One pipe central heating -- improve and add a rad

I have an ole 1 pipe system which is performing quite poorly downstairs. The issues are the lounge and hall take a long time to heat up. I'd also like to add a small rad to a downstairs loo.

The system is sketched in the attached. The area under the red box is inaccessible due to wood flooring / tiles.

My proposed modifications to the system are in the 2nd attachment. These are:
1) Drop another flow and return from upstairs to feed the hall rad and the new one in the downstairs loo. This will be done as a 2 pipe branch rather than 1 pipe! I'll probably do this in 15mm copper unless there is a good reason not to? Distance is around 3m each way, and around 3Kw, so I think 15mm should be OK?

2) remove the bypass under the lounge rad to improve it's performance, essentially making the lounge a 2-pipe system too (at least sort-of, in that all the flow will have passed through a radiator before going back to the boiler).

I understand the issue with point 2 is that if the lounge TRV activates, the other 2 dining room rads will also go off, but I think this is an acceptable price to pay in order to get some heat in the lounge, and I'll probably just remove the lounge TRV as it is always on max anyway.

The boiler is a 30KW condensing combi, so I'd also perhaps expect a little efficiency improvement too by removing the bypass?

My question is, does the proposed changes seem reasonable, and do I need to be weary of any other factors (I understand it will need re-balancing)?

Thanks!

Current layout:


Proposed layout:

Current:

It's more than likely, that you'll screw everything up. I had a one pipe system with a gravity feed hot tank, and when I got it changed to a two pipe system, the improvement was absolutely dramatic.

Thanks for the warning! Can you elaborate a little as to why it might screw it up though? As far I see, this is basically a poor-man's conversion to a two-pipe system, with the caveat that there are 3 rads in series??

I agree a proper conversion would be ideal, but with the tiled/wood floors, it's not really an option, unless desparate.

You have no guarantee that the water will flow in the direction you want; you would be better off cutting into the main pipe, rerouting it to the new rad, and then routing it back. If you've got tiled floors etc, then consider running the flow and return pipes under the 1st floor rooms, and then dropping the pipes down the walls in the corners, then routing them along to the rads.

Yes, drops from above would be much better. Your altered system above looks a nightmare to balance with rads piped in 3 different ways. Downstairs rads always suffer due to heat rising and favouring the upstairs rads. One pipe just makes the situation worse.

I see your point re balancing -- thanks!

How about this: if I drop from above for the hall and downstairs loo (i.e. proposed mod 1) directly from the main 22mm flow and return, close to the boiler, and just leave the rest alone for now (leave the lounge bypass as-is), would that be a reasonable thing to do in the short-term? In other words, my proposed layout in the sketch above, but with the bypass intact.

I think I will take your advice them all fed from above eventually, but would like to do one at a time as time permits. So is a hybrid 1 pipe / 2 pipe likely to mess things up?

Cheers.

ASKI said:

. So is a hybrid 1 pipe .

Click to expand...

Hmm. 'hybrid'. I could think of other words for it.

Yes, leave the by pass pipe in place and put in the f and r for the two rads for now. Removing that bypass may have an even more detrimental affect on the other 2 rads that would still be on one pipe. Then run drops to the one pipe rads as time allows.

Well, If you go for a hybrid system, then new rads will work, and there's every chance that the old rads will be okay as well, and the beauty of this idea, is that it'll show you just why you should then finish the job off properly.

ASKI said:

I have an ole 1 pipe system which is performing quite poorly downstairs. The issues are the lounge and hall take a long time to heat up. I'd also like to add a small rad to a downstairs loo.

Click to expand...

Нi
I will be glad if it is useful to you.
I did this system in different ways.

Thanks, Zmicer. I think I'll be gradually replacing it with a 2-pipe. But something like that might help a bit in the short term...

One thing that bothers me, especially having a condensing combi, is I suspect quite a lot of the flow is just going round the loop without entering the rads, so goes back to the boiler fairly hot. Starting up from cold, the boiler is modulating almost straight away.. I don't think that any one pipe modifications can stop that (other than removing the bypass; but as doggit & Regan say, that is probably a very bad idea for other reasons!)

Cheers all. Much appreciate the good advice.

First thing I’d say is that most TRVs are not suitable for use on a 1 pipe system.
Secondly just bite the bullet and repipe fully into a 2 pipe system.
Comfort levels and warmup times will be greatly improved so in the long run will be a more cost effective option

ASKI said:

Thanks, Zmicer. I think I'll be gradually replacing it with a 2-pipe. But something like that might help a bit in the short term...

One thing that bothers me, especially having a condensing combi, is I suspect quite a lot of the flow is just going round the loop without entering the rads, so goes back to the boiler fairly hot. Starting up from cold, the boiler is modulating almost straight away.. I don't think that any one pipe modifications can stop that (other than removing the bypass; but as doggit & Regan say, that is probably a very bad idea for other reasons!)

Cheers all. Much appreciate the good advice.

Click to expand...

One pipes are really relics of a bygone era. They will undoubtedly kill most of the efficiencies of a condensing boiler, because condensers work best with return temps being low. Your return temps will be too high. But poor economy is a secondary consideration to comfort.

Оne-pipe systems are seems simple, and it has many options.
So you need to act thoughtfully.