RogerS":gtw8ho9d said:
he most hassle free route is to replace with a condensing normal boiler.
What eee said.
We have a somewhat 'exotic' system, of solar heating plus a condensing 'system' boiler (it's the trade term for ornery ones), and it's all installed in a side attic on the top floor. There is a separate pressurised hot water tank. I don't think there's a limit to how low you can have them on the inside wall (ours is almost at floor level), BUT the pipes enter and leave at the bottom usually, so fitting them low down is a PITA for access later. Flues can be ducted quite long distances, both horizontally and vertically, they're also pretty narrow, so you can run one inside an old flue from an earlier boiler if you want to. As mentioned, you do have to have a drain nearby, to deal with the acidic condensate, but ours just comes out at the eaves of the house and drains into the plastic gutter - it was a bit of a fiddle to organise but it works fine.
You can either put a rad in the airing cupboard, or a hot water tank as before. You shouldn't need a header tank in the attic, which is a good thing on balance.
I wouldn't worry unduly about pressurizing the heating system: it will work fine as low as 1.5bar, it's never at mains pressure, and leak sealer (goes in the water along with the corrosion proofer) will deal with small leaks. You can see how much water is being lost by the pressure gauge on the boiler. If the system is <20 years old it ought to be OK.
The hot water will be pressurised if you have a combi, as will the bath cold tap (if you take the cold water header tank out), but that still shouldn't be more than 3bar, and you can dial that down on most pressure reducers. Combis do usually need a pressurised cold feed though to work correctly. If you fit a 'system' boiler, the hot water can be at the same pressure it is now.
My only concern would be that your water probably comes from Dartmoor or Exmoor, and is thus acidic. The pipes don't fur-up like ours do, but they do corrode, so I'd give all brass fittings a careful check - they lose the zinc and go cheese-like in the wrong conditions ('de-zincification'), usually due to electrolytic corrosion. Your plumber ought to give you some idea of the health of the rest of the system, and can pressure-test (using air) to check for leaks. I think Roger mentioned replacing the tank: in hard water they fill with sludge, and in soft water they corrode - I'd do yours if you're keeping that system.
Our house still has some 30-year-old rads, thermostats and piping in it, which are fine. If yours is old enough to have 1/2" and 3/4 piping instead of 15mm & 22, there might be a problem, but that would date it around 1970 or earlier.
Sadly, boilers are no longer a DIY job, so you probably have to budget at least £2,000 for boiler+fittings+labour (and that's optimistic). There are grants for eco-friendly new boilers though.
One final tip: find out which brand of boiler the local plumbers prefer. The modern ones need regular servicing and spares, and parts can cost £loads (I'd avoid Vaillant for that reason alone - ours has been horribly expensive already, and Vaillant's customer support is rubbish as far as I can tell). If you get a popular one, chances are spares will be available.
HTH,
E. (I'm only a DIY plumber but I've been doing it for 30 years, and have put in combis in the past, when it was legal to DIY it).
