Check valves

[rotadays]

Would appreciate some guidance please.
I am fitting a thermostatic shower bar above a bath (Bristan Zing) I want to tee off the 22mm bath feed to 15mm for the shower, the old plumbing had check valves into the bath hot and cold not sure if they were double or single. Is it necessary to fit check valve for this set up and where should I fit them if necessary. I have a conventional boiler with an unvented heating system Mega Flow Heatrae Sadia.
Thanks in advance

 

[PrenticeBoyofDerry]

The manufacture's Instructions will state that certain backflow prevention should be maintained or installed.

Table relating to:
Size of tap or combination fitting.
Vertical distance of outlet above spill-over level.
.
1. Not exceeding 1/2 inch =20mm spill-over level
2. Exceeding 1/2 inch but not exceeding 3/4 inch= 25mm spill-over level
3. Exceeding 3/4 inch= 70m spill-over level

For backflow protection in domestic or installations up to, and including, Fluid Category 3.
If the fitting cannot be installed as indicated in the table above it shall be installed as either
a or b below:
a: with an approved double check valve assembly or some other no less effective backflow prevention device immediately upstream of the inlet.
b: so that it draws water by gravity only from a cistern, or cylinder having a permanently open vent pipe, and the distributing pipe supplies no other fitting (other than draining tap) at a lower level.

For backflow protection in premises or installations up to, and including Fluid
Category 5.
The vertical distance of the outlet above the spill-over level shall be not less than 20mm or twice the diameter of the inlet pipe to the fitting, which ever is the greater. If the fitting cannot be installed as indicated it shall be installed with a backflow prevention arrangement suitable for the Fluid Category.

 

[TeaTime]

It is nessasary to have check valves installed on the shower inlets, since the shower head is not fixed and can draw waste water back into the hose when detached.

Alternatively, you may be able to just have a check valve fitted on the hose connection to the bar, which hopefully will be explained in the MI, as explained above.

 

[rotadays]

Thanks for your replies.

So are you saying that the only reason check valves need to be fitted is it is technically possible for the shower head to syphon water back through the system from dirty bath water?

would check valves significantly affect pressure to the shower with my hot water system? the pressure seems to be very good with the hot and cold?