The signs of an undersized boiler are easy to spot: the bath’s never warm enough and the house is always a little cold. Part of the problem might be heat distribution (e.g. one room is always cold because it’s a long way from the boiler), radiator issues (too few in the room, or the radiators are too small), or even your house insulation. But if everything looks good and you’re still having heating issues, your boiler might not be sufficient for your needs.