[park381]

Originally Posted by derekgas

Long piperuns should be avoided where possible,

That may be all well and good in a new build house, but not always possible in an existing house. Most boilers are sited in a kitchen with the bathroom not directly above.
Have seen installations where combi boiler in installed in a garage, and the cylinder (first floor) has been removed, pipework linked so the hot water flow is to the first floor before coming down to the kitchen, that is a lot of water to waste, may be a wrong application/position of the combi boiler.
In most cases where a normal system boiler/cylinder is replaced by a combi boiler that is what happens.

With a combi there is always the drop in temperature of the incoming mains cold water in winter, all combi boiler manufacturers use the 35* temperature rise as a yard stick, so in winter the temperature of the hot water produced is relative to the reduced temperature of the incoming cold water. That is unless the combi boiler is way oversized.