Blocking up an old Flue is Lime Mortar Needed?

[michaeletc]

Hi all,

I'm planning on having my old central heating and hot water boilers replaced with a new combi boiler in a new position. The old boilers had large square flues which will need bricking up. I've heard that cement is a killer for old houses like mine (1897), so do I need to insist that the plumbers use lime mortar? I'm not sure if they are really set up for that given that they are plumbers rather than builders.

Thanks for the advice.