Suitability of Unvented Cylinder

[johnbusby]

Hi

I've read through dozens of threads on here about unvented systems but was wondering if someone might provide some advice on an upgrade we are planning as part of a bathroom refurb

Current Setup: Gravity fed system with cold water tank in the loft and hot water tank in the hot press (1st floor). Everything works okay although pressure obviously isn't great so we've got electric showers which are mains fed. Heating is provided by an oil-boiler which is in the garage. Our mains comes in on a 22mm pipe and then goes to 15mm straight after the stopcock (which I know isn't ideal)

Proposed Setup: Move to hot an unvented system and 2 mixers showers fed off the cylinders (bath bathroom are on 1st floor). It's not essential that both showers will work at the same time although would be nice. I've a plumber lined up to do the work and have just asked him whether he has the G3 ticket. He hasn't replied to that direct question yet which isn't promising, and hasn't actually done any flow / pressure tests which is a bit concerning. For the avoidance of doubt I'll not be letting him near the installation unless he has the G3 accreditation.

Mains water readings: To satisfy myself that I'm not going to end up wasting a load of money I bought a pressure gauge and Weir cup. The readings I have so far:

Static Pressure (measured at outside tap) : 2.2 bar
Dynamic Pressure (Kitchen tap open supplying 10 litres): 1.6bar
Dynamic Pressure (Kitchen tap and utility open supplying 18 litres): 1.2bar
Flow rate: 26 litres/min

Question: Is this setup likely to work well with an unvented system? Are there any other readings that I need to take to ascertain if it's suitable?

Many thanks in advance to anyone who takes the time to reply and help us out.

All the best

John

 

[Ric2013]

Your line of thinking re standing and working pressure is spot on. Bear in mind that if you have taken pressure readings on the ground floor, you need to reduce the result by 0.2-0.3 bar to allow for the fact that the showers are 2-3 m higher up, and also there may be a slight further loss due to the flow resistance of any pipework in the house (and the cylinder, valves etc.) between the stopcock and the showers.

2.2 bar is rather low as a standing pressure, but it seems to retain a reasonable pressure when supplying 10 litres per minute. That said, 1.6 bar (which you won't quite have at your shower inlets) will not give a particularly high flow through a shower mixer, but (with the right shower mixer) should still compare favourably with electric shower flow rates. Have you checked these results at peak times when neighbouring houses are also running water?

I'm a bit confused as to how you are measuring 26lpm. What is this figure based on?

I'm wondering how the 'upgrade' to an unvented cylinder compares favourably with a vented cylinder with, perhaps a pumped hot water distributing pipe to your showers, but then I think it's fair to say I am firmly in the camp that can't really see the point of unvented cylinders, so I accept I may be biased. (FWIW I have 22lpm to my hot bath tap, running on an unpumped vented cylinder).

 

[johnbusby]

Many thanks for the reply, and glad to hear that I'm not completely barking up the wrong tree with my measurements! I've checked these measurements at various times throughout the day and they seem to be fairly consistent. They are indeed taken at the ground floor so thanks for pointing out they will be a bit lower by the time the water gets up to the shower.

In terms of the flow measurement - The outside tap was completely overwhelming the Weir cup on it's own, so instead I opened all three main-fed taps at the same time (outside, kitchen and utility) and measured the flow at each of them before adding them together (from memory think it was 18+4+4 litres/min respectively) to get the total. Does that seem reasonable?

Our initial plan was to just pump from the vented cylinder but we'd need to buy a larger hot water tank (and potentially cold water tank?) plus an expensive pump (was initially specing the ST Monsoon negative head 3 bar) so it wasn't too much extra to go unvented with the added benefit of taking the pump noise out of the picture, getting rid of the loft tank etc. I'm not averse to reverting back to that plan if the unvented isn't a good choice for our circumstances.

We had picked some rather large overhead shower heads (12x12 inches) alongside a separate smaller handset on a rail - I get the feeling from what you are saying that this system won't provide the skin tingling showers we were hoping for. With that being said we've got a fairly poor electric shower at the minute, so our expectations are somewhat tempered by that! Our shower mixer is made by Vado who are at least a British company so do seem to be geared towards slightly lower pressure systems. The specification sheet for the valve shows a minimum operating pressure of 0.2 bar and about 25 litres flow per minute at 1.5bar 'for mixed measurements' but afraid I don't have the necessary knowledge / experience to understand what that will mean in real life with our mains supply...

 

[John.g]

"Static Pressure (measured at outside tap) : 2.2 bar
Dynamic Pressure (Kitchen tap open supplying 10 litres): 1.6bar
Dynamic Pressure (Kitchen tap and utility open supplying 18 litres): 1.2bar
Flow rate: 26 litres/min"

Can you measure the dynamic pressure at the kitchen tap with it closed but any/all the other taps open as you may get a more accurate dynamic pressure with the pressure gauge in "still" water (kitchen tap closed), after measuring the flow rates then confirm static pressure again at 2.2 bar or whatever.

 

[johnbusby]

Thanks for your reply. I think I'm probably misunderstanding what you mean but I've no way to test the pressure at the kitchen tap as the (cheap) pressure gauge I bought from Screwfix screws on to the end on the outside tap.

 

[John.g]

OK, maybe misunderstanding myself!. but something a bit strange, a dP of 0.6bar (2.2-1.6), is giving a flowrate of 10LPM and a dP of 1.0bar (2.2-1.2) gives a flowrate of 18LPM?. By calculation, since dP is proportional to the square of the flowrate then the dP at 18LPM should have been (18/10)^2 X 0.6, 1.94bar which leaves a dynamic pressure of 0.26bar, (2.2-1.94) and not 1.2bar and theoretically the max flowrate (at 2.2bar dP) is 19.1LPM.

 

[johnbusby]

Apologies, the math has slightly gone over my head! I would say that I'm a novice with the Weir cup, and it seems to only be useful to give approximate flow measurements anyway as it's inherently difficult to read. I tried to be conservative in reading it (i.e. err on the side of underestimating the flow) - could that be responsible for the theoretical discrepancy that you're seeing? More than happy to get a bucket and stop watch out if need be to try and measure the flow more accurately.

The pressure gauge is obviously much easier to read so I'd be more confident that those figures are spot on. Thanks again for replying - it's much appreciated.

 

[John.g]

Can't beat the bucket and stopwatch IMO, pressure gauge should be accurate, presuming that there isn't a non return valve installed on the supply to the outside tap?.

 

[Ric2013]

Can you measure the dynamic pressure at the kitchen tap with it closed but any/all the other taps open as you may get a more accurate dynamic pressure with the pressure gauge in "still" water (kitchen tap closed), after measuring the flow rates then confirm static pressure again at 2.2 bar or whatever.

You've confused me too, to be fair. I'm assuming you mean test the pressure at the tap itself. But the tap would have to be open. Or do you mean test the pressure directly below the closed tap, adding a means of connection to the pipework if necessary?

Or do you mean, connect to the tap, run other taps, and then close the tap you are measuring at before taking a pressure reading? Are you suggested using the kitchen tap to avoid the potential problem of a check valve on the outside tap which would lead to an inaccurately high reading?

 

[John.g]

If the PG was teed off any of the running taps, (before the tap) then the dynamic reading will generally be lower than the true reading taken say from before a tap that's not running, taking the pressure from a (outside) tap downstream of any nrv isn't a good idea either.
There is definitely something strange with the above readings, I took a set of readings some time ago from a mains fed upstairs 3/4ins bath tap and it flowed 14.4LPM at a dP of 2.65 bar (3.6/0.95) and when throttled in to give a dP of 0.9bar (3.4/2.5), a flowrate of 8.4LPM, almost exactly as the theoretical calculation showes, 14.4*sqroot(0.9/2.65), 8.39LPM.

Above, post#1 gives a flowrate of 10LPM at dP of 0.6bar (2.2-1.6) and 18LPM at dP of 1.0bar (2.2-1.2) which IMO cannot be correct as it should only give a flowrate of 10* sqroot(1/0.6), 12.9LPM or else, if the 18LPM is the flowrate then the dP is 0.6* square(18/10), 1.94bar so dynamic pressure of 0.26bar (2.2-1.94)?.