Hot water going cold

Hi had new toilet and sink fitted all old pipes used from floor and shut off valves fitted to hot and cold and toilet.the sink hot water is fine but the bath starts hot then goes lukewarm the bath plumbing has not been touched. The boiler has been checked out all ok it's a combi.the boiler chap said it's the cold water leaking in the bath mixer.could someone explaine what's going on as the mrs is drivin

 

Turn the heating on and leave till hot, then open the bath tap and feel the hot and cold pipes below the boiler and make sure you havent crossed the pipes over

 

Thanks,pipes all good.the water starts hot then goes lukewarm without touching the cold tap on bath mixer

 

Are you able to isolate the cold under the bath and test the hot? I wouldn't think cold pushing the hot as they're near on balanced.

Edit: has this only started since new toilet and sink?

 

How many litres per minute of hot water is the bath tap delivering?

Fill a bucket and time it.

 

Hi yes this has only started since wc and sink fitted but cannot see why as the pipes tails coming through the floor are the same all that's changed are new isolation valves fitted as the old ones would not shut of properly and new mixer tap fitted on sink all old bath plumbing untouched

 

Could be an issue with new tap. Isolate hot and or cold and run bath, if it stays hot, id say you've found your fault

 

there will be an isolation valve on the cold water inlet to your boiler close it and open a hot tap till the water stops, then your bath mixer move it to max hot and open and see if any water comes out

 

Or it could be a bath hot outlet is 22mm and a basin 15mm, with the outside mains flow much cooler this time of year and a low kw combi would produce luke warm water from hot outlet.

 

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