LPG to Oil boiler swap

Firstly having looked at the pictures and read the following is it
1 possible
2 economic

Situation is this. lpg gas fired condensing boiler fitted internal to house in France. It eats gas and therefore money. It runs central heating over three floors totalling 350 sq meters. It also keeps what I believe is a hot water storage system, well , hot.
What I would like to do is install, not by me, an exterior oil fired boiler located outside where the gas boiler is installed inside.
My preferred solution would be run whatever pipery is required through wall and connect up to what the current system is having disconnected the gas boiler first.
There is a wall about 2.5 feet high about 6-7 feet away from proposed site of boiler where I was going to locate a storage tank for gravity feed and ease, off a driveway, of refilling it.
So is this doable?
Would the oil boiler be content sitting on the granite slabs shown or would I have to put a fixed concrete base down first.
Would the storage tank be o.k situated that close to a house/ doorway. fire risk?
Is 2.5 feet enough head to gravity feed the proposed boiler?
The wall is granite which gas proved very difficult to penetrate for other things. I believe the interior structure is random rubble /lime mortar/ mud and clay whats the wisdom in passing pipes through?
At present I can find no other independent circulation pump so I believe the boiler pump pushes water around quite a distance and up three floors? Can the oil boiler do the same?
Is there a boiler that can continue to service the existing hot water storage system. This system currently provides for 2 en-suites, kitchen and utility room. These are already on quite long runs and a combi boiler would have a long run time before hot water arrived at a tap.
I have seen 'boards' advertised with a combination of valves prv's etc which give the facility to connect say a gas boiler to a wood burning system or it appears any other combination of heat sources. There is, after removal of worktop etc, room in the utility room to fit such a board if leaving the gas boiler fitted as an auxillary backup heat source would be a better solution. The lpg tank and pipery would be left as this also services a bungalow for its heating.
If this project isn't just a pipe dream what boiler would you recommend to do the job?

You'd need to consult the French authorities on siting of the oil tank, as regulations will almost certainly be different there to here.

What you suggest looks eminently doable. Is it economical? Probably not. I'd budget around £4000 to do the conversion. Do you think you'll save that much in fuel costs?

At 700£ a tonne which we seem to use at 10kgs a day in winter so possibly 2 to 3 years. I agree with your comment about French regulations, although they seem to be more ignored than obeyed, its a national pastime.
Any preferences for boiler?

hi1 said:

At 700£ a tonne which we seem to use at 10kgs a day in winter so possibly 2 to 3 years. I agree with your comment about French regulations, although they seem to be more ignored than obeyed, its a national pastime.
Any preferences for boiler?

Click to expand...

I'm not convinced your maths is correct on that. 10kg/day = £7 a day in Winter, assume that's for a third of a year, that's £850/year, add 15% for the rest of the year makes it £980 a year on LPG, so that's in excess of four years assuming your oil boiler uses zero oil, which clearly it won't. Based on that rough calculation, I reckon you're using about 17,900kwh a year

Heating oil in France is about 65p/litre according to the Internet. One litre will give you about 12.4kwh of energy, so to match your gas consumption you'll need to buy 1445 litres a year at a cost of £938. That of course assumes that the efficiency stays the same. If your gas boiler is old and inefficient, then you might get away with 1200 litres of a oil a year at £786/year with the improved efficiency of the oil boiler. 980-786= £194/year saved. Call that £200 for simplicity and that's a 20 year payback based on a £4000 conversion cost.

As for which boiler, Hounsfield or Navien. Not sure how much warranty support you'll get out there though.