On 9 Feb, 13:05, "Ian_m" wrote:

Apparently a condensing boiler should never
have been fitted in this location, a detached'ish garage, due to having have
a long run of condensate drain pipe.

We had that, fixed it with a small plastic bucket as a tundish
(Farmers' shop calf-feeding bucket - they're thick plastic and pretty
coldproof). If it did freeze, the bucket was several days reserve
capacity.

Another fix is one of those foul drain air admittance valves, with the
flappy plastic bag valves, on the outfall from the condensate drain.
If you can stop air whistling up the pipe, you've solved most of the
freezing problem.