freezer or fridge freezer and can save you a fortune over the genuine item which, to be brutally honest, are very often the same thing anyway. The hardest part is knowing which one to use and that requires a little detective work on your part or, you get a professional repairer to look at it for you.
These thermostats are the ones that almost every single fridge engineer will always have in his van as it will solve 90% or more of thermostat failures. They're cheap as chips and miles cheaper that the genuine thermostat on almost every occasion.
In normal domestic use there are three basic types of fridge or fridge freezer thermostat all of which you can see from this link that opens a new window or tab. Although more then three are shown we'll break that down for you in a moment, the general three types are as follows:
Icebox fridge with push-button (manual) defrost
Icebox fridge with no defrost
Fridge or fridge freezer with automatic defrost (see below for more detail)
The first listed, an icebox fridge with push-button defrost, is fairly self-explanatory. If you have a red (usually) button on the thermostat fixed to a thin shaft that comes out the centre of the thermostat knob shaft, this is the one you need and it will fit almost any fridge with a push-button defrost.
The second, for a fridge that has an icebox inside at the top, just like the one above, but it doesn't have the manual defrost button. Again, this thermostat will fit virtually any fridge that has this configuration.
The third type is where things get complex.
For a normal auto-defrost fridge or fridge freezer things used to be simple and there was the VT9 thermostat, it fitted them all pretty much. The code, VT9, stems from those days when the most common replacement thermostat was the Ranco VT9 and, the name stuck.
This thermostat will fit both auto-defrost fridges and fridge freezers where it is suitable.
The VT9 has the connection on it for a drain heater or defrost heater. This is the aluminium based heater that was fitted to clear the ice off the evaporator and also to clear the drain channel of ice allowing the water, built up inside the fridge as ice, to be melted and drained off. To this day the VT9 remains in use and is still popular on many low-cost fridge freezers as it's simple, cheap and just works.
But in the late eighties/early nineties we started to see the introduction of what we call "wet wall" fridges and fridge freezers. This is a type of fridge where the evaporator (the bit that cools) inside the fridge was buried behind the rear wall of the fridge and was no longer visible. The rear wall with have a smooth plastic back if you have this type of fridge freezer and you will not be able to use the VT9 thermostat in all likelihood, on some you can but it's better to check with us if you're thinking on trying that.
If on the back wall you have an aluminium plate with what looks like little tubes running all round it, it's an older type of design and, in virtually every case the VT9 will be fine in these types.
The reason that you can't use a VT9 in a wet wall cabinet is because the cut-in and cut-out temperatures that the thermostat is pre-set to operate within are different and you would fit it and it would either not cool or not cool very well or, more commonly, it would run constantly and over freeze. Don't be fooled by the "K59" monicker as we've seen many thermostats with that code that are not a VT9 thermostat. The golden rule is, if you're not sure CHECK FIRST!
Also the capillary length is stated as being 1200mm, this can be coiled (remember not to kink or cut) up at the thermostat housing with no effect on performance.
Fridge Freezer Thermostat Types
You might find it a bit odd that you use the same thermostats in a fridge and a fridge freezer but, in most every domestic unit there is a fridge with a separately controlled freezer or a fridge with a thermostat that makes the freezer also cool. How this works is beyond the scope of this rather large article and actually needs one to itself as a subject so, suffice to say for now, that's the way it is.
As above, a lot of fridge freezers can use the trusty old VT9 but most of the more modern wet wall cabinets can't use that. So, for the trade especially, alternatives were needed so that we didn't have to carry a plethora of different thermostats and to make the ones we needed cheaper through volumes.
So, we've ended up with a range that will fit, again, 90% or more of the machines out there that use a traditional thermostat, most of which are intended for use in a fridge freezer.
Now, it is very important to note that while many of these thermostats look the same, they're not! They operate on different preset temperatures and it is essential to get the correct one, if you're not sure then ask us.
For wet wall fridge and fridge freezers you need the Ranco VXO thermostat, which you can see here. Although it's billed as being used on Hoover and Candy primarily that's only because they were the first ones most people came across that use the much higher warm cut out temperature of -5˚C, whereas the traditional VT9 used a -11˚C warm cut out.
Now, remember the bit at the start of this article about temperature working in reverse, well here's one many people struggle to get their heads around, because the VT9 was traditionally attached to a metallic evaporator plate the temperature cut out (to tell the compressor to start running) is rated as cutting out 6˚C higher than the VXO type thermostat. The cold cut out (to tell the compressor to stop running) is -15˚C as opposed to -26˚C on the old VT9 type.
These are MASSIVE differences in temperature range and, what it means is that if you fit a VT9 where you should have used a VXO then you would have the fridge running much longer than it should and virtually turning it into a freezer. Conversely, if you fit a VXO where you should use a VT9 then it would not work, the machine would "short-cycle" and not cool effectively.
It is therefore important to get the right thermostat.
Normally, on any decent thermostat, the temperature cut in and cut out points are stamped on the thermostat body and this should enable you to choose the right one.
If you don't have that then, as a general rule:
- If there's a defrost heater, use a VT9
- If there's no defrost heater, use a VXO
If it's totally different of has any oddball characteristics, such as special connections or weird capillary lengths then it's liable to be a special for that manufacturer or model and you'll need to get the pukka part.