Boiler cutting out when cold water introduced???

[cooper.s]

We got a call to a caravan the other day basically when the customer used the shower it was heating the caravan after going to diagnose the problem we found that what was actually happening was the boiler was cutting out and dumping the hot water into the CH, the park that sold the caravan told the customer they could have a new boiler so that was then fitted but the problem carried on we have narrowed the problem down so basically the DHW works fine untill any cold water is introduced into either the same tap or a different 1? There is a thermostatic tap in the shower which we thought it could be so that was replaced and we have put a non return valve on the DHW outlet but now were lost as problem is still persisting.
Any help is much appreciated

 

[jtsplumbing]

What make boiler is fitted ? sounds like you have a crossd hot & cold water supply or linkd together !

 

[cooper.s]

What make boiler is fitted ? sounds like you have a crossd hot & cold water supply or linkd together !

a Worcester came out and a Morco went in and got the same problem on both, we have checked the full installation all the colds are fine and all hots are fine no crossovers etc

 

[Gary71]

Are you losing flow rate through the boiler when the cold is turned on?

 

[jtsplumbing]

This seems strange, is the filling loop on the cold main / return ?

 

[Gary71]

we found that what was actually happening was the boiler was cutting out and dumping the hot water into the CH,

Do you mean water from the mains that has been heated is being drawn in to the heating circuit or that when a cold tap is opened heat for the plate heat exchanger is passing in to the heating circuit.

If it's the second then I have had it before where the cold was robbing from the hot supply pulsing the flow switch allowing heat in to the heating circuit.

 

[jtsplumbing]

Just give it a go with filling loop disconnected,and see what happens ! worth a try.