What size/power boiler do I need to heat my house? ...will I need a new pump too?

Hi, You will need to find out the BTU output of your boiler. Then measure the size of each radiator and make a note if it is a single or double radiator and if a double whether it has one or two sets of fins between the panels. Find a web site that sells radiators and find radiators that are similar in size to yours and note down the BTU output. Add all these BTUs together, add on 5000 BTU for the hot water and you will have the total BTU requirement of your system. If it is more than the output of the boiler then your boiler is probably too small. If it is ot then your system nay need balancing. You can try this your self, try and find out which radiators are nearest and which farthest from the boiler and make a list. Close all the lockshield valves on each radiator, these are the valve that normally have a cap covering a spindle, you may need to use a spanner to close them. Starting with the radiator nearest the boiler open the lockshield valve just a fraction, if the radiator heats up okay then go to tthe next radiator and do the same and so on, if a radiator does not heat up a lot then open the lockshield valve a little more. What you are trying to achieve is a balanced system as the radiators nearest the boiler take the most water so you need to restrict the flow to them to allow more flow to the furthest whos valve will need to be open a litle more. It can be a little hit and miss and it could take a while to get the right balance. It could also be that the extended pipework for the additional radiators is not large enough and you may never be able to get sufficient flow.