In other words, the model ‘number’ is not always an indication of heating output, although it occasionally is (confusing we know) to make sizing clearer, we have included a separate column called ‘heating output (size)’ which lists the actual heating output. Replacing your boiler is a big purchase decision, one which you shouldn't consider without understanding what's best for you and your home here are a few things to consider when booking a quote with us. Replacing.