Cylinder pipe sizing for new hot water system

[bureaucrazy]

Current system: Combi boiler with direct cold water all on 15mm pipes. 28mm outside diameter lead pipe from supply converted to 15mm copper to kitchen tap and then onwards to bathroom.

Proposed new system: No more instant hot water from combi. Replace with hot water cylinder with 2 heat exchangers -1 from heating circuit and the other from solar thermal.

I spoke briefly to a plumber and it left me a bit confused so I hope to get some answers here.

He seemed to recommend fitting an unvented system with indirect cold water. He said the pipes would need to be 22mm for both hot and cold coming from the tank and cylinder as it would not have enough pressure for 15mm pipes.

Q1, is there any reason why I need to change to indirect cold water when I already have a good direct cold water system? So can I just run a new 22mm hot pipe and keep the old cold pipes?

Q2, once the 22mm hot pipe reaches the bathroom can it branch off to 2 basins and a seperate mixer shower with 15mm branches? How long can the branches be?

Q3, I have a 15mm pipe already going to the loft feeding an electric shower. Can I branch off this to fill the header tank/water cylinder?

 

[Stigster]

Q1, No good reason I can think of other than if you have a very poor incoming mains water supply. If that's the case simply installing an unvented cylinder as an open vented system will not gain you much in the way of performance. (If I understand correctly by what you say about "Indirect cold water".)

Q2 Yes that's fine and they can be pretty much as long as you need but for very long runs then maybe a secondary hot return system might be wise.

Q3 Yes but if your mains is good enough e.g. 3 bar + dynamic pressure and 22 -25 litres per minute flow rate you won't need a cold water cistern.

 

[bureaucrazy]

Q1, No good reason I can think of other than if you have a very poor incoming mains water supply. If that's the case simply installing an unvented cylinder as an open vented system will not gain you much in the way of performance. (If I understand correctly by what you say about "Indirect cold water".)

Q2 Yes that's fine and they can be pretty much as long as you need but for very long runs then maybe a secondary hot return system might be wise.

Q3 Yes but if your mains is good enough e.g. 3 bar + dynamic pressure and 22 -25 litres per minute flow rate you won't need a cold water cistern.

Thanks for your answers. I think he meant a traditional system with a header tank in the loft, but it seems I can have this tank + cylinder just to supply hot water and carry on with the cold as is.

RE Q2, the longest 15mm branch off the 22mm hot pipe would be about 4m to reach a basin in the next room. By secondary hot return, do you mean the hot goes back to the cylinder in a loop to keep the standing water warm?

Your third answer suggests installing an unvented pressurised cylinder? From what I understand, I would need to run a 22mm cold pipe from the stopcock to the cylinder to give it adequate pressure. (So, I cannot make use of the existing 15mm shower supply pipe that goes up into the loft) However, the incomming pipe is lead, about 28mm wide (externally) but is adapted immediately to 15mm copper. Despite that, I can fill a 10 litre bucket in about 20 seconds from the nearest tap so I guess the pressure is good. I might have to get the adapter taken off and a 22mm put on there. Send the 22mm to the cylinder and branch off a 15mm into the existing cold pipe run to the taps?

Clearly a job for a professional but it helps to be able to speak the same lingo.

One more question: Q4, if I go for a pressurised cylinder, can the hot water output run in 15mm and connect into my existing 15mm hot water pipes instead of the combi boiler to supply the hot taps that way? Or do I need to run a 22mm as with the open vented arrangement?

 

[bureaucrazy]

He seemed to recommend fitting an !unvented! system with indirect cold water. He said the pipes would need to be 22mm for both hot and cold coming from the tank and cylinder as it would not have enough pressure for 15mm pipes.

Sorry, I meant to say he was recommending a VENTED traditional system with a standard cylinder (albeit with 2 heat exchangers) and a header tank in the loft.

I got muddled up with all this plumbing lingo vented unvented etc.

Sorry for confusion, does this change any of your answers?

 

[Stigster]

Scratch that, I see you've asked some more questions. I'll have another read and see if I can answer.

 

[Stigster]

Q1 If you have mains fed cold and tank fed hot you will have unbalanced pressures which will likely cause problems with showers and mixer taps if you have any. Best to feed hot and cold from the same systems to keep pressures equal.

Q2 That would be fine, 22mm to bathroom and branch off to outlets with 15mm. As for the distances, in any normal domestic setting 15mm branches going for 4 metres is fine. Again, if you are using mixer taps you would want the hot and cold coming from the same water supply to keep the pressures balanced.

Q3 I think branching off the electric shower feed will be fine for filling the cistern although I would like the input of others here on that one to confirm but I think it would be ok.

You need 22mm pipe to feed the hot water cylinder and 22mm cold feed from the cold water storage cistern to the bathroom before branching off to outlets in 15mm for performance reasons so the plumber is correct there.

As for the secondary return, the performance at the hot outlets is the important factor. I don't think there are an actual figures on distance but your hot water needs to reach 50C within one minute of opening the tap. In most normal domestic situations that is achievable without a secondary return (you described that correctly BTW).

 

[Stigster]

Q4 The outlets on an unvented (pressurised) cylinder are 22mm diameter and it would be preferable to run the hot distribution pipe in 22mm to the bathroom and then branch of with 15mm to the outlets. The bath would fill more quickly with 22mm to the outlets and is the usual way of doing things but 15mm may be adequate. It's good practice to run 22mm feeds to bath really though.

Provided you've got 3 bar dynamic pressure and around 25 litres per minute flow rate I really would be looking at an unvented system as performance is better all round in comparison to an open vented "traditional" system.