Weird hot water issue

[Riley]

Scratching my head a bit with this one. I don’t know if anybody has come across this before. Bath in a property has very basic nonceramic taps when she runs h & c, hot runs cold after about 10 mins. It’s off a combi boiler sonoy a lack of water. When she run just the hot tap it runs continuously hot.

Hope somebody might be able to offer a pearl

 

[ShaunCorbs]

Low water pressure???

 

[Gasmk1]

is there a mixer tap or shower could be draing the cold water through one of them

 

[Harvest Fields]

What boiler is it and what Is the hot water flow rate.

 

[Riley]

I did think mixer tap or shower but it’s so strange that it runs perfectly for 10 minutes and then goes cold but only when the cold bath tap is on. Boiler is a new Biasi combi ( not fitted by me I hasten to add). Flow belts out Of both the hot and cold taps And hot does need to be throttled back to achieve piping hot

 

[Harvest Fields]

Could the cartridge be closing slightly when bothbtaps are on?

 

[Riley]

Cartridge on what mate?? There are no thermostatic mixers?

 

[Riley]

Sorry just realised I didn’t say that the hot and cold taps are completely separate.

 

[Harvest Fields]

Sorry just realised I didn’t say that the hot and cold taps are completely separate.

No worries. What is the flow rate on the tap after he 10 minutes?

 

[Riley]

Identical doesn’t drop off. If full open then easily 15/16 litres if throttled back to achieve piping hot then 10/11 litres at good temp

 

[Harvest Fields]

Could he flow switch on the boiler be cutting out?

 

[Riley]

As I said mate when cold is off hot water tap runs constantly hot at a good temperature. It’s only when the cold tap is on the hot runs cold about 10minutes later

 

[Harvest Fields]

When it goes cold does the boiler cut out or stay alight?

 

[Riley]

Stays alight it’s really bloody weird. The only recent change is the new bath taps but these really are nothing special contract variety

 

[Harvest Fields]

Very strange. Worth getting BIASI out and you being on site at the same time??

 

[Riley]

No I’m just trying to find a solution. It’s almost like there is a crossed pipe somewhere but all other outlets work fine

 

[ShaunCorbs]

Does the boiler stop firing ? When both are open?

 

[Riley]

No mate carries on firing as normal

 

[Riley]

And if I open another hot outlet it runs hot as normal. Albeit the flow reduces because two hot taps are open

 

[Harvest Fields]

Does the boiler stop firing ? When both are open?

Asked that earlier mate.

 

[Riley]

It’s a proper head scratcher isn’t it

 

[armyash]

Interested to read the outcome of this.

 

[snowhead]

It's almost as though whoever fitted the new boiler and or taps, also fitted a TMV (under the bath) which is playing up.

 

[Riley]

Nope thought TMV but nothing there

 

[Allgoode]

Hi,
You could put a check valve on the hot supply to the bath to eliminated cross-flow.
might fix or will allow you to rule out the issue for little cost.
Cheers,
Andy