Double check valves

[long ironer]

Are all double check valves the same, or do some allow more flow than others?

These valves are being fitted in both 22mm H & C feeds to to a Mira shower to comply with water regs & with Mira installation instructions. Potential loss of pressure from over-restrictive valves is an issue.

TIA

[John Stumbles]

If both H & C are fed from a gravity system I don't think you need check valves as there'd be no risk of contamination of potable water by 'unwholesome' water (at least as far as the mixing of the two supplies in the shower valve is concerned: you still have to ensure that the shower hose can't be immersed in waste water in the shower tray or bath or fall into the loo).

If C is from the mains (and H from mains pressure system e.g. unvented or thermal store, or pumped gravity system) then you do have a potential for contamination (e.g. fire engine turns up in the street, sucks water out of the mains and draws water from your hot water pipework into the cold supply main which then is fed to other users when the fire engine stops pumping). A check valve on the cold inlet to the shower will protect against this, but the pressure drop in the cold water supply will be negligible.

[long ironer]

/snip/

Many thanks for your comments. Made me carefully check back to the water regs. Even if the shower H&C supply is from a separate tank, a means is required to prevent back syphoning.

Restraining the shower hose so that it can't drop into the shower tray is acceptable and is the standard Mira solution (Mira supplies a restraining loop for the hose support). An alternative is a double check valve either on the outlet connector (Mira has an accessory to do that) or on both supply pipes. It is the last I am wanting to do & hence need to keep loss of pressure to a minimum.

Since posting I have discovered that there are indeed 'high flow' and 'normal' check valves. Normal check valves lose around 0.5bar; high flow check valves lose 0.1bar.

BTW the above need for check valves applies (and is usually overlooked) on those bath taps which have a switched shower option as the shower hose will always be able to drop into the bath.

Thanks again.