Condensing boilers have a white plastic pipe, about as thick as your thumb, that runs out of the bottom of the boiler and into an internal or external drain.
Those people with a condensate pipe that runs externally may have issues with those pipes becoming blocked with ice.
Condensing boilers are more efficient because they recycle the heat/exhaust from the boiler but as a consequence drops of water are formed that need to drain somewhere. If your boiler is gurgling and bubbling and the weather is very cold outside then it probably has a blocked condensate drain pipe.
Depending on just whereabouts the block is, unfreezing a blocked condensate pipe can be as easy as applying a hot water bottle to that area. The block is likely to be in a bend and likely to be exposed to the outside air rather than insulated against the winter with good lagging. You do, however find blocked condensates inside unheated garage installations without a frost stat.
Fault Codes That Suggest Your Condensate Pipe Is Frozen
Vaillant boilers display the error codes f28 or F29 for a blocked condensate pipe. Glow-Worm boilers could display F1, F4, F28 or an F29 code. Worcestor Bosch display an EA code with a slow blue flashing light. Baxi display a E133 code. Pottertons display E133 E1 or 33.