What is the heat output of my autoclave?

To assist with this question we have compiled a Power Ratings and Heat Dissipation Table which can be found under downloads/instruction-sheets/ on this website.

The output from the autoclave is cyclic and varies throughout the cycle with the heat being put into the system during heat up and sterilising time and with this stored heat being released into the area around the autoclave mostly during cooling – a large amount at first, decreasing as the autoclave cools. Some heat will also be retained by the load and much of that will be released into the room on unloading.

To some extent the absolute amount of heat released depends on many factors not least the type and size of load being processed and the type of cycle but a good rule of thumb for the thermal loading placed on the air conditioning and ventilation system by the autoclave is approximately one third of the total heater power of the autoclave for each cycle.

If the autoclave is steam heated from an external supply then a good indication can again be gained by checking the heater power of the corresponding electrically heated model in the products section of the website.

Models with internal steam generators, although these are heavily insulated, will give off a little more heat as the generator is running throughout the cycle and even when the autoclave is not in use.

Models with a steam jacket will give off a little less heat as the jacket is insulated and if water cooling is used the heat is taken out to the drains rather than going into the laboratory.

In the case of a direct steam heated unit simply look up the wattage of the corresponding electrically heated unit.

Do I need a drain to connect my autoclave to?

Yes. If used for processing contaminated materials or the autoclave has any form of active air removal system. Exceptions are rare. Please refer to our Guide to Autoclave Installation (found under downloads/instruction-sheets/ on this website) for more details.

Do I need an extractor hood for my autoclave?

Always an advantage, but not essential. An extractor hood will help to reduce steam and heat build-up in the room, and help to reduce odours. If disposable plastic ware is being processed then the possible build-up of potentially harmful fumes is eliminated.

Do I need to have my autoclave examined regularly?

Yes. It is a requirement of the UK Pressure Systems Safety Regulations that all pressure equipment is regularly examined by a surveyor working for a competent body (usually arranged by your building insurer). This inspection should take place at a maximum of 13 month intervals i.e. annually.

Although as the manufacturer of the equipment Priorclave are not permitted to carry out these inspections, for customers with a maintenance contract one of the routine visits can be organised to coincide with the surveyor’s visit so that the autoclave has been serviced in preparation for the inspection and our technician is available to assist during the inspection.

This can also be arranged as a one-off if required. Please contact Priorclave Service service@priorclave.co.uk for details and a quotation.

For autoclaves installed outside of the UK regulations vary from one country to another but usually require some form of routine inspection. Check with your insurer to find out what is required in your jurisdiction. Your local Priorclave distributor may also be able to assist you with this.

Which products are suitable or recommended for cleaning the inside of the Priorclave autoclave? Are there any particular types of cleaning product you can recommend?

However careful you are, autoclaves can get dirty inside after a time and need a bit of a clean-up, it really depends on how dirty and how often as to what the best approach is.

What we say about this in our manual is as follows: “Grade 316L stainless steel is employed to reduce the corrosive effects of substances such as hydroxides and chlorine. However we recommend that the interior of the vessel is kept free of such potentially harmful substances and is regularly cleaned out with softened water. The use of chlorine based or other aggressive cleaners is not recommended. Exposure to such chemicals could damage the surface finish and the integrity of the pressure vessel and door. Care should also be taken not to routinely introduce such chemicals where they are used to pre-wash items that form part of the load. In such cases the items should be thoroughly rinsed before autoclaving.”

Practically speaking though, if you intend to regularly clean up the inside of the autoclave you really should avoid using chlorine or ammonia based cleaners as over time they will have an effect on the inside surface of the autoclave vessel, effectively making it appear dirtier more quickly. Over longer periods they could corrode the pressure vessel and door. For regular cleaning some water and a plastic scouring pad is best. Do not use a metal abrasive pad as this will leave rusty residues in the pressure vessel. You may find that using a wet/dry vacuum cleaner will be useful to change the water during rinsing. Alternatively we can supply a small syphon pump to assist in this process.

If it’s a clean up after a long period of time or after a spillage then initially we would suggest a mild acid like phosphoric acid (kettle de-scaler) to help with a big clean up but be sure to thoroughly rinse out the chamber afterwards as unless you have a cycle running that drains the chamber at the end then electrolytes will tend to concentrate in the water reservoir over time.

Our service department (service@priorclave.co.uk) can offer a cleaning and de-scaling service if required, which is useful to remove water scale from the autoclave vessel and heaters if that is the issue.

I have a drain condenser system fitted to my autoclave and I hear the occasional banging noises, usually during the cooling stage of the cycle. Should I be worried?

There is no need to be too alarmed by these types of noises.

Within the condenser and pipework there will be a constantly changing mix of hot Steam and cool or cold water, so the mix will create differing gaseous pockets at different stages within the pipe-work. It may not be consistent between different autoclaves as it very much depends on a host of variables such as pipe-work length, route, size, temperature of water and steam and even minor manufacturing differences with the plate heat exchanger etc. There are no moving parts or excessive pressure within the system, so it is perfectly safe.

The Factory set thermal lock is set to 80°C but the autoclave will not open until the control panel shows a temperature is below 50°C. How I can reset this to 80°C?

The thermal lock is fitted as a safety measure and was incorporated into the UK standard for laboratory autoclaves (BS2646) after a number of accidents where glass bottles containing liquids or growth media exploded on exposure to cold air when the autoclave door was opened. Although generally bottles with liquids should not be fully sealed when autoclaving, if one is sealed in error or becomes sealed during autoclaving, as the autoclave cools the large thermal mass of the bottle and liquid does not cool as quickly and the bottle becomes pressurised and is pressurised even when the autoclave chamber temperature is below 100oC.

The standard requires that the door cannot be opened when the temperature of the load inside the autoclave is above 80°C but a liquid autoclave load cools much more slowly than the autoclave chamber. Since it is impractical to routinely monitor the load temperature with any degree of certainty we make a calculation based on experience and measurement of a number and variety of autoclave loads that the Control Probe will be approximately 30 degrees below the load temperature so the safety lock releases at 50°C.

With the more common use of borosilicate glassware, or where plastic bottle are used or where no glassware is involved the thermal lock temperature can be changed by a trained Priorclave technician or by uploading a new setting file to the autoclave. If required, different thermal lock temperatures can be set for different programmed cycles.

However as the thermal lock is essentially a safety guard and we set it at a level to ensure safety for all load and conditions we require customers to provide a written disclaimer before we can carry out this work.

When customers are considering doing this we strongly recommend carrying out a risk assessment the Health and Safety Executive's guidance note PM73 (found under downloads/safety information on this website) can give some assistance on this. Also advisable is arranging a Performance Qualification test to determine the actual load temperature.

Please contact Priorclave Service (service@priorclave.co.uk) for more details or a quotation.

When I press START the autoclave beeps and fault sign lights up. No codes are displayed. I've checked all the obvious things but please could you advise?

This scenario sounds like one of the START criteria has not been met. Can you please check against our information sheet No Start Lamp on Control Panel (found under downloads/instruction-sheets/ on this website) to see if you can identify anything that may have been missed. If you can’t see anything, please contact Priorclave service (service@priorclave.co.uk) for further assistance.

We own one of your 40L autoclaves. It has never been used. We are now wanting to use it, but are a little confused over the connection of the device to the drain.

Basically, the Safety Valve Outlet should remain open at all times so that, in the event of the valve blowing, the exhaust is visible to any operator (to identify a fault and take action). The VENT outlet should be connected to a drain system (sealed drain if used for waste loads, but open is fine for standard sterilising). You could opt to simply pipe this down to a ‘catch bottle / tank’ to allow steam to condense and then you periodically empty the container. Please note that the vented steam could be hot (typically 104 degrees during a process called Freesteaming). Further information on autoclave installation and positioning can be found in our datasheet A Guide to Autoclave Installation (found under downloads/instruction-sheets/ on this website).

We have a Priorclave autoclave and have bought a water softener. We were wondering if it’s convenient to connect this softener to the drain condenser system as well as the water-fill tank, or maybe only to the water-fill tank?

In many areas, in order to protect the autoclave heaters from the effects of lime scale we recommend the use of a softened water supply. See our Data Sheet found under downloads/instruction-sheets on this website for recommended water quality levels.

Since the water runs through the drain condenser and if fitted a vacuum pump it is acceptable to use mains water, though there is no harm in using a softened water supply. However due to the flow rates the volume of water used will be fairly high.

Also a decent head of pressure is needed to keep the vacuum pump feed tank topped up while it is running and this is not critical for the water filling system.

It is for this reason that two water inlets are fitted to many Priorclave Autoclaves; one for the autoclave itself and the second for any ancillary equipment.

From your sales information there are two types of models: Electrically Heated Priorclaves and Steam Heated Priorclaves. Surely all autoclave sterilisers are steam heated? What is the difference between these two?

The only difference between the Electrically Heated and Steam Heated models is the way in which they are heated. Both models are suitable for the same range of applications.

Steam Heated models require an external steam source such as steam from an on-site boiler house or sometimes a steam generator built into the autoclave - separate from the main autoclave pressure vessel. On these models steam is injected into the autoclave to heat and pressurise the autoclave and its contents.

Although these direct steam heated models can give a slightly quicker cycle time by up to around 10% they do have disadvantages in terms of initial and ongoing expense:

• you need a steam supply available and to have installed or purchase all the regulation equipment required to provide steam at the correct pressure and condition (dry and saturated), or you have to purchase a steam generator

• if the “house” steam supply is down for maintenance so is the autoclave

• built in steam generators add complexity to the autoclave and therefore require more maintenance and possibly will experience more downtime.

• generally they use more energy due to either the heat losses in transmitting the steam from a central boiler or because internal steam generators are running all of the time even when there is no demand for steam (over 50% of a normal operating cycle and all the time the autoclave is not in use)

The Electrically Heated models use electric heaters fitted inside the autoclave pressure vessel to generate steam inside the vessel. These heaters are only running and using electricity during the heat-up and sterilising parts of the cycle.

To answer your question regarding needing water and steam together this is not always the case but for autoclaves with steam generators, autoclaves with vacuum systems or if a condenser system is fitted to cool the exhaust from the autoclave, then water is required for these systems to operate.

For further information can I refer you to page 17 of our Autoclave Knowledge Bank booklet (found under downloads/datasheets on this website), which runs through all the different variations of heating systems, vessel types and vacuum systems and other general information to assist in the correct specification of a laboratory, research grade autoclave.

Why are there two water connection points on my autoclave?

In many areas, in order to protect the autoclave heaters from the effects of limescale we recommend the use of a softened water supply. (See our Data Sheet -found under downloads/instruction-sheets - for recommended water quality levels).

Since the water runs through the drain condenser and if fitted, the vacuum pump it is acceptable to use mains water, though there is no harm in using a softened water supply. However due to the flow rates the volume of water used will be fairly high.

Also a decent head of pressure is needed to keep the vacuum pump feed tank topped up while it is running and this is not critical for the water filling system.

It is for this reason that two water inlets are fitted to many Priorclave Autoclaves; one for the autoclave itself and the second for any ancillary equipment.

What level of water quality do you recommend for your autoclaves?


Priorclave autoclaves use water in two ways, to produce steam for sterilising with electrical heaters inside the autoclave chamber and for ancillary equipment such as vacuum pumps and cooling condensers.

In general mains / tap water can be used directly for the ancillary equipment as the volume and water pressures required (a minimum of 2Bar) make the use of or need for treated water an unnecessary and impractical expense. Where the autoclave is heated by external steam then specific feed water requirements will be covered in the manual for the steam boiler.


To avoid unnecessary and costly maintenance and in some cases possible contamination of autoclave loads, close attention should be paid to the quality and condition of the water used to generate steam in electrically heated models. In these models heaters are fitted to the bottom of the autoclave chamber and sit in a reservoir of water from which the sterilising steam is generated. The major cause of maintenance issues with Laboratory Research Grade Autoclaves is water hardness and particularly hardness due to calcium carbonate. Over a relatively short period of time lime-scale from hard water will start to coat the heaters, initially reducing their effectiveness and ultimately causing them to fail.

Lime-scale build up will also affect the operation of water level sensors and the efficient operation of valves.

A less common issue is water supplies containing a high level of particulates, which can also affect the operation of valves. This can often be an issue in new buildings for a short period while newly installed pipes flush through. Unless the water supply is defined as drinkable (potable), soft water (in the UK soft water is defined as having a total hardness in terms of Calcium Carbonate of up to 50mg/L or 50ppm) then water treatment is strongly recommended. Although to some extent distilled and ultra-pure water supplies are often abundantly available these are unsuitable for Priorclave Laboratory Research Grade Autoclaves unless they have been specially adapted to run from these types of water supply.

There are two main reasons for this; the first is that extremely pure water prematurely corrodes the copper pipework and brass valves and fittings used in the autoclave and the second is that heater protection and water level controls use the conductivity of the water in the chamber to detect it.

Priorclave recommend the use of a sodium ion exchange water softener, which removes Calcium Carbonate, reducing lime-scale formation, while keeping conductivity levels at a detectable level.

Although electrical and magnetic softeners are available which operate by changing the nature of the calcium carbonate so that it is less able to form lime-scale, these are not recommended as over a short period of time the denatured calcium carbonate becomes concentrated within the autoclave and forms a thick white powder in the bottom of the chamber.

If you already have a water supply that you feel may be suitable the key figures for suitability are as follows:

Total hardness in terms of CaCo3: < 50mg/L (50ppm) Conductivity: > 15 micro Siemens

Investing in proper water treatment goes a long way towards trouble free operation but this not a substitute for a proper draining and flushing regime.

I am presuming Autoclaves need cleaning/water changing completely at various intervals. What is the best/recommended practise for doing this?

Although water does get used during a cycle in the steam generation process, we would recommend exchanging water on a weekly basis. You can use a simple hand-operated syphon pump or wet / dry vacuum cleaner if available.

While empty, inspect the vessel surface and Water Level Monitoring Probe(s) and wipe down if necessary, then re-fill with fresh clean water.

We have an autoclave that hasn't been used for some time and would like to have it up and running again. Have you any advice on the appropriate site to install it and some of the considerations that need to be taken into account (health and safety).

Please refer initially to our Guide to Autoclave Installation (found under downloads/instruction-sheets on this website) to give you some of the information that you want.

This sheet gives general information on what is required but depending on circumstances and what is being autoclaved and how often solutions can be found for most locations and uses.

For more detailed information please contact our service department (service@priorcl;ave.co.uk) who can advise you further. For new installations in the UK we can often arrange for one of our Service Technicians to come and look at your proposed installation site and give practical advice.

For health and safety aspects if you are unfamiliar with running autoclaves you can refer to the Health and Safety Executive's guidance note PM73 (found under downloads/safety information on this website)which covers all aspects of laboratory autoclave safety.

I was wondering whether you are able to provide any on-site training, for your Laboratory Autoclaves?

We can certainly offer training on the use of Priorclave Autoclaves and on Autoclaving Principles. Please contact Priorclave Service (service@priorclave.co.uk) for a quotation.

Should I connect the safety valve outlet to a drain?

Unlike some other steam heated systems the safety valve does not operate during normal running. It should actually never open at all except during routine annual testing.

We strongly recommend that the safety valve outlet of the autoclave is left OPEN so that, in the event of a malfunction, the excess pressure is expelled without hindrance and the main pressure vessel is safe and avoids potential catastrophic failure.

This arrangement also gives an audible and visual indication to operators that there is an issue with the autoclave.

We suggest that if any pipework is fitted to the valve outlet that this is only to direct the output of the valve to the floor with the minimum number of turns possible so as to ensure free flow.

How much water will my autoclave use?

Determining water use is a little problematic as it does depend greatly on the cycles run since the major variable users of water in the cycle are the vacuum pump and the drain condenser where these are fitted.

Firstly the autoclave should have 2 water inlets, one for the water fill for the autoclave chamber and one for the vacuum pump. This is because in many areas we recommend the use of softened water for the autoclave chamber.

However this is not strictly necessary for the vacuum pump and / or condenser and customers may not want to flush the more expensive treated water down the drain through these. Also a decent head of pressure is needed to keep the vacuum pump feed tank topped up while it is running and this is not critical for the water filling system. If required the two inlets can be joined to a common feed outside of the autoclave.

A while ago we conducted some tests on one of our front loading 150L models with a vacuum system and a drain condenser fitted but the data gathered from these tests can give us at least an approximate idea of water use.

Autoclave Use: On average for a normal type cycle around 3.3L will be lost from the autoclave chamber over a cycle, which is replenished by the water fill system at the end of the cycle. A total fill of the water reservoir at the base is around 12L.

Vacuum Pump Use: This again is very dependent on the cycles run but on a cycle with the standard 2 pre-heating vacuum pulldowns and 12 post cycle vacuum pulldowns the use is around 35L per cycle. If only a pre-heating vacuum is used then the use is around 10L per cycle.

Drain Condenser Use: This again is very dependent on the cycles run as the condenser is thermostatically operated so that it only runs when it needs to but on a cycle with the standard 2 pre-heating vacuum pulldowns and 12 post cycle vacuum pulldowns the use is around 10L per cycle.

The service light on my autoclave has just come on. What does this mean?

The autoclave service light will come on after either six months or 500 cycles since the last service.

The service light is a warning and the autoclave will carry on working correctly.

The service light will be re-set as part of the routine service by a Priorclave trained technician.

I am looking for guidelines on what chemicals, agents or materials that should not be autoclaved. Do you have a list of things that should not be autoclaved?

Although this is not an exhaustive list, the following things need to be considered when deciding if items can be autoclaved.

The first thing to look at is whether you want to re-use the item. If you do then you must consider the material that the item is made from. Any material that will not withstand the set sterilising temperature (usually between 121oC and 134oC will obviously not be suitable but since steam sterilising works by delivering a large amount of heat over a short period of time care should be taken with items that might perhaps withstand high temperatures in an oven as the effects of heat are much more intense inside an autoclave. Plastics such as nylon, polypropylene and silicone rubber can be autoclaved provided that they are of a high grade without the inclusion of too much recycled material. Materials such as PTFE, peek and other high temperature plastics should be OK within the temperature range of a standard laboratory sterilising autoclave.

Where materials are being sterilised for disposal we highly recommend the use of metal discard containers to prevent the melted plastics (and their contents) from falling into the bottom of the autoclave chamber. If this happens then heaters and fittings within the chamber can be damaged and drains and inlets blocked, affecting the performance of the autoclave. At best this could lead to an expensive repair bill and at worst it could affect the sterilising performance of the autoclave. We recommend metal containers over plastic ones because of the heat insulating characteristics of plastics which can make it more difficult to sterilise items within the containers, leading to longer cycle times.

Secondly, volatile inflammable items should not be autoclaved in a standard laboratory autoclave, especially those with heaters within the autoclave chamber. We did once have a customer who was working on the growth of bacteria in aviation fuel and was autoclaving the aviation fuel before disposal. Fortunately there was no explosion but the fuel interacted with the silicon rubber door seal and Viton valve seals, causing extensive damage and a potential hazard as the autoclave could not be vented effectively. Likewise even if chemicals are non-flammable, attention should be paid to the effects they might have on silicone, Viton and other high temperature rubbers to avoid damage.

Thirdly, regarding corrosive chemicals the chambers of all Priorclaves and those of most other European manufactured laboratory autoclaves are made of corrosion resistant grades of stainless steel though nonetheless our operating manual gives the following advice:

“Grade 316L stainless steel is employed to reduce the corrosive effects of substances such as hydroxides and chlorine. However we recommend that the interior of the vessel is kept free of such potentially harmful substances and is regularly cleaned out with softened water. The use of chlorine based or other aggressive cleaners is not recommended. Exposure to such chemicals could damage the surface finish and the integrity of the pressure vessel and door over time. Care should also be taken not to routinely introduce such chemicals where they are used to pre-wash items that form part of the load. In such cases the items should be thoroughly rinsed before autoclaving.”

One off exposure to these chemicals should not be much of an issue but, as mentioned above routine and constant exposure from spillages of these materials or residues from cleaning or disinfection processes prior to sterilising will cause damage and potentially shorten the life of the autoclave.

Finally, great care and consideration should be given if thinking about autoclaving radioactive materials as there can be accumulations of material in the valves and pipework which may cause a hazard when servicing the autoclave and for disposal of valves and pumps that may need replacement during the life of the autoclave. In addition to this radioactive material may be carried over in the autoclave exhaust and drain into the drainage system. It would not be impossible to use a laboratory autoclave to sterilise radioactive materials but it would need to be dedicated to this particular purpose and require special precautions for its installation, operation and maintenance along with ultimately the disposal of the entire autoclave at the end of its working life.

Why does the thermal lock not let me open my autoclave until it cools to 50oC wasting a lot of time. Can it be changed?

Let’s start with a little bit of background history.

Laboratory autoclaves started to be fitted with thermal locks as a safety mechanism from the late 1970s following a number of bad accidents in the UK where glass bottles containing liquids exploded after opening the autoclave door when the autoclave was hot but not under pressure.

Although the autoclave itself was at below 100oC and not under pressure the problem was that the contents of the bottles were still at a temperature above 100oC and under pressure because the caps were sealed, or had become sealed during autoclaving and contact with colder outside air (especially with the soda glass bottles commonly in use at the time) cracked the glass making the bottles explode.

The thermal lock was invented to prevent these accidents and certainly in the UK fitting of thermal locks as standard has eliminated these accidents over the past 20 years or so. Where this safety feature was not introduced, for instance in the USA, these incidents continue to happen which is why a quick internet search (in English) for this type of accident brings them up only for the USA. If you want to read up on these the following links are for two of these incidents. https://www.reddit.com/r/labrats/comments/1br3ow/bottleexplodingafterautoclave/ https://www.osha.gov/pls/imis/accidentsearch.accidentdetail?id=200800530

Our Health and Safety Executive here in the UK defined the thermal lock as a mechanism to keep the autoclave locked until the temperature of its contents is below 80oC. The autoclave chamber temperature (the temperature displayed on the control panel) will fall more quickly than bottles of liquid within it and so as a default and following load temperature tests we set the thermal lock to release at a chamber temperature of 50oC, which allows a safety margin for all sizes and types of liquid loads.

Although this safety lock if very effective it does, as you have observed, significantly increase the cycle time, even when cooling systems are used to help to speed the process.

For autoclave loads that do not include liquids in sealable glass containers the explosion hazard is not an issue, although you would still want to consider the physical heat of the load for safe removal, but as manufacturers we have to apply the thermal lock as standard to all cycles as we cannot assume that all autoclave operators fully understand what they are doing.

There are two ways around this however.

Firstly you can use the Thermal Lock Override, which is fitted to all Priorclaves.

This is usually in the form of a Key Switch can be used to unlock the door so that it can be opened at higher temperatures (although not while the autoclave is under pressure). Good practice is for the key to be held by the Laboratory Manager or Supervisor so that a responsible and informed decision can be made to open the door, rather than a student or other user wanting to get home early and ending up with a visit to hospital instead.

The second way is that the thermal lock temperature can be set higher by an approved and trained Priorclave Autoclave Technician. In the UK we would require a signed disclaimer from the Laboratory Manager to do this. We would strongly advise carrying out a risk assessment before doing this, taking into account your working procedures and the level of training and responsibility of autoclave users. You could also carry out thermal tests on liquid loads to assess the correct setting levels. If you would like to change your thermal lock settings please contact Priorclave Service.

Is Overheat Protection (Fault Code F003) fitted to my Priorclaves or how I could find out if it is?

Overheat Protection (fault code F003) is a standard feature on all Priorclave models with Tactrol control systems.

This functions from the main autoclave controller and takes its temperature feed from the autoclave temperature control probe. If the control system detects a temperature above 142C then the program is aborted and the F003 overheat error code is displayed.

In addition to this there are other safety systems fitted.

Firstly, in the case of the heater not being under control the safety valve will prevent the autoclave going above the design pressure of the pressure vessel. This will vent and close around 2.4 Bar rather than blowing down all the steam in the autoclave. This is to give a measure of protection against any biological hazards which may be present within the autoclave load.

If left unattended and not manually switched off the water in the autoclave will be used up by the heater and the low water system will shut the autoclave off.

In the event of a failure of the safety valve and low water system the F003 would be the next point at which over-temperature /pressure would stop the autoclave.

Finally in the event of a failure of all these systems then the heaters are fitted with a non-resettable cut outs in their individual electrical power feeds which will cut off the power to the heaters if their trigger temperature is reached.