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Building Services Engineering 5th Edition Handbook

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96 Heating<br />

Table 4.1 Classification of hot-water heating systems.<br />

Type Flow temperature ( ◦ C) Pressure<br />

LTHW 80 Open to atmosphere, 1 bar<br />

MTHW 120 7 m head above atmosphere, 2 bar<br />

HTHW Over 120 In excess of 7 m head, 3 bar<br />

Note: LTHW, low-temperature hot-water; MTHW, medium-temperature hotwater<br />

(refer to Fig. 4.17); HTHW, high-temperature hot-water.<br />

Pressure controller<br />

Pressure sensor and gauge<br />

Motorized valve<br />

Pressure-reducing valve<br />

Pump<br />

High-temperature<br />

hot-water pipework<br />

to convectors,<br />

radiant panels,<br />

air heaters or<br />

heat exchangers<br />

Boiler<br />

Discharge<br />

from spring<br />

safety valve<br />

Expansion<br />

vessel<br />

High-pressure<br />

nitrogen cylinders<br />

4.17 Pressurization equipment for a high-temperature hot-water heating system.<br />

Hydraulic gradient<br />

+ve pump pressure<br />

+ve<br />

Overflow<br />

Feed and<br />

expansion tank<br />

Vent<br />

–ve<br />

Main<br />

Water level<br />

Cold feed<br />

Pump<br />

Flow<br />

Boiler<br />

Return<br />

Neutral point<br />

4.18 Neutral point in a heating system.

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