If a condensate pump is used check it is clear, working correctly and that the output pipe is free of any kinks.

Drains shared with other appliances, especially washing machines, susceptible to build up of debris and blockages.

During cold weather ensure that the build up of condensate in the trap or pipe is not due to the external drains or condensate pipe freezing.

If a condensate pipe has frozen, consider the installation of appropriate measures to prevent recurrence such as lagging, Condensulate, a trace heater or rerouting the pipework.