Combi boiler issue controlling non-thermo shower

[Jimmy Gear]

I have a combi-boiler and no wet loft. Water feeds in through the mains into a serpate 200 litre tank (made by grundfos with a small pumup) wherein it goes to the boiler. Reason for this is that I'm on the top floor of a house with one mains supply.

Anyway, issue I have is that the temperature of the shower fluctuates between v cold and v hot without any notice. The shower is non-thermostatic therefore believe issue is with the boiler - this said, if my cold water comes from the tank at the same pressure all the time, unsure why the temperature difference is occuring.

Can anyone lend some advice?

 

[stevetheplumber]

I have a combi-boiler and no wet loft. Water feeds in through the mains into a serpate 200 litre tank (made by grundfos with a small pumup) wherein it goes to the boiler. Reason for this is that I'm on the top floor of a house with one mains supply.

Anyway, issue I have is that the temperature of the shower fluctuates between v cold and v hot without any notice. The shower is non-thermostatic therefore believe issue is with the boiler - this said, if my cold water comes from the tank at the same pressure all the time, unsure why the temperature difference is occuring.

Can anyone lend some advice?

had similar problem as this on a break tank system the pump cutting in and out cased the diaphram to jump about causing boiler to go on and of had to feed combi direct from main leaving cold water supplied by break tank

 

[Jimmy Gear]

OK. Sounds like Grundfos might need to give me a new bit of kit? Or replace the pump...is there any reason, in your opinion, (apart from defective pump) why water would not enter the boiler at a constant pressure - and thereby leave the boiler at a constant rate enabling the boiler to remain on (assume good flow rate through the shower, bath).