Hot bath water when shower's on cold...

[co]

Hello All,

I have a mixer bar type shower connected to the same hot and cold water pipes as the bath, fed from a combination boiler. The problem is this:

The hot water coming from the bath tap is only ever luke warm. The actual pipe before the joint to the shower is too hot to touch, but then after the shower feed the pipe goes warm. The only way to get very hot water in the bath is to run the hot tap but also to have the shower on cold going straight down the sink! Wasting a lot of water, but very annoying.

Could there be cold water being sucked into the hot pipe from the shower supply?
Missing a non return valve maybe?

I know nothing about plumbing, but need to be an expert by the weekend!!

Cheers in advance,

co

 

[JohnD]

What sort of bath tap have you got?

 

[co]

Separate taps for cold and hot water. Traditional style.

 

[JohnD]

As you have a combi boiler...

Do you get hotter water if you turn the bath-tap half on (i.e. to get a slower flow)?

Also, does the shower mixer have service valves/little stop cocks like this

so you can isolate it?

Shower mixers do, sometimes, let water leak between thge hot and cold sides. You can detect this by feeling the hot and cold pipes that ssupply the shower when it is turned off (they should be about room teperature, and neither should change temperature with teh shower turned off - and the cold one should not get hot, ever, nor the hot one get cold.

 

[ChrisR]

All that's happening is that the increased flow out of the bath tap is "diluting" the available energy from the boiler. More water, means cooler water, perfectly correct for a combi. As the water mains cools with the season, you'll notice it more.
By running cold water into the sink you're slowing down the HW into the bath a bit, so it comes out hotter.

So the solution is to not open the bath tap as far!

 

[co]

Thanks for the replies both.

Yes I do get hotter water if the flow is reduced. But why is the hot water pipe scolding, and then after a 2cm joint where the connection to the shower is, the pipe is warm?

Yes there are service valves nearby.

The pipes to the shower do not seem to change temperature when the bath tap is on.

Hope this makes sense.

Cheers,

co

 

[JohnD]

...The pipes to the shower do not seem to change temperature when the bath tap is on...

Then I don't think it's passing at the shower mixer. If it was, you would feel the cold water flowing down the hot pipe.

It's just one of the effects of having a combi that high flow = low temperature

 

[co]

ok... but then what could be making the very hot water turn to warm water?

The hot water is getting right there under the bath, but somehow it's being cooled...

The boiler is definately getting scalding water to under the bath, but then it's being cooled...

You guys must be thinking I've completely lost it... but honestly I haven't

co

 

[JohnD]

Sorry, don't know. I would need to feel the pipes to understand what's going on.

Basically, I think the poor temperature is caused by high flow on a combi , and the colder winter incoming water.

It is possible for cold water to come from a mixer if it has an internal leak. This might be the shower mixer, or a sink, basin or bidet, but you should be able to detect it by feeling the cold water travelling down a pipe.

 

[co]

Thanks for your help John.

I'll have a feel again later on.

Cheers,

co

 

[TicklyT]

Sounds like a check valve in the shower feed isn't working/isn't fitted on the hot feed to the shower. Something like this.....
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=15139&ts=58840 , or maybe there's an integral one within the mixer that's not working as it should.

....So when the hot tap is opened the pressure drop through the boiler circuit is enough to allow cold water from the shower mixer cold feed through the mixer, the wrong way down the hot feed and out to mix with the hot water.