Eurocal oil fired boiler - HOT water for 60 seconds them only warm water?

Hi,

I’m hoping you can offer some assistance with a 5 years old (Approx) Eurocal Oil-fired combi boiler

Problem

Hot water comes out hot to start with and then cools down until its just luke warm this takes only a couple of mins tops (No other water is being used at the time of the test)

Background

Boiler has been regularly serviced

Boiler is set to 80oC

My girlfriend has had this problem with her boiler for a couple of years and has at considerable expense replaced a number of parts none of which has made any significant improvement, the heat exchanger being the last part replaced earlier this year. Since then she has just lived with the problem as she ran out of money to carry on trying to solve the problem. She has spent in excess of £600 replacing parts and having the boiler serviced

Parts replaced in the past year or so in order

Expansion Vessel
Thermostatic mixing valve
Plate Heat exchanger



Some trouble shooting I’ve done

There are 3 pipes that go into the thermostatic mixing valve

They start

Top - Warm
Back - Warm
bottom – Warm

When Tap is turned on

Top - Changes from hot to ice cold
Back – Stays Warm
bottom – Boiling Hot

Not sure if this helps or wher to go from here

any trounleshooting advice would be appreciated

Simon