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)?.

 

[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).

 

[johnbusby]

Thanks gents - you've convinced me that I shouldn't go for an unvented cylinder and I've an underlying uneasiness that flow/pressure of my mains could deteriorate in the future which would turn my unvented system from 'just about okay' to 'completely useless'. There are plans to build a couple of thousands houses in my relatively small town so who knows what could happen in the future.

Therefore I'm going full steam ahead on the vented cylinder plan with a pump. I've been in touch with Stuart Turner support who are as good as everyone says there are, but was wondering if either of you could advise what size of cold and hot water cylinders I should be looking at to give me a bit of headroom as the family grows up (currently 1 toddler and one on the way - no plans for anymore!). I've just been up in the roofspace as the current cold water tank is 50 gallons, as I've said before the hot water cylinder is currently 140 litres. We are looking to be able to run decent showers off these, maybe around 15 litres / minute

 

[John.g]

Simple HW cylinder calc, 2 X 10 minute showers @ 15LPM = 300 Litre.
Your existing 230 litre (50 gall) CWST may be sufficient bearing in mind that the mains @ even 10LPM will have replaced 100 litres in 10 minutes so effectively, CWST is 330 litres. Also the roof space may need extra supports if you increase the CWST capacity.

 

[johnbusby]

I certainly don't want to get above my station here (!) but wouldn't a 15L/min shower only be made up of 10L of hot water? Great to hear that I can probably get away with leaving the cold water tank as it is as the cost of this entire enterprise is going through the roof!

 

[EvilDrPorkChop]

Personally if it were me i'd stick with the unvented. I've done them in lower pressure situations and they work fine. You might not have the full performance, but it will be a lot better than your electric showers plus it would be as good as/better performance that what you'd get off a combi.

You'd be pushing it for two showers at once, but as long as you understand that then I don't see the Unvented an issue.

Also considering your flow rate is good too, I wouldn't pay too much attention to pressure. You'd need to upgrade the supply to 22mm to the cylinder from your stop tap though.

 

[John.g]

I certainly don't want to get above my station here (!) but wouldn't a 15L/min shower only be made up of 10L of hot water? Great to hear that I can probably get away with leaving the cold water tank as it is as the cost of this entire enterprise is going through the roof!

A very good point and yes correct, if you use a average of 10C cold mains and HW cyl at 60C then a mixed flow at 40C requires 0.6LPM of HW mixed with 0.4LPM to give 1 LPM at 40C, so HW requirements for a 15LPM shower is 0.6*15, 9LPM, total HW requirement 2X9x10, 180 litres. so HW cylinder maybe 200 lires? and CWST as is at 230 litres?.

If you retained your existing HW cylinder and raise its temperature to 70C, then you will get almost 280 litres at 40C so almost there,

 

[Timmy D]

I knew John would be here. Love reading your calculated posts.

If you’re happy with having one decent shower running at a time, and will be slightly influenced/noticed whenever someone opens another tap/flushes a toilet, install a 250L unvented.

You will want to upgrade the mains supply to the unvented and will need to install safety discharge line etc.
Also, you ideally want to connect all of the cold supplies to the balanced output of the combination valve to ensure hot/cold supplies are the same pressure.

This doesn’t seem to be a major issue for you at the moment as your supply is less than the pressure reducing valve setting of 3 bar. But if in the future, your mains supply from the street gets boost to say 4 bar, this could cause issue in the property as the hot would be limited to 3 bar.

Other option, leave it open vented, add a pump. Pretty much run what you want as long as the pump/storage is sized correctly.

Perhaps upgrade your current cylinder to 250L and rather than replace your current cold water tank, add a second one.

Then you can run two showers and forget about it. If/when you have a teenage daughter spending an hour in the shower you’ll thank yourself.

Edit: The general public think that unvented cylinders are the solution to flow/pressure problems but it couldn’t be further from the truth. They can only put out what you put (unless using accumulators). If you’ve got less than 20lpm, no real difference than a large combi.

 

[Ric2013]

Simple HW cylinder calc, 2 X 10 minute showers @ 15LPM = 300 Litre.
Your existing 230 litre (50 gall) CWST may be sufficient bearing in mind that the mains @ even 10LPM will have replaced 100 litres in 10 minutes so effectively, CWST is 330 litres. Also the roof space may need extra supports if you increase the CWST capacity.

Clarification for the OP: John is referring to the capacity of the Cold Water Storage Cistern here but we plumbers often use that term to refer to what is somewhat pedantically more properly called the Combined Cylinder Feed and Cold Water Storage Cistern (i.e. a cistern that, in this case, would feed both the cold and the hot side of the shower). So the fact that the shower will use a mix of hot and cold water still impacts on the size of cistern required.

It can be argued that it is better to combine the cisterns rather than have 2 separate ones as it is then preferable to make the connexions to the pump such that you always run out of hot first, thus removing any possibility of being scalded.

On the other hand, it may be worth considering that with the climate situation meaning that water supply may be restricted in coming years and the price of heat going up, it seems that the responsible thing to do may not to be using such vast quantities of water and energy in the first place. Surely the point of a shower is to use less water than a bath would? My opinion, but FWIW, 6l per minute is a decent shower, with 10l per minute being a thoroughly good one. My mother had 'a quick shower' the other day and used a total of 18 litres of water, admittedly she probably wasn't running the shower at 6 litres per minute.