While Nick_B's info may be correct about being able to only heat 2-3 radiators, that leaves out some important facts:
- How powerful the fire is (Usually measured in KW)
- How well designed the system is. i.e. v.important especially in NZ to ensure your plumber/engineer has experience in designing and installing such systems. I have heard way too many stories about central heating performing badly becuase it appears it was designed badly from the outset.
- How well insulated the house is. Ask your builder if you can over-spec the insulation - we did.
Any good seller of fires or manufacturers (see Harris Flame for their "Woodsman" range, I hear they are good) will be able to quote you the kilowatt (KW) heat equivalent output of the fire.
As an example, we have 6 beefy old cast iron 4 column radiators in each room of our new replica villa. They are heated directly from a wood-fired Kitchen Range (A reconditioned Rayburn - check out classiccookers.co.nz) which, depending on the type of wood used and how well seasoned it is can chuck out well over 16kW of heat which has been *easily* enough to do the rads, boost our 400L water cylinder (it's primarily heated via a 30-tube solar collector). *and* heat water enough for x2 300W equivalent towel rails in the 2 bathrooms.
So you should therefore have no problem with 3-4 rads and the underfloor providing:
- The system is well designed and installed
- The fire is powerful enough.
As Nick_B says you could also use a boiler which are designed especially for just such a case, and there a multitude of types, sure it would need to be stoked, but so would any fire.
Also, find out exactly how much heat you actually require to heat any one room. Sounds logical now that you think about it. You can then use a calculator online to calculate the amount of heat required to heat any room, once you punch-in the room's dimensions, glazing type, insulation, orientation (North, South etc)
It sounds like a lot of work, but it really isn't and you learn so much about your home that you simply would not have done otherwise.
I found an immense amount of satisfaction having done all the legwork beforehand and it paying off last winter. Sure, there are things about the house as a whole I would have done differently if I had my time again, but doesn't everyone!?
Online heat calculators:
I used the calculator here: http://www.theoldradiatorcompany.co.uk/btu-calculator
I also just found this: http://www.theradiatorcompany.co.uk/heatoutput/
You can use this site: onlineconversion.com to convert BTUs (British Thermal Units) to Watts/Kilowatts.