The combi boiler needs to produce enough hot water to supply a shower, so minimum size is around 20 kW, so new boilers do have an advantage over old boilers in the amount they can modulate is higher, so we are seeing boilers that can modulate to 6 kW before they have to revert to a mark/space ratio, but looking at £800 for a boiler without fitting, so clearly not cheap.
So with a tank of hot water we often had 22 mm pipes, the water pressure was low, we needed special mixer taps where water only mixed as it leaves the tap, with the combi you can't use a power shower and really you want smaller pipes so less hot water needs running off before hot arrives at tapes, as often there is a delay between tape turning on and boiler firing, some do have a small reservoir, but my point is you don't NEED to re-pipe, but if you don't you loose some of the advantages, as a result the cost is variable depending on what is done.
I went to direct water heating, (two boilers not a combi) because I wanted the room taken up by the cylinder, but my father-in-law had solar powered DHW once that tank has gone, then you also loose the ability to use solar power, and also ability to have a power shower, takes a lot longer to run a bath, and you can't run two showers at the same time.
However plumbing is time consuming, and there are so many factors, when I wanted to fit a power shower to mothers house there was no header tank cold water to the wet room, clearly both hot and cold need to be same pressure, so I had to run an extra cold water feed, I would not have needed this if using a combi boiler both hot and cold are same pressure, what ever pressure the supply is, some taps can't take the higher pressure, this house bathroom has no direct cold water, not really a good idea as cold water could be contaminated as held in the loft in an open tank, but that is how it's done.
Also when talking about pressure, using an expansion tank the pressure is often higher than with a header tank 30 foot is one atmosphere so max pressure is around 14 lbs/in² or 1 bar, a combi boiler between 0.5 and 3 bar, so the pressure can be 3 times higher, so some old lock shield valves may leak.
So it does not matter what work you have done, there are two ways to price the job, one is time plus materials and the price varies depending on how long it takes and what is required, installer may give you an estimate, but price not fixed, i.e. you take the risk, the other is a fixed price where the installer gives you a fixed price, and they take the risk, in the main former is the cheaper, but just 2.5 days for two men is a weeks work, so labour is going to be likely £500 plus, boiler £800 and then you have extras at EvoHome 8 TRV pack is around £600 so would be easy to have a £2000 bill, and if new radiator or pipe work required could raise to double that.
Yes you could do it on the cheap, but then is it worth doing? Most people charge a boiler because the old one has failed, the cost of a new boiler is unlikely to be recovered in lower bills when condensing boiler already fitted. My son is just about to have new boiler fitted, the reason is old one leaking, they have been in some 30 years now, and have been very reliable, I do not expect the new boiler will last 30 years.
