In this cold weather we’ve heard quite a lot of news about condensing boilers breaking down, and condensate pipes freezing. An example article is linked here:
Thisismoney about condensing boilers failing during the cold snap.We thought this article was a bit OTT for what is essentially occasional installer error on condensing boilers – which are in our experience usually good and reliable. It is however a useful article in highlighting this issue and gives us the opportunity to describe the solutions we’ve come across. Do feel welcome to comment with ideas / perspectives which may be useful to future readers.

If your pipe freezes, chance is that it is too small (20mm is common) or not steep enough. Ideally the pipe should be 32mm or more, and should fall quite steeply. Even so it may still freeze.

Further solutions we’ve come across people using successfully are to:
1 – put the pipe into an internal waste pipe (eg connected to sink pipe). This is probably the best solution.
2 – extend the pipe down into a drain that will be warmer due to bathwater / flushing loos / etc., being careful to ensure that it is positioned in a way that it won’t become easily blocked.
3 – install pipe heaters (final option as these will use additional electricity – example product here). About 10W should probably do the trick.

The point about return temperatures is correct, and there is a related issue that old radiators will have been sized for a flow temperature of 85, whereas it’s recommended that a condensing boiler should use a circulation temperature of 70. This can occasionally lead to under-sized radiator problems and/or a drop in efficiency, but even so the non-condensing cycle of a condensing boiler is more efficient than older boilers running at optimum efficiency, and any decent heating engineer will check as part of the installation.

There is no data available on reliability of condensing boilers – we hear that British Gas collects this data but it isn’t currently published, and no one else seems to have any data. We’d love to see some research on this if anyone is aware of any existing, or chooses to commission some.