Unvented Undersink Water Heaters. For D.I.Yers.

So why don't they fit one internally?

Wall hung combis usually do have an internal expansion vessel - floor standing ones often don't. I think the problem is that the expansion vessel need to be sized according to the overall heated water content (e.g. including all the radiators and pipework, not just the boiler internals). A small (wall hung) boiler is likely to serve just one ordinary sized house so the same relatively small expansion vessel will serve 99% of situations (although the instructions usually say to add an external one as well if the heating circuit exceeds a certain capacity). Large (floor standing) boilers are likely to serve much larger and more varied areas - so you might need a large, very large or huge expansion vessel depending on the installation - so better let the installer select whatever is needed rather than include one that'll either be too small or too expensive/bulky for many situations.

    - Andy.

So why don't they fit one internally?

Wall hung combis usually do have an internal expansion vessel - floor standing ones often don't. I think the problem is that the expansion vessel need to be sized according to the overall heated water content (e.g. including all the radiators and pipework, not just the boiler internals). A small (wall hung) boiler is likely to serve just one ordinary sized house so the same relatively small expansion vessel will serve 99% of situations (although the instructions usually say to add an external one as well if the heating circuit exceeds a certain capacity). Large (floor standing) boilers are likely to serve much larger and more varied areas - so you might need a large, very large or huge expansion vessel depending on the installation - so better let the installer select whatever is needed rather than include one that'll either be too small or too expensive/bulky for many situations.

    - Andy.