I have recently purchased a Hansgrohe Focus S mixer tap for a downstairs WC re-furb. The installation instructions state that equal pressure is required on both hot and cold feeds, though the suppliers claimed it would work with mains cold and gravity hot. My concern is that, not understanding the internals, it may be possible for the mains water to back fill the hot water cold storage tank. So fit a non-return valve on the hot feed? I have done that in the bathroom, and the hot flow is seriously compromised! Is there a non-return valve available with a weak spring so that flow is far less affected? Do I actually need a nrv in this situation?
I have the same situation as you know, although with a different model. I am not sure about the Focus S, but the Uno has a single spout and the mixing occurs in the body of the valve. Some sink mixers have a concentric arrangement that ends at the spout exit, so no possibility of water being passed back unless some kind of blocked tube were attached.
The only way that you could get back filling with the Hansgrohe tap is to put your finger over the spout and open both sides.
More to the point would be the requirements to avoid cross contamination. AIUI, the mains water is fluid category 1 and the HW supplier sourced via the roof tank is fluid category 2. For showers with a flexible hose, there are supposed to be check valves on both supplies because of the possibility that the shower head is dropped into the shower tray or even the toilet (category 5).
I am not sure that there is a requirement to protect the supplies to the basin tap. At any rate, what I have done is to put a check valve in the cold supply only plus a service valve with small aperture. There is a need to reduce flow anyway. The hot is connected directly at present. I am going to check the point with WRAS, but I believe that this meets the cross contamination requirements.
I don't think that the risk of cold mains being fed to the roof storage tanks is an issue. The warning pipe of the tank would highlight that and it does require someone to deliberately block the spout
Don't forget that most monoblocs have a sparkler or whatever on to mix the hot/cold as it leaves the tap. This is equivalent to a thumb over the tap, particularly on cheap taps. It caused me months of trouble with the roof tank overflowing until the combi was installed. I was fine with it but everyone else in the house seemed incapable of not overflowing the tank out the overflow pipe.
This one has such a fitting but it's adjustable by removing a component for low pressure.
Up to a point. The effect would depend on the resistance to flow at the spout for the mains flow and the difference in head. I would expect it to be worse on a first floor tap. Reducing the flow on the mains side would also reduce the effect. It isn't really a thumb over spout situation because there is still flow.
On some you can remove the spring altogether. Since all you are seeking to prevent is back flow pushed by positive pressure from the cold then it should still do this without a spring. Using one designed for 22mm pipe would be another way of reducing the impact.
Yes, thanks for the recommendation. Assuming it performs OK, I have a smart mixer which appears of good quality for £73 - the alternative supplier's offerings, some of dubious quality, were in the 100-160 range.
What's to stop the cold water entering the valve body and leaving via the hot feed route rather than the spout? I suppose a lot will depend on the bore of the inner channels relative to the spout size. EricP's experience suggests that it doesn't take much if any restriction to cause this issue.
I know it is kind of overkill, but I was considering 2 service valves on the cold feed. The downstream one would be used to throttle the flow with the upstream being on-off only. The reason being that ball valves are not intended for flow control and running them partly open can cause seat wear. The downstream valve would be in a "really easy to switch" location.
Please post new thread if you get a response from your WRAS enquiry
Thanks once again Andy, help as always greatly appreciated
Because the way out through the spout presents a lower resistance than the way back up the hot supply plus the head from the cold tank in the roof.
This is a pressure/flow issue. It will partly depend on the location of the tap - i.e. likely to be worse upstairs where the head from the tank is less - the cold is less as well but this is a matter of pressure difference.
I think that it could be resolved by reducing the cold flow to the same rate as that of the hot when the tap is fully open.
You could do that. It would save readjusting if you need to turn off the service valves.
I'm going to call them tomorrow. I've done so before on the showers issue because I couldn't think of a reason why water from a shower could enter the plumbing.
They are in Gwent and a very excitable Welshman with a high voice (do you remember Clive Jenkins, the union leader?) explained that there are "sections of the community" who remove the shower head from the hose and use it to clean "where the sun don't shine" and then drop the hose into the toilet. Then somebody downstairs turns on the hot tap rather than the cold for drinking water..... It seemed implausible - not the ablutionary aspect - toilets in the Arab world are generally fitted with a little trigger operated shower with cold water - yes... but the siphoning. Nevertheless, I fitted the valves.
One trick is to fit larger ones than the nominal pipe size - i.e. 22 or even 28mm to the hot.
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