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Frequently Asked Questions

This page provides a list of recent frequently asked technical questions and suggested responses or sources of answers.  If you have a general technical question to pose to the IOR Technical Committee of experts please use this contact form.

What are the considerations regarding the venting of Pressure Relief Valves to atmosphere? (June 2010)


It depends on the quantity of refrigerant in the system and the size of the room into which the refrigerant would vent. If the total refrigerant charge could exceed the practical limit for the room -defined as the refrigerant concentration limit for the refrigerant -then there would need to be gas alarms and remedial action - in this case that would mean emergency ventilation. This requirement is common to EN378, ASHRAE 15 and ISO5149 (although phrased differently).

A further consideration is that the most likely cause of a relief valve lifting is in the event of a fire in the vicinity of the vessel. As the products of combustion of HFC refrigerants are highly toxic (much more so than ammonia) the concept of a relief valve lifting while someone is trying to fight the fire is a significant concern. If the design of the machinery room ensured that this was not possible then that would be OK. However the easiest way to achieve this requirement is to pipe the valves to atmosphere, or at least into an extract ventilation duct. This point is not covered by EN378, but would be taken into consideration by HSE in the event of an accident investigation. Since the size of the relief valve is calculated on the basis of the heat from a fire around the pressure vessel it is not a valid defence to say that fire was not foreseeable.

If the system is sufficiently small, and the room is sufficiently large and the vent from the relief valve would not pose any danger to personnel then it may well be acceptable to vent within the machine room. NB: Outlet piping should not be added to CO2 relief valves because there is a danger of dry ice formation which could plug the pipe and prevent safe release of the system pressure

What is a Positive Displacement Compressor?  (November 2010)


A positive displacement compressor is one which encloses a defined volume of low pressure gas and compresses it to a higher pressure by reducing its volume. A piston compressor is positive displacement because the intake volume is defined by the cylinder volume at the bottom of the stroke. The gas is then compressed until the discharge valve opens (automatically) when the high pressure is reached.

A screw or scroll compressor is also positive displacement, but in this case the gas filling the low pressure pocket is compressed by a pre-determined amount before the discharge port opens. All the gas is then expelled as the pocket volume continues to reduce to zero. The design is usually such that the pressure in the pocket is close to the required high pressure when the port opens.

Non-positive displacement types are centrifugal or turbo compressors. The strict definitions are given in EN378:2008 part 1 as follows:

  • 3.4.6 positive displacement compressor – a compressor in which compression is obtained by changing the internal volume of the compression chamber
  • 3.4.7 non-positive displacement compressor – a compressor in which compression is obtained without changing the internal volume of the compression chamber

Where is it recommended that the emergency showers be located for use after an ammonia leak from a refrigeration system? (Nov 2010)

The highest risk of being in contact with or suffering exposure to ammonia is when in the plant room. Therefore a shower external to the plant room but near the exit is the most sensible place as specified in BS EN 378 2008. This means that the shower should be located as close as is reasonable to the plant room, in a position which is not likely to be affected directly by fumes from a leak, it must also be capable of being used for say 0.5 to 1 hour after someone has been exposed.

In the case of a catastrophic or major leak personnel should not enter the plant room itself to rescue others unless they are trained in the use of respirators or breathing apparatus and follow the requirements of the Confined Spaces Regulations SI 1713 of 1997. Breathing apparatus should also be available external to the plant room in a suitably locked cabinet with access by the trained personnel who would also be a key holder.

Where can I get information about compliance with the F Gas Regulations?

The UK Government Department responsible for the F Gas Regulations (DEFRA) have set up a helpline which can answer specific questions and provide a range of guidance notes. Contact details:

  • F-Gas Support Helpline 0161 874 3663 0161 874 3663 (9-5 Mon-Fri)
  • email -

 Why does a domestic fridge freezer kept in an unheated area sometimes stop working?

Standard domestic fridge freezers are designed to work in an ambient temperature of 10°C to 32°C, ie normal kitchen conditions. When temperatures drop below 10°C the fridge freezer will start having problems holding temperature in the freezer. This may result in the freezer turning off. This information is usually stated in the handbook that comes with the appliance.

It is possible to buy fridge freezers that will work in an ambient of 0°C but they are normally more expensive and use an elaborate compressor and temperature sensing system.

Problems are very common with fridge freezers kept in unheated areas such garages or out houses. There is only one way to overcome the problem and that is to have the appliance sited in ambient temperature that does not drop below 10 degrees. This would mean either heating your garage or moving the appliance inside the house.


This information is provided as general guidance to legislation and standards only. IOR accepts no liability for information provided on this website. It is recommended that enquirers contact the relevant authority or a reputable contractor or consultant to seek a formal expert opinion.