20mm MDPE okay for unvented?

[benraz69]

Hi, I have a 20mm MDPE incoming main, which reduces down to 15mm copper straight after the stopcock. Will this be adequate to supply to an unvented cylinder?

(Family of three, with two shower rooms, though both are never/very rarely used at the same time. Would be easy enough to change the 15mm pipework between stopcock & cylinder, if that would improve things).

Tests below done using an outside tap (with kitchen tap fully open for dynamic pressure/flow readings). Stopcock is at the front of the house & kitchen/outdoor tap are at the back. Cylinder will be on the first floor, almost directly above the stopcock.

10th Mar @ 6pm:
• Static pressure = 3 bar
• Static flow = 22+ lpm
• Dynamic pressure = 2 bar
• Dynamic flow = 12 lpm

16th Sep @ 8pm:
• Static pressure = 2 bar
• Static flow = 12-14 lpm
• Dynamic pressure = 1.6 bar
• Dynamic flow = 10-12 lpm

 

[Madrab]

Not sure what you mean by static flow - there isn't such a thing TBH

Dynamic pressure and flow are the key elements. I always state that the entry level for a good unvented system is 2bar @ ~ 20 L/Min dynamic.

As a standard, a suitable supply for an unvented would be a 22mm feed to the cylinder and a 22mm feed from the cylinder out to the branches to the outlets. The dynamic pressure is lowish @ 1.6bar, what does that drop to if 2 outlets are open, one being upstairs? Same with a dynamic flow reading, run the outside tap and then have 2 outlets running inside with the upstairs one being the measure.

This would be a truer real life test, so if say the shower was on, the washing machine kicked in and the sink tap is used. It's the impact to the shower that's the important one as that is what would be more noticed. You don't want to go to the effort and expense of installing an unvented for it then to not perform as you would want it to.

 

[Scally-Ho]

What size main connection is your water supply company charging for? If it's only a 15mm supply, then the rest of the pipework is mostly irrelevant.

 

[Madrab]

I presume the 15mm is internal after the mains stop tap, with the incoming main from out in the street being 20mm MDPE?

 

[benraz69]

Not sure what you mean by static flow - there isn't such a thing TBH

Dynamic pressure and flow are the key elements. I always state that the entry level for a good unvented system is 2bar @ ~ 20 L/Min dynamic.

As a standard, a suitable supply for an unvented would be a 22mm feed to the cylinder and a 22mm feed from the cylinder out to the branches to the outlets. The dynamic pressure is lowish @ 1.6bar, what does that drop to if 2 outlets are open, one being upstairs? Same with a dynamic flow reading, run the outside tap and then have 2 outlets running inside with the upstairs one being the measure.

This would be a truer real life test, so if say the shower was on, the washing machine kicked in and the sink tap is used. It's the impact to the shower that's the important one as that is what would be more noticed. You don't want to go to the effort and expense of installing an unvented for it then to not perform as you would want it to.

Sorry, by “static flow” I meant flow with one outlet open & by “dynamic flow” I meant flow with two outlets open.

Having looked at the internal pipework, it appears to be as you said - 22mm hot & cold everywhere, going down to 15mm at the final branches (with the exception of the 15mm incoming main ofc). 22mm pipework will be run all the way to both shower locations as part of the renovations (if they aren’t already).

I will try those tests you suggested this evening, although I’m not sure there are any other outlets at mains pressure (apart from the kitchen sink & the outdoor tap).

Yes, you are correct in your second reply - 20mm MDPE incoming from street, then 15mm internal after the stopcock. Would changing the internal stopcock & running 20mm MDPE/22mm pipework up to the cylinder improve things?

 

[Madrab]

Sorry, by “static flow” I meant flow with one outlet open

Ah ok

although I’m not sure there are any other outlets at mains pressure

Is your whole system not mains fed? Or is upstairs fed from a Cold water cistern?

Would changing the internal stopcock & running 20mm MDPE/22mm pipework up to the cylinder improve things?

It may - I say may because 15mm pipework will flow plenty of water, the trouble is forcing more water through the narrower pipe tends to need more pressure due to friction, therefore if the dynamic pressure isn't high to start it can have a greater impact on the dynamic figure. That and to maintain flow @ higher rates the water velocity needs to increase and that causes more noise and wear - not ideal.

The best approach IMO would be from your 20mm MDPE, increase the stop tap to 22mm full bore and run that all the way up to cylinder. Then take a balanced feed from the control set back out in 22mm to feed all the cold outlets with the 22mm supply from the cylinder out to all the hot. That would optimise the system.

 

[benraz69]

Ah ok

Is your whole system not mains fed? Or is upstairs fed from a Cold water cistern?


It may - I say may because 15mm pipework will flow plenty of water, the trouble is forcing more water through the narrower pipe tends to need more pressure due to friction, therefore if the dynamic pressure isn't high to start it can have a greater impact on the dynamic figure. That and to maintain flow @ higher rates the water velocity needs to increase and that causes more noise and wear - not ideal.

The best approach IMO would be from your 20mm MDPE, increase the stop tap to 22mm full bore and run that all the way up to cylinder. Then take a balanced feed from the control set back out in 22mm to feed all the cold outlets with the 22mm supply from the cylinder out to all the hot. That would optimise the system.

Sorry, I probably should’ve mentioned I have a gravity fed system with conventional boiler & copper cylinder. Looking to swap to an unvented cylinder to improve the showers & have drinking water at all outlets.

I tested again at 4pm today. I opened the kitchen tap, outdoor tap & bath tap (I appreciated the bath is gravity fed, but opening it would’ve caused the tank in the attic to fill, which is mains fed obviously). I took the readings at the outdoor tap and got a dynamic pressure of 1.7 bar & a dynamic flow of 12 lpm.

Your advice sounds good r.e. the pipework, although you mentioned a 22mm full bore stop tap - I assumed all stop taps were full bore, is that not the case?

 

[muggles]

With numbers like that you'd likely be better off going with a 35kW-ish combi boiler. An unvented cylinder wouldn't give you any more performance than a combi would

 

[Madrab]

I assumed all stop taps were full bore, is that not the case?

Nope - unfortunately not.

Whilst I wouldn't necessarily disagree with the combi approach the only thing I would add to that is that a combi probably wouldn't give you 12L/min dynamic hot water @ ~50deg @ multiple outlets

With the mains supply at that level then I wouldn't usually recommend an unvented as it really is borderline, usage would need to be managed quite carefully. That could also be said about a combi as they do struggle to supply more than one HW outlet at a time.

 

[benraz69]

Nope - unfortunately not.

Whilst I wouldn't necessarily disagree with the combi approach the only thing I would add to that is that a combi probably wouldn't give you 12L/min dynamic hot water @ ~50deg @ multiple outlets

With the mains supply at that level then I wouldn't usually recommend an unvented as it really is borderline, usage would need to be managed quite carefully. That could also be said about a combi as they do struggle to supply more than one HW outlet at a time.

Think I might change the stopcock over as a matter of course, as it’s quite stiff anyway. Struggling to find one online that confirms it’s full bore. Would it be okay to swap it out for a 22mm lever valve instead?

 

[Madrab]

Would it be okay to swap it out for a 22mm lever valve instead?

Absolutely, just ensure it's WRAS approved.