Page 431 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 6 Hot water systems

Protection against backflow and back siphonage

This subject is dealt with in Chapter 5, Cold water systems, where different types
of basic backflow prevention devices and air gaps are discussed. That theme
is continued in this chapter as hot water is itself categorised as fluid category
2 simply because heat has been added to the cold wholesome water. Other
considerations here are that many of the bathroom appliances that are connected
to the hot and cold supply are also at risk from fluid categories 3 and 5.
Appliances that may be at risk from backflow are listed in Table 6.14.

 Table 6.14 Appliances that may be at risk from backflow

Taps for use with washbasins should discharge at least 20 mm above the spill-over level of the
appliance (AUK2 air gap). Mixer taps should be protected by the use of single check valves on the
Washbasins hot and cold supplies. Twin-flow mixer taps do not require any backflow protection as the water
Fluid cat. 2 and 3 risk mixes on exit from the tap.
No backflow protection is required as the height of the outlet is well above the spill-over level of the
appliance. This is classified as an AUK3 air gap. If a mixer tap, where both hot and cold water mix
Kitchen sinks in the tap body, is installed then single check valves must be fitted on both hot and cold supplies.
Fluid cat. 5 risk Twin-flow mixer taps do not require any backflow protection as the water mixes on exit from the tap.
As for washbasins, except that the air gap should be 25 mm. Bath/shower mixer taps, where the
water is fed from the mains cold water supply and there is a risk of the showerhead being below the
Baths water level in the bath, should be protected by double check valves or a shower hose retaining ring
Fluid cat. 2, 3 and 5 risk that maintains an AUK2 air gap above the spill-over level of the bath.
There are two types of bidet that are at risk from backflow. These are:
1 the ascending spray type – special consideration must be made when fitting this type of bidet
(see Figure 6.60); these cannot be used with mains-fed hot and cold water systems; fluid cat 5 risk.
2 the over rim with shower hose connection – with this installation there is fluid cat. 5 risk as well as
a fluid cat. 2 risk.

The cold water supply must
only supply the bidet and the WC

Flexible hose and spray
Single check
valve Zone of backflow

Bidet represents
both ascending
spray and flexible
hose types

Hot supply to all
Bidets other hot outlets
Fluid cat. 2, 3 and 5 risk p Figure 6.60 Installation of an ascending spray bidet and bidet with flexible hose
➜

419

9781510416482.indb 419 29/03/19 9:02 PM