Replacing gun barrel heating

My parents house is from the late 60's with the original pipework (gun barrel) for most of the downstairs. They did a loft extension ~35 years ago and all the plumbing upstairs is with Qual-Pex. My father wants to replace the heating system downstairs but leave upstairs as is. Downstairs consists of 7 rooms with 9 radiators but the bathroom rad and pipe work are new (they cut the upstairs plex and fed the rad from there). The living room does have a back boiler that has never really worked. Downstairs is all suspended wooden floor except for the kitchen. The oil boiler is ~35 years old.
He got a quote about 6 years ago for €17k to replace the heating system and the contractor who did the bathroom was meant to back with a quote but never did (however my father made contact with them again this week).
The reasoning for upgrade is to make the house warmer and only heat the rooms that are being used ie put the house into 4 zones (one for hot water).
I am going to lift a few floorboards downstairs to look into insulating underneath them but I know myself I am going to very tempted to pull in new plex pipe when I've the floorboards up.
So I've two questions on this:

1. Is there that much value in replacing the heating system rather than just running it all the time in the winter as how many years will we need to see a return on investment?
2. Would any plumber connect into my pipe work if I fit all the rooms with new pipe work and radiators? I would be just leaving the new pipes unconnected in the boiler room?

Just on the second question I'd like to think I'm pretty competent as I've already replaced two rads upstairs with their corresponding TRV/Lockshield valves and done other work that requires you to be competent i.e. electrical work.

There is also gas available but I'm not sure if it's worth the hassle to switch.

If a plumber did connect into your pipe work would you stand over any leaks/faults if they arouse ?
For the sake of a few lengths of insulated copper pipes and an extra hour or so labour I’d get the plumber to run the pipes in.
At 35 years old the efficiency of the boiler will decrease with age.
The back boiler not working correctly is another sign too get a completely new system in for peace n mind.

done4now said:

Would any plumber connect into my pipe work if I fit all the rooms with new pipe work and radiators? I would be just leaving the new pipes unconnected in the boiler room?

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I’ve done something similar a few times, plumbers have never batted an eye lid.

Others can probably comment on how to do it ‘right’ while you’re at it, but the things that jump out to me are to make no joints under the floor if it can be avoided (bring all runs back and into a manifold), use pre-insulated pipe in any cold areas, be aware there are various types of PEX (barrier PEX etc) so figure out which is most appropriate for your usecase, for bathrooms try to come up inside the wall rather than out of the floor so any leaks don’t pour down rad pipe holes, if you kink PEX badly best pull that run again.

If you know what you're doing and do a decent job of it the plumber should have no issue. They can pressure test it to be sure it's all good.

Thanks for all the replies. We've since had the contractor who did the bathroom in, we've asked them to quote all the work including the work I was planning on doing on insulating under the floorboards. He did mention replacing all the floorboards with MDF as the surface area of the downstairs area is ~130 square meters which might work out cheaper but not worth it.

Yeah understanding which pex to use will be interesting as even in the house there is already three types the standard brown qual-pex (it's pretty much all this), black (no idea what that is) and the new bathroom has some grey pex. Another plumber had mentioned using pre-insulated pex with crimped fitting.

I hadn't thought of having continuous runs back to the manifold. I had just thought of running 3/4" pipe for each zone with 1/2" for each tails of the radiator.

Ideally I'd like to have an actually plumber design the system and then try and do some of the more physical work but let a professional do the technical work.

done4now said:

He did mention replacing all the floorboards with MDF as the surface area of the downstairs area is ~130 square meters which might work out cheaper but not worth it.

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OSB? I wouldn't be using MDF on a floor, if not fully supported it will sag over time and is prone to significant expansion if exposed to water.

Leo said:

OSB? I wouldn't be using MDF on a floor, if not fully supported it will sag over time and is prone to significant expansion if exposed to water.

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This appears to be the standard approach now? I had a big leak and the insurance contractors replaced the floorsboards with OSB in the impacted bedroom. It is still sitting on joists, so don't see an issue with sagging to be honest.

DublinHead54 said:

This appears to be the standard approach now? I had a big leak and the insurance contractors replaced the floorsboards with OSB in the impacted bedroom. It is still sitting on joists, so don't see an issue with sagging to be honest.

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The original posters' contractor was suggesting MDF though , not OSB/ply. Miscommunication I'm sure.