The arithmetic is simple and it is unforgiving. A combi makes hot water on demand through a single heat exchanger. Ask it for one shower and it is excellent. Ask it for two showers, a bath and the kitchen tap on a February morning and it physically cannot — not because it is a bad boiler, but because no combi can. Houses in Hale routinely have four or five hot outlets that get used at the same time, and that is a system boiler and a properly sized unvented cylinder, every time.
Sizing the cylinder is where the thought goes. The question is not how many bathrooms you have, it is how many run at once and for how long. For most large Hale houses that lands between 250 and 300 litres. Undersize it and you have swapped one disappointment for another.
Before any of that we measure the incoming main. An unvented cylinder needs a decent flow rate to be worth having, and some of the older properties round here still have a supply pipe that will not deliver it — sometimes because it is old and narrow, occasionally because it is lead. That test takes two minutes at the stopcock and it changes the whole design, so it happens at the survey rather than on the day.
One thing worth thinking about if you are replacing a boiler here now: if a heat pump is anywhere in your ten year plan, the radiator upsizing and the cylinder are the same work either way. Doing it once with the boiler, rather than twice, is a genuine saving.
