Gravity system - Move Hot Water Cylinder to Roof Space?

[arjc1]

We've had conflicting advice from two plumbers - I wonder if anyone here can clarify.

We have a gravity fed system system, currently with the hot water cylinder alongside the boiler which is in a separate 1 story room with some roof space. We're looking at moving the cylinder into the roof space of that room but all the piping to the rest of the house (including the feed from the cold water tank and the hot water feed to the rest of the house) enters at floor level in that roof space and it's not practical to move it. One plumber is quite happy to move the cylinder up there, the other says its not possible because the plumbing to the rest of the house will be at floor level.

Very simplified diagrams of before and after attached below.

I've thought about this now and I can't see the problem with moving the cylinder because the header tank is well above everything - but I'd appreciate any expert thoughts.

 

[steadyon]

1. In a gravity fed system, the pressure, hot and / or cold at non-mains fed outlets is determined solely by the vertical distance between the outlet and the cold water storage cistern (CWSC - what you have called the header tank). Most people measure from the base of the CWSC, as that is the most reliable measure as the cistern can never be worse than empty. Strictly speaking the measure should be from the top as that gives the actual pressure from a full cistern, but of course as it empties the pressure becomes less.
2. The flow rate at non-mains fed outlets depends on the pressure, but is subject to the resistance to flow of the pipe work. So narrow pipes, restrictive valves and numerous bends / elbows will all reduce the flow, even though the static pressure remains the same.
3. Given the above, your proposed scheme would make no difference to the available pressure at the outlets shown. Provided there are no overly restrictive pipe runs, I see no reason why it shouldn't work.

 

[Chuck]

The 'inverted-U' you've created in the DHW outlet section is likely to cause potential problems. Over the course of time air will collect in there reducing the effective diameter of the pipe and your flow rate will drop noticeably.

 

[snowhead]

What happened to the vent from the top of the cylinder to the CW Storage?

 

[Ben-gee]

As long as you maintain the vent, in a continuous rising manner, there is no problem moving the cylinder up a floor - the usual attention to pipe runs that one should consider with gravity systems will obviously apply.

 

[arjc1]

Thank you all for your comments on here. I had missed the point about the vent pipe to the CWSC. That will also have a U in it in the proposed installation in the roof space because the pipework to CWSC also comes into that roof space at floor level. I presume that will give an issue with trapped water in the U.

 

[steadyon]

It mustn't have a U in it. It must rise continuously from the hot water cylinder to the inverted J with no obstructions or valves of any sort.

 

[Ben-gee]

So not like this then!

someone swapped a 36” cylinder up to a 60” and this is how they altered the take off/vent. AFAIK it’s been there for years like this, but I have now taken it out.
Really lazy as they could easily have done it better even if it looked bad , it’s in an 18” service void that gets covered in.