Low output pressure from combi boiler to thermostatic shower

Joined
22 Feb 2012
Messages
3
Reaction score
0
Location
London
Country
United Kingdom
I have recently had a new thermostatic shower/bath mixer (Grohe rapido T), fitted, which is fed by a Vaillant ecomax boiler (c. 6 or 7 years old).

The problem is that the pressure is terrible and the mixer cannot sustain hot water for any length of time.

Now i understand that combi boilers cannot always keep up with hot water demand and so temperature can waiver, but it wasn't this bad under the old configuration with manual mixer.

My builder assures me that the plumbing is correct and is suggesting that the boiler might have a limescale problem.

But I don't understand why this would suddenly have got so much worse and so am more likely to blame the thermostatic mixer.

As the new mixer only has one flow rate (on/off), could the problem be linked to the above issue re boiler not being able to keep up with demand?

Should i remove the thermostatic mixer and replace with a manual where I can adjust down the flow rate?

Why would the hot water pressure be so much better in the kitchen sink?
 
Sponsored Links
I have recently had a new thermostatic shower/bath mixer (Grohe rapido T), fitted, which is fed by a Vaillant ecomax boiler (c. 6 or 7 years old).

The problem is that the pressure is terrible and the mixer cannot sustain hot water for any length of time.

Now i understand that combi boilers cannot always keep up with hot water demand and so temperature can waiver, but it wasn't this bad under the old configuration with manual mixer.

My builder assures me that the plumbing is correct and is suggesting that the boiler might have a limescale problem.

But I don't understand why this would suddenly have got so much worse and so am more likely to blame the thermostatic mixer.

As the new mixer only has one flow rate (on/off), could the problem be linked to the above issue re boiler not being able to keep up with demand?

Should i remove the thermostatic mixer and replace with a manual where I can adjust down the flow rate?

Why would the hot water pressure be so much better in the kitchen sink?

If you can turn on the basin tap in the bathroom and get a good flow
of hot with the hot tap fully open there is nothing wrong with the boiler.
More than likely the plumber has plumbed the thermostatic show in back
to front. Hot to cold and cold to hot. Or there is a problem in the thermostatic valve of the shower.
You should be able to lookup the water requirements of the shower you have and measure that you can get that at the basin taps.
 
The basin flows a little better but is that linked to smaller pipe? The basin will also suffer from loss of hot, but not as readily as shower, again I assume linked to size of pipe.
 
The basin flows a little better but is that linked to smaller pipe? The basin will also suffer from loss of hot, but not as readily as shower, again I assume linked to size of pipe.

If you aren't getting it to the basin there is a problem with pressure
or piping or the boiler.

If you have a good cold main flow and pressure at the bath tap
then the problem would appear to the boilers delivery of hot water.
 
Sponsored Links
Try a spray head with smaller holes in it, for fewer litres per minute.
 
It had occurred that something with smaller holes might help - currently have a big drencher rose installed.

But the bath suffers the same problem as the shower (they're both connected to the Grohe Rapido T).

I was testing the kitchen tap and the bathroom basin last night and they do both have better hot flow than the shower/bath mixer. Though temp tends to waiver through all outlets.

Slightly better pressure for cold in bath/shower mixer.

My builder has assured me that the mixer is connected correctly (hot to hot / cold to cold).

My theory is that there is a fundamental pressure problem, potentially linked to the boiler being scaled up. The reason that the kitchen tap and bathroom basin work better than the bath/shower mixer is due to large pipes requiring greater demand from the boiler that the boiler can't match. But I also suspect that the thermostat in the bath/shower mixer is adding to the problem, due to it's inability to work as designed due to varying temp and pressure.

does that all sound plausible?
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Back
Top