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I'm looking at putting in a central heating system into a house (2 up and 2 down) with no current heating system. I like the idea of a combi boiler as there is no hot water storage issues. Can anyone tell me if the install is any more difficult ? Does the boiler have a separate set of outlet pipes for hot water as opposed to central heating water ? Thanks. Bill

Installation is easy.

Pipe work as follows: -

Central Heating Flow.
Central Heating Return.
Mains Cold Water In.
Hot Water Out.
Pressure Relief Valve Discharge Pipe.
Gas In.

It does have separate pipes. Because everything is included in boiler electricians and plumbers have very little to do compared with the non combination types and they include anti-cycle and many more software bits to ensure a good system. But have one to two draw backs. Should the water be cut off you have no reserve. You can't use power showers although standard thermostatic showers work better because everything at cold water pressure although only one shower can really be fitted where with non combi you can use en-suit without one shower affecting the other. Take ages to fill a bath. You can't use other water heating methods like solar panels. I live in a soft water area not sure how they work with hard water. All boilers can now be condensating and also both open and closed systems can be installed although most new systems are closed. The closed system has a limit to how much water can leak, is unlikely to have problems with legionnaires and once running unlikely to need bleeding etc. But harder to bleed in first place. They can run hotter but unless the radiators have some protection against direct contact unusable. And of course have no header tank. Although a combi boiler does not require the tanks and cylinders used in non combi systems it is a bigger boiler as it contains the heat exchanger/storage tank, the expansion tank, the pump, and sometimes time clock as well. Because new boilers don't just turn on/off but the flame size changes with demand and is controlled by the return water temperature in turn controlled by radiator valves TRV's it no longer uses the room thermostat in the same way as before. The room stat if fitted only turns system off in the summer and does not control room temperature and is often fitted in entrance hall etc. often unheated they no longer control room temperature. Sometimes zone valves are used so some rooms are only heated at night and others in morning and evening as extras like these are added the combi loses favor. I am sure I have missed something I have a system very like the combi two boilers but hot water instant gas installed before combi's were common and I like it but the old style does have some advantages.

Thanks Guys, a very comprehensive answer ! Sounds like an easy install of the hardware but I'd probably need an plumber/electrician to commission it. Very helpful, Cheers.

As Ericmark says they have good and bad points.
If there are only two of you in the house and, you always know what the other person is doing, then possibly fine.
If not, then running a shower can be unpleasant as only one person can use the water at a time. Even then, you have to allow for washing machines and dish washers taking the water - these things can be very irritating.
If you have a young family, or intend to have one, then having no emergency store of water, when the mains go down can be very upsetting and may need an urgent trip to the supermarket to buy water, expensive water for washing baby etc.
Final point. While water companies try to supply water at 1.5 bar, they often don't. Low pressure can mean everything takes for ever to fill up, even the toilet wont flush if its empty.