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

 

[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.