Compliance Regulations

Regulations applicable for boilers

The Technical Discussion – Boilers

This article has been written to provide helpful information to any person who has an interest in learning some basics about the Water and Building Regulations to do with how the PRV and the condensate and its associated tundish & pipework are installed compliantly for BOILER installations.

It could provide the seasoned expert with facts and links that were not previously known about, but it is generally written for an installer who maybe has limited knowledge and is on their path to learning within the industry.

It is not intended to provide any exhaustive or legal information and as such, the person wishing to pass exams or qualifications should treat this as a “research” type article. That said, it has some very useful links to Regulatory Documents that the learner may like to know about

hotun was developed and designed to specifically meet all the Water and Building Regulation requirements and is the first, Single Product Solution on the market, to connect a PRV discharge to a tundish and/or condensate and thereafter to an internal drain or soil pipe, achieving the correct performance and regulatory requirements.


hotun – it’s not just a tundish

This article has been written by the inventor of the hotun dry trap tundish,

founder and Managing Director of RA Tech UK Ltd, Russell Armstrong (MCiphe)

Boiler PRV Regulations - Overview

A boiler Pressure Relief Valve (PRV) discharge is specifically different to that of a combined Temperature and Pressure Relief Valve (T/PRV) in that they only operate according to system pressure and NOT temperature. The boiler PRV only ever discharges under excess pressure, once the pressure has been dissipated, the valve then closes, stopping the flow.

However, on a T/PRV under excess temperature, the valve will open fully between 90 – 95°C and discharge large amounts of very hot water (the PRV on the unvented will still vent excess pressure the same way that a boiler PRV does).

Therefore, for boilers, we are not worried about high volume flow at temperature so the “rules” for pipework sizing and pipework suitability are more relaxed with Boiler PRV’s than it is for Unvented T/PRV’s.

The Building Regulations (Approved Document) that deal with the installation of Boilers comes from Approved Document

However, this document does not mention how to compliantly install the PRV discharge.

Likewise, when we turn to The Gas Safety (Installation and Use) Regulations 1988 there is also no guidance on how the PRV discharge pipe is to be run or terminated.

It is only when we turn to BS6798-2009, that we find some relevant installation guidance.


There is an argument that as filling loops should be disconnected, thereby isolating the heating system from the incoming mains cold water, that water regulations (backflow contamination) are not needed to be considered, however, should a boiler be permanently connected to an incoming cold water main, the use of an air brake to drain type tundish MUST be used when connecting the PRV to a foul drain, to comply with the Water Regulations, such as the hotun dry trap tundish.

This is where the hotun dry trap tundish provides an elegant solution. It solves all the regulatory requirements in a single product and was the first dry trap tundish on the market to do this.



Suitable pipework!

This is the most often misconstrued “Regulation” for boiler PRV discharges.

BS6798-2009 says that “the pressure relief valve should be located where its discharge is unlikely to cause damage to the premises” But it makes no mention of the pipework itself and what it need to be to be fit for purpose.

Good practice would dictate that the discharge should not be located where it is likely to cause a danger to a person or persons in or about the building and that (fit for purpose) the pipework should be made from a material, run and adequately supported such that it can withstand the effect of temperatures and effects of water expected from a failure condition.

PRV operation considerations

In a sealed system, there are four main reasons that the PRV may operate

  1. The filling loop has been left open and/or the isolation valve has failed
  2. The PRV itself has become faulty and has weakened 
  3. There is a rise in system pressure causing the pressure to go over 3 bar
  4. (combi boilers) There is a failure in the plate heat exchanger allowing cold main water to enter the heating system water

In 1,2 & 4 the water potentially discharging into the PRV will either be at ambient (system off) or set point (system on heating/hot water mode) temperature. Therefore any pipework capable of handling hot water would be suitable.

At 3, is where we get the misunderstandings. Many installers are under the impression that “heat rated” pipework must be used because the heating system could be as hot as 100°C and in that respect, it should be considered in the way that “D2” hot water heater pipework is specified from G3.

This is not so because of the operation of the PRV and how it works.

Many installers still use a copper D2 pipe for the PRV discharge, the question of material suitability comes when the installer combines the PRV with the condensate below the tundish that the issue of water temperature comes into question as this would normally have to be some sort of plastic pipe to handle the acidic nature of the condensate.

Pressure and heat

The main driver of system pressure increase will be caused by the water expanding due to temperature rise. In the instance that (for example) an expansion vessel has lost its pre-charge pressure, system pressure will climb with temperature. However as we get to a certain temperature and if at that temperature the system pressure is at 3bar (the water temperature does not have to be 100°C) the PRV will open and allow a small quantity of water to escape, reducing the pressure to below 3bar.

As the water continues to heat, the pressure will rise once again to 3bar and at that point the PRV will once again eject a small quantity of water. This goes on until the system is at set point. The water would only ever reach any higher temperature if all the boiler controls and interlocks have failed. Can you imagine the danger of a system with water at 100°C in the radiators?

However, even here, the pressure would rise to three bar and the PRV would eject such water that will reduce the pressure back below 3bar and then close.

Ejected water quantity

The amount of water ejected is very small and, as such will have very little stored energy in the form of heat. Therefore this small amount of water will rapidly cool.

It is because of this that it is our belief that any pipework suitable of handling “normal” hot water will be suitable for use in this application.



Tundish - Discharge Water Visibility

One of the main benefits of using hotun is bringing the point of visibility to inside the building.

This makes determining that the PRV has actually failed simplicity itself. hotun has been specifically designed to allow easy visibility of water both when the PRV is dripping and after the PRV has stopped dripping! In fact it is this latter benefit that most installers and customers will find hotun the most useful

Its unique open sided design makes it really easy to determine if the PRV has been dripping (see picture)

When the PRV drips, a small puddle of water forms above the valve. When sufficient water has gathered the valve opens and the water passes into the downstream discharge pipework. However a small quantity of water is always retained above the valve (due to surface tension) and this sits there for a few days before evaporating (if the PRV has stopped dripping). This gives the repair engineer a really easy way of checking to see if the PRV has actually discharged.

No other dry trap tundish on the market has this benefit! – We call it #seePRVeasily

So no more bagging of the PRV outlet outside, re-pressurising the system and coming back next day to check the bag! Fitting hotun eliminates all of the guesswork that used to be associated with external points of discharge.

Get In Touch

Registered Address

RA Tech UK Ltd
6 Nottingham Road
Long Eaton, Nottingham, UK

T: 01332 702678



In one simple fitting, hotun solves the problem of compliantly running a PRV valve discharge from a pressurised hot water cylinder relief valve or boiler pressure relief valve to a local waste or soil pipe.

hotun combines a tundish and a non-return valve to make a dry trap tundish which acts as a visible early warning device and prevents foul odours from escaping from the waste/soil pipe.


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