Hot Water Options - Likely Fall in Mains Pressure

[NealHarman]

Hi, currently renovating/extending, and will need a new HW/heating system. The heating is not a problem, but HW may be. Currently, we have 4bar mains pressure and a basic flow rate test (tap+bucket) gives us 27l/min.

I was intending to get an unvented cylinder put in, as we will have two showers and it'll at least partly defeat the point if we can only use one. One of these will be the attic (about 8-9m up).

Now... I've found out from a friend that Welsh Water are likely to reduce our mains pressure to about 2.5bar (as part of a leak reduction strategy). Since he would be partly responsible for implementing this, I suspect the information is accurate - I'm trying to confirm it but as you can imagine this is not easy. I guess further reductions can't be ruled out - they're only obliged to provide 1bar, apparently.

So... is 2.5bar going to be enough? I'm sure 1bar isn't. I don't want to spend a lot of money on a solution that might become useless in the near future. The only really 'safe' option I can think of is a traditional vented cylinder, but there's absolutely no room for a header tank.

I did put in some 10mm T+E 'just in case' so one electric shower is a possibility.

Thanks in advance,
Neal

 

[ArtfulBodger]

All of the unvented cylinders that I've put in have had pressure reducing valves supplied set to 3 bar, so I wouldn't worry too much about being reduced to 2.5.

So what if you can't run 2 showers similtaneously? you'd soon run out of hot water anyway!

 

[Nige F]

Consider a pressurising pump set. Or u/v. cyl with boost vessel...cost half as much again but.............

 

[NealHarman]

All of the unvented cylinders that I've put in have had pressure reducing valves supplied set to 3 bar, so I wouldn't worry too much about being reduced to 2.5.

Thanks - that's reassuring!

So what if you can't run 2 showers similtaneously? you'd soon run out of hot water anyway!

Then I might as well only have one OK, I know that's arguable, but there are five of us and some friends find it irritating that they can only use one shower simultaneously.

Consider a pressurising pump set. Or u/v. cyl with boost vessel...cost half as much again but.............

Thought you could only put pumps on vented? What's this boost vessel though - never heard of one before and sounds interesting! (At least something I should know about even if it turns out we can't afford one )

Thanks,
Neal

 

[Agile]

I think that you are approaching this from the wrong direction!

Pressure reductions have been talked about for a long time and may not be implemented for some time.

Remember the flow rate you have measured is the open pipe rate, you should really measure the dynamic flow with say 0.5 Bar remaining in your supply pipework. That pressure is required to operate showers etc.

Where you should be looking is at the shower heads, with a view to reducing their flow rate to what your system can handle. It will also mean the stored hot water will last longer!

Probably not much need to do it now but if the pressure is reduced and the combined flow rate is too high then you could replace the head or just block up a ring or two of holes to reduce the flow to what the system can provide when both showers are in use.

Tony Glazier

 

[ChrisR]

An electric shower could well turn out to be a Good Thing. They give you a backup, only draw about 4 litres/min and you can go on showering forever.

 

[Agile]

Pretty measley shower though!

In any case if he is having an unvented then he has a backup from the electric heating element if the boiler fails.

That is of course IF its been connected! Went to one last month where the boiler had failed and the immersion had never been connected! I had to sabotage an extension lead to connect it until I could get the obscure part for the Alpha boiler.

Tony

 

[ChrisR]

BIt of a long wait between showers though, tank may only give a 15 min shower!

 

[NealHarman]

I think that you are approaching this from the wrong direction!

Pressure reductions have been talked about for a long time and may not be implemented for some time.

Remember the flow rate you have measured is the open pipe rate, you should really measure the dynamic flow with say 0.5 Bar remaining in your supply pipework. That pressure is required to operate showers etc.

Yeah I knew it was a bit rough-and-ready, but given all I had was a cheapie static pressure gauge, that's all I could do.

Where you should be looking is at the shower heads, with a view to reducing their flow rate to what your system can handle. It will also mean the stored hot water will last longer!

Probably not much need to do it now but if the pressure is reduced and the combined flow rate is too high then you could replace the head or just block up a ring or two of holes to reduce the flow to what the system can provide when both showers are in use.

Tony Glazier

OK - so given the right choice of shower/head, then something workable could be arranged? And I should stop being paranoid?

An electric shower could well turn out to be a Good Thing. They give you a backup, only draw about 4 litres/min and you can go on showering forever.

Pretty measley shower though!

Kids can have that one Seriously, we're not too bothered about super-powerful showers - if we want to flense flesh from bones we'll get a pressure washer I was just doing the arithmetic - 2.5 bar minus 0.9 bar to get to the top shower head, minus pipe losses (even though they've got the smoothest bends I could arrange) - and wondered if it would work at all. Also, even if we went with - say - a big combi running one and an electric, both would still be ultimately supplied by the same incoming supply pressure, no?

BIt of a long wait between showers though, tank may only give a 15 min shower!

...which brings me to something I'd forgotten - what about a heat bank/thermal store? No limit to hot water and I could do more of the work myself - what's the catch?

Any idea of relative running costs of these things (unvented, electric, combi, etc.?)

Thanks for all the help so far!
Neal

 

[Agile]

"""...which brings me to something I'd forgotten - what about a heat bank/thermal store? No limit to hot water and I could do more of the work myself - what's the catch? """

Er... physics ?

"no limit to hot water" well yes if your thermal store is infinitely sized.

Whether you have an unvented or thermal store your storage capacity is the heat stored by that volume of water.

Its true that most thermal stores are designed to operate at an increased temperature compared with an unvented so they store a little more heat , about 35%. But the higher storage temperature means the boiler will not be condensing much of the time when heating water so its less efficient.

You can easily measure the dynamic flow rate using the parameters I listed above.

Tony Glazier

 

[NealHarman]

Er... physics ?

"no limit to hot water" well yes if your thermal store is infinitely sized.

Whether you have an unvented or thermal store your storage capacity is the heat stored by that volume of water.

Tony Glazier

Drawing hot water takes heat out - I can't believe that wasn't obvious

You can easily measure the dynamic flow rate using the parameters I listed above.

Just worked out how to do that one too..

Thanks for all the help,
Neal