The first question is not which boiler. It is which type. A combi is genuinely the right answer for a three bedroom semi with one bathroom, and it is genuinely the wrong answer for a five bedroom house with three bathrooms and a family who all shower at eight in the morning. In a house like that a combi will always disappoint, whatever badge is on the front, because it is trying to make hot water on demand for more outlets than any single heat exchanger can serve. That is a system boiler and a properly sized unvented cylinder.
The second question is your incoming main. A combi is only as good as the water going into it, and there is no point hanging a 35kW combi on a wall if the main can only deliver twelve litres a minute. We measure the flow rate at the survey. It takes two minutes and it settles the argument before it starts.
One local point worth knowing, because it saves people money: Trafford water is soft — about 30 mg/l. Limescale is not your enemy here the way it is in the south east. You do not need a scale reducer, you do not need a softener, and if someone has quoted you for one on top of a boiler in Altrincham, ask them why. What we do look at on the older stock is the incoming supply pipe, because soft water and Victorian lead are not a good combination.
Every install gets a system flush, a magnetic filter and an inhibitor dose, and the reason is not upselling. It is that the black sludge that kills heat exchangers is magnetite — iron oxide off the inside of your radiators — and it has nothing to do with water hardness. Soft water areas get sludged systems just the same. The filter is what keeps the new boiler alive.
