"Andy Dingley" wrote in message
...
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.

What usually happens is residual water at the bottom of the pipe freezes, he
next batch of condensate runs over this ice and some stays. That freezes.
Then there is a build up of ice in the pipe. If there is drip it will
freeze up, as happens to dripping Pressure Relief Valve pipes. Then the
boiler blows the expansion vessel.