What lesson can be learnt from this?

[doitall]

Sorry Doitall, don't know your name

If mixer shower is on and cold is at lets say 1.5 bar and hot 1.2 bar after the boiler, then would'nt cold fight hot to 0.3, hot winning by 0.9 bar...... or is the drink saying go to bed Craig, golf at 6.30am

John thescruff Craig

Yes the cold would be dominant, but must showers allow for small pressure differentials.

Provided the shower was turned off, non of the other outlets would be affected.

If the mains pressure is low, then the kitchen sink for example could rob the water from the shower, but isn't that the case with any combi

 

[Balenza]

Chrishutt wrote

It is only the non-return valves that stop cold flowing across to hot pipes and vice versa.

Depends on the temperature setting. You can shut off the cross flow by turning the adjustment to either cold or hot or vice versa.

 

[clf-gas]

Sorry Doitall, don't know your name

If mixer shower is on and cold is at lets say 1.5 bar and hot 1.2 bar after the boiler, then would'nt cold fight hot to 0.3, hot winning by 0.9 bar...... or is the drink saying go to bed Craig, golf at 6.30am

John thescruff Craig

Yes the cold would be dominant, but must showers allow for small pressure differentials.

Provided the shower was turned off, non of the other outlets would be affected.

If the mains pressure is low, then the kitchen sink for example could rob the water from the shower, but isn't that the case with any combi

Got your train of thought now John.

Cold to shower can push down hot feed from boiler forming Static pressure, thus boiler shut down thinking no demand. now Iam stuck. How does the water run lukewarm. Was the boiler running at min thinkinking low demand

 

[chrishutt]

Depends on the temperature setting. You can shut off the cross flow by turning the adjustment to either cold or hot or vice versa.

I don't think shower valves ever completely close either the hot or cold inlets. Otherwise there would be problems using twin-ended shower pumps with them for one thing. It may be that the residual flow of cold when the valve is set to the hottest position (and vice versa) is so low as to sometimes appear to stop.

 

[DP]

Did you bother to check the bod's CORGI registeration? Have you talked to the dogs to check if he is registered?

 

[EliteHeat]

But I usually give free advice on this web site although recent developments might cause me to consider reviewing that.

Has something happened? what have I missed?

 

[chrishutt]

Cold to shower can push down hot feed from boiler forming Static pressure, thus boiler shut down thinking no demand. now Iam stuck. How does the water run lukewarm.

Static pressures only apply when there is no flow. With a hot tap running lower, dynamic pressures will apply. If the flow from the tap is approaching the maximum flow capacity of the combi the pressure of the hot water will be very low.

At the shower valve the cold inlet pressure will be slightly reduced from static due to the flow of cold to the combi, but the pressure on the hot side of the shower valve will be reduced far more, so a substantial pressure differential will be created, resulting in flow from cold side to hot side. This cold flow will continue along towards the hot outlet, mixing with the hot flow from the combi and resulting in warm water at the hot tap.

 

[doitall]

Cold to shower can push down hot feed from boiler forming Static pressure, thus boiler shut down thinking no demand. now Iam stuck. How does the water run lukewarm.

Static pressures only apply when there is no flow. With a hot tap running lower, dynamic pressures will apply. If the flow from the tap is approaching the maximum flow capacity of the combi the pressure of the hot water will be very low.

At the shower valve the cold inlet pressure will be slightly reduced from static due to the flow of cold to the combi, but the pressure on the hot side of the shower valve will be reduced far more, so a substantial pressure differential will be created, resulting in flow from cold side to hot side. This cold flow will continue along towards the hot outlet, mixing with the hot flow from the combi and resulting in warm water at the hot tap.

So there would be a backflow problem if another outlet was open at the same time as the shower

 

[baxpoti]

But I usually give free advice on this web site although recent developments might cause me to consider reviewing that.

Has something happened? what have I missed?

Please enlighten us

 

[Agile]

Unless the cold to the boiler is restricted, then the hot and cold would be balanced, or am I missing something

Actually John the DHW route through the boiler has considerable resistance on most models. Apart from the resistance of the plate HE there is usually a flow restrictor to limit the flow to what the boiler can heat.

The cold water supply to the taps and mixer shower is 15 mm pipe so thats quite a low resistance. Sometimes its necessary to use an isolating valve closed somewhat in the cold to get a better shower when fed from a combi.

As Chris says, most thermostatic showers seem to always let about 10-20% of cold water to pass, presumably to reduce the risk of scalding.

Tony

 

[chrishutt]

As Chris says, most thermostatic showers seem to always let about 10-20% of cold water to pass, presumably to reduce the risk of scalding.

Tony, can you confirm that the hot inlet will also always remain slightly open (when shower is set to cold)? If it shut off completely then the hot side of a twin-ended shower pump would be pumping against a dead end.

 

[Agile]

I am less sure about that aspect Chris.

With no flow perhaps the pump would not turn on? Do all pumps have two flow switches? And are they always connected in parallel or series? Monsoon is parallel so perhaps they say it does not matter if there is no flow?

I dont deal with showers as they are so problematic. I only encounter them as a component of an apparent combi boiler problem.

Tony

 

[doitall]

As Chris says, most thermostatic showers seem to always let about 10-20% of cold water to pass, presumably to reduce the risk of scalding.

Tony, can you confirm that the hot inlet will also always remain slightly open (when shower is set to cold)? If it shut off completely then the hot side of a twin-ended shower pump would be pumping against a dead end.

Of course they are closed, assuming they comply with BS & water regs.

The water must mix in atmosphere not the body, or you need check valves, also all shower valve must fail safe, e,g, no cold water = no hot

In addition the pump need zero flow to shut down via the pressure switch, any movment of water andd the pump will hunt on & off.

 

[Balenza]

That would explain why no water passed through a Mira excel I pressure tested yesterday to 6 bar. The cold port was used iniatially for the test with the other ports open with pipes connected.
I acheived the 8 bar through the shower hose outlet. Though either the cold or hot leg was shut off by adjustment of the temperature control.
I fitted 4 in total and Im just hoping I didn't damage the first one with the 6 bar as MI,s state a maximum of 5 bar.

 

[Agile]

The water must mix in atmosphere not the body, or you need check valves, also all shower valve must fail safe, e,g, no cold water = no hot

Thats the problem many need check valves but few people ever fit them!

To get the valve to fail safe perhaps a flow of CW is given to prevent the possibility of hot without cold?

Tony