Check Valve and Overflow Pipe Questions

[voicey]

Hi there - it's been a while since I've been on here!

I've just got home after been called out to an "emergency" at the flat we rent out. Our tenant noticed water flooding out of the overflow and also leaking inside the airing cupboard. After a bit of faffing around I have identified two problems:

1. The kitchen mixer tap has a flexible nozzle for rinsing plates. If the tap is left on but the nozzle is left closed the hot and cold water mix in the hose. As the water is mains pressure it back fills the hot water system. The answer is to switch the tap off properly but I was thinking of fitting a check valve to hot water supply to the kitchen tap (15mm). Is this a suitable fix? What's the pressure loss like over such a valve? As the hot water tank is only one floor up the hot water pressure is not great - should I step up/down to 22mm and fit a larger check valve?

2. The overflow from the cold water storage tank leaks at a join. On closer inspection is appears the pipes are of different diameter. On one side there is a white (21.5mm?) overflow pipe glued to a straight connector (join is good). The other side is connected to a grey overflow pipe which is slightly too small a diameter to fit inside the connector properly. This has meant the join has failed and is now separate after I gave it a wiggle. As both pipes are slightly different diameter (and replacing either is not an option) is there an adapter I can buy?

Apologies for the long post but I know you guys will have the knowledge to point me in the right direction. Cheers!

 

[seco services]

1. double check valve, its a cat 3 risk a flexi hose over a sink.

(and replacing either is not an option)

2 why is it not an option ?

easier to do it properly than bodge two different makes of pipe.

 

[voicey]

1. double check valve, its a cat 3 risk a flexi hose over a sink.

(and replacing either is not an option)

2 why is it not an option ?

easier to do it properly than bodge two different makes of pipe.

Thanks. One pipe disappears into the concrete floor, the other into the cavity between the ceiling and the floor in the loft (access is very tight). If it came to it I could replace the white pipe (which I think is 21.5mm that I've seen in the shops before) but I've not seen the smaller grey pipe for sale anywhere. I was wondering if there was an adapter available.