If multiple mixer taps may be getting cold going back up the hot

[WaterTight]

Can you just put a non return valve on the hot under the combi where it comes out? Rather than put one on the hot pipe to each and every tap? Or is that no-no for some reason?

Ta

 

[Ric2013]

If balanced pressures to mixers, no legal requirement for check valve on taps.

I can't see why having one on the boiler outlet would be a problem as surely, it wouldn't be any different from having an isolation valve in that location.

 

[WaterTight]

Good point, as iso's are meant to be facing one way. It's balanced pressures (combi) but I suspect cold is getting back up hot as it's the classic works fine for a few mins then gets cold, until you turn down/briefly off the cold then turn it back on again and boiler fires up again. And only at outlets where hot and cold mix

 

[Scott_D]

Try isolating the hot to each tap (if they have ISO’s)

 

[WaterTight]

They do not

 

[WaterTight]

Although how would that help? It would produce the same effect: No hot.

 

[Ric2013]

Hmm. If the issue is no hot with tap in mixing mode, I read it this way:

Cold water working pressure higher than hot even if theoretically balanced. Tap outlet restrictive. Hot is prevented from issuing as pressure in tap body exceeds working pressure of hot supply pipe. Check valve would prevent cold from backfeeding hot, but would remain closed in this situation so mixing would not necessarily be improved, though I suppose the fact that it doesn't backfeed may make it easier to find the sweet spot on the mixer control.

Might it be better to use an equalising valve to balance the pressures?

However, I have a doubt: if the cold if backfeeding the hot, where is it backfeeding it to unless other taps are in use? Confused.

 

[Scott_D]

Although how would that help? It would produce the same effect: No hot.

Sorry
I meant isolate the cold to see if the tap runs hot with no cold

 

[WaterTight]

Everything runs fine with just hot outlets open. Although now and then, at high-usage-in-the-street times of day, the flow, which has never been massive anyway, reduces to the point the boiler cuts out until I open two hot taps, increasing the total flow, which seems to persuade it to fire up again. That's my theory anyway.

So it could be two issues at same time I guess. The main annoyance is the shower. Full hot, fine. Mix in cold and it will struggle to maintain a good temp, varying from too hot to too cold depending on my struggles. But it's just a bath shower mixer tap with hose of it, no cartridges. It's not thermostatic. I'm guessing a thermostatic one wouldn't solve the issue if it's cold back feeding which I suspect.

I'll bung a non return valve somewhere and see what happens and report back

 

[WaterTight]

Might it be better to use an equalising valve to balance the pressures?

However, I have a doubt: if the cold if backfeeding the hot, where is it backfeeding it to unless other taps are in use? Confused.

I guess it just needs to push back down the hot pipe just enough to resist the hot coming out and register as no demand for hot water at the boiler at which point it cuts out...?

 

[WaterTight]

I would like to understand it as even though it's my flat I bet it won't be the last time I see the problem.

 

[Ric2013]

I would like to understand it as even though it's my flat I bet it won't be the last time I see the problem.

I know the feeling. Logically, IF it is actually backfeeding, a check valve alone won't solve the problem in that the backflow will stop but the hot flow will still cease. Having said that, it's not a problem I have actually come across, so I'm not speaking from experience - so please let us know your findings!

 

[WaterTight]

Why would the hot flow still cease?

 

[Ric2013]

Because if the cold pressure is high enough to backfeed the hot from the tap mixing chamber then this shows two things (among others):

1. Cold working pressure > hot working pressure
2. Hot working pressure < tap body pressure

For hot water to flow into the tap and then out of the spout, the hot working pressure needs to be > the tap body pressure (flow is always from high pressure to lower pressure area) and also high enough to open the check valve. The check valve will prevent backfeeding, but cannot, surely, reverse the direction of flow?