Suitability of Unvented Cylinder

[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.