Shower looses a lot of pressure when tap is used?

If there's no storage tank or hot cylinder then that suggests you have a combi boiler which heats the hot water instantly?

In which case - sorry - but only one hot source can be fully used at a time.

The only solutions are costly - fitting an unvented hot storage tank, for example.

Most peeps chust cope. I do...

 

Or a more capable combi boiler, but equally as expensive as above suggestion.

 

Only DIY here, but surely the problem is just as likely to be poor flow rather than weak boiler? Perhaps 15mm main, restrictive isolators etc? High pressure doesn't mean high flow!

Why not run a test? Open first tap on the system after the main stop and see what flow rate you can get - time filling a bucket. Post here will give an idea what you have. Also post your boiler model. Typically a mid-range combi will give you about 12-14l/min into a shower. Big boilers up to double that or more. A shower under 9-10l/min is getting weak.

In our house we only have 1 shower and a 13l/min boiler. One additional tap drops pressure a bit, tap + toilet cistern filling drops shower to just about useable. Add in a washing machine at the same time as a tap and a cistern and the shower near enough stops. We are on a 15mm main.

 

Quick look on line. That boiler is rated 10.7l/min at 35 degrees rise for the smaller 24kW version and 14l/min off the 32kW version. That's pretty low. Don't think you are going to run 2 showers off either with any great success. If you have the 32kW, you might just about get 2 weak showers at the same time with nothing else open.

As CraigMcK said, new bigger boiler may be the solution, but if you go that route, make sure the supply and pipework is suitable for the flow you want. Still worth checking the available flow on the first tap - the one nearest the stop cock - often kitchen - to see what you can get. I get about 22l/min from my kitchen tap. If you get the same then it would be worth thinking about a bigger boiler rated 20-25l/min. That would drive 2 decent showers at the same time, but that's a big combi. Alternative is system boilers with storage.

Not a plumber, so I'll let others recommend particular solutions.

 

OK. be interesting to see what you come back with. But you're going to need someone else to help with the practical solution. I can only offer the theory

 

You didn't say it was a new house. I would be inclined to point out that the boiler has to be fit for the purpose of supplying two showers as that is what the house has. If they don't play ball ask them for written confirmation that the small boiler is suitable. They may say it is but be reluctant to put it in writing. Depends how hard you want to push but if they really have supplied a boiler with a 10.7l min flow rate for a house with two showers it's very shoddy and cheapskate.

 

Caution...

Most combi boilers cannot provide a proper hot flow to more than one tap at a time. This is normal, and is recognised as one of the main drawbacks of having a combi.

You need to bear in mind that ALL the water supply in your house - hot and cold - is ultimately fed by a single 15mm supply once inside your house. If you check the flow rate at the cold sink tap, you'll likely find your house is supplied at around 20 lpm at the most.

The average combi can supply around 12lpm hot water, but if you open a cold tap at the same time, you'll see that this can only hugely affect that rate.

15mm pipe is fine as it's delivered at a highish pressure, but once you open a tap that pressure will drop quickly.

You will only get a combi to supply two showers at the same time if (a) your mains flow is comfortably in excess of 20lpm and (b) the boiler is rated at - ooh - well over 40kW.

Basically, you'd be nuts to even try.

A much better option is a pressurised unvented cylinder. These will store heated water under mains pressure, with a type of expansion vessel to keep the pressure going as the mains replenishes it.

What you are experiencing is normal - it is not down to an installation issue.

(That's not to say the installation ain't pants, tho' - I don't know...)

 

Ask the GasSafe for his thoughts when you next see him.

It is possible to help the situation a bit by keeping the mains pipework in 22mm right through the mains stopcock inside the house, and only splitting it into 2 of 15mm as it approaches the boiler. One feeds the boiler and t'other the cold taps.

That might help, but it still won't cure. There is no cure . They chust cannae do it, cap'n.

 

Boy you have had a merry time.
I butted in coz just weeks ago I had my heating changed, from an Elson tank that's a big
vented copper tank system . Its worked for 35 years with a Potterton Flamingo 50.
It was rated 19kw. But was heating a tank of hot water not instantaneous like a Combi.

It was all changed and radiators removed and all cleaned and an Intergas 36kw-30kw fitted.
Its working fine and the CH is of course much better that the old system.
But it only took days to understand that one needs at least the 36kw domestic for a good
bath flow. I have left the electric shower as it is now.
But opening a cold tap does reduce flow as you would expect.
So that's my 2 pence worth.
I was a plumber for 15 years. By golly gosh plumbing has changed in the 55years since I left it.
Keep at the NRBC its needed.

Johnny M

 

Jeepers, how did you measure that? You didn't 'time' for 2.2 seconds, did you?!

(I usually run the tap for, say, 10 seconds, and then times the litres by 6)


Anyhoo, assuming your measured flow is accurate at ~14, I have to say that it isn't great. It's adequate, but not amazing.

It still won't provide two showers simultaneously.

Nothing will - other than a 'stored' system either pressurised or vented.