These are very good questions and their isn't one simple answer there are two opposite answers!
Reasons for Replacing an Old Boiler Which is Still Working
A new moel could be far more efficient but you need to check out your model of boiler on the Zedbuk list to find out how much more efficient.
New boilers pretty much will turn 90% or so of gas into heat but old boilers utilised less. This might be just a bit less like 80% or it could be a lot less, like 55%. So if your gas bill for heating high you could be saving over £100 per year by switching to a new boiler. BUT check out arguments against replacing the boiler, below.
If your boiler is 12 years old then replacement parts may become obsolete, so when it breaksdown, that is it.
For older boilers break downs may become more frequent and parts and labour becomes expensive, the money spent on keeping the old appliance going could have made a hefty contribution to installing a new model with a 2, 3 or 5 year parts & labour warranty.
Here is someone from British Gas talking through the issue:
Reasons for Keeping the Old Boiler, Not Getting a New One
The cost of a new boiler is high. Depending on the needs of your house and the existing heating system a new boiler fitted could cost anywhere between £1,500 to £3,000.
So even if the new boiler is more efficient the saving in fuel bills per year hardly contributes to the cost of installing a new boiler.
There is a GREEN case which argues the cost to the environment of manufacturing and transporting new appliances is not justifiable when old appliances could keep going with just a few small parts.
When To Replace An Old Boiler?
You can expect a gas boiler to last more than 10 years, 15 years is very good. An oil boiler has a much longer lifespan.
- Replace it when repair bills become high.
- Replace it in the summer if possible, certainly in warmer months not in between December and February if possible.