I know alot comes down to price - but also personal preference, so it's really what you want/need.
Changing radiators alone is not going to save much if any money.
Best bet would be to upgrade the boiler. You could get a 80kw system boiler. You could zone the house, so that you were only heating the main living space, upstairs, or the whole house.
Fit 2 boilers, 2 system boilers maybe ? or one system and one combi ?
The combi could heat the water for the kitchen and en-suite, instant hot water, and the system boiler could supply water for the main bathroom.
One boiler could heat the upstairs radiators, and the other downstairs ? or one could heat the main living areas, living room, kitchen, passage, bedroom and bathroom - the other boiler could do the rest and the water ? not sure how many rooms you have in your house. Or what sapce you have for the boiler/s.
Has your boiler ever broken down and been off for a couple of days ? with 2 boilers it's unlikely they will both go off together, so you'll always have some form of heating.
The laws of physics are quite simple heat always moves to cold.
A given space will always require the same amount of heat however its delivered.
Insulation works, at cutting heating bills.
The only way to reduce your heating bills, are a change of lifestyle or insulation.
If you turn off the heating the problem goes away.
If you insulate the outside walls and the top floor ceilings with 5 inches or more polystyrene your bill may be reduced by 90%.
The biggest loss of heat is through holes in ceilings, light fittings and holes in walls and floors, if you have a ventilated space under your ground floor, open chimneys, open windows these are all costing. Damp walls transmit heat out into the garden at a fantastic rate.
Buy, borrow or steal an infrared camera, photograph the walls, roof, ceiling find out where the heat is disappearing and fix it.
A infrared camera will be yours for just over Â£2k.
You will save the price in no time.
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