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My house was fettled with 10mm microbore feeds to each of the radiators, but they are fed off a heating 'ring main' which uses 22mm copper for the flow and return to the boiler itself. According to my plumber, not only is it the optimal way of feeding radiators for a gas or oil installation, but works rather well with alternative heating sources too. The 22mm 'ring' runs through the space between the downstairs ceiling and upstairs floor in an insulated duct with access points to get to the radiator feeds. Not only that, but he fitted a few additional tap-off points too in case I decide to finish the internals of the barn properly and want to fit a couple of extra radiators out there. He's also fitted a couple of 15mm hot and cold pipes for water in case I fancy an en suite or kitchenette out in the office in the future.
Most systems are plumbed with radiators in series, but mine are effectively in parallel if you use an electrical analogy. I've lived in too many houses where the first radiator in the chain gets hot first (often the kitchen which needs heat the least) and then they eventually warm up in sequence. Mine all get hot at the same time, which makes for a far more even and efficient heat distribution and allows me to be comfortable at a lower, but far more consistent, 16°C downstairs and 14°C in the bedrooms.
It was a bit of a work up in an 1840s stone built cottage, but as the place was pretty much an empty shell, lifting floorboards to access all areas was easy to do with the house unoccupied. The sparkies also ran new electrics and I had access to run my CAT6 and CT100 feeds tor my networking and AV needs.
The external boiler was a clever move too as, if I ever decide to go down the heatpump route in future, there's no internal work to do. But heat pumps, I'm afraid, are not on my radar just yet. The boiler has a 15 year warranty and oil is much cheaper than gas and will be for the foreseeable future, so it will probably see me out.