Shower just gives boiling water

[Mmf]

So I have a mixertap/shower.

The tap works great, but when I change it to come out of the shower it just comes out boiling hot, regardless of how much I fiddle about with the hot/cold water taps.

The strange thing is that if I remove the shower head the water that comes out from the shower hose is perfect. So the problem is clearly with the shower head.

Any suggestions as to why this may be??

 

[rpm]

Try another shower head then.

 

[Mmf]

Clearly something needs to be changed. When I moved in, there was a different shower head on (basic white plastic one) and the water was boiling hot even from then. The shower head I have on it now I changed because even though I couldn't use the shower, I just didn't like the look of the cheap plastic one.

So that's two different shower heads that have caused the exact same problem. I don't want to keep spending money on different shower heads that are looking like will keep causing the same issue.

I was more wondering what the cause is (maybe something like the pressure) so that I can get one that will specifically work under the circumstances.

 

[rpm]

Didn`t see the shower head being the problem but you seemed too in your first post.

Thinking the balance between hot & cold supply is unbalanced however normally the cold wins.
Suggest you tell the guys what sort of system you have to start with.

 

[Mmf]

 

[rpm]

Suggest you tell the guys what sort of system you have to start with.

Sorry, I meant system in the house for heating and hot water.
Also I don`t do mixer taps but maybe the problem is within that mixer, sure others can advise better later on.

 

[Mmf]

Sorry, I meant system in the house for heating and hot water.

Sorry, I have a Logic Combi 24 boiler

 

[Last Plumber]

Sorry, I have a Logic Combi 24 boiler

I would expect an imbalance in Hot and Cold to be honest but as RPM said, Cold normally wins if it's mains cold and tank fed hot.

In your case with a combi, Both hot and cold pressures should be the same or at least too similar to cause a difference.

You need a plumber to come and check pressures and flow rates in my opinion.

 

[rpm]

The part I don`t understand is it works fine if the shower head is removed? Mind you that head looks like it has big holes for the water jets to come out of.

 

[ShaunCorbs]

TBH as you have a combi you should have a thermostatic shower / bath mixer fitted

 

[Last Plumber]

TBH as you have a combi you should have a thermostatic shower / bath mixer fitted

Very true indeed Shaun.
Very dangerous set up in all honesty.

 

[Last Plumber]

Sorry, I have a Logic Combi 24 boiler

As mentioned, bit dodgy that. No thermostat, you could easily get burnt if the temp changes suddenly. Lot less likely with a Thermostatic. If you do change it for one, don't go for cheap.

 

[Mmf]

I've had a plumber out before, who I don't think was that bothered with helping me. He told me to just fiddle about and try turning down the temperature on the boiler whenever I have a shower. It can be turned down to the lowest and still not be ok.

I'll try and get someone out again to check the pressure and flow rates.

There's also a fair bit of limescale on and around the tap. I live in London, wondering if the hard water has affected anything at all.

So my mixer shower tap isn't a thermostatic one?

I do have to add, the main reason I think it may be pressure related, is that while I have the shower head off, the water comes out perfectly from the hose. If i try putting the shower head back on (whilst the water is still on and running through the hose) it's as though any resistance against the water flow coming from the hose causes the water coming out to overheat. Same thing happens if i place my thumb partly over the end of the hose.

 

[Last Plumber]

So my mixer shower tap isn't a thermostatic one?

No it isn't.

 

[Chuck]

Any suggestions as to why this may be??

It's consistent with having the cold water supply to the bath coming from a storage tank in the roof and the hot water coming from a combi. If so, whoever installed the combi cut corners as they should have made sure that the supply pressures to the mixer tap were balanced.

Do you have a water tank in the roof?

 

[rpm]

Find a Plumber, Tiler or Electrician | Plumbing Forum | Gas Engineers Forum | Plumbing Advice - DIY and Professional
Few guys here in and around London.

 

[Last Plumber]

I do have to add, the main reason I think it may be pressure related, is that while I have the shower head off, the water comes out perfectly from the hose. If i try putting the shower head back on (whilst the water is still on and running through the hose) it's as though any resistance against the water flow coming from the hose causes the water coming out to overheat. Same thing happens if i place my thumb partly over the end of the hose.

I think you're on the right track. Pressure related.
If there is a slight imbalance somehow on the system so Hot has a bit less restriction than Cold it could emphasise it when you restrict the flow. I'm Guessing because I haven't tested it.

A good Plumber will be able to test things for you and tell you the cause. It needs someone who is on site and can look at the Hot and Cold water installation. If you have good flow on hot and Cold, the answer could be as simple as replacing it for the right type of Mixer.

 

[ShaunCorbs]

It's consistent with having the cold water supply to the bath coming from a storage tank in the roof and the hot water coming from a combi. If so, whoever installed the combi cut corners as they should have made sure that the supply pressures to the mixer tap were balanced.

Do you have a water tank in the roof?

Simple way to test

Open just the hot tap into the bath

Then just open the cold only pressure look the same ?

 

[Chuck]

Simple way to test

Open just the hot tap into the bath

Then just open the cold only pressure look the same ?

We know that when the pressure at the tap is near zero (no shower head) the rates are about the same because the temperature is warm not hot. With the shower head on the pressure at the mixer will be higher and this will suppress the flow of cold if my guess is correct.

 

[ShaunCorbs]

We know that when the pressure at the tap is near zero (no shower head) the rates are about the same because the temperature is warm not hot. With the shower head on the pressure at the mixer will be higher and this will suppress the flow of cold if my guess is correct.

It will give you an idea if it's cold tank fed tho

I don't think it is tho

 

[Mmf]

It's consistent with having the cold water supply to the bath coming from a storage tank in the roof and the hot water coming from a combi. If so, whoever installed the combi cut corners as they should have made sure that the supply pressures to the mixer tap were balanced.

Do you have a water tank in the roof?

Not too sure. I live in 80+ year old flats on top floor (4th) it's quite a small bathroom with an additional wall/section built behind where the bath and taps are, wondering if there is maybe a tank or something inside there? It's council maintainedand I'm constantly finding problems within my flat where they have cut corners.

If you have good flow on hot and Cold, the answer could be as simple as replacing it for the right type of Mixer.

I'm hoping to get the taps changed to see if that makes a difference.

Then just open the cold only pressure look the same ?

There is definitely lower pressure on the hot.

 

[ShaunCorbs]

That's great mains system so thermostatic mixer should sort your problem

 

[rpm]

Back to the council then
"This mixer tap is unsuitable and dangerous I could get scolded!"