On 04/01/2021 13:28, Andrew wrote:
On 03/01/2021 23:22, Roger Mills wrote:
On 03/01/2021 14:06, Dave Liquorice wrote:
On Sun, 3 Jan 2021 12:56:56 +0000, Roger Mills wrote:

Unless the boiler is in an unheated outhouse or attic, why would
that be
any more likely to freeze than the pipes?

Boiler has balanced flue that could allow cold external air to
circulate through the heat exchnager.


Why would it do that without the fan running? "Balanced" means equal
input and output pressures.

There are 'old fashioned' balanced flue boilers that are not
fan-forced. I suspect they are the most ineffient types going
and ?most have now been recycled into Nespressos. I regularly
see properties with a modern 4 inch balanced flue adjacent to
a bricked up hole that was about a foot square where the old
balanced flue came out.

Yes, indeed. But the non fan-assisted flues required some thermal
activity in the boiler in order to expel the products of combustion and
draw in fresh air. There would be negligible circulation without that.
--
Cheers,
Roger