I have a Halstead Boss 50 boiler that for no reason keeps locking out, does not do it at a set time it can be up to 2 or 3 times a day.

When you switch the boiler off and re fire it up it's fine.

Cannot find user manual anywhere - anybody know why it is doing this?

I have bled the system there is no air in any radiators, only had it serviced a month ago???

Susan Hacknet, December 2009

I've the same problem - have found turning the boiler down seems to reduce frequency of cut outs. Got mine set on 2 - only draw back is the radiators don't get so hot so it can take longer to heat house from cold.

Keith Thomas, January 2010

download servicing manual http://www.halsteadglendimplex.com/(A(ElxT86-1ygEkAAAAOWQwMzQ1NDgtMDM5Zi00YTVkLTgxOTAtYTFmMDE2MjQ2NmM4xsLisJOGMhCG_yghhwIMLlQ18iY1))/downloads/SEBoiler/BEST%20db%20Inst%20Manual.pdf

steve edge, December 2009

The fact that it locks out only after it has been off for a while points to machanical faulty
the most common fault for lock out is dirty fan drum and slightly blocked venture' causing failure of air pressure switch could also be aps itself
Halstead parts are expensive,not advisable to replace willy nilly

steve edge, December 2009