Do check valves work to prevent backfeeding on mixers with unequal pressures?

[WaterTight]

Once both feeds are open the double checks on the feeds are open.

So how would the double check then prevent back feeding with unequal pressures.

Seen this fix suggested a bunch of times, mainly for kitchen mixers or bath fillers as showers should have equal pressures normally. But don't get how it would work?

Thanks

 

[Ben-gee]

If the pressures are significantly different the higher one will flow back down the low pressure pipe and close the non-return valve.

 

[Ric2013]

They actually spring shut and water pushes them open. So long as pressure in the mixing chamber<pressure in the pipe the water will not backflow anyway. Only if the pressure in the mixing chamber>pressure in one of the pipes will you have the symptoms that you get full hot or full cold only (whichever is higher pressure) and backflow. With the check valve fitted, backflow is prevented. The check valves may help even up the pressures a little (?) but they won't make the mixer perform significantly better, but they do make the installation legal.

 

[WaterTight]

If the pressures are significantly different the higher one will flow back down the low pressure pipe and close the non-return v

The fix I heard mentioned on here numerous times in the past for when mains cold was back filling into tank fed hot and so the mixing of hot and cold coming from the tap just became cold was to fit a check valve on the hot. But this can't work right? Becauase if the check valve stays open then cold will back feed anyway and if it shuts due to cold backfeeding you get the same problem.

So if it's a tap where hot and cold mix in the tap before they come out of the spout and hot and cold pressures are not equal there is no fix for this issue other than fitting a tap where they don't mix in the spout?

A check valve may make it legal but it won't make it work, right?

----

A possibly related thing:

On my kitchen mixer and bath shower mixer the pressures should be equal as it's a combi but the boiler cuts in and out when mixing hot and cold (hot on full, cold on a little.) it doesn't do so when just hot is on with reasonable flow.
To "solve" the problem I've found that if I turn on a separate hot tap somewhere else a tiny bit at the same time as the mixer tap or mixer shower is mixing hot and cold the problem stops. So it seems like insufficient hot flow can be achieved out of one mixer tap or mixer shower to fire up the boiler if cold is being run too, solved by increasing the flow a little by opening another hot tap a little.
What does this mean? What's a proper solution?
I believe I saw a customer recently who may have the same problem. And a friend of mine (bit of a plumbing newbie) just swapped a bath shower mixer for another bath shower mixer on a combi and it's now failing to stay hot reliably when having a shower. I'm wondering whether turning on the basin hot a little would solve that too. If so, there's a pattern to these problems. Tap design? Pressure was never quite sufficient to run a combi but they got lucky before? How do you work it out?

Cheers

 

[Last Plumber]

If so, there's a pattern to these problems. Tap design? Pressure was never quite sufficient to run a combi but they got lucky before? How do you work it out?

There could be a number of reasons for that problem.

What size is the combi - What temp rise will it give to DHW?
What is the minimum DHW flow rate (to fire the boiler)?
What flow rate is the tap giving? (hot side when being used as you described - not full flow)
Is the boiler reaching temp and shutting down?
What temperature is the water leaving the tap? (hot only)
What temperature is the water leaving the boiler?

Those are a few initial questions I would be looking at just to gain some idea of what's happening.

 

[Ric2013]

The fix I heard mentioned on here numerous times in the past for when mains cold was back filling into tank fed hot and so the mixing of hot and cold coming from the tap just became cold was to fit a check valve on the hot. But this can't work right? Becauase if the check valve stays open then cold will back feed anyway and if it shuts due to cold backfeeding you get the same problem.

So if it's a tap where hot and cold mix in the tap before they come out of the spout and hot and cold pressures are not equal there is no fix for this issue other than fitting a tap where they don't mix in the spout?

A check valve may make it legal but it won't make it work, right?

Cheers

Hmm. If the tap is not restrictive to flow then the pressure in the body will be low and it may only be in certain conditions (e.g. cold 90% open, hot 10% open) that there may be a risk of backfeeding as the outlet struggles to allow the quantity of water going in to get out and so pressure builds up. I'm assuming in other conditions (e.g. cold 20% open, hot 20% open) pressure does not build up in the mixing chamber and mixing takes place. In fairness, I have fitted mixers on unbalanced pressures (always with check valves as per Water Regs) and they did work.

I suppose if backflow if eliminated at least you have the option to turn down the cold a little and normal service will immediately resume as the hot water pipe is not full of cold water that then has to be evacuated. So while a mixer on unbalanced pressures and with check valves fitted may work differently from the same mixer on balanced pressures, it may still be reasonably functional.

The only problem is that spring check valves (the only type the Water Regs actually recognises for this application) tend to restrict the flow and the lower pressure water flow will be reduced even further. I have found some (from BES) that are less restrictive than the normal type (which, at 0.1 Bar or less can actually fail to open altogether once the grease applied during manufacture has washed out) though I'm not sure they are actually to a required and certified standard.

The key is to check MIs: some manufacturers recommend balanced pressures and some insist on it. Obviously if pressures are unbalanced, separate taps or 'mixer' taps with separate waterways (so-called 'biflow') are a better choice.