Vokera compact 28 hot water isn't fluctuating cold and hot

[Inverness]

went to see about a problem. I've never dealt with these kind of boilers. I'm new to it so be gentle..
Any advice what the issue could be? Blocked flow ?

 

[ShaunCorbs]

Hot water is fluctuating ???

If so check the temps at the plat heating side if there not moving much I would be looking at plate

Unless the boiler isn't staying fired then could be flow switch

 

[Inverness]

When hot tap is turned on it goes hot then cold. I was thinking might be the primary thermistor??

 

[ShaunCorbs]

does the boiler flash on and off?

 

[Inverness]

Green light is on when it's producing hot water then after a few seconds the light goes red, then green again.

 

[ShaunCorbs]

sounds like it cant get the heat away

heating ok?

 

[Inverness]

I checked the expansion vessel was fine and I topped up the pressure as it was at zero.
What might it be?

 

[ShaunCorbs]

could be a few things but i think dirty plate or system (hex)

if its doing the same in heating then its the hex

 

[Inverness]

Also I went to that other job today. It was one 40 cdi Worcester heat only boiler 35mm gas pipe.
Max ratio 0.0007 min ration 0.0001
Ppm less than 100.
I did a gas rate imperial meter timed for one relvoution of a turn.i used my phone app.

 

[Inverness]

This boiler doesn't have a plate heat exchanger. It old style.