Suitability of Unvented Cylinder

[johnbusby]

Good morning everyone and thanks again for the responses so far. I've gone away over the weekend and bought a bucket to try and measure flow a bit more accurately! I did this by opening all three mains tap on the bottom floor (outside, kitchen, utility) and collected water for 10 seconds from each of them. I then measured how much water was in the bucket by decanting into a measuring jug and multiplying by six to give a flow measure per minute. Doing this has largely confirmed the measurements above (albeit with slightly less flow than I'd originally thought):

Flow Rate: 23 litres/min (down from 26 litres when measured previously using the Weir cup)
Static Pressure: 2.2bar (measured from outside tap)
Dynamic Pressure: 1.6 bar when taking 10 litres/min off the kitchen tap
Dynamic pressure: 1.2bar when taking 16 litres/min off both the kitchen and utility tap down from 18 litres when measured previously using the Weir cup)

Question: I realise that these aren't stellar numbers, however am I likely to get reasonable performance out of an unvented cylinder?

All the best

John

 

[Ric2013]

The question is what sort of pressure drop you would experience through the cylinder, installation and associated pipework itself. My hunch (and no more than a hunch) is that you'd lose perhaps another half a bar, so you'd still have 1 bar working pressure at 10 litres per minute. Which means it would be safe to say you could have one reasonably powerful shower off that unvented, or two piddly showers. You'd need to check with tap and shower mixer manufacturers to see what sort of flow rate they might anticipate at that kind of working pressure at tap/mixer inlets. Mira does some shower mixers which have useable flow rates down to 0.2 bar, which is why I suggest you may get a quite reasonable shower at 1 bar.

But certainly worth checking my hunch with someone with experience of unvented cylinders and how they actually perform.

 

[John.g]

10 LPM through the shower head may be be about the size of it.
If you allow 0.5bar static head (to bathroom), 0.26bar pipework loss, 0.25bar cylinder loss and shower head pressure drop of 0.5bar, total 1.51bar, dP of 0.69bar, (2.2-1.51). Based on above [email protected] dP gives [email protected] dP.
Easy to say of course but would be far happier with a static head of 3bar or more.

You can also do a test with the electric shower, especially the one upstairs, measure its flowrate with the temperature (flow) control valve to fully cold with both shower head attached and unattached.

 

[johnbusby]

You can also do a test with the electric shower, especially the one upstairs, measure its flowrate with the temperature (flow) control valve to fully cold with both shower head attached and unattached.

Interesting - had wondered if I could measure the flow at the electric shower but worried it was restrictive in some way. With the shower head off I'm getting just less than 10 L/min flow, this drops to 8 L/min if I open up the kitchen top fully. Does this represent what I'll actually get with the new installation, or do these figures represent a lower bound of whats possible (if the electric shower is restrictive in some way)? Is this kind of flow likely to lead to a reasonable shower - I'm guessing that running a couple of showers at once is completely off the table!?

The valves we've speced are from Vado (Phase model), which I think are geared more towards lower pressure systems and the minimum pressure is only 0.2 bar which might help squeeze the most out of system?

Many thanks for your help again

 

[John.g]

My electric shower flows 8.4/9.6LPM with shower head on/off but these showers are probably restrictive as the they normally have a flow control valve.
I don't think realistically you have a hope of running two showers except you install 5LPM flow restrictors in each but only advantage with that is that you can have 2 showers running together but at not much greater flow rates that your existing electric shower flows, a 9kw gives ~ 3.7/5.2 LPM winter/summer, at 40C. You will have no PRV on the incoming mains to the unvented cylinder.

Some people who only require high shower flows install integrally pumped showers with a cold & hot feeds from the CWST and the vented HW cylinder, Triton make a Novel SR (was the AS2000XT) which give flow rates of up to 14LPM, I know of several of these very satisfactory installations. Of course you can also consider whole house pumped hot&cold from the same CWST & vented HW cylinder, a good quality 3 bar pump should give years of service.

 

[johnbusby]

To be honest the idea of having two showers running at the same time was always a bit of a pipe dream (pardon the pun). Is it safe to assume that we'll get one decent shower out of this new tank, and that it will still be decent if someone opens a tap elsewhere or flushes a toilet? I think I could live with that...

 

[John.g]

I wouldn't care to bet that someone opening a tap won't have a serious effect on shower flow rate, just don't know, someone else on here might have experience of relatively low mains pressure.
Generally, unvented cylinders are supplied with water at a relatively constant pressure of ~ 2bar maintained by a PRV which also supplies the cold water via the balanced tapping on a combination valve, this only works if the upstream dynamic pressure is at least 2.5bar.

 

[johnbusby]

mmm...all of this is making me think that heading back to a vented pumped system is probably the way to go. I know I've already asked a lot of questions so far and very thankful for the response. However would I realistically need to upsize my hot water tank (currently 140 litres, plenty of physical space to go larger) to service two showers? Is a ST Monsoon pump complete overkill to pump the two showers and the bath and will that just drain the hot and cold tanks too quickly (i.e. a fantastic but very short shower)?

 

[John.g]

Don't think a ST Monsoon (universal, if negative head required) is overkill but the cylinder is definitely too small as you will get around 210 litres at 40C so two showers at 10LPM each would exhaust it in 10 minutes.

 

[Ric2013]

Given that mixer showers will use hot and cold water, both from storage, you'll 'need' to upgrade your combined cold water storage and cylinder feed cistern (loft cistern) if you are wanting pumped showers. As well as your cylinder which would generally be considered too small (and indeed is too small, unless, like myself, you take short showers).