why not install a combi in a new house?

Another possible reason for installing a 'heat only' boiler is poor mains cold water pressure! With regards to the previous post even with an unvented cylinder hot water flow rate & pressure are completely dependant upon good incoming mains cold water too!

There isn't a regulation against installing a combi, but a heat only boiler combined with a few valves and a decent cylinder, especially an unvented cylinder is a much better system. The downside to a non-combi system, especially in a terraced house is space, as a combi is much more compact than having a cylinder, expansion vessels, a separate boiler and all the valve gear etc. On the opposite side, if you have the space available to install a heat only boiler with an unvented cylinder, you'd end up with mains pressure hot water at an excellent flow rate, much better than any combi could offer. You'd also benefit from the back up of an immersion heater, so should your boiler break down etc, you'd still have hot water. Finally, and I suppose personally, I'd much rather look at a nicely installed unvented cylinder and all it's valvework than a metal box in the kitchen any day!