Reducing 22mm pipe to 15mm

[GC2]

Hi,

We've recently had a pressurised hot water cylinder installed. When we use the kitchen tap it's quite a long run so the hot water takes a long time to run hot. I could fairly easily reduce about 7 metres of 22mm pipe down to 15mm (it already reduces to 15mm upstairs anyway before dropping down to the ground floor). Would you recommend this?

Thanks

 

[Ric2013]

7 metres of 22mm pipework is only around 2.1 litres. Reducing to 15mm will roughly halve that volume.

Is it going to be a lot of effort to do?

 

[steadyon]

1. If you already have some 15 mm in the run, replacing 22 with further 15 won't make any difference to flow.
2. Also, with only 7 metres to play with, it's not going to make a huge difference to the hot water wait.
3. If you could use the difference in diameter to install better insulation, that would make some difference as well, depending on frequency of use.
4. The only genuine (but expensive) solution would be to install a secondary circulation system.

 

[Scott_D]

Can you run a dedicated 10mm pipe from the cylinder to kitchen sink?

 

[GC2]

1. If you already have some 15 mm in the run, replacing 22 with further 15 won't make any difference to flow.
2. Also, with only 7 metres to play with, it's not going to make a huge difference to the hot water wait.
3. If you could use the difference in diameter to install better insulation, that would make some difference as well, depending on frequency of use.
4. The only genuine (but expensive) solution would be to install a secondary circulation system.

Thanks for the replies. Hmm not sure if it's worth it but it would be easy to do.
I could insulate a fair bit of the run but the hot tap is not used all that often; I tend to fill a basin 2 or 3 times a day. So again not sure if that's worth it. I looked at installing a secondary return pump but access in the kitchen is extremely tricky so lagging it fully would be difficult/impossible. Should I just put up with it and accept the waste of energy/water?

 

[john the plumber]

Hi,

We've recently had a pressurised hot water cylinder installed. When we use the kitchen tap it's quite a long run so the hot water takes a long time to run hot. I could fairly easily reduce about 7 metres of 22mm pipe down to 15mm (it already reduces to 15mm upstairs anyway before dropping down to the ground floor). Would you recommend this?

Thanks

Easier to add a secondary return

 

[GC2]

Can you run a dedicated 10mm pipe from the cylinder to kitchen sink?

Hadn't thought of that. Probably could. How close to the unvented cylinder are you allowed to T off? Access to the kitchen taps is stupidly difficult but possible, of course.

 

[green grant]

This happens to everyone, its what it is. Water in pipe goes cold, cold water is pushed out ahead of hot water..its not rocket science is it!. So you can have a source of hot water right next to the sink so heating on demand. Costs around £150. Next option is to move the slug (is that a plumbing term?) of cold water around the pipes so it goes back into the tank and is replaced by hot water. Same process applies but this time you preempt the demand and have as mentioned before, re-circulation. This re circulation can be either a dedicated pipe from the furthest outlet back to the tank/boiler OR you can have a bypass valve fitted at the furthest outlet. Here is a video example. Forget the brand names, many do this.

 

[green grant]

i

Hadn't thought of that. Probably could. How close to the unvented cylinder are you allowed to T off? Access to the kitchen taps is stupidly difficult but possible, of course.

if everyone has already said that 15-22 will make no difference, how will 10mm make any difference?

 

[Ric2013]

if everyone has already said that 15-22 will make no difference, how will 10mm make any difference?

Good question. I suppose we are assuming there is other pipework other than the 7m of 22 that needs to be purged when you open your kitchen tap. If 7m is the total length of run from the cylinder to the tap, then you're "only" wasting 2l of hot water per time with 22, and 1l with 15. 10mm will probably save 50% compared with 15mm, but it's diminishing returns.

The secondary return idea would save water, but would of course waste more heat.

The other thing you can do if you are genuinely considering the environment (and ongoing costs?) is fully reconsider your pipe runs from the cylinder onwards and how your taps are used. What I did in my own house was run a dedicated 22mm run (had to be 22mm to get enough flow as not a mains pressure system) to the bath tap (as only used once a week) and a separate 15mm run to the washbasin (used several times a day) and continue the 15mm to the kitchen sink. This way, the kitchen tap only had a small deadleg from the bathroom washbasin pipe (which was almost in constant use) onwards.

Out of interest, what's the litres per minute from your hot kitchen tap and how long are you having to wait for hot water?

 

[green grant]

Good question. I suppose we are assuming there is other pipework other than the 7m of 22 that needs to be purged when you open your kitchen tap. If 7m is the total length of run from the cylinder to the tap, then you're "only" wasting 2l of hot water per time with 22, and 1l with 15. 10mm will probably save 50% compared with 15mm, but it's diminishing returns.

The secondary return idea would save water, but would of course waste more heat.

The other thing you can do if you are genuinely considering the environment (and ongoing costs?) is fully reconsider your pipe runs from the cylinder onwards and how your taps are used. What I did in my own house was run a dedicated 22mm run (had to be 22mm to get enough flow as not a mains pressure system) to the bath tap (as only used once a week) and a separate 15mm run to the washbasin (used several times a day) and continue the 15mm to the kitchen sink. This way, the kitchen tap only had a small deadleg from the bathroom washbasin pipe (which was almost in constant use) onwards.

Out of interest, what's the litres per minute from your hot kitchen tap and how long are you having to wait for hot water?

flow rate from the tap of say 12l/m and 2l water in the pipe (your claim) then that means water comes out at 200ml/second. So it would take 2/.2 = 10 seconds to run hot. I would have though that is not a long wait.

 

[Scott_D]

i

if everyone has already said that 15-22 will make no difference, how will 10mm make any difference?

Minimal cold will be drawn off before you get hot water.
No flow reduction to other outlets fed from the existing pipework.

 

[Ric2013]

flow rate from the tap of say 12l/m and 2l water in the pipe (your claim) then that means water comes out at 200ml/second. So it would take 2/.2 = 10 seconds to run hot. I would have though that is not a long wait.

But, in practice, some kitchen taps do NOT have a high flow rate, particularly mixers. Hence asking the question.

 

[green grant]

But, in practice, some kitchen taps do NOT have a high flow rate, particularly mixers. Hence asking the question.

so in essence, all of these questions can be answered by math and a few simple experiments

 

[gpbeck]

so in essence, all of these questions can be answered by math and a few simple experiments

Doctor Dolittle. As I live and breathe!

 

[GC2]

Thanks again for all the replies. I've used the tap today so difficult to test at the moment. Flow rate is 18l/m and I would guess about 25-30 seconds to get hot. It's more the waste of water I'm bothered about so the 10mm dedicated run appeals. It could bypass a bathroom currently on the same run that is fed with 22mm. and take a shorter route to the kitchen and feed a downstairs toilet basin on its way

 

[Ric2013]

Thanks again for all the replies. I've used the tap today so difficult to test at the moment. Flow rate is 18l/m and I would guess about 25-30 seconds to get hot. It's more the waste of water I'm bothered about so the 10mm dedicated run appeals. It could bypass a bathroom currently on the same run that is fed with 22mm. and take a shorter route to the kitchen and feed a downstairs toilet basin on its way

Ah yes. That does change things. Throwing out 7.5-9l does indeed feel wasteful, and it's annoying to have to pour away water for 30 seconds!

You'd then be bypassing far more than the 7m of pipe then, I take it.

 

[GC2]

Ah yes. That does change things. Throwing out 7.5-9l does indeed feel wasteful, and it's annoying to have to pour away water for 30 seconds!

You'd then be bypassing far more than the 7m of pipe then, I take it.

Yes there is about 7 or 8m of 22mm feeding the bathroom before it reduces down to 15mm and tees down to the downstairs toilet and kitchen. Annoyingly it reduces to 15mm before the bath anyway but the flowrate is fine

 

[Ric2013]

Yes there is about 7 or 8m of 22mm feeding the bathroom before it reduces down to 15mm and tees down to the downstairs toilet and kitchen. Annoyingly it reduces to 15mm before the bath anyway but the flowrate is fine

22mm should be fine for a bath now you are mains pressure. 22mm was probably used when it was a low pressure system.

In any case, even when installed that probably wasn't strictly wrong: a short section of smaller bore pipe does not have the same effect as the entire length being in a smaller bore. Long story, but take my word for it