unvented system shower flow

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I am having a loft conversion (including extra bathroom) which will end up with a flat roof and needs an unvented system as there is no room for tanks.
The plumber assures me that the shower will be no problem and similar to the pumped aqualisa quartz I had before. One of the reasons for the conversion is to have a good shower.
I've had an electric shower put in the existing bathroom and the flow from that is very poor even on cold. Will the new mixer shower upstairs have much better flow/pressure than this electric shower?
 
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All comes down to flow and pressure of the cold mains, Unvented cylinders can only output what is input. Undoubtably it should be better than the electric unless the mains is miserably poor
 
Thanks - I’m hoping it will be better.
Doesn’t the electric shower also take the cold mains pressure?
I’m guessing that the hot water from the cylinder will mean greater flow? But how much?
How much extra do you get from an accumulator and can they be placed anywhere? The boiler and cylinder are going to be put in a cupboard upstairs with not much extra room, might have to redesign a bit if that’s needed.
 
Your plumber should take some flow tests but your electric shower will/should give some idea (cold).

Turn the temperature control to minimum which is full flow and see what flowrate you are getting.
My one, with static pressure of 3.5bar (at ground level) flows 9.6LPM with 2.4bar dynamic (ground level) which translates to ~ 2bar dynamic at the shower.

Electric hot flow will only be 3/5LPM winter/summer at 38/40C but thats because you throttle the flow ("temperature") control knob to give the correct temperature.
 
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Thanks. He did some quick checks but I didn’t write down what he said but he said it wouldn’t be a problem. I’m not sure he understands what I expect or how important it is.
I am testing this on cold. The shower is 9.5 kw and has two elements. I’m guessing that the temperature control just controls the flow. When both elements are on it’s too hot even when set to cold. With one element I have to reduce the flow to get the temperature up which isn’t good. Even on cold the flow isn’t good. I should have got a lower power shower maybe.
I guess when the weather is colder I can use both elements but should have the other shower then.
I’m planning to have an aqualisa upstairs but will get something cheaper if it’s not going to work anyway.
 
You havn't a prayer IMO of running with a unvented cylinder off that mains pressure, the electric shower trips the power to the heating elements at 48C, so a 9.5kw shower must flow > 5 LPM to avoid it reaching this temperature from a present mains temp of 20C, on one element the flow required for a comfortable shower at say 38C is only 2.5LPM and that's why you must reduce the flowrate. Only other reason apart from very poor mains pressure is that the shower filter is blocked, have you checked this. May be worth buying a 0 to 10bar pressure gauge and taking a few readings yourself.
 
Thanks.
Pressure gauge sounds a good idea, I've just ordered one - need to find somewhere where it will connect when it turns up tomorrow.
The shower was installed on Saturday so I wouldn't have thought the filter would be a problem.
I measured the flow and it's about 7 litres / minute at the kitchen tap and the shower. I tried with the shower head on and off and it's the same. I thought I might be losing a lot between the kitchen and shower but seems not.
I've asked for larger pipes when the upstairs is plumbed- I hope that isn't a waste of time - presumably won't hurt.
 
A flowrate of 7LPM will result in a showering temp of 39.5C from a cold mains of 20C which is borderline comfortable.
You should be able to install that PG on your outside tap.
 
Reading - a shower takes about two thirds hot and one third cold water - does that mean my unvented system should get a max of 21 l/m?
That sounds like a lot and two thirds hot water also sounds like a lot.
The aqualisa site says their showers give 13 l/m on normal and 18 l/m on boost. I felt that normal was too low so usually used the boost. Don't know if I was getting 18 l/m but would expect something similar to what I had.

My outside tap gives about 12 l/m (difficult to measure as too fast). Could something be limiting the flow?
I have a water softener but I'm told that shouldn't affect things.
 
The Aquialisa has a integral pump that takes gravity hot and cold water? So why are you not replacing g iyt with something similar like a Triton that will give - 14LPM, not good enough??

I have read that water softners can have a large pressure drop.
 
There's no room for the tanks in the loft conversion. It's almost a new floor with a flat roof. It's a bungalow and getting what my sis-in-law calls a shed-on-the-roof.
The aqualisa was at my previous property.
I don't remember the flow being much greater before the water softener was installed but it may have been.
If I was getting 13-18 l/m before I'd like something towards the 16-18 mark. I could maybe live with 14 though.

It's probably the water softener manufacturers that say they don't reduce flow.
 
Checked the outside tap from a hose pipe and that gives about 14 l/m.
So something is causing it to reduce to 7 l/m by the time it gets to the kitchen and bathroom.

I guess that needs to be sorted out otherwise my mixer shower won't be any good? What should I expect if I'm getting 7 l/m from the cold plus the hot water cylinder - which I guess will be under the same pressure? Should I expect about 14 l/m.
The pressure also seems low at the bathroom downstairs - my pressure gauge didn't turn up today. I'm assuming this will be even lower upstairs but hopefully not by much.
 
I guess that needs to be sorted out otherwise my mixer shower won't be any good? What should I expect if I'm getting 7 l/m from the cold plus the hot water cylinder - which I guess will be under the same pressure? Should I expect about 14 l/m.
7 LPM from the cold + 7 LPM from the unvented HW cylinder still requires a total cold water feed of 14 LPM.

Your pressure gauge when installed downstairs will tell a lot when you measure the flowrate upstairs from as many outlets as possible.
 
Thanks
I was thinking that the cylinder would be full and would supply on top of the cold but I guess what is taken out of it will be replaced as it’s being used so I won’t get anything above the mains flow. Just have to hope that I can get mains flow and that the outside tap is restricted so I’ll get more than that.
Reading articles it seems that the minimum flow rate for an unvented system is 20 l/m and 1.5 bar. Hopefully my plumber has checked this but I don’t think any of the taps show that - maybe he knows what’s available from the mains here.
 
There are a lot of factors to take into consideration when it comes to flow and pressure. Pipework size, number of fittings, length of pipe runs etc. Your kitchen tap could have an aerator water saver and it's fed by narrow flexi's which will affect it's output.

An unvented will need 22mm pipework from the mains to the cylinder and out to the outlets first branches to maximise flow. Dynmaic Pressure needs to be understood as that's the driving force behind the flow and the dynamic figures of flow and pressure need to be understood, so you can understand the drop off at an outlet when more than one is being used at the same time. You need your plumber to give you those figure and compare them with what you find.

If you are achiving 14LPM cold at the outside tap then open up another couple of outlets inside and measure again and see what, if any, the change is. Do the same when you receive the pressure guage, that'll give you a starting point and whether an unvented is viable or not.

20L/min @ 1.5bar dynamic would be the lowest readings you want to get the best out of an unvented.
 

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