Grant 90 MkII Oil Combi Boiler - Hot Water Only When Heating

[m60vxr]

Hi guys, I have a problem with our Grant 90 MkII Oil Fired Combi boiler in that there is only hot water from the hot water tap if the central heating is on or made to come on by turning up the room stat till it clicks the heating on.

I tested the microswtich that is pressed by the pin attached to the diaphram in the diverter valve and the boiler fires up when the microswitch is pressed manually. So I then I tested the movement of the pin and although it moves outward when the hot water tap is turned on, it doesnt do so far enough to operate the microswitch. I overcame this for testing purposes by packing out the microswitch with a spacer so that even though the pin doesnt come out far enough it still operates the microswitch so the bolier fires on demand whenever the hot water tap is on.

Soon as I did that I thought that would be a temp fix however, the odd thing (or the thing that seems odd to me) is that even now I have managed to get the boiler to fire when the hot tap is turned on, the water still runs cold until I go to the room stat and turn the heating up far enough to cause the rads to heat up and when I do this the water runs perfectly hot.

The moment I turn the room stat down so the central heating clicks off again the water starts running cold again even thought the boiler is still firing and continues to fire until I turn the hot tap off again.

Anyone any idea whats happening here and whats at fault?

Many thanks.

 

[dcawkwell]

Diverter valve isn't actually moving stuck half open or similar.

 

[m60vxr]

Thanks dcawkwell,

Do you know how the diverter valve internals link to the diaphragm if at all?

I have investigated the pin movement a little further and it does push in and return to the correct position and it pulls out further (with the help of a pair of pliers) than it moves when you turn the tap on but seems to stick in the fully out position and then needs a helping hand to push it back in before it returns to normal position of its own accord. Does this sound normal or is this indicative of a diaphragm issue too?

If the diverter internal valve(s) is siezed, is it repairable/serviceable or is a new diverter valve usually the only way to go?

If it were stuck half way, would we get perfect hot water when turning the heating on? The fact it works fine when flicking the central heating on makes me think that the valve must be moving fully or we wouldnt have perfect hot water at all? I am wondering what turning the heating on is doing to rectify the issue or if that gives any more clues.

 

[Johnwayne2]

Probably a new diverter valve,sounds like the valve is stuck mid way and only heating water when heating is on,so the heated water is passing to the hot water side.Making the switch manually may indeed bring on the boiler,all you are doing by making the switch manually is kidding the boiler that the motor or diverter valve has moved or diverted to the relevant position when in fact, it has not.

 

[m60vxr]

Ok cool thanks, that makes sense.

Just so I understand the mechanism here, does that mean that the diaphragm and valve mechanism are joined and because the valve is only moving half way thats why the pin only comes half out?

What makes the valve move? Is it purely the water pressure created by the demand when the hot water tap is turned on? I dont see any motor on the diverter valve.

Sorry to be geeky but for some reason I am interested

 

[Johnwayne2]

Although I`d love to go into the intricate nature of your boiler and it`s diverter valve, tell you how it works etc,the fact is,I ain`t getting paid, so go on an expensive long course like I did,serve an apprenticeship for many years or renew your diverter valve.

 

[m60vxr]

lol, fair enough JW.

Anyone else like to help me in my quest for knowledge?