Bath hot feed running cold unless turned right down

[djdd]

Morning guys, first post!

I've done a fair bit of noddy plumbing, but nothing too taxing; just refitting a bathroom and changing taps here and there.

Basically a friend of mine (which doesn't mean me! I have an unvented cylinder system ) has just had a combi boiler installed and they're not getting any hot water to their bath unless they turn the hot tap right down so it's trickling out.

I took a look at the pipework and it's 22mm from the bath hot water tap to the airing cupboard where the hot water tank used to be. This looks like it's now fed with 15mm pipe from the new combi.

Am I right in thinking that replacing this section with 15mm pipe and modern taps will make a big improvement to both flow rate and temperature? Trickling the water out of the current taps will reach 50oC+, but it'll take about an hour to fill!

If I am right, why is this? If I'm wrong, how can I help them fix the problem?

Just to note, in case it's useful, the rest of the taps in the house get nice and hot. It's just the bath hot water that runs cold.

 

[Armo74]

Combi's are really supposed to run off 15mm pipe,
The reason you have to turn the tap down is to enable the boiler to keep up with the flow rate demand from the 22mm pipe.
Changing the pipework to 15mm should solve the problem.

 

[ALEC1]

how many KWs is the combi.. a 24kw combi will raise the temperature of the incoming main by 35c at about 9l/m

bathing temperature is about 37-42c, so if the main is 6c you will only get 41c at a 9l/m...a trickle for bath filling...

 

[ALEC1]

replacing pipes won't do anything much to the hot water temperature..

 

[djdd]

Changing the pipework to 15mm should solve the problem.

replacing pipes won't do anything much to the hot water temperature..

So who's right?

From what I remember from yesterday, I think the combi is 24kw. Also, the taps are about 20 years old if that matters.

I did read somewhere someone saying that fitting flow restrictors to the taps would solve the problem, but surely it's better to get some taps specific for combi boilers and feed them with the correct pipework in the first place?

 

[ALEC1]

smaller pipes do lower flow rates, but unless the pipe run is very long it will be imperceptible.

flow regulators will do the trick and are the mor scientific way of restricting flow..

 

[gigz]

It doesn't matter what size the pipework is, if the cold is 4 degrees the hot will only be 35 degrees more at say 10 litres. So reducing the pipe size is the same as closing the tap down. So even with 15mm pipework you will only get the same result (apart from heat loss from a greater surface area from 22 mm pipe) which will be negligible. Two things to check :- make sure the stop tap is open enough (preferably measure the flow/pressure) and make sure the temperature dial is set high for hot water (if it has one). It will get better as it gets warmer (at the mo 4+35=39 degrees. In the summer 20+35=55 degrees. At the same flow rate. ) Then they'll have to add a bit cold to the bath water

 

[gigz]

Blimey alec1 post wasn't there when i started replying, ten mins to type that out Have to speed up methinks

 

[ALEC1]

combis are a disaster for anything other than a one bedroom flat or studio...