Wireless doorbell

[JohnW2]

Not a chance. Firstly, the boiler has no involvement with the cold supply to the mixer - they are not related, and there is no reason why they should remain either constant or at the same ratio.

Of course they are very much related. In the absence of any gravity pressurisation, the pressure in the hot water system is a direct consequence of the cold water supply pressure.

Secondly, if the pipe taking cold water to the bathroom also serves the WC, then causing the cistern to fill will without doubt reduce the flow to the bath taps.

Sure, but that's a totally different problem from what the OP is experiencing and what we have been discussing, and is a problem with any plumbing system if some other 'load' like a loo shares a pipe run with the feed to a shower. The (common) problem which the OP is talking about will/can exist even if both cold and hot feeds to the shower are totally 'dedicated'.

Kind Regards, John

 

[JohnW2]

If a shower has it's own dedicated feed pipes from cold water storage tank and hot water cylinder, there will usually (but not always) be no problems of the type we are discussing. I presume that failure to do that is the 'bad design' to which you were referring.

It's not really a "failure", unless you consider the fundamental choice of having a combi instead of stored hot water a fundamental failure. ... It may well be that the OP's plumbing is badly designed for using with a combi shower, and that things could have been done differently (eg not have the WC share the same 15mm pipe as the bath taps. ....

You seem to have lost track of the discussion. My comments to which you are responding were about what can happen if one does not have a combi boiler.

Kind Regards, John

 

[bernardgreen]

If one does NOT have a combi boiler one can have equal pressure hot and cold supplies to the mixer / thermostatic valve so reasonably good temperature control is possible.

Even mains pressure showering is possible by using a heat exchanger to heat the hot water to the shower with hot water in the cylinder as the heat source. My heat exchanger is a second coil in the top of the hot water cylinder. Works well with constant temperatures even when other taps are turned on and off.

 

[flameport]

How d'you know that? Surely it might just be that the combi is too small.

This:

This is so when water needs to be turned off in kitchen (flow is rubbish) family can sound the bell

Ps you're right it's cold and hot water from downstairs that can mess up shower flow not just hot

If the problem was just the boiler, the consequences would be that the shower was not as hot as desired, or the flow through the shower was inadequate all the time.

Using a hot outlet elsewhere will always be a problem, as combination boilers are single outlet devices and cannot properly supply two outlets at the same time.
Using cold elsewhere should not be a problem if the cold flow rate is sufficient to supply several outlets at the same time.

 

[JohnW2]

If one does NOT have a combi boiler one can have equal pressure hot and cold supplies to the mixer / thermostatic valve so reasonably good temperature control is possible.

Sure - as has been said, provided that neither hot nor cold feeds to the mixer are shared by anything else (taps, loos) which could disturb the balance.

Even mains pressure showering is possible by using a heat exchanger to heat the hot water to the shower with hot water in the cylinder as the heat source. My heat exchanger is a second coil in the top of the hot water cylinder. ...

Is that not essentially the same as when one is using a combi-boiler, the (gas/water) heat exchanger in that case being in the boiler?

Kind Regards, John

 

[bernardgreen]

Is that not essentially the same as when one is using a combi-boiler, the (gas/water) heat exchanger in that case being in the boiler?

No because the cylinder of pre-heated hot water is a heat store that can transfer enough heat ( water to water ) to the water feeding the shower's hot supply. The cold feed splits into two feeds next to the cylinder, one feed goes direct to the shower's cold inlet and the other feed, via the heat exchanger coil in the cylinder, goes to the shower's hot supply. Any fluctuations in the mains supply pressure affect both hot and cold feeds to the shower.

This arrangement would work without a thermostatic mixer but after a long shower (or a couple of short ones ) when the cylinder has given up a lot of heat the hot supply to the shower is not as hot as it was at the start of the shower(s) and the thermostatic mixer compensates for this. The important thing is that there are no sudden changes in temperature of the hot water to the shower.

Hot water for bath and wash basins is taken from the water in the cylinder at low pressure from cold water storage tank, cold supplies to bath, wash basins and toilet cistern are also low pressure direct from the cold water tank. Hence the hot and cold pressures at the bath taps are equal and constant irrespective of what ever taps are in use.

 

[ban-all-sheds]

Not a chance. Firstly, the boiler has no involvement with the cold supply to the mixer - they are not related, and there is no reason why they should remain either constant or at the same ratio.

Of course they are very much related. In the absence of any gravity pressurisation, the pressure in the hot water system is a direct consequence of the cold water supply pressure.

Not as directly consequential as when the head of the cistern is providing the pressure for both the H&C water. Depending on what pipes run where, what is connected where, and the quality of the supply, it's quite possible for the cold pressure at a point of use to change and the hot to remain the same.

Sure, but that's a totally different problem from what the OP is experiencing and what we have been discussing, and is a problem with any plumbing system if some other 'load' like a loo shares a pipe run with the feed to a shower.

A problem which can be minimised by good design, and mitigated by a good thermostatic mixer.

The (common) problem which the OP is talking about will/can exist even if both cold and hot feeds to the shower are totally 'dedicated'.

Do we know that more cannot be done to separate the feeds?

 

[JohnW2]

Is that not essentially the same as when one is using a combi-boiler, the (gas/water) heat exchanger in that case being in the boiler?

No because the cylinder of pre-heated hot water is a heat store that can transfer enough heat ( water to water ) to the water feeding the shower's hot supply. The cold feed splits into two feeds next to the cylinder, one feed goes direct to the shower's cold inlet and the other feed, via the heat exchanger coil in the cylinder, goes to the shower's hot supply. Any fluctuations in the mains supply pressure affect both hot and cold feeds to the shower.

I must be misunderstanding something since, as I said, I would have thought you could say essentially exactly the same as that for what happens with a combi boiler - i.e. that the mains feed splits into two in the vicinity of the boiler, one branch going directly to the shower's cold inlet and the other branch going to the shower's hot inlet via the boiler's heat exchanger.

Kind Regards, John

 

[ban-all-sheds]

If a shower has it's own dedicated feed pipes from cold water storage tank and hot water cylinder, there will usually (but not always) be no problems of the type we are discussing. I presume that failure to do that is the 'bad design' to which you were referring.

It's not really a "failure", unless you consider the fundamental choice of having a combi instead of stored hot water a fundamental failure. ... It may well be that the OP's plumbing is badly designed for using with a combi shower, and that things could have been done differently (eg not have the WC share the same 15mm pipe as the bath taps. ....

You seem to have lost track of the discussion. My comments to which you are responding were about what can happen if one does not have a combi boiler.

Not at all.

Read what I quoted again.

You regard the shower not having it's own dedicated feed pipes from cold water storage tank and hot water cylinder as a bad, failed, design.

My point is that if you make the fundamental decision to remove the cold water storage tank and hot water cylinder and install a combi, you obviously cannot have dedicated feed pipes from cold water storage tank and hot water cylinder, so do you regard the fundamental choice of having a combi instead of stored hot water a fundamental design failure?

 

[JohnW2]

Not as directly consequential as when the head of the cistern is providing the pressure for both the H&C water.

Maybe not, but that doesn't make it any more correct to say that the pressures are "unrelated".

Depending on what pipes run where, what is connected where, and the quality of the supply, it's quite possible for the cold pressure at a point of use to change and the hot to remain the same.

Indeed (or vice versa) - but if the (hot and/or cold) supplies to that point of use are not in 'dedicated' pipework, then exactly the same is true of a gravity fed system. Indeed, that's the (potentially dangerous) problem that some people have with gravity-fed showers without thermostatic mixers when the pipework is not properly designed.

The (common) problem which the OP is talking about will/can exist even if both cold and hot feeds to the shower are totally 'dedicated'.

Do we know that more cannot be done to separate the feeds?

As I said, the problem that the OP seems to be describing is one that can occur with a combi boiler even if the feeds to the shower are totally separate/dedicated.

Kind Regards, John

 

[bernardgreen]

one branch going directly to the shower's cold inlet and the other branch going to the shower's hot inlet via the boiler's heat exchanger.

but the hot water from the combi heat exchanger also feeds other taps. Opening any of these taps will reduce the flow of heat to the shower. It might be less water at the same temperature or the same amount of water at a lower temperature. In my system the heat exchanger feeds only the shower and nothing else.

 

[JohnW2]

Not at all. ... Read what I quoted again. ... You regard the shower not having it's own dedicated feed pipes from cold water storage tank and hot water cylinder as a bad, failed, design.

I do. I think anyone would - it's a potentially dangerous design if it is the cold feed to the shower which is not 'dedicated'.

My point is that if you make the fundamental decision to remove the cold water storage tank and hot water cylinder and install a combi, you obviously cannot have dedicated feed pipes from cold water storage tank and hot water cylinder, so do you regard the fundamental choice of having a combi instead of stored hot water a fundamental design failure?

No. Whilst, with a combi-boiler, one obviously cannot have dedicated feeds from (non-existent) cold water tank and hot water cylinder, but one can have dedicated feeds from the equivalents - the cold mains supply and the outlet of the heat exchanger - which is correct design. IF, with a combi, those feeds are not dedicated (e.g. if the loo shares the cold cold supply pipe to the shower), then one has exactly the same potential problems (due to shared pipework) that one would have with a gravity-fed system (in addition to combi-specific issues).

Kind Regards, John

 

[JohnW2]

one branch going directly to the shower's cold inlet and the other branch going to the shower's hot inlet via the boiler's heat exchanger.

but the hot water from the combi heat exchanger also feeds other taps. Opening any of these taps will reduce the flow of heat to the shower. It might be less water at the same temperature or the same amount of water at a lower temperature. In my system the heat exchanger feeds only the shower and nothing else.

If the heating capacity of the boiler and/or the incoming water pressure is inadequate to service both 'loads' satisfactorily, then that would indeed be true.

Kind Regards, John

 

[bernardgreen]

If the heating capacity of the boiler and/or the incoming water pressure is inadequate to service both 'loads' satisfactorily, then that would indeed be true.

This is the problem with combi boilers. To supply a shower with a reasonable quantity of hot water they have to be able to put around 20 kW of heat into the water to the shower. Yet to heat the house the same boiler has to throttle down ( modulate ) to a few kW at most since the radiators have a limit to how much heat they can disipate into the house.. So combi boilers have to work in two different modes. Yes they can be designed to be efficient at both levels but the complexity of control and construction makes them less reliable.

 

[ban-all-sheds]

Anyway - talking of design failures...

Bell/button is to go in shower room upstairs

Sounder/speaker is to go in kitchen so people don't turn tap on if people are showering cos flow gets too low.....

It would require gaining access to the hot supply to the bathroom somewhere, to install a flow switch, but what about using such a switch to control a motorised valve, so that when the shower is in use there simply is no hot water supply to the kitchen taps? Unless you have a house full of teenagers it shouldn't be much of an inconvenience.