Increase pipe size?

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Could I have your advice on this matter please.
There a 7 radiators split between 5 rooms on the ground floor of my house and they are all slow to heat up. From looking under the downstairs floorboards it appears that the entire system is made up of 15mm pipework (not just the pipework to the rads).
The central heating flow and return pipes from/to the pump situated in my upstairs airing cupboard are also in 15mm. From the pump they run across the landing into a bedroom, then downstairs via the wall of the cloakroom below, to beneath the floorboards. What I would like to know is: Would the radiators heat up quicker if increased the diameter of the flow pipe (to 22mm) from the pump to just below the downstairs floorboards, and if I did this, would I also have to increase the size of the return pipe from the floorboards to the pump? Also, if the above is possible, are there any other problems this may cause?
 
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Could I have your advice on this matter please.
There a 7 radiators split between 5 rooms on the ground floor of my house and they are all slow to heat up. From looking under the downstairs floorboards it appears that the entire system is made up of 15mm pipework (not just the pipework to the rads).
The central heating flow and return pipes from/to the pump situated in my upstairs airing cupboard are also in 15mm. From the pump they run across the landing into a bedroom, then downstairs via the wall of the cloakroom below, to beneath the floorboards. What I would like to know is: Would the radiators heat up quicker if increased the diameter of the flow pipe (to 22mm) from the pump to just below the downstairs floorboards, and if I did this, would I also have to increase the size of the return pipe from the floorboards to the pump? Also, if the above is possible, are there any other problems this may cause?

It certainly seems that the pipes are undersizwhat boiler have got, ie what output. radiator sizes would also be helpfull
 
Close half the radiators. If the others DONT heat up you have other problems as well, such as sludge.

Traditionally, about 4.5kW was the max on 15mm.
 
The current rads are mostly 20+ years old and I intend to replace most of them - like for like soon. The total output of the new rads will be: Upstairs 6kw, downstairs 11kw, plus the existing emersion tank of say 3kw.
The boiler is a Vaillant 428, rated down to 21kw at present.
 
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Chris, the heat does eventually get to the rads but slowly. I took 5 of the 7 rads off and flushed them out about 6 months ago with slight improvement. The system was also flushed with a cleaner at the same time. I am pretty sure the small pipework is the main problem but I don't fancy replacing all of it, only what I can get at without too much problem.
 
Sounds like you have no choice but to put bigger pipe in, then. On average, rads are about 1.5kW so that's about 3, on 15mm, max.

While you're at it, use PROPER insulation on pipe in unheated spaces (under floors downstairs, eg). It'll save a lot of energy.
 
Thanks for that Chris, but do you think it will do any harm to just increase the section of flow pipe to 22mm from the pump to just under the downstairs flooring (from then on it will remain at 15mm) . And if I do this, will I have to increase the return pipe to 22mm from just under the flooring back to the pump? Or can I just leave the return pipe at 15mm?
 

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