Help wanted-position for rads in extension

I need to plan the location of the radiators in an extension I am putting to the side of our house. You can see in the attached images (ground floor plan, side elevation) that there will be a kitchen/dining room with a study to the front of the extension. The kitchen will have a flat ceiling as it is in the existing house and the dining part of the kitchen/dining room will be in the extension so it will have a pitched roof (side wall of extension will be a gable end as in attached image). Study part of extension will have a flat ceiling.

Total area of kitchen/dining room is 27 sq metres (290 sq ft) and its volume, including the pitched ceiling in the dining room part is 90 cubic metres (3179 cubic foot). I estimate a total of ~9,000 BTU required for this room.
Total area of study is 10.64 sq metres (114 sq ft) and its volume is 27.6 cubic metres (974 cubic foot). I estimate a total of 4,000 BTU required for this room.

I’m currently planning on putting a vertical radiator at point A in the kitchen part. I’m also thinking of a second vertical rad at point B in the dining part with a horizontal rad at point C under the bay window in the study. There will be a dresser and shelves running along the wall at point D, but a vertical rad could possibly fit in a point D.

I suppose I could run feeds to all four rad points in the image (A, B, C, and D) which would give me the option to decide on location of the rad in the dining room (B or D) when the walls are built. What do you think of the proposed locations and would it be wisest to run pipes to both point B and D?

There are ¾ inch copper pipes running from the boiler to Rad A (red lines). I am joining ¾ inch qualpex barrier pipe to these copper pipes and running it to point D (pink line – showing one for simplicity). I would then branch of this with ½ inch qualpex to run pipes to point B and point C (green lines). Is this the best way to do it or should I use ¾ inch pipe all the way to point C?

Seriously indebted to anyone who can give some suggestions or advice on this.

would you not consider employing a plumber?

Had really bad experience with plumbers so have been doing the plumbing myself since then.

I now reckon I'm going to run 3/4 inch pipe to all rad locations and only break out into 1/2 inch for the 1 foot final run to the rads - should avoid problems balancing the rads.

In terms of actually doing the plumbing I've no problem there, just wanted advice on what is the best solution, as opposed to how to do it.

Any advice on the location of the rads is much appreciated.

I have a few comments regarding your proposal. For rads to work best they should ideally be fitted beneath a window or alternatively on an external (cooler) wall. You're stuck with location A (this rad should be sized to take account of heat gains from cooking etc. in the kitchen) and the rad at C is ideal. However I think you should forget about rads at B & D and put a rad on the western wall of the dining room instead. You will have better convection of the heat if you adopt this route. As for piping it is normal practice now to have flow and return manifolds at the boiler/ hot press location with individual 1/2" flow and returns to each rad but I presume this option is not available to you. Therefore I would suggest the following (all in qualpex barrier pipe, except for 1/2" copper tails to and from each rad valve): 3/4" feed from boiler to A, and C, run feed from here to rad at western wall in 1/2". 1/2" return from rad at western wall to rad C, 3/4" from here back to boiler, picking up return from A and back to boiler all in 3/4". Fit TRV on all rads and lag pipework in Cozytube or Tubolit insulation. There's no need to pipe it in 3/4" all round- the runs are short enough and the rads won't be huge anyway.

Thanks for the info Carpenter.

If I put a rad at point D it would be on the western wall (external insulated cavity wall) though it would be vertical to leave space along the rest of the wall for furniture. I am also considering putting a rad in the breakfast bar separating kitchen area from dining area. Would this solution sound ok? I really don't want a full width rad on that western wall.

As for the piping - I was advised that if I ran all the pipes from point A to points B, C, and D in 1/2 inch qualpex barrier pipe I would have no problems. The pipes already run to point A are 3/4 inch.

I bought the gear this morning and its all 1/2 inch stuff that I got. I was told that using 3/4 inch pipes for the main runs as you describe would make it easier to balance the rads, but other than that running all 1/2 inch beyond point A would not be a problem. Do you agree or do you think that its a show stopper and I have to use 3/4 inch for the main runs?

Original plumber was a cowboy and it seems that all of the upstairs heating pipes are 1/2 inch. We have been having non stop problems with pressure dropping (though I can't find any leaks) and trapped air in the highest point (vertical towel rad in upstairs bathroom) - is there any reason to think that using 1/2 inch downstairs would compound this problem?

BTW I'm not a plumber but I do my own plumbing and I work for a large developer and read Mech. & Elec. drawings all the time so I consider myself to be reasonably competent in the area of small domestic installations such as yours. Back to your questions, I think 1/2" would be fine throughout- the pipe runs are pretty short, the rads won't be huge so the water volumes involved are small- that's my reasoning anyway. So use the 1/2" gear you have, shouldn't be a problem. I only saw your note re: dresser and shelves on western wall after, so go ahead with the vertical rad on this wall. As for the rad at the breakfast bar location, is this necessary? I don't know your other rad sizes, but I suppose it would be better to put it in now rather than have your new accommodation too cold. Enjoy the plumbing!

Thanks for that Carpenter, it tallies with other advice I got. Pro plumber or not you're clearly well up on this subject so your advice is much appreciated.

Based on my BTU calculations I think 2 ~4,000 BTU rads in the kitchen plus similar in the study would be ok, so the breakfast bar rad idea would be a luxury almost - nice heat when having brekkie at the bar. Concrete won't be poured in the kitchen part for about a month so I've time to decide on this.

Thanks again.

If you do decide on a rad at the breakfast bar you could go for (if you haven't already considered this) a low profile rad- they come in approx. 6" and 9" high formats. This could be integrated into the design of your counter unit (unless you're going Cash and Carry option). If the heat proves too much you can always turn down the TRV. The low profile rads are a special order and not cheap but they are very useful in a situation like this.