Mixing 10mm microbore and 15mm pipes in CH

[Paul_Thomas]

Further to one of my recent posts discussing manifolds and distributing the how water around the house:-

Our existing 3 bedroom house has 10mm microbore throughout for central heating. This is now over 17 years old and still works 100%.
We are building a large extension to the side of the house which will require an additional 4-5 radiators (Inc en-suite).

We are having a new boiler fitted in the extension to supply the whole house. This will have a 22mm output pipe which will join the existing flow and return pipes.

Should I use 10mm microbore in the extension or 15mm instead?

Bear in mind the extension will get the hot water first as it's closer to the boiler, it will also be far more insulated than the existing house so will need less heat to bring it up to temperature.

Looking around at copper and connecting prices there doesn't seem to be much in it between them.
Microbore would be easier to run as I can go straight through the joists and there will be no joins under floorboards to leak. (Although the furthest radiator on the 1st floor will be 10m away from the 22mm pipe)

Thoughts/Opinions?

 

[andymarshal]

Im not a plumber but i did my own central heating system, using a 22mm send and return, branching off to 8 decent output rads with 10mm microbore, works a treat, also looks really neat as the pipes come out the wall behind the rad and not up from the floor.

One tip though, use inserts and pressure test before plasterboarding, also if drilling through joists allow plenty of clearance around the pipe, ie for 10mm use a 15mm drill.

Andy

 

[kevnurse]

Bear in mind the extension will get the hot water first as it's closer to the boiler, it will also be far more insulated than the existing house so will need less heat to bring it up to temperature.

Your rads should all have lockshield valves, each set slightly differently to ensure a balanced system. i.e, the rads nearest to the boiler will have the least open valves and the rad furthest will have a fully open valve.

The lockshield valves are the ones on the outlet from the rad and are normally housed in a plastic cover. The idea is that they are not used daily.

Its been a while since I last installed CH, but I don't think micro-bore and small bore are compatible. The pump is the key issue. In micro-bore systems the pump runs faster (due to smaller pipes) so any larger bore pipes in the system will unbalance it. The associated lockshield valves will have to be almost closed. I foresee squealing valves as the fast water squeezes through the narrow openings

 

[Paul_Thomas]

Its been a while since I last installed CH, but I don't think micro-bore and small bore are compatible. The pump is the key issue. In micro-bore systems the pump runs faster (due to smaller pipes) so any larger bore pipes in the system will unbalance it. The associated lockshield valves will have to be almost closed. I foresee squealing valves as the fast water squeezes through the narrow openings

It will be a combi boiler so the pump is integrated, I certainly haven't seen anything on forums or in documentation saying that Combi boilers need tweaking in some way to work with 15mm or microbore?

Seems it is best for me to run a 22mm feed and return into the extension and branch off that with 100m microbore. Certainly easier to route the pipework

 

[Paul_Thomas]

Im not a plumber but i did my own central heating system, using a 22mm send and return, branching off to 8 decent output rads with 10mm microbore, works a treat, also looks really neat as the pipes come out the wall behind the rad and not up from the floor.

One tip though, use inserts and pressure test before plasterboarding, also if drilling through joists allow plenty of clearance around the pipe, ie for 10mm use a 15mm drill.

Andy

What's an insert?

 

[ringer]

I also fitted my own heating, 22mm then 10mm microbore running from manifolds to rads. running with vailiant combi boiler. All working ok.
Also need to know what inserts are as just about to start plasterboarding.

 

[LeakyOldBoiler]

Ringer, guess this may be too late but you need to know!

Inserts are metal or plastic fittings which go into the end of the plastic pipe to ensure it does not get crushed or deformed, particularly when you use compression fittings (the pipe has little integral protection against compression at the ends). You can retro fit them and personally i would if you can still get to them.

For info Hep uses metal inserts and Speedfit/Guest uses plastic ones. All cheap and essential!

 

[stevesey]

Have a very similar arrangement to your planned extension. House was 22mm + 10mm, then for the extension I moved the boiler and used 22mm + 15mm + 8mm. The main extension feed is 22mm this splits into 2*15mm each for which then split into 8mm to feed the rads. This was done 7 years ago and all has worked fine since.

Couple of tips - separate the extension from the house with a couple of 22mm full bore valves. Very useful during the work and when you come to any work later you can easily isolate half the system - just found this useful today as I need to drain the system down to move rad pipes in bathroom, by closing the valves I only needed to drain half the system.

Also as Andy says allow plenty of space around pipes and insulate the joists to avoid ticking/creaking.

 

[Paul_Thomas]

Cheers guys.

I will take your suggestions on board.

Need to think where I can put the isolator valves for the extension now. (Maybe next to the boiler?)

 

[Paul_Thomas]

I've thought about this a bit more:-

My initial plan was to T off the main 22mm pipes from the boiler to the house and run a langth of 22mm for flow and return the length of the extension. Then T off as required down to 15mm and then reduce to 10mm for each radiator.

But, doesn't this defeat the original idea of 10mm microbore which I was told was to have less volume of water in the system and therefore heat up quicker? Or am I analysing this too much? (Perhaps with a modern combi condensing boiler the heating ability is so great that a large volume of water doesn't really matter?)

Another point would be that by having a manifold type arrangement in the roof above the boiler splitting from 22mm to multiple runs of 10mm (1 for each radiator) it would mean no joins in roof area/under the floor boards to leak at a later date. 10mm pipes could easily be run through the centre of joists etc.

Thoughts Gentlemen?

 

[stevesey]

If you go for the manifold arrangement you may end up with 10mm pipes running in parallel - which wil hold a similar volume of water. As as someone pointed out on a different thread - the heat loss is greater for 10mm pipe (surface area/volume = higher ratio).

Soldered joints shouldn't be an issue - as it's an extension you can easily plan appropriate hatches in the floor if you are that worried. If at all possible make sure you fill/test the pipework before the the ceilings go up (or at least before the artex) - that cives you a chance to address any issues.

 

[croydoncorgi]

Certain amount of 'uninformed' advice offered so far.

eg. the fact is that a circulating pump runs FASTER when the output isolator is completely closed than when it is pumping water around a tight loop at its maximum design rate. Moving the water counts as 'work' in physics terms; the more work done the more energy required. But since the pump has an induction motor, it SHOULD NOT slow down much under load anyway. Note also that running a pump against a closed valve is a Bad Thing because it uses the water flow (to some extent) for cooling.

In your case, I can't see why there would be any problem at all combining two different pipe designs, so long as the flow to each part of the system are balanced (ie. you don't want a situation where the lower resistance of the bigger pipes 'steals' all the water so that the rads on smaller pipes are starved. If you have microbore plumbed into a manifold, that shouldn't happen. Put an accessible gate valve on the Flow pipe to the 15mm part of the system and you'll be able to adjust it. Put another gate valve on the Return and you'll be able to isolate it too!

 

[mk2phil]

Im not a plumber but i did my own central heating system, using a 22mm send and return, branching off to 8 decent output rads with 10mm microbore, works a treat, also looks really neat as the pipes come out the wall behind the rad and not up from the floor.

One tip though, use inserts and pressure test before plasterboarding, also if drilling through joists allow plenty of clearance around the pipe, ie for 10mm use a 15mm drill.

Andy

Andy - how did you manage to get the pipes to come out the wall behind the rad? Does your house have a solid floor, and are the pipes sunk into the wall and plastered over?

Thanks

 

[andymarshal]

Hi, when i did the heating, the ceiling was down so i chased the pipes down the wall and then plastered over, all the pipes drop from above and come out behind the rad.

Cheers

Andy