Unvented Hot Water Installation Regulations

Regulations applicable for pressurised Unvented Hot Water Installations (UVHW)

The Technical Discussion

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 T/PRV and its associated tundish & pipework is installed compliantly.

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

It is not intended to provide 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 not otherwise 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 T/PRV discharge to a tundish and thereafter to an internal drain/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)

The 7 Regulations applicable for full compliance

What are we trying to achieve with a dry trap tundish?

We are wanting to comply with all the requirements to enable an installation to be fully compliant if you choose to discharge the PRV to a local foul soil or waste pipe!

In doing so we have to concern ourselves with both Water and Building Regulations at the same time (plus any additional regulator fittings standards).

Whilst G3  & H1 sets out the requirements in the confines of the Building Regulations, when we connect the (T)/PRV discharge to a foul drain we also have to comply with the Water Regulations to prevent backflow or back-siphonage contamination from category 5 risk foul drain water and the wholesome water coming from the cold main.

In all, there are 7 major requirements applicable for full compliance, they are;

  1. You should use a device as visible means of discharge and have an air break to drain facility. The tundish must be manufactured in accordance with the WRAS test code sheet number 221210 (date of issue No2 July 2000) for all fittings incorporating a connection to drain facility. (Derived from prEN 1717. Section 9).
  2. Water Regulations section 6.3 Regulator’s specification for backflow prevention arrangements and devices for the requirement for fittings and section 6.4 Guidance clauses related to schedule 1: Fluid categories; and Paragraph 15 of Schedule 2: Backflow prevention Come into effect as we are directly connecting the wholesome water supply with a fluid category 5 hazard. Therefore any such connection must have an air break to drain facility (see 1. above)
  3. Water Regulations Section 8 Schedule 2 G19 Discharge pipes from safety devices; G19.1; G19.3; & G19.4.
  4. Building Regulations G3 (2). Constructed and installed so as to resist the effects of temperature
  5. Building Regulations G3 (3)(b), ensure that any discharge from safety devices is safely conveyed to where it is visible
  6. Building Regulations G3 Guidance notes 3.50 discharge to tundish; 3.54 tundish requirements and Note; 3.55 visible means of discharge; 3.58 tundish outlet size; 3.60 a. a connection to a soil discharge stack and must contain a mechanical seal not incorporating a water trap, which allows water into the branch pipe without allowing foul air from the drain to be ventilated through the tundish.
  7. Building regulations H1 Guidance: c.prevents foul air from the drainage system from entering the building under working conditions and Section 1: 1.3.

In addition to this there is one other factor that needs to be considered, that of diagnosis and visibility even after the (T)PRV has stopped dripping!

Whilst the regulations call for any water to be “visible” what they dont do is what happens after the PRV has stopped dripping. We have got that covered at RA Tech. With hotun’s open-sided design, the installer/engineer can easily see (and touch to confirm) any residual water!

We call this


The hotun dry trap tundish is the most versatile and useful dry trap tundish on the market bar none

RA Tech UK Ltd understands the principles of the requirements and has specifically developed hotun to comply with the above regulations as an all-in-one unit, a true one-product solution!

Therefore when you fit hotun you know you’ll have the peace of mind that hotun will be the most cost-effective way of routing the (T)/PRV to drain, saving you time and money!

Regulations Overview

When installing a pressurised Unvented Water Heater (UVHW) connected directly to the water undertakers (Water Boards) water supply, the contractor has to install the unit in accordance with The Building Regulations (2010) Document G, (Sanitation, hot water safety and water efficiency 2015 edition, Approved Document),  The Building regulations 2010 Document H (Drainage and Waste disposal 2015 Edition, Approved Document) and with The Water Supply (Water Fittings) Regulations 1999 (legislation.gov.uk) And a guidance document that will help you interpret the “Regulations” (which you can find here) the Water Supply (Water Fittings) Regulations 1999 (Guidance notes)

It is important to note that whilst the “Building Regulations” deal with the safe installation and use of the equipment and how that is achieved, the “Water Regulations” mostly deal with the “5 tenants” of the undertaker’s obligations to provide wholesome water to households to prevent;

Misuse, Waste, Undue Consumption, Erroneous Measurement and (most importantly) to Prevent Contamination of Drinking Water.

These two separate Regulatory requirements (Building and Water Regs) deal with separate (but sometimes overlapping) matters. But within the two documents, the “guidance” notes go over similar topics and use similar, but not exactly the same wording. Therefore, the two Regulations can have differences in interpretation. This makes it difficult for installers to get it right!

To get an internal discharge solution, if an installer connects the T/PRV discharge to drain, there is an increase in risk factor for backflow contamination which then needs a different Backflow Contamination solution than if it wasn’t directly connected to drain. Raising the installation risk from a category two risk (double check valve protection acceptable) up to a category 5 Risk and, as any professional installer will know, we need to install an air break to drain type device to cater for that increased risk – the unassuming standard tundish provides that air break and makes discharging water visible (both important to comply with Regulations.

However, if we just use a standard tundish, other installation and regulation compliance factors are introduced into the equation to do with Building Regulations not Water Regulations! The installer now needs to include a way of stopping foul odors and gasses from escaping from the drain or soil pipe back into the building and they are not allowed to use a standard wet trap because, due to the infrequent nature of use of the (T/PRV), the trap could dry out!

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.

An Introduction to G3 and Water Regulations

The “G3 Regulations” and “Water Regulations” (full titles in the links above) give good guidance for the safe installation and use of Unvented Hot Water Heaters (above 15 litres in storage capacity) for use in (but not exclusively to) domestic situtations.

However, as we started to allude to above, because the Water Regulations were similar but not identical to the Building Regulations, situations arose where the two documents could be confusing and interpreted in different ways. It became apparent that this would create issues and confusion between regulatory bodies and lead to differing approaches to defect and regulatory enforcement.

Therefore, if there was any conflict, the Water Regulators defer authority to the G3 Building Regulations (which then takes precedence). In order to fully understand an installer’s and thereafter building users’ obligations to meet the Building Regulations, we need to dissect quite carefully how the Regulations are written and how they guide an installer to carry out thier work.

An aside on the subject of boiler Regulations

It must be said that Regulation surrounding Pressure Relief Discharge in boilers, is separate from that of “G3 Regulations“.

Once again, confusingly, the rules for boiler PRV discharge look, at first glance, to be very similar to that of Unvented Hot Water Heaters, but because of several important factors, they should NOT be confused. (We will deal with boiler PRV Regulations in a different topic)

What is an Approved Document?

Page 3 of the Approved Document gives us an insight into what the document is for and how it should be used, we can focus on two paragraphs

The two paragraphs on page 3 of the “G3 Approved Document” tell us that there is a presumption of compliance if the guidance is followed, but it tells us that this presumption is not conclusive! 

The two paragraphs (shown in the image below), effectively say that, what is written in the “guidance section” is “Guidance”. This is not the actual “Requirement” it is just one method of potentially reaching compliance with the “Requirement“.


Of course, as we have said above, for the installer and building user, it is a sensible approach to follow this Guidance where possible. It has been written so that there is, by and large, safe conformity of installation standards which is a good thing for the industry because it creates a level of understanding for the installer of what is expected and what needs to be done.

As experts in this field, RA Tech UK Ltd largely endorse the guidance written in the G3 document, BUT, very often Regulation Inspectors take the “Guidance” as the “bible” and if they see an installation not installed in line with the “Guidance“, they will reject an installation without a thought! They should not necessarily do this!

In reality, all the installer needs to do, in order to comply with “The Requirement” in G3 Regulations (for Relief Valve Discharge),  is to provide a safe and visible way of discharging the T/PRV water.

Thats it!

The Guidance is the Guidance, not the Requirement!

Responsibility of the installation

The installer carries the responsibility for the installation and all compliance matters, and if they are part of a Competent Persons Scheme can self certify the installation before handover to the customer/Building user (see part 5 excerpt below) otherwise the installer has to notify the local Authority Building Control Office of the installation.

The Requirement vs The Guidance

There are many installers (and regulators even) who might not fully understand the subtle difference between “The Requirements” and ” The Guidance” in the “G3 Approved Document

The Building Regulations

The difference between the Requirement and The Guidance in the G3 Approved Document might be better explained as the Law being written down in its literal form (The Requirement) and then having a separate document interpreting that Law (The Guidance). However, the interpretation is still only one way of looking at the law and not necessarily the only way! (Law doesn’t work like that…) and in this instance, The Requirement is in the same document as The Guidance and is called “The Approved Document

Let us first look at The Requirement itself, everything else stems from here. The Requirement is written up in the Government website document here The Building Regulations 2010 (legislation.gov.uk) and then repeated in the Approved Document

For T/PRV Discharge we are interested in just a part of the overall “Requirement” from

(2) A hot water system including any cistern…..(see the extract of the document right)

However, we are still only interested in parts of these paragraphs, suffice to say (and to paraphrase) the installer is “required” to;

“Provide a system that is fit for purpose (withstands the temperatures expected in failure operation) and ensure that any discharge is safely conveyed to where it is visible but not cause a danger to persons in or about the building”

Or to put it in layman’s terms;

“Safe and visible and doesn’t collapse with several cycles of 95°C water!”

How an installer does this is up to him!

The Guidance is just a suggestion, that said, if an installer decides to, or is forced to depart from The Guidance, then it would put a greater emphasis on the Installer to prove that the work is fit for purpose and that the installer has done the work in a “workmanlike” manner.

The installer, however, has the responsibility to carry out the work in compliance with the “Regulations” and if the installer is deemed to be a competent person, can then also sign off the work via the self-certification route found in Part 5 of the Building Regulations (see right)

Notification & G3 Qualification

Building Control Notification

Persons carrying out installation, servicing, and repairs to Unvented Water Heaters (larger than 15 liters) need to hold relevant qualifications to legally carry out that work. This is known as an “Unvented Hot Water Qualification” or “G3 Qualified” The installer, once qualified (and valid for 5 years) will be issued a certificate and/or card that the installer can then show as proof of his qualification.

That installer can then apply to a Competent Persons Scheme, to then be qualified to self-certificate their work

Quote from www.legislation.gov.uk/uksi/2010/2214/made – Explanatory Notes

Membership of self-certification schemes exempts persons carrying out relevant work from the normal requirements under the Building Regulations to notify the local authority of an intention to carry out the work.”

If the installer is not part of a Competent Persons Scheme then they may have to give notification of the work to the Local Authority Building Control Office. Please contact your local office to see if you need to give notice before you start work.

See Page 8 of Approved Document G, listed under “Notification of Work” 

Water Regulation Notification

As with Building Control, there are very similar requirements for Notification and Water Regs UK Limited – Notification states that

  1. In England and Wales, if an Approved Contractor, (such as a WIAPS, TAPS, Aplus, WaterMark or SNIPEF approved plumber) is used in these circumstances, the water supplier does not need advanced notification.
  2. In Northern Ireland, if an Approved Contractor, (such as a WIAPS, TAPS, Aplus, WaterMark or SNIPEF approved plumber) is used in these circumstances, the water supplier does not need advanced notification.
  3. In Scotland, if an Approved Contractor, (such as a WIAPS, TAPS, Aplus, WaterMark or SNIPEF approved plumber) is used in these circumstances, the water supplier does not need advanced notification.

if an installer is not part of a competent persons scheme they may have to give advanced notification of the intended work and when they do so:

The Water Supply (Water Fittings) Regulations and Byelaws in Scotland, allow 10 working days for water suppliers to grant their consent. You should not start work until consent has been granted. A water supplier may not withhold consent unreasonably. They may however, grant consent subject to conditions, which must be followed.

If consent is not given within the ten working days, then the Regulations and Byelaws deem the water suppliers’ consent to have been granted. However, any work must still comply with the Water Fittings Regulations and Byelaws in Scotland.

For further information please read the Water Regs UK notification leaflet. 

Building Regulations - Guidance - G3 (3) a. Safety Devices on Unvented Water Heaters

Performance (Pages 19 & 20)


G3 (3) a. Safety Devices on Unvented Hot Water Heaters


Within the Approved Document, there is some really useful Guidance that sets certain performance requirements for the safe use of unvented water heaters. These requirements are really aimed at the cylinder manufacturer, but they have installation implications thereafter;

In order to ensure unvented cylinders were as safe as possible, Hot Water Storage Heaters (larger than 15 Litres capacity) were accepted in the UK, provided that they were to have

a. “……at least TWO independent safety devices such as those that release pressure and so prevent the temperature of the stored water at any time exceeding 100°C in addition to any thermostat.”


The Guidance in G3 (page 20) states;

3.18 An acceptable approach might consist of

a. a non self resetting energy cut out to disconnect the supply of heat to the storage vessel in the event of the storage system overheating: and

b. a temperature relief valve  or a  combined temperature and pressure relief valve to safely discharge the water in the event of serious over-heating.”



Unvented Water Heater manufacturers determined that providing a combined Temperature and Pressure Relief Valve (T/PRV) mounted on the cylinder would provide the required second safety device. The first being a non-resettable high limit cut out on the electric immersion heater.

The part that interests us is the implications of discharging the T/PRV water.

Interesting side issue – The implications (if any) of an indirect heat source (boiler/heat pump)

Controls found on boilers means it would be virtually impossible for indirectly heated water (for example from a gas fired boiler or heat pump) to reach 100°C under normal, or even failure scenario conditions.

The maximum sustained flow temperature from most boilers (before thermal overheat cut-outs operate) are between 80-85°C and even if this climbed to near 100°C it could not ever heat the water in the cylinder via the secondary heating coil to higher than that. That said, indirectly heated water heaters can be deemed to have the secondary energy cut out incorporated within the boiler

G3 (Page 21) Paragraph 3.21  states;

If an indirect supply of heat to an unvented hot water storage system incorporates a boiler, the energy cut-out may be on the boiler.”

But because the Water Heater Manufacturer does not know what (indirect) products are going to be used in the installation, it is standard to provide both energy cut outs on the water heater itself.

It is a fact that energy coming from a “failed” electric heater coil could heat the water to very high and dangerous temperatures (up to 130°C). If there was no secondary safety device. It is therefore largely recognised that issues with temperature, upwards of 95°C, only occur when the water if there is a direct electrical coil heating the cylinder.

Therefore, water heaters that have both an energy source from a boiler AND a backup electric immersion heater, will still need two independent forms of energy cut outs on the water heater itself.

One being the thermal (high limit) cut out on the immersion heater, the other normally the T/PRV on the cylinder.

Building Regulations - Guidance - G3 (3) b. Visibility and Safety of discharging water

Performance (Page 19)


G3 (3) b. Visibility and Safety of discharging water


Visibility – Whilst discharging

Coming back to page 19, the performance guidance gives thoughts about how the discharge must be

G3 (3) b. “…visible at some point and safely conveys it to an appropriate place open to atmosphere where it will cause no danger to persons in or about the building”

The Guidance for dealing with the safety and visibility of the discharge pipework is written from paragraph 3.50, on Page 23 through to a final single paragraph 3.63 on page 26.

It is generally accepted that a satisfactory solution to providing a visible means of discharge will be to site the tundish where it can be noticed and is visible. This would mean installing the tundish within the space where the cylinder is located and within 600mm of the cylinder itself. 

NB – The Guidance gives some interesting dimensional requirements for the positioning of the tundish. Its states that the tundish must be

…..fitted as close as possible to, and lower than, the safety device with no more than 600mm of pipe between the valve outlet and the tundish (see diagram 1)”

However, tests have shown that fitting the tundish as close to the T/PRV outlet as possible, may not necesserily be the best thing to do. (see the section on “safety” below

Visibility – After discharging!

Whilst much thought is given in the regulations to making sure that any DISCHARGING water is visible, how can the diagnosis of a (T)/PRV be achieved after the (T)/PRV has stopped discharging?

As is obvious, a dripping PRV is a sign that there is something wrong with the appliance or the system. But how can a homeowner/installer/engineer tell what has happened after the PRV has stopped dripping?

With hotun, we have designed it so that it has open accessible sides (very much like a traditional tundish) but with one added advantage!

The valve requires a small amount of water to form (as a small puddle) on top of the valve before the weight of the water overcomes the tension in the spring, thus allowing the valve to open. However, once the water is discharged, the valve holds a small residual amount of water, which the person inspecting any issues can see and touch very easily! There is no guesswork or taking the tundish out or apart to determine if the PRV has passed water. Its sitting there. we call this


As can be seen in this picture

No other dry trap tundish on the market has this unique feature, it makes diagnosing quick and simple.


The Regulations demand that any water is discharged “where it will cause no danger to persons in or about the building”

In terms of safety, in regards to tundishes, this means that the water must pass through the tundish without splashing or spilling over.

We have studied this and hotun has been designed to allow both drips and high volume flow to pass through the tundish.

We have carried out extensive independent tests to establish design volume flow rates to ensure that the correct hotun is selected for the correct application. (see Product Selection Guide here)

Generally hotun and hotun sf (15/22) are suitable for use with low volume applications, such as appliances that have low volume discharge Pressure (only) relief Valves (PRV) eg boilers and unvented hot water heaters under 15L in capacity, 

The “hiflo” versions, hotun hiflo and hotun hiflo SF and hotun XLSF, (15/32 & 22/32) are designed for ALL applications and especially those with high volume possibility (such as a T/PRV found on Pressurised Unvented Hot Water Heaters over 15L in capacity).

In terms of flow dynamics, in tests, it has been shown that, in a high volume failure scenario, as the water exits the relief valve it is highly turbulent. If the water enters the tundish in a highly turbulent state, it is possible that adequate flow is maintained and an unwanted amount of spillover could occur.

It is for this reason, that RA Tech recommends that the tundish is located directly below the relief valve outlet and have at least 200mm of straight pipe between the outlet of the relief valve and the inlet of the tundish (see diagram), with the branch pipe from the inlet control group PRV coming in horizontally. 

If this cannot be achieved the installer should have at least 200mm of straight pipe upstream of the tundish before the last change of direction or branch.

D2 - Suitable pipework!

What type of plastic pipework can I use for the D2?

This is one of the most common questions we are asked!

Unfortunately, as we do not manufacture plastic waste or soil pipe, and are not waste pipe material experts, we cannot give definitive answers. What we can do is give a rationale for the installer to follow so that the installer can ask the correct questions and decide on his/her own understanding what pipework should be used.

The Requirement

Reminding ourselves what the installer must achieve from G3. (2) & (3)

(2) A hot water system, including any cistern or other vessel that supplies water to or receives expansion water from a hot water system, shall be designed, constructed and installed so as to resist the effects of temperature and pressure that may occur either in normal use or in the event of such malfunctions as may be reasonably be anticipated, and must be adequately supported.

(3) A hot water system that has a hot water storage vessel shall incorporate precautions to:

           (a) prevent the temperature of the water stored in the vessel at any time exceeding 100°C; and

(b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building.

It is therefore incumbent on the installer to make sure that the installation is fit for purpose and will withstand water flowing (potentially) at 95°C for several minutes and over several cycles of discharge.

What we must make obvious is that the entire system receiving that flow must be capable of withstanding the temperatures, so that will include, D1, the tundish, D2 and any subsequent soil pipe.


Generally, all D1 pipework is made from metal (copper) so is suitable for use


Many tundishes on the market are made from a thermal co-polymer (Acetyl) and are suitable for use. The hotun range of dry trap tundishes use Acetyl and are ultrasonically welded for superior continuity of all joints in the reassembly of the tundish. (we do not use any click jointing methods that can lead to leakage of dripping water through the tundish).


If an installer uses metal (copper) pipework for the D2 it would be suitable for use (please see our tips and tricks for flow performance on 22mm copper pipework, critical flow performance and chocking/flooding over), however, installers are moving towards the use of plastic D2 pipework

It is recognised that pipework made from the following materials are generally suitable for being capable of withstanding the relevant flow conditions

They are;

HDPE (High Density PolyEthylene)

PP (Polypropylene)

MuPVC (Modified unplasticised Poly Vinyl Chloride)

It must be said that HDPE is not readily available in small quantities to consider as a readily available alternative. Therefore the installer is best to use PolyPropylene (push fit system) or MuPVC (solvent weld system). This pipework should be supported every 300mm.

Soil pipe

New Build

On new build, we would encourage housebuilders to use the latest and highest spec materials that are suitable for this application. They are available and should be used when discharging high temperature waste from the T/PRV to a soil pipe.

Please refer to soil pipe manufactures literature to determine the suitability of the soil pipe to withstand repeated high-temperature discharges.


To our knowledge, since the acceptance of this method of discharge, there have been no reported fatal injuries caused by failures of soil stacks or joints caused by high temperature discharges running through soil stacks.

The route to failure should be viewed through the lens of risk mitigation and risk assessment. If the installation has been carried out in a manner that should any failure of any sort happen, and the failure of the D2/soil stack is the consequence of that failure, if the pipe fails, will it fail in an unsafe or an at-risk way to persons in or about the building?

If not, it would be down to an insurance claim for damages or repairs to the fabric and waste system and the installer should ensure that his work has been carried out in a professional and workmanlike manner and he has taken all and every necessary precaution and made a due risk assessment before undertaking the work.

Water Regulations - Visibility and undue consumption

Visible discharge and Undue Consumption 


The Water Regulations demand that any fittings or installations do not lead to undue consumption or contamination of the wholesome water supply. This is an important consideration when we are looking at discharging water from safety devices.

As such, if something goes wrong and the water heater develops a fault, which results in an over pressure situation, water will be discharged either via the T/PRV or the PRV to drain. With a connection from the (T)/PRV made to a local foul drain, it is important to comply with the issue of backflow or back siphonage from that drain!

It is very important that a building user has the opportunity to see that there is water being discharged because it acts as a warning that there is something wrong with the system. If the installation does not have a visible discharge and a failure is left unresolved, not only will there be undue consumption but a catastrophic failure could eventually occur and the building becomes flooded (or worse).

Also,if the home owner or building user cannot see any discharging water, then the continual drip left un-noticed could go on to waste a lot of water and the energy needed to heat that water – literally money being thrown down the drain. So not only does this cost you, it is also against the Regulations! (We talk about the usefulness of our alarm unit (hotun detect) in the fight against waste, here.

This is why the tundish must be located in a position that is easily seen and for the tundish itself to have easy visibility of any discharged water

hotun, is the only dry trap tundish on the market that provides this level of visibility at the point of discharge, we call this



The fitting of hotun to your unvented hot water heater then ably satisfies all requirements of the Water and Building Regulations in regards to visible discharge and being able to reasonably prevent undue consumption.

As discussed above, it is accepted that the discharging water could be terminated to a local soil pipe provided that it can be demonstrated that the pipework could withstand the effects of the temperatures expected and that an odour trap containing a mechanical seal (not incorporating a water trap) was used – For example a sprung valved dry trap tundish, such as hotun.

When a discharge from the T/PRV or PRV from a mains fed Unvented Hot Water Heater, is routed to a foul drain, we have to consider the Building Regulations AND Water Regulations concurrently, enabling an installation to be fully compliant.

Both Water and Building Regulations stipulate that the water discharging has to be visible, but it is only the Water Regulations that are relevant to backflow or back-siphonage contamination. 

Either way, the hotun dry trap tundish provides the perfect solution for full compliance!

Examples of non-compliancy!

(Unvented Water Heaters)

What is non-compliant?

What is non-compliant?


Did you realise that hard piping the pressure relief valve to the waste pipe (like the one in the picture) is non-compliant and does not conform to the Regulations?

Do you know WHY this particular installation contravenes the Regulations?

1/ It is hard piped to drain

Why is this important? The incoming mains cold water to the water heater is in direct fluid communication with the category 5 foul water drain. If the drain ever blocks contaminated water, bacteria and pathogens can backflow/backsiphon into the potable/wholesome water and contaminate the cold water supply.


To rectify this, an air break to drain must be fitted. Due to the category of the could drain (category 5 hazard) a single or double check valve (on the cold water feed) is not deemed to be suitable protection. A suitable device to provide a solution would normally be a standard tundish. The standard tundish provides two solutions in one, it becomes a visible means of discharge and has the backflow and back siphonage protection. However, if the “D2” is connected to waste then a further problem is introduced which then needs a solution.

A standard tundish has no way of stopping foul odours or gasses from entering back into the building, a wet trap cannot be used as it would dry out. Therefore a dry trap (such as a hotun) has to be used.

2/ It has no visible signs of discharge

Why is that important? Without being able to see if water is dripping from the PRV, a building user will not know there is something wrong with the system and the water will continuously be wasted without any remedial repair being undertaken. In terms of Water Regulations, this will be deemed as undue consumption and a misuse of water.


Fitting a tundish provides the visibility required 


Consequences for the homeowner or building user

If an installation is non-compliant a building owner/user, upon investigation, would normally be issued with a non-compliance notification. If the building owner/user fails to rectify the notice they would then be served with an enforcement order/notice, if they still fail to comply they could be criminally prosecuted and if found guilty, receive a conviction and a fine.

We see many, many water heater pressure relief valve discharges installed like this one, in fact the problem is so widespread that Severn Trent and Yorkshire Water have stated that well over 50% of installations are installed non-compliantly! 


But if so many are fitted this way non compliantly Why are they fitted this way?

Is it because the installer;

1/ Had no knowledge of the requirements?

2/ Had a lack of cost-effective credible alternatives?

3/ Was unaware of the current water and building regulations?

4/ Had simply always done it that way and no one had said anything was wrong?


Now there is no excuse for a non-compliant installations!

Typical Small Water Heater Installation

hotun has been invented so that installations can pipe their unvented hot water PRV discharge to a local drain compliantly – quickly and easily.

How to make the installation compliant

How to make the installation compliant

hotun allows the installer to pipe the discharge from the PRV, locally to a foul waste or soil pipe.

What the hotun solution provides

In order to make this example compliant, we need to look at what the installer needs to achieve

  1. Provide a visible means of discharge
  2. Allow the PRV water to be safely conveyed to a point of discharge
  3. To ensure that the category 5 risk contamination water (in the foul drain) cannot back siphon or back flow into the wholesome water supply
  4. Ensure that no foul odours can escape back into the building from the foul drains

This is what the hotun dry trap tundish has been specifically designed to achieve – an all in one product solution

Fitting a standard tundish provides the first three protections but, up until hotun was designed and marketed, the installer would have had to have fitted a separate type of dry trap to stop the foul odours. This takes time and adds extra expense

Therefore if you cant discharge pipe to a safe point of discharge outside or it’s not cost-effective to do so, fitting hotun is cost-effective, quick and easy;

Why fit a hotun dry trap tundish?

Be fully compliant, easier, quicker and less costly!

Now there is no excuse to be non-compliant


No other dry trap solution provides compliancy at the point of

installation and visibility at the point of diagnosis!

Note – High volume T/PRV discharge pipes (as opposed to PRV discharge) should NEVER be connected to a waste pipe if that waste pipe is connected to a sanitary appliance. The high-temperature full flow discharge could back up into the sanitary appliance (a basin for example) overwhelm the appliance capacity and spill uncontrollably onto the floor. This is a very dangerous situation and must be avoided.

Discharges from appliances with Pressure Relief Valves (not combined T/PRV’s), are not a high volume flow risk

Due to the nature of operation of a PRV, it is RA Tech’s opinion that a connection from a PRV could be connected safely to a local waste pipe.

Severn Trent Water and Yorkshire Water estimate that at least 50% of all under-counter unvented water heaters are installed non-compliantly!

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Registered Address

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

T: 01332 702678
E: info@ratechuk.co.uk



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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