Hot tap on bath not hot, unless just slightly on...... Combination boiler

[gregzapper]

Hi, advice please..

Combination boiler.
Hot water to the shower is very poor.
Hot water to the bath is hot if just a very slow trickle is made
Kitchen sink is red hot so boiler is just fine..

Everything used to be fine so the pipework must be ok..

How is cold water mixing with the hot supply...

Why is the hot water cold by the time it gets to the bath or the shower.

They are normal bath taps..

The shower is separate and probably thermostatic..

Can the shower be causing this?

Might there be a one way valve that is broken?

Combination boiler so hot and cold should be at the same pressure...

I'm going under the bath tomorrow to check the pipes, to see which is hot or not etc..

Anyone give me any possibilities before I turn up?

Any advise welcome...

Thanks in advance

 

[Puddle]

What make/model of combi boiler is it ?

 

[armourer]

At a guess when the combi was installed the flow restrictor was left out.

Temperature fine when run through 15mm pipework and basin taps but flow rate is too high when run through 22mm to bath taps?

Probably exaggerated as the pipe run to the bath is further than to the kitchen sink.

Check the mi's for the boiler and take a flow rate reading at the bath.

 

[gregzapper]

Hi, I have reduced the flow rate on the cold using service valve to the bath shower and basin. It works! Hot water to the bath and shower hot...

Now I would like to fix it properly. Ball 'o' fix not good..

I would like to use a one way valve on the hot but were talking mains pressure, does such a valve exist?
If not then pressure reducing valve on the cold, even though in theory the hot and cold are the same pressure...

Any ideas people..

All help appreciated...

Thanks in advance...

 

[jtsplumbing]

Remember combi will only rise hot water temp about 35 degrees above cold water temp, what temps are hot and cold ? and what size combi have you got ?

 

[stevetheplumber]

Remember combi will only rise hot water temp about 35 degrees above cold water temp, what temps are hot and cold ? and what size combi have you got ?

thats not true 35degrees is the industry standard temp rise at a given flow rate so we can compare flow rates, slow the flow down and you can see a greater rise then 35
boiler hot water output is purely dependent on size all 24kws will give more or less the same output as will all 30kw if measured at the same temp rise

 

[mylesy]

flow rate and pressure are differant as im sure your aware. i dont see a problem with the ball valve throtling down the flow rate. pressure will be the same. (ish, due to the venturi principal) its that or pipe it all up in 15mm, any really long runs you could lag. sounds like you already cured it. and depends on boiler on the delivery. say it raises 10ltrs by 35. check cold in (say 6 in this weather) put a flow cup under bath tap and turn it until you get 10 ltrs (it may flow more with tap open fully but put it to 10ltrs or whatever the manual says) and see what temp you get, (in this senario around 41) it prob be higher than that if all ok. you can get poor hot water if the boiler is under gassed, but that a differant kettle of fish. hope this helps pal.

 

[mylesy]

24kw (9-10 ltrs/min) 30KW (11-12 ltrs/min) 37kw(12-14 ltrs/min) these are average, depends on make though.

 

[AWheating]

At a guess when the combi was installed the flow restrictor was left out.

Temperature fine when run through 15mm pipework and basin taps but flow rate is too high when run through 22mm to bath taps?

Probably exaggerated as the pipe run to the bath is further than to the kitchen sink.

Check the mi's for the boiler and take a flow rate reading at the bath.

with modern boilers the flow rates are not determined by the size of the pipework to the taps normally so a 22mm feed to the bath being changed to 15 is unlikely to effect the flow rate. It will just save you water as the flow rate is determined by the appliance and its supply.Unless the flow rate through the boiler is stupidly high of course or the boiler has no ablity to adjust the flow to a recomended amount. This is why it would be helpful to know what boiler it is and the flow rate to each tap.

 

[AWheating]

One example is the potterton puma which had a allen screw on the cold inlet to adjust flow into the boiler, but it was a poor idea. with todays modern taps its more likely to be the tap causing a difference in flow rates between say the kitchen sink and the bath.

 

[mylesy]

heres a link on loading units. its like gas pipe sizing but for water hot and cold
http://www.calgonpump.com/pdf/Loading%20unite.pdf