combi boiler.no hot water unless cold tap is running?

[fabio]

called to a job today.sauniar duval thermo classic or compact condensing boiler.The hot water tap in the bath will only come through with hot water when the cold tap is turned on in the wash basin?iam thinking this has soemthing to do with the water pressure?iv had a look futher and they have a water meter fitted.Would it help if i was to fit a expansion vessel on the cold feed? cheers guys......................

 

[aggis]

Could possibly be excessive water pressure, water passing thro' boiler too fast to heat. Ideal water pressure supply for a combi 2-3 bar. Try a presssure reducing valve.

 

[SafeGasInstall]

might be flow sensor ,or not !?

 

[mrlennie]

From what I've read on here it sounds like a flow switch is the pressure particularly high? As in does it shoot out of the hot water tap? Because if that was the problem I would imagine that just turning the hot water down to half a gentle flow not trickle would sort it. I would imagine they have tried this so again sounds like flow sensor.

Just my thoughts.

 

[RoryD]

If the boiler fires when the hot tap is on,check any pressures /gas rates. I'm wondering if a mixer is passing somewhere. I put a gate valve on the DHW pipe just out of the boiler and open the hot tap, if there's water then a mixer valve is passing cold water(usually a shower). If there are no mixers / showers then every thing i've just written is 'bolleaux' as they say in France

 

[plumberdarren]

Sounds like a non return valve in a mixer i.e shower is letting bye to me.