Dhw pipe and cylinder sizing

[Hammertime]

Hello - just wanted to sense check my copper pipework and direct unvented cylinder before I install it. It's a one bedroom bungalow for two people. Incoming pressure and flow rate is 2 bar and about 25lpm.

 

Unfortunately due to space, the cylinder and the shower it will feed will be at opposite ends of the bungalow, so about 18m run with about six 90 degree bends including the one coming out of the cylinder. I'll do them as pulled bends to reduce friction. 

 

Having lived in a succession of houses with rubbish (electric) showers, for once I'd like a great shower experience, so I'm thinking 12-15lpm.

 

1) I'm thinking of doing the run in 22mm rather than 15mm. It'll be a longer wait for the shower to warm up, but it looks like the pressure drop in 15mm would be too great. Does this make sense? 

 

2) If the hot is done in 22mm then should the cold also be done in 22mm?

 

3) Manufacturer guidance for cylinder sizing seems conservative - 150l for a one bed house with shower. But two 10 minute 15lpm showers with 65 degree water mixed down to 40 would need 175l. And if we had guests... Is there any reason not to oversize the cylinder as much as possible - 250l or beyond?

 

Thank you for any advice you can give.

 

Edited 12 hours ago by Hammertime

[JohnMo]

31 minutes ago, Hammertime said:

I'm thinking of doing the run in 22mm rather than 15mm

Don't. We are about 20m as the crow flies and it's 15mm Hep2O. Zero flow issues and gets hot water quick enough. 22mm the delay getting hot water will drive you made.

 

31 minutes ago, Hammertime said:

but it looks like the pressure drop in 15mm would be too great. Does this make sense? 

So how are you calculating this?

 

31 minutes ago, Hammertime said:

 

2) If the hot is done in 22mm then should the cold also be done in 22mm

It should be balanced feed either take from the cold outlet on the cylinder combination valve or add an additional pressure regulator valve to the cold inlet to the house. If you do the additional pressure control valve you need to add a check valve to the DHW outlet.

 

Cylinder size upsize and don't heat as hot. Less chance of scalding anyone. There are two of us and we have 210L, we heat to 53 degs and we can get 3x showers out of it if we have guests

[Nickfromwales]

32 minutes ago, Hammertime said:

Hello - just wanted to sense check my copper pipework and direct unvented cylinder before I install it. It's a one bedroom bungalow for two people. Incoming pressure and flow rate is 2 bar and about 25lpm.

 

Unfortunately due to space, the cylinder and the shower it will feed will be at opposite ends of the bungalow, so about 18m run with about six 90 degree bends including the one coming out of the cylinder. I'll do them as pulled bends to reduce friction. 

 

Having lived in a succession of houses with rubbish (electric) showers, for once I'd like a great shower experience, so I'm thinking 12-15lpm.

 

1) I'm thinking of doing the run in 22mm rather than 15mm. It'll be a longer wait for the shower to warm up, but it looks like the pressure drop in 15mm would be too great. Does this make sense? 

 

2) If the hot is done in 22mm then should the cold also be done in 22mm?

 

3) Manufacturer guidance for cylinder sizing seems conservative - 150l for a one bed house with shower. But two 10 minute 15lpm showers with 65 degree water mixed down to 40 would need 175l. And if we had guests... Is there any reason not to oversize the cylinder as much as possible - 250l or beyond?

 

Thank you for any advice you can give.

 

Ola.

 

From the stopcock to the new UVC you need 22mm pipework, allowing the supply to the cylinder pressure reducing valve (Inlet Control Group) to be unaffected by other cold mains draw off's aka "cold mains priority".

 

Alter the house plumbing to have things like the dishwasher and washing machine fed off the 22mm cold mains, T'd off in 15mm as close to the stopcock as is possible. Do NOT take these off the balanced cold outlet at the UVC!!

 

At the control group you get balanced (3 bar pressure controlled) feeds to the cold input of the UVC, which becomes your 'hot out', and a balanced cold draw of to give you balanced hot & cold supplies to mixer devices; showers / monoblock taps etc. You cannot have the raw cold mains on one side of a mixer / monoblock and then the balanced hot, as you can get cold forcing its way back to the UVC and damaging it (backflow). This is all in the G3 installation guidelines with every UVC sold in the UK. It's at the control group that you can get starved of flow and pressure.

 

Off the top of the UVC you can take a hep2o 15mm pushfit line directly to the shower for hot, and same for cold from the control group, and then you will have long radii bends vs 90's, but then also zero joints on the way there. The hep2o inserts are very thin, stainless liners, so flow doesn't get strangled like it can with other systems using thick plastic inserts. You can put these alongside one another and instantly see the difference. You can convert to copper at the shower, or use hep2o all the way, just let us know what you have / are going to have, and we'll advise accordingly as to how to best terminate it. FYI I use hep2o in every job, and my typical projects run from just under to well over 7 figures. Never had a single issue, other than an odd "Friday" fitting that I changed on the same day.

 

For your cylinder size, go 210-250L if on gas, or 250-300L if on an ASHP. Better for everything to be storing more water at a lower temp for the same performance, imho. With gas you'll be fine for recovery / guest use, plus you'll be able to turn the immersion on for any time you are really under duress.

[Hammertime]

Thank you both for your detailed replies - much appreciated and very useful information. I'll be doing the pipework but getting a g3 qualified installer to connect up the cylinder, so this will help me check he's done it right. 

 

@NickfromwalesThe incoming mdpe is connected to the meter by 15mm copper - should this be upgraded to 22mm too or is the meter itself a bigger constraint so it doesn't matter? I don't have an external stopcock, which might complicate upgrades.

There's also a basin and toilet before the cylinder - if I understand correctly, I should take the toilet's cold from the raw incoming, but 'double back' for the sink by taking its cold feed from the balanced cold. 

 

@JohnMo My thinking for doing the shower run in 22mm was because the pressure drop tables suggest a loss of about 75mbar/m at 15mm, 25lpm. Over 18m this means 1.35 bar loss even in copper. At 2 bar incoming, the shower would be 0.65 bar max. Is your pressure perhaps better than mine?

 

 

 

[Hammertime]

EDIT: just realised I should have measured the pressure drop at 15lpm (shower flow rate), not 25lpm (incoming mains). But it still looks to be a chunky drop in pressure even before taking the bends into account and 2 bar isn't a lot to play with. And maybe velocity would become an issue too.

Edited 2 hours ago by Hammertime