My combi boiler goes to lockout everytime I draw hot water or use the heating,although once the heating is running it will run for ages if the thermostat is set low .The problem started when new switches were fitted after draining down,many whooshes and gurglings were heard but have disappeared after a few months .Two heating and hot water switches have been changed,the heat exchanger has been cleaned ,spark gap been reset.The problem, when using the heating is only on startup ,if the thermostat is set too high it will cut out but if set low it will run for ages if it does,nt have to stop and restart.This has been going on for about a year now.I am confused about the reference to "flow switch" is this the same as the dhw switch? After the second restart,after cut out it will run fine until it switxches aff after not being used and then the problem reoccurs.