Wet systems have gone all compact and microbore over the years. This must have lowered the costs and speeded up installation considerably. It will probably have increased the number of leaky radiators – have you ever noticed how easy it is to bash the flimsy pipes with a hoover? Microbore is great for compact living and fast and responsive heat and great for the installer who will be in and out in no time at all.
But if you have biggish old house you will probably like the slow warming / slow cooling luxury of Cast Iron Radiators, not just the look. Your copper bill will be higher, so you are not going to save money here, unless you re-use some of the material from the old system (except 8mm will not be re-useable!). Your boiler may well need a bit more umph to cope with the extra volume of water. Discuss all of this first with your plumber and tell him what you are planning to do, especially if like me you are doing the job on a budget and over time. You can save money on cast iron rads. No need to buy them new or to buy the fancy ornate ones – school radiators look good too. To save the most just buy them pressure tested and primed and paint them yourself. Shop around as prices vary a huge amount and it is easy to get ripped off.
Consider zoning the system. The larger the property the more likely it is that there will be times when the heating isn’t necessary upstairs and downstairs. Personally I wouldn’t bother with thermostats and just manage things as you see fit. The radiator stats are ugly. Room thermostats are probably fine if your home envelope is completely draughtproof and you’re not the micromanagement sort, your family demands temperatures in winter higher than in summer when the heating will be off or you have the heating on all year round. I run the system at a lower temperature but for longer and turn it off if I feel a little too warm. If you are on gas, your condensing boiler is more efficient if you run it this way. Designing the system to hang out in a T-shirt all year round is not recommended for old houses. It is going to play havoc with your woodwork, the panelling on walls and doors will shrink before your eyes and worse panels may split, even though it has been there for 2 hundred years or so. Maybe move to a modern house instead.
Where to put the radiators? This is not just an aesthetic consideration, but the common wisdom is to locate them under your windows which will use otherwise dead space. Ever noticed how some rooms are decorated with radiators making it impossible to arrange furniture? If you have nice wood panelling in your window recesses it is better to have a radiator complement them than have book shelves built in and obscure them entirely (why would you ruin panels like that?). There’s plenty of other reasons why you should locate them underneath windows. The warm air should circulate better in the room envelope and improve your experience of warmth. That and the fact that you’ll be able to sit on them comfortably. It obviously will cost a little more to place them in windows, as more pipe is needed. You’ve got shutters, so use them. Don’t hang your curtains in front, unless they are mainly there for aesthetic reasons and don’t obstruct the rads.