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The Barbour Guides are intended to provide a short introduction to, or overview of, a given subject area. They should

not be viewed in any way as being complete or comprehensive. Barbour recommends further reading, starting with

the references listed in the Guide and other information held within the Barbour services.

MEDIUM TEMPERATURE HOT WATER HEATING

HEALTH AND SAFETY

Temperatures at which MTHW systems operate represent a health risk. Measures should beprovided to prevent direct contact with heated surfaces. The requirements of NHS EstatesHealth Guidance Note 1Safe hot water and surface temperatures should be considered andare a statutory requirements in many government establishments and buildings.

GUIDANCE

Design water temperatures for MTHW are 1000C to 1200C (see CIBSE Guide B1).

This covers the complete distribution system, covering everything but the boiler plant.Therefore, this will cover equipment such as pumps, pressurisation plant, pipework, valvesand emitters.

Pressurisation:Where systems operating above 1100C are pressurised by means of a header tank, anexpansion vessel should be incorporated in the feed and expansion pipe. This vessel shouldbe adequately sized to take the volume of expansion of the whole system so that boiling willnot occur in the upper part of the feed pipe. On no account should an open vent be providedfor this type of system.

Additional safety measures are required in closed or sealed systems to avoid excesspressure building up. These include high and low temperature cut-outs, and also a highpressure cut-out.

If reliability is an important factor, then pressurisation plant should be selected with duty andstand-by motors or pumps. They can also be provided with the facility to be controlled via aBMS, and also to give remote fault reporting.

Pipework:When sizing pipework, keep pressure drops to manageable levels as high resistances resultin high pump heads and excessive energy consumption. Such systems can be noisy, andalso difficult to balance and commission properly.

Pipework must be adequately supported in both the vertical and horizontal applications, andshould not have its weight borne by plant or equipment.

When routing pipework, ensure that the pipes are laid to fall to facilitate filling and venting,and adequate fittings are provided for draining down in suitable locations to avoid dead legsand stagnant lines. Make sure that valves and other equipment are accessible once installedfor future commissioning or testing.

Emitters:Carefully consider the use of the space before selecting heat emitters. In some specialistapplications such as health care premises or primary schools, emitters must have low surfacetemperatures to avoid the risk of injury to the occupants.

However, due to the low surface temperature usually being achieved by using lower flow andreturn temperatures within the distribution system, the radiator output is reduced for a given

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surface area. This could result in increased costs and a greater space requirement.

Check the performance data produced by the manufacturer very carefully. The performancefigures quoted by them will have been tested at very specific conditions which may not be thesame as for the system under consideration, and correction factors must be applied to getaccurate output information. Typically with radiators, for example, the performance data hasbeen produced from tests using a mean water temperature in the radiator of 800C and a roomtemperature of 200C, thus giving a difference of 600C. In practice, however, if designconditions of 1100C flow and 900C are used, these will give a mean water temperature in theemitter of 1000C. With a room temperature of 210C, the temperature difference becomes790C, and the appropriate correction factors must be applied.

Similarly, the manufacturers data should also state the connection arrangement, withcorrection factors for variations. The normal arrangement for testing may be tboe (top andbottom opposite end), where in practice boe (bottom opposite end) is more commonly used.

Fan convectors provide greater output than radiators but care has to be taken in selection toavoid excessive noise.

Emitters in high spaces need to be carefully selected to heat the occupied space rather thanwasting energy by heating up to the roof. To this end, radiant heating has been usedsuccessfully in large spaces such as workshops and warehouses.

Commissioning:Ensure that the division of responsibility between the designer and the commissioningspecialist contractor is clearly defined to avoid duplication of work, and possible sitedisagreements.

Select pipework components and layouts to ensure that the system is as inherently stable andself-balancing as can be economically justified. This will reduce the time required to regulatebranch flow rates and will often make it unnecessary to balance flow rates through terminalunits.

Consider the requirements for two and three port valves, gauges and fittings for measuringsystem performance.

Check that design pressure losses across terminal units fed from the same branch are notsignificantly different, e.g. within around 20kPa of each other. Mixing low resistance terminalunits such as radiators with high resistance units such as fan coils should be avoided.

Where possible ensure that terminal unit design pressure losses on reverse return branchesare the same in order for flows to these terminal units fed from a reverse return branch toachieve some degree of self-balancing.

Check that flow regulating and measurement valves are properly located. Usual locationsinclude risers, branches, sub-branches, terminal branches, by-passes from three way controlvalves, pump sets, individual boiler and chiller circuits. Only where a degree of self-balancinghas been achieved might they be omitted.

Check that design flow rates are achievable with regulating valves more than 25% open.Select regulating valves such that at their design flow rates the required pressure lossesacross them can be achieved without the need to close below the 25% open position. Valvesclosed beyond this point can be prone to dirt and air blockages. Low flow 15mm valves havea minute orifice and should be considered with caution.

Check the accuracy of the specified flow measurement device is adequate to achieve therequired flow balance tolerance. Depending on the device selected, accuracy can varybetween +/-3% and +/-10%.

Flow measurement devices should be selected such that at design flow rates the pressure

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signals across them are greater than 1kPa. Anything less than this becomes difficult tomeasure with site instruments which affects accuracy.

Tolerances of final flow measurements should be assessed and specified, and thecommissioning specialist given a realistic flow rate tolerance within which to establish a flowbalance. (see CIBSE Commissioning Codes and BSRIA Guides). Note that the total oftolerances may affect pump selection.

Valve and flow measurement device locations should be indicated on design schematicdrawings and contractor installation drawings with commissioning data.

Units such as fan coils can be grouped together to reduce the number of commissioningvalves required.

REFERENCE DOCUMENTS

The following documents provide further guidance:

BSRIA TM 1/88 Commissioning of HVAC systems. 1988BSRIA AG 2/89.2 Commissioning of water systems in buildings. 1989BSRIA AG 20/95 Commissioning of pipework systems. 1995BSRIA AG 1/01 Pre-commissioning cleaning of water systems. 2001CIBSE Commissioning Code B: 2002 Boiler plantCIBSE Commissioning Code W: 2003 Water distribution systemsCIBSE Guide B1: 2002 HeatingNHS Estates Health Guidance Note 1: 1998 Safe hot water and surface temperatures

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