Building Services

By John Bradley– licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License

http://creativecommons.org/licenses/by-nc-sa/2.5/

John Bradley

INTRODUCTIONINTRODUCTION

Hot water

E = Expansion (m3)C = Volume of water in system (m3)ρ1 = Density of water before heating (kg/m3)ρ2 = Density of water after heating (kg/m3)

CONVENTIONAL VENTED SYSTEMCONVENTIONAL VENTED SYSTEM

Cold water supply from rising main

Cold water storage cistern

Boiler

Cylinder

Coil heat exchanger

to hot water outlets

Cold feed pipe

Immersion heater

Vent pipe (at least 19mm)

Overflow pipe

Primary circuit

Secondary circuit

Heating and hot water systemHeating and hot water system

CWSCCWSC

Normal expansion space

Additional expansion space

Overflow pipe

Vent pipe

Rising mainCold feed pipe

80mm

Support for CWSC

Flat, level, rigid platform to extend > 150mm beyond cistern

Conventional vented system: directConventional vented system: direct

• An alternative to the indirect system that is not now commonly used, but occurs in many older properties, is a direct system.

• In direct systems the water in the cylinder is heated directly, either by an electric immersion heater in the cylinder, or by the water being circulated around a boiler. This was a common arrangement with back-boilers behind open fireplaces and ranges such as Agas and Rayburns, but is not generally used with central heating boilers.

• The hot water from the boiler mixes directly with the water in the cylinder. Water that has circulated in the boiler and primary circuit is drawn off through the taps and can therefore be contaminated.

• If used in a soft water area the boiler must be rust-proofed. This system is not suited to hard waters. When heated the calcium in the water precipitates to line the boiler and primary pipework, eventually furring up the system making it ineffective and dangerous.

Immersion heatersImmersion heaters

Immersion heater

LegionellaLegionella

• There is an increased risk of bacterial growth in water held at temperatures between 20°C and 46°C for prolonged periods. This can cause Legionnaires’ disease. The elderly are particularly vulnerable. Control of the bacteria is therefore vital in settings such as hospitals and care homes.

• The following measures are recommended for use with hot water systems:– Stored hot water temperature 60 to 65oC throughout the storage vessel– Pipework ‘dead-legs’ to be minimal– All pipework to be insulated to reduce water temperature losses– Distribution temperature to outlets >55oC

UNVENTED SYSTEMUNVENTED SYSTEM

Unvented SystemsUnvented Systems

• Until the 1985 Building Regulations and the new Model Water Byelaws of 1986, a domestic hot water storage system in the UK was required to have an open vent pipe (a vented system).

• The majority of new houses now built in the UK are designed with sealed, unvented mains pressure hot water systems: norm in Europe and USA.

• Expansion of water dealt with by the use of an expansion vessel. This replaces the CWSC. The system is normally supplied direct from the mains and is sealed to the atmosphere (rather than being vented to the atmosphere). There is therefore no need for a vent pipe. Hence the term unvented system.

• A full description of the system would therefore be a mains pressure, unvented sealed domestic hot water system.

• The installation of an unvented system is notifiable building work. Installers registered with a competent person scheme can self-certify that the work complies with relevant Building Regs and the owner/occupier will be given a Building Regs certificate of compliance .

Unvented hot water and space heating systemUnvented hot water and space heating system

No roof space required for CWSC or feed and expansion tankNo roof space required for CWSC or feed and expansion tank

Expansion vessel replacing CWSC for hot water system

Expansion vessel replacing feed and expansion tank for heating system

Unvented SystemsUnvented Systems

• Water in the unvented cylinder comes directly from the cold water main and is at (nearly) mains pressure.

• To contain this pressure the cylinder has to be much stronger than in a gravity-fed system. Unvented cylinders are therefore made of thick copper or stainless steel.

• The outlet of the cylinder is to hot water taps which are normally closed. The inlet is from the cold water main which may incorporate non-return (check) valves or other devices preventing expansion back into the supply pipe.

• Therefore measures have to be taken to accommodate the expansion of the hot water which could otherwise give rise to enormous pressure in the cylinder.

• These take the form of some type of container of gas which can be compressed as the water expands. This may be arranged as a bubble of air in the cylinder or a separate expansion vessel.

Unvented Systems: expansion vesselUnvented Systems: expansion vessel

• An expansion vessel contains a diaphragm and a volume of air or nitrogen to absorb the expansion. It should be able to accommodate >4% of the system’s overall water content.

• The photograph below shows an unvented hot water cylinder and two expansion vessels: one for the primary circuit and one for the secondary circuit.

Diaphragm expansion vessels

Unvented Systems: air gapUnvented Systems: air gap

• A purpose made hot water storage cylinder designed with provision for an air gap is an alternative to installing a separate expansion vessel.

• As the water expands on heating, the volume of trapped air is compressed to provide adequate delivery pressure and flow.

• Some manufacturers fit a floating baffle between the water and the air, to reduce the effect of turbulence.

Hot water cylinder incorporating air gap Baffle

Safety of unvented systemsSafety of unvented systems

• At atmospheric pressure water boils at 100oC. At higher pressures boiling point increases so that pressurised water can be heated to over 100oC and remain liquid. However if the pressure is released (when a tap is opened) it will turn to steam, expanding and causing a steam explosion.

• Therefore unvented systems must have safety systems to control the temperature and pressure of the water.

• The expansion vessel is fitted with an expansion relief valve in case the vessel should fail. Beyond this there are 3 levels of safety :

1. Thermostat, set to operate at 60 to 65oC

2. Non self-resetting energy cut-out, set to operate at 85 to 90oC, to disconnect the supply of heat to the cylinder in the event of the thermostat failing and the storage system overheating, by turning the boiler off

3. Temperature/pressure relief valve, to discharge water to a safe and visible place open to the atmosphere, through a tundish (a small funnel with a pipe discharging into it to provide an air break in the overflow) if the water temperature reaches 95oC

Unvented system with safety features Unvented system with safety features

Pressure reducing valve (to keep pressure less than the expansion valve opening pressure)

Line strainer (to remove dirt)

Cold water main

Stop valve

Check valve (to stop water returning to cold water main)

To cold tapsExpansion

vessel

Expansion relief valve

To drains

Tundishes

To hot taps

Boiler

Thermostat

Energy cut out

Hot water cylinder

Temperature/pressure relief valve

Temperature and pressure relief valveTemperature and pressure relief valve

Safe discharge from T&P relief valve

Discharge from T&P relief valve• The diagram shows the method prescribed in AD G for the discharge of water from safety devices

• The tundish should be:– Vertical;– Located in the same space as the

cylinder; and– Fitted as close to the valve with no

more than 600mm of pipe between the valve outlet and the tundish.

• The discharge pipe from the tundish should:– Have a vertical section of pipe at

least 300mm long below the tundish before any elbows or bends in the pipework; and

– Be installed with a continuous fall thereafter of at least 1 in 200.

MAINS PRESSURE VENTED SYSTEMMAINS PRESSURE VENTED SYSTEM

Thermal storesThermal stores

• Unvented systems are normally at mains pressure. Thermal store systems have been developed that are at mains pressure but are vented.

• A container of water (the thermal store) is heated by the boiler via a heat exchanger coil at the bottom of the store. Mains pressure cold water passes through a second heat exchanger coil in the top of the store where it is heated by the stored hot water surrounding it and supplies the hot water outlets at mains pressure.

• Expansion of hot water in the store is accommodated by a feed and expansion cistern located just above the store (ie it is a vented hot water system).

Thermal store

• The thermal storage system is supplied with primary water from the boiler which heats the store via a primary coil.

• Secondary water flows directly from the cold mains into a secondary coil where it is heated by the store before being delivered to the taps at mains pressure.

Thermal store (configuration for a sealed central heating system)

Primary coil heat exchanger

Thermal store

Secondary coil

Feed and expansion cistern for DHW system

INSTANTANEOUS SYSTEMSINSTANTANEOUS SYSTEMS

Multi-point instantaneous heaters: Combi boilerMulti-point instantaneous heaters: Combi boiler

• The ‘combi’ gas boiler functions as an instantaneous water heater only heating water as required. Water supply is from the mains, providing a balanced pressure at both hot and cold water outlets. Ideal for showers.

• System is sealed and has an expansion vessel which is normally included in the manufacturer's pre-plumbed, pre-wired package.

• Saves space: no need for cisterns in roof space, no hot water storage cylinder and associated pipework

SOLAR HOT WATER SYSTEMSSOLAR HOT WATER SYSTEMS

0 – 50 7.37 – 10.07 23.50 – 25.15

Solar irradiationSolar irradiation

Contours show solar irradiation in kWh/m2 pa

811 kWh/m2/year

1115 kWh/m2/year

30o – 40o

System efficienciesSystem efficiencies

Solar irradiation 100% 60% 45% 30%

Useful energy to taps

Losses

GlazingCollectorReflectionConductivity

Losses

PumpPrimary pipes

Losses

StorageSecondary pipes

Solar collectors

Flat plate collector

Evacuated tube collector

Flat plate collectorsFlat plate collectors

• Insulated metal box with either a glass or plastic covering and a dark absorber plate usually made out of copper or aluminium. This absorber plate transfers the heat to a tube where the heat transfer fluid flows, picks up the heat from the plate, and returns it to the storage tank.

• The main distinction in the types of solar collector is between glazed and unglazed:

• The most common type of unit for DHW purposes is the glazed type

Evacuated tube collectorsEvacuated tube collectors

• Evacuated tube collectors are more efficient than flat plate collectors and can provide higher output temperatures, but are more expensive. They are more efficient because the absorber is mounted in an evacuated and pressure-proof glass tube which reduces conductive and convective heat losses.

Evacuated tube collectorsEvacuated tube collectors

• There are two main types: direct flow and heat pipe:• In a direct flow collector, cold return fluid passes through the manifold and circulates round the

absorber tubes in series and is heated in the process, returning to the flow stream of the manifold

Evacuated tube collectors: heat pipeEvacuated tube collectors: heat pipe

• Heat pipe evacuated tube collectors contain a copper heat pipe, which is attached to an absorber plate, inside a vacuum sealed solar tube. The heat pipe is hollow and the space inside is also evacuated.

• Heat pipe contains liquid. Vacuum enables liquid to boil at low temperature.• When sunlight falls on surface of absorber, liquid in heat tube turns to hot

vapour and rises to top of pipe. Water or glycol, flows through a manifold and picks up the heat. The fluid in the heat pipe condenses and flows back down the tube.

Evacuated tube collectors: heat pipeEvacuated tube collectors: heat pipe

Heat pipe collectors must be inclined at an angle of > 25o to allow internal fluid of the heat pipe to return to the hot absorber

Heat transferHeat transfer

• The system contains water and low toxicity polypropylene glycol (anti-freeze for frost protection) and corrosion inhibitors as a heat transport mechanism.

• If the solar collectors can be mounted below the level of the solar store circulation will occur by natural circulation (the thermo-syphon effect). This system is often used in Mediterranean countries.

• In the UK, the solar collectors are usually mounted on the roof so circulation in the solar primary circuit is pumped.

• Pump controlled by a differential temperature controller which compares the temperature at the collector panel outlet with the temperature of the water in the solar store and switches pump on when sufficient thermal gain.

Heat storageHeat storage

• A solar DHW system requires the solar heat to be stored in a vessel to allow the heat to build-up during the day.

• Because of the pattern of solar gain, a back-up heat source will also have to be integrated into the system. This can either be in separate storage cylinders or in a combined cylinder.

Separate solar cylinder Combined cylinder

Solar coil at bottom of cylinder pre heats water

Combined cylinder heat storageCombined cylinder heat storage

• The solar coil should be the below the boiler coil, allowing the solar pre-heated water to rise up the cylinder for any top-up heating from the boiler coil.

Solar coil

Boiler coil

Safety of solar systemsSafety of solar systems

• Solar energy can quickly accumulate enough heat in a high performance collector to convert the circulating liquid to steam under significant pressure.

• Sometimes a high temperature situation arises during a circulation failure, eg a faulty pump, power cut and both the primary and secondary system may overheat. The state in which there is no net heat extraction from the collector is described as ‘stagnation’.

• This can be dealt by either:– Vented system: hot water vented to the atmosphere into a CWSC– Unvented sealed system, either:

• An expansion vessel and safety valve• Drain-back: switch-off pump and drain away from a collector into a

vessel

Vented systemVented system

• A solar DHW open vented primary system has features that distinguish it from conventional open vented heating systems:– If antifreeze is used, the feed and vent cistern is not normally connected to the cold water mains

via a float valve, in case of dilution due to long-term evaporation. Hence, re-filling of the header cistern with antifreeze has to be undertaken manually.

– Cistern needs to be fitted high up to gain the greatest static pressure (head) above the collector. It is not always practicable to achieve sufficient height.

Schematic pipework layout of open vented solar primary system

Unvented systemUnvented system

• An unvented system contains a vessel capable of holding the primary fluid contents of the collector at all the permitted pressures in the system. This vessel can be of the membrane expansion type or of the drainback air-pocket type.

Schematic pipework layout of unvented expansion vessel type

Unvented system: drainback

• The location of a solar collector normally above the store, allows the possibility the ability to drainback. The system is only partially filled, leaving an air pocket permanently present in the circulatory circuit. When the pump is off, the fluid does not fill the collector but instead rests wholly within the lower part of the circuit. When on, the pump pushes the air out of the collector replacing it with fluid, the air is then displaced down to inside a ‘drainback’ vessel

Schematic layout of drainback solar primary

Solar hot water with a combi boilerSolar hot water with a combi boiler

• Because a combi boiler heats the hot water directly from the mains, there is no hot water cylinder. It is therefore more difficult to install a solar system. It is possible to pre-heat the mains water inlet to a combi boiler but the boiler must be solar compatible ie have approved temperature blending equipment and be able to monitor the temperature of water entering the system and modulate the flame accordingly.

• An alternative arrangement is shown below whereby a twin-coiled cylinder supplies the majority of outlets and the combi directly supplies a shower for example.

FITTINGSFITTINGS

Fittings: taps Fittings: taps

• About 20% of domestic water flows through sink and basin taps.

• Long, un-insulated pipe runs (dead-legs) waste water whilst waiting for the tap to run hot.

• Spray taps (aerators) can save about 80% of the water and energy used for hand washing

• The amount of water used by a tap is related to frequency of use, flow rate and duration. Flow restrictors that reduce flow rate do not necessarily reduce water use, since some functions of taps are volume dependent (e.g. filling sinks, kettles) rather than duration dependent (rinsing, hand washing).

Fittings: showers and bathsFittings: showers and baths

• In new houses, showers and baths account for around 45% of water In new houses, showers and baths account for around 45% of water used. used.

• Showers can use a third of the water of a bath but people tend to take Showers can use a third of the water of a bath but people tend to take them more frequently and power showers and mains pressure systems them more frequently and power showers and mains pressure systems have increased flow rate: long shower can use more water than a bath.have increased flow rate: long shower can use more water than a bath.

• Showering is single largest use of hot water in modern homes. Water Showering is single largest use of hot water in modern homes. Water saver showers introduce air or atomise the water drops to improve saver showers introduce air or atomise the water drops to improve wetting for a given flow rate. Feels like a ‘power shower’ but uses less wetting for a given flow rate. Feels like a ‘power shower’ but uses less water. water.

• Or, flow rate can be limited. Minimum acceptable 4 litres/minute.Or, flow rate can be limited. Minimum acceptable 4 litres/minute.

• Typical standard bath capacity is 225 litres. Reduced capacity (shallow) Typical standard bath capacity is 225 litres. Reduced capacity (shallow) baths are available with a capacity of 140 litres which retain a standard baths are available with a capacity of 140 litres which retain a standard bath footprint.bath footprint.