Water Regulations Tutorial #3 - Filling Heating Systems
Steve Hockley Grace - Technical Director - Arrow Valves
Updated - 10 November 2011
With reference to the “Water Regulations Guide” ISBN 0-9539708-0-9
Open and Closed Systems
There are two types of heating system – vented and unvented. Vented – also known as “open” - means the primary system is pressurised by gravity. Filling and expansion is via a primary feed and expansion cistern. Backflow prevention is afforded by a “family A” (type AA or AB) air gap in the cistern.
Unvented systems – also known as “sealed” or “closed” – have been widely used in the UK for more than a decade. A cistern (feed and expansion tank) at high level is not required as the expansion is normally accommodated in an air charged expansion vessel. A higher system pressure reduces air lock problems and pump cavitation. The initial fill and any top up of water from a supply pipe must be via an appropriate backflow protection device or arrangement.
The fluid in a heating system is often contaminated with dissolved metals and some form of chemical inhibitor. Whilst the chemicals and concentration in a house primary circuit may be similar to those in a “other than a house” (non-house or commercial) system, the quantity is likely to be substantially more in the latter case. Commercial systems may have a higher static head and there is a greater duty of care. Consequentially the overall risk is considered higher. The regulations deem the primary circuit in a house (including a flat or bungalow), irrespective of boiler size, is Fluid Category 3 (Table G6.1d).
The Water Regulations classify a primary system in “other than a house” to be Fluid Category 4 (Table G6.1d). Whilst not in the Regulations or Guide, the water industry unanimously regard a heating system with a boiler output of less than 45 kW to be Fluid Category 3 – see below -
Topic B22 states -
“Heating system appliances of any size in purely domestic premises, and single appliances or combinations of appliances rated at an output of not more than 45 kW (150,000 Btu/hr) in other types of premises (such as small offices), should be regarded as a fluid category three backflow risk. For such appliances, a temporary filling loop with a double check valve for backflow protection is permitted, the filling loop being disconnected when it is not in use. The double check valve must be installed before the filling loop and remain on the supply pipe connection when the loop is disconnected. For appliances above this heating output, in other than a house, the filling system would require Fluid Category 4protection.”
The idealistic concept of first identifying the Fluid Category and then choosing an appropriate device has its limitations. Whilst a house primary circuit is Fluid Category 3, it is not acceptable to protect with a Double Check Valve on a permanent basis.
House Backflow Protection – Vented Primary
The feed and expansion cistern in a house should have a type AG air gap (formerly known as Type B). In practical terms this is achieved with a BS1212 part 2 float valve. ½” BS1212 part 1 float valves are not allowed in any application because the close level cannot be adjusted (G16.1d). BS1212 part 2 valves have a brass body, level adjustment and an “up & over discharge”. The warning pipe (overflow) should have a 19 mm minimum bore and be positioned below the supply pipe to the float valve.
Non-House Backflow Protection – Vented Primary
The open primary feed cistern is classed as “industrial” (Table G6.1e) and the Fluid Category is 5. For safety reasons, a lid should be fitted and therefore a Type AB air gap is required with a weir slot arrangement to BS EN13077: 2003 (formerly BS6281 part 1). The float valve opening level should be towards the bottom of the cistern in order to leave capacity for expansion. When fully expanded, the fluid should be at least 25 mm below the overflowing level of the warning pipe (G21.1). To allow at least 4% system capacity for expansion means the opening level could be 1 m or more below the float valve. Clearly a 300 mm “drop arm” fitted to a float valve is of little benefit; besides this is mechanically unwise and further degrades an already unreliable device.
If there is insufficient expansion volume available – e.g. using a conventional float valve, the fluid will overflow through the warning pipe. This is an offence under Regulation 3(3)(i) (waste), contravenes clause G21.1 and will lead to rapid corrosion and scaling of the system. Having expelled the fluid – together with its inhibitor - the float valve will open upon contraction and admit raw water with its dissolved oxygen and calcium carbonate (scale). The resulting damage can be phenomenally expensive and disruptive.
The Water Regulations are not retrospective providing the installation took place before the 1st July 1999 and it conformed to the Byelaws. The Byelaws had the same requirements as the new regulations for an industrial feed and expansion cistern (formerly type A air gap) so every existing system must be altered if necessary to comply with the Water Regulations. Furthermore it is an offence to contaminate the supply pipe at any time.
House Backflow Protection – Unvented (closed) Primary
Many houses and individually occupied flats have a sealed heating system, e.g. “combi” boiler. G17.1b shows a diagram where the Double Check Valve is now upstream of the temporary filling loop. The filling loop must be disconnected after filling or replenishing. This method ensures backflow protection should the flexible hose be used for other purposes.
An automatic filling and top-up solution is provided – CA device (clause R24.2) with a pressure regulating valve. The CA device offers Fluid Category 3 protection (table S15.3) – and unlike a double check valve – it may be left permanently connected and turned on. Arrow Valves manufacture a filing valve assembly model CA15C297PRV. This solution is useful for sheltered housing and similar where the occupier would be required to monitor the system pressure and manually fit a filling loop.
A small pressurisation unit can be used with a type AB air gap. A temporary filling loop and Double Check Valve may be legally used for filling a house system provided the filling loop is removed after filling (R24.2a).
“Non House" Backflow Protection – Unvented (closed) Primary
Many Consultants design sealed heating systems for “other than a house” applications. If the system output capacity is less than 45 kW the industry considers this to be a Fluid Category 3 risk – see above. An Autofill with a CA device (model AFCA) is available, which does not require annual testing.
Systems with an output greater than 45 kW are Fluid Category 4 and should be filled via a BA device or family A air gap (clause R24.2). Conventional pressurisation unit are not suitable for filling a system. Small pressurisation units often feature an electro-magnetic piston pump and most units have a low capacity float valve. For longevity reasons, these should not be used for filling the system. The primary circuit should be filled through a BA device (RPZ valve) or from a Break Tank (and typically a multistage centrifugal pump) with a type AB air gap and various controls. Where a break tank and pump is proposed to be used for filling the system, the pump must be continuously rated and capable of sustaining the flow rate – e.g. Midi-Fill Digital (model MFD) – see Type AB Pressurisation Unit below.
The BA device could be fitted in parallel with a conventional pressurisation unit. Here, a “filling loop” could be used (and left permanently connected) because the BA device offers the required fluid category 4 protection. The pressurisation unit would normally incorporate a type AA or AB air gap. The filling loop need not be a flexible pipe because it is permanent. The BA device requires the mandatory resilient seat valves, fine strainer and a Pressure Regulating Valve (PRV) or other pressure control arrangement.
The use of a filling loop with a double check valve (ED device) for filling a non-house heating system with chemical inhibitors is an offence even if the system was installed before the introduction of the regulations (July 1999). Table G6.1d guidance on 6.6 states – “some typical applications of fluid category 4 are: water in primary circuits and heating systems in other than a house irrespective of whether additives have been used or not”. The act of connecting the loop today will be deemed the time of the offence. Similarly it is no defence to argue the filling loop was already connected because that too would be an offence! In this sense the regulations could be described as retrospective. Remember, it is an offence to contaminate the supply pipe at any time.
Type BA Pressurisation Unit
Since a BA device can be permanently connected and permanently turned on, it can be used to fill and pressurise the system with mains water without the need for a conventional pressurisation unit. It is important not to over pressurise the system, so a Pressure Regulating Valve (PRV) or other arrangement is necessary. Mains pressure can range from 1.2 to over 10 bar at ground level. The water companies are obliged to supply 10 m of head at the boundary at 9 litres/min. The minimum static head is therefore assumed to be 1.2 bar. However pressures below 1.5 bar are rare and 3 bar is typical. Many Combi boilers, electric showers etc require at least 1.5 bar and water companies aim to achieve this with a margin to avoid complaints. The water company can advise the pressure for a particular location. Most water company distributing networks are gravity controlled and the pressure should be consistent on a long-term basis. If the pressure drops at some time during the day, it should not matter because pressurisation can take place when the pressure recovers at some stage every 24 hours.
Pressure Reducing Valves perform well with minimal differential when the fluid is clean. PRVs can “creep” - viz. fail to close tightly at the set pressure - when debris (brass & copper swarf) becomes trapped between the seat and rubber washer. Most PRVs have plastic seats, which are more prone to damage. Model PRV536 has a stainless steel seat. Applications with no regular draw off, such as a sealed heating system, could be over pressurised and the high cut-out switch may stop the boiler. A fine strainer upstream of the PRV reduces this potential problem. Note, many PRV’s have an integral strainer but these are often far too coarse. An alternative method is a solenoid valve controlled by a pressure switch. The solenoid (especially servo types) will have a high closing force and small debris is often accommodated in the rubber seal. This type avoids water hammer. Again, a fine filter assists greatly. This option is available on the Autofill in place of the PRV – Autofill model AFS.
BA devices are designed to meet the Regulators’ Specification for water fittings Test Criteria. As such all makes will have a pressure drop of about 0.6 bar. This pressure is required to open the device. The system pressure is normally the static head plus 0.2 bar. A mains pressure of 3 bar can therefore fill 22 metres, whereas 1.5 bar would be suitable for a head of 7 metres. A BA device can therefore be used for all ground floor and most first floor applications.
Where a coldfill pressure higher than that possible from the mains is required, the Autofill should be supplied from the boosted cold water service if available. The booster set will normally provide at least 2 bar at the top floor for the domestic services, so there is plenty of pressure for the BA device. Remember, there is no problem supplying the Autofill with wholesome (drinking) water from the mains or boosted circuit because it provides the required backflow protection.
Where there is no boosted cold water circuit, the Autofill can be supplied with an integral pump model AFP. Again this is a change of philosophy. A Break Tank is not necessary if the pump is incapable of drawing more than 0.2 litres/second (regulation 5, table 4d). The Autofill is available with an optional electro-magnetic pump to boost up to 5 bar. Fast filling is provided - without a filling loop - and the pump is only switched on once the system is filled using mains pressure. This design avoids the unreliability problems of a float valve and having no cistern eliminates the risk of airborne legionellae. Furthermore open cisterns can accumulate debris, which can cause electro-magnet pumps to seize.
BA Device (RPZ Valve) Requirements
WRAS or KIWA UK approved BA devices that are correctly installed, used and maintained will conform to the Water Regulations. They are reliable and compact (Autofill is 300 mm square x 200 mm deep). BA devices need to be commissioned and tested annually. This needs to be done by an accredited tester (Arrow Valves has testers throughout UK mainland). Normally testing the BA device will be done at the time of the annual system check - such as recharging the pressure vessel, testing the pressure switches and safety devices. There are a number of recommendations for BA devices - refer to WRAS document Water Suppliers' Approved Installation Method AIM 08-01 Issue 1. This document supersedes IGN 9-03-02. The requirements include the provision of resilient seat isolating valves, appropriate strainer, sufficient rear clearance, discharge tundish and a tamperproof environment or cabinet. The Autofill complies with all these requirements. The remaining installation requirement is the height of 300 – 1500 mm from the floor, which is a typical installation height anyway.
For large non-house heating systems - both vented and un-vented – it is advisable to have a water meter connected to the supply. It is an offence to waste water (Regulation 3,3(i)) due to fittings being damaged, worn or otherwise faulty”. Most systems only require a DN15 or DN20 supply. The meter should detect low flow rates (e.g. class C or D) with per litre increments and have a pulsed output linked to the “Building Management System” (BMS). The meter can be used to determine the system capacity, which is essential for knowing the quantity of inhibitor to add. Secondly a meter will indicate any leaks in the system and again this information can be used for calculating the quantity of inhibitor to add. Note – the Midi-Fill Digital and Dose & Fill Pressurisation units incorporate electronic water meters with adjustable BMS alarm.
Type AB Air Gap Pressurisation Unit
Most conventional pressurisations units incorporate a Type AB air gap, which provides protection during “make–up” only. Generally they are not suitable for the initial fill – or partial fill after maintenance. In the light of the water regulations some manufacturers now produce units that are continuously rated with a high flow rate suitable for the initial fill and any make-up. The Arrow Valves “Midi-Fill” Digital (model MFD) is an example of this new type of wall mounted unit.
The 22 mm version has the mains water supplied through fast-fill solenoid valve and Type AB air gap provides Fluid Category 5 protection. To avoid pump seizure – historically a problem is due to infrequent use – the pump features silicon carbide seals and an anti-seize daily pulse feature. Pressure switches are another common weakness of any pressurisation unit and these have been replaced in the MFD with a pressure transducer together with - digital controller, LCD display, setting buttons and button lock.
The system pressure is at its lowest when the primary circuit is cold, which typically for schools and offices would be early Monday morning. If the pressurisation unit has failed to make-up due to a seized pump, isolated supply etc., the low level cut-out device will activate stopping the boiler. Inoperative heating can be hugely disruptive and costly in the case of primary schools. The MFD performs a weekly self-test whilst the system is at normal pressure to ensure it is fully functional. Any fault is immediately reported to the BMS system well before system shut-down.
A Low Temperature Water (LTHW) heating system must be corrosion resistant. Schedule 2, paragraph 3 requires any fitting to be immune or protected from corrosion that could lead to a waste of water. It is generally impractical for heating system to be constructed from inert materials. An “inhibitor” is frequently used to protect systems with a multitude of materials.
A Dosing Pot is normally used in non-house heating and chilled circuits systems for adding liquid chemicals – such as corrosion inhibitors and anti-freeze solutions. For safety reasons, an anti blow back valve is recommended to prevent operative scalding (model DP). Note - the Dose & Fill pressurisation unit incorporates automatic and manual dosing pot facilities.
Pressurisation Unit with Automatic Dosing
One significant problem with old heating systems - particularly ones converted from low pressure open systems – is the leakage rate. Every time the pressurisation unit makes-up with raw water, the inhibitor is diluted. Modern systems frequently have aluminium heat exchangers, steel panel radiators & thin copper tubing, requiring adequate levels of inhibitor to avoid perforations, blockages and to validate the manufacturers’ warranties.
The Dose & Fill (model DF) automatically injects inhibitor into the heating system according to the concentration set via the LCD digital display. The correct concentration is therefore maintained.
Water Regulation paragraph 21 requires an expansion vessel to accommodate any expansion water from a primary circuit during normal operation. The thermal expansion must be calculated according to the system volume and temperature change. An undersized or wrongly pressurised vessel(s) can cause the system pressure to increase excessively and activate the high cut-out safety device or safety relief valve.
Model EVCP is available up to 700 litres, although it is more common to use several small vessels in parallel. A bespoke locksheild valve (model BVEV) with drain tap is recommended for each vessel to facilitate annual servicing to check the pre-charge.
- Filling loops (with a double check valve) must not be used for non-house heating systems.
- Type CA devices may be used for filling non-house sealed heating systems with less than 45 kW output.
- Type BA devices or Break Tank with type AB air gap may be used for filling a non-house system.
- A water meter on the supply to a pressurisation unit is recommended – with BMS alarm.
- Unvented systems should have adequately sized, charged and maintained expansion vessel(s).
- All systems should be protected from corrosion – e.g. by the use of corrosion inhibitors and the concentration should be maintained after any make-up.
Thank you for your interest