Pressure/Flow requirements for unvented cylinders?

[indus]

I understand there are pressure and flow requirements for unvented cylinders to work properly.

I have measured my pressure and flow from an outside tap. This comes off the mains pipe quite close to where it enters my house (approx 15 feet), it is however a 15mm pipe whereas my showers etc have 22mm.

The pressure was 3bar and the flow approx 18litres/min.

Will this be adequate for an unvented cylinder system?

Thanks

PS, the outside mains pipe has been upgraded from the old lead job to 22mm already

 

[amuiphoto]

Hi,

Having just spent the last week trying to improve the pressure and flow to my unvented cylinder I'll let you know what I've discovered.

Pressure at my outside tap was similar to yours at around 3 bar and peaking at 4 bar, flow at the outside tap was around 20 l/min.
In the cellar I took measurements off the incoming main (22mm) which gives a better reading than kitchen/garden taps and got circa 3 bar with flow of around 30-40l/min depending on what time. Pressure and flow vary a lot through the day/night.

From the cellar to the first floor where the unvented cylinder and main shower is, the pressure had dropped to 2.5 bar due to pipe (22mm), fittings and gravity, I believe you lose around 0.1 bar for every metre in height you go up. flow was around 30 l/min. There is a pressure reducing valve fitted to all unvented cylinders and mine was fixed at 2.25 bar. I was surprised when I checked the pressure going in to my cylinder to find it was only 1 bar. This I believe is being caused by a faulty PRV - I'll find out tomorrow when I fit the replacement.

However, having spoken to the PRV manufacturer tech support they confirm a pressure drop of 0.5-0.7 bar across the valve is normal.

Therefore to answer your question as best I can, it depends how high you are going up. You will likely lose 0.5 bar to gravity and pipe fittings plus 0.5 bar on the PRV, maybe more. If your showers are on the second floor then there will be a further pressure drop. Flow rate at 18l/min is good for one shower running at a time but not great to run 2 showers, try to get a flow reading from your incoming 22mm pipe, hopefully it will be better.

I did a quick test on mine with the PRV removed and got 2 bar at the shower
head with a flow of around 20l/min - this gave me a very enjoyable shower!

Angus

 

[indus]

The new system I plan to put in will have the unvented cylinders on the ground floor but then showers on a first and second floor ie loft.

This would mean a height of say 12m to the top of the loft shower. So this means just under 2 bar at that height.

Would this mean (as a result of my low pressure and flow that I would need an accumulator or home booster pump to allow an.unvented system to work properly?

Thanks

 

[amuiphoto]

12m will set you back around 1.2 bar at best, not including fittings etc. Allowing 0.5 for the PRV and I reckon your pressure at the top of the house will be around 1 bar. This is fine but not exactly a power shower.

At first floor I would guess (finger in the air) a pressure of just under 2 bar, which is perfectly respectable - assuming the flow is OK!

An accumulator of say 300-500L will sort out your flow problem but bear in mind you only store around half that in the vessel as the rest is made up of pressurised air. And a pump will sort out the pressure issue - if there is one. You could certainly pump the water up but bear in mind the noise of water pumps.

A flow reading off the incoming main would be very useful in making the best decisions as I suspect you may have more available than the outside tap. (any experts out there wish to comment?)

The cheapest accumulators I've seen come from Anglian Pumping and I've heard a number of people on this forum recommend the Grundfos 15/90 pump. There are also some well respected mains boost systems from GAH and TWS, and a good bit of info on there websites.

BTW, I'm a photographer by trade, not a plumber so please take advice accordingly.

 

[Agile]

Take care anyone hoping to glean useful information from the above posts.

The only relevant readings are the dynamic pressure and flow.

That means NOT the static pressure and NOT the open pipe flow rate!

Tony

 

[Agile]

The Grundfoss 15/90 is designed to boost pressure from a gravity tank in the loft.

Its excellent for that !

But its not relevant to unvented cylinders!

Tony

 

[indus]

Thanks Amui

Tony, yes I recall you mentioning dynamic flow and pressure rate on another thread.

Could you kindly tell me how this is measured and how it differs?

Thanks very much

 

[amuiphoto]

Take care anyone hoping to glean useful information from the above posts.

Hmm, I seem to have supplied quite a bit of useful information and you appear to have supplied next to nothing. ''

The only relevant readings are the dynamic pressure and flow.

That means NOT the static pressure and NOT the open pipe flow rate!

So that means anyone who's ever done a bucket filling test and taken a static pressure reading is wasting there time then?

 

[Agile]

I have explained what is important. I would suggest the minimum for an unvented cylinder is 22 li/min @ 1.5 Bar and increased by 0.33 Bar for each floor above the cylinder that HW is required for showers.

Obviously if you cannot understand this then thats up to you. Good luck to you with your buckets! I hope that you work in another field.

Dynamic pressure and flow has been explained many times on this forum and is easily searched and I cannot repeat the same basic information on each thread.

Tony

 

[amuiphoto]

Thats good information for the OP and I have to defer to the experience you clearly have on this subject. I was just offering my own perspective on a post which hadn't had any replies.

Angus.

 

[ubuntu_83]

Howdy

I am aware that this thread is rather old but I will still try. I am new to these forums, and I have tried to search this around but couldn’t find answers. If there is info on this out there please point me in the right direction. With that said…

I am planning to replace my existing 20+ years old gravity fed system w/Potterton system boiler and a pathetic (to say the least) electric shower with either a combi or an unvented cylinder and need some help with the decision. My question is regarding unvented systems.

The property is a 3 bed house with 1 family bath/shower and 1 downstairs w/c. I am considering extending an en-suite bed in the loft and hence considering the unvented route to future proof the system. The unvented cyclinder will have to go in the loft. Mains supply is 10-12 ltrs/min @ 1.3-1.5 bar so it does not meet the requirements for the Megaflo cyclinder as for now.

I have been told I could replace the lead supply pipe with a broader blue pipe to improve the flow rate but there is no way to guarantee it will up it to 22 ltrs/min as required for an unvented pressurised system and therefore it “may not perform" as Heatrae Sadia said; perhaps not until the main supply pipe is upgraded at least...

or “I will not benefit from the additional expenses from the cylinder” as I read on the forum.

but IT WILL WORK I understand. so if I still go down the unvented route to have the headroom and with an aim to upgrade my supply pipe at the later date, can someone tell me what can I expect in terms of the performance now until then:-

Will I be able to run the shower + simultaneous taps and get my current mains pressure/flow i.e. 10-12 ltrs/min at both outlets without the pressure or the temperature drop?

Can 38 kWH combis be upto the job for 2 showers?

Any opinions on storage combis like Vaillant ecoTech plus 938 or Glow Worm 35 store? Will they be able to handle 2 simultaneous showers? Any recommendations on the make/model?

Would it make sense to go for a combi now, and see how it performs and add an unvented cylinder as the demand grows? Is this possible?

Sorry for asking a lot of questions – but I believe I could get some good answers here.

Thanks.

 

[Agile]

I think you need professional advice.

If not then you MUST try to understand that dynamic pressure and flow are essential to be measured and understood.

Measured at ground floor 1.3 bar static pressure will hardly get any useful pressure in the loft.

An open pipe flow rate of 13 li/min is adequate to fill a loft tank ( slowly ) but will be totally inadequate for normal living with an unvented cylinder.

You need a tank and booster set at ground floor or basement level.

Even a new supply pipe will give pretty poor performance because of the low pressure.

No combi can run two proper showers at the same time even if you had adequate mains supply.

Tony

 

[ubuntu_83]

Thanks Tony. I am going to get a professional to inspect the flow and the pressure (static & dynamic) at the outside stop tap and also try find out what is actually being supplied from the community pipe.

One more question though - I still go down the unvented route as things stand at the moment with an aim to upgrade my supply pipe at the later date, can I expect I will be able to run two showers or taps at a uniform rate of 10/12 ltrs/min and probably slightly reduced pressure?

I think you need professional advice.

If not then you MUST try to understand that dynamic pressure and flow are essential to be measured and understood.

Measured at ground floor 1.3 bar static pressure will hardly get any useful pressure in the loft.

An open pipe flow rate of 13 li/min is adequate to fill a loft tank ( slowly ) but will be totally inadequate for normal living with an unvented cylinder.

You need a tank and booster set at ground floor or basement level.

Even a new supply pipe will give pretty poor performance because of the low pressure.

No combi can run two proper showers at the same time even if you had adequate mains supply.

Tony

 

[Agile]

A good shower is at least 10 litres per minute and requires a dynamic pressure of about 1.0 bar to give a nice tingly feeling.

You can run two showers but only at 5 li/min each or even three at 3.3 li/min each.

But your supply seems totally inadequate to me.

Be aware that many plumbers don't understand these things themselves and will only measure static and open pipe flow rates.

Tony