Combi boiler and mixer shower - flow rates?

Jimbo Mahoney · 27 Apr 2019 at 13:35

Hi all,

Currently, we have a combi boiler and electric shower. My brother in law lives on the same street and had made a comment about the water pressure not being great (we're quite high up on a hill) and therefore maybe causing issues for a mixer shower fed from the combi.

The electric shower is pathetic - 8.5 kW giving 3 L/min measured output.

My concern is the pressure / flow rates in our house in general and using the combi boiler to supply a mixer shower.

I've measured the flow rate in the kitchen from the (small) hot tap (fed from the combi boiler) and it gives ~6 L/min. With the cold tap on too, it's ~ 10L/min but obviously not hot enough for a shower.

In the bathroom (from the bath's large hot tap fed from the combi boiler downstairs) gives ~8 L/min.

If I estimate that the mixer shower would therefore give about 6L/min, this is double what our electric shower gives, but is this enough for the mixer to function properly?

(Hoping I can answer this question online, but realise it may need a plumber to investigate).

bremen1874 · 27 Apr 2019 at 13:47

Would the wiring support a 10.5kW shower? My 10.5kW isn't fantastic, but it manages 7 l/min at a decent temperature.

Jimbo Mahoney · 27 Apr 2019 at 13:47

Oh, and we have a Worcester 30si boiler and some more research *suggests* we'll be fine as long as we get a specific thermostatic mixer with a "combi valve" to minimise the "fighting" between the shower and the boiler?

Jimbo Mahoney · 27 Apr 2019 at 13:51

bremen1874 said:
Would the wiring support a 10.5kW shower? My 10.5kW isn't fantastic, but it manages 7 l/min at a decent temperature.

Not sure. My *guess* would be no. MCB has a 40A fuse fitted to that circuit?

Had thought about a higher rated shower, but would prefer to run it on gas (cheaper).

bremen1874 · 27 Apr 2019 at 14:00

Mines on 10mm cable with a 40A MCB. In theory, the shower could pull a 43A but it's never been a problem.

What's the flow on the shower on full cold as you'd never get more than that?

One thing that did make a difference to my shower was buying a shower hose with a larger internal diameter.

Usel · 27 Apr 2019 at 15:25

I believe theres a certain water pump you're allowed to use on the mains water to boost the pressure before the boiler.
This is probably what you need to look into.

Jimbo Mahoney · 28 Apr 2019 at 20:14

Usel said:
I believe theres a certain water pump you're allowed to use on the mains water to boost the pressure before the boiler.
This is probably what you need to look into.

Thanks!

It seems there is. Salamander comes up on Google.

fezster · 28 Apr 2019 at 21:42

You are allowed to pump up to 12L/min from the mains, which the Salamander and other pumps will do. It might be worth asking your water company to see if they can find any reason for such poor flow - do all your neighbours have the same? Might be some blockage in your mains or internal pipework. Just a thought.

Jimbo Mahoney · 5 Jul 2021 at 18:08

Resurrecting this thread as we're about to go ahead with the bathroom upgrade.

Here are my current thoughts:

1) Salamander HomeBoost to increase the flow rate / pressure to the whole home to 12 L/min (currently estimating it's 8 - 10 L/min).
2) Thermostatic mixer shower with exposed thermostatic valve (would prefer concealed, but gather that replacing the valve will be a tiles off job).
3) Shower with both rain and standard style heads. (Was going to go with rain-only, but fear that it might be a disappointing trickle if we're running off a combi boiler!)

I figure that this will give us the best chance of having a decent shower with easy maintenance?

Advice appreciated!