FAO people with unvented systems

Participant · 13 Dec 2015 at 10:36

Hi

We're a young family of four in a fairly large detached house, with growing water demands.

We currently have a combi boiler, which we were just going to upgrade. However I'm exploring unvented systems, as I want to be able to use multiple outlets simultaneously without it affecting the flow of the shower, for example.

We've had a plumber round who just measured the flow of the garden tap (20L/min) and said unvented will be fine. However I don't want to spooge £3k and it be 'fine', I want it to be blasting at 3bar from every orifice at once!

With that sort of flow, what performance can I expect? Surely total cold flow in cannot exceed total cold flow out? What if two people are having a shower and the washing machine and dishwasher are going at once, will the showers slow to a trickle?

Are my expectations of unvented unrealistic? Do I need to look at upgrading my water main, which I think is probably 15mm from the street? Is it easy to check?

Lots of questions I know...in essence I'm asking just how good are unvented systems?

Thanks.

bazzabear · 13 Dec 2015 at 10:59

If you're so worried, could you not move to stored hot and cold water instead of a combi?

Would allow you to have a power shower too, which is a lovely thing.

Dangerous Dave · 13 Dec 2015 at 14:12

I have a Vaillant unvented system with the Vaillant controls to allow weather compensation on the heating circuits, the system is superb I can run three showers simultaneously with no drop in performance, it wouldn't be able to supply a 4th shower as the hot water cylinder pressure is reduced to 3.5bar but you can run another hot or cold water tap at a sink etc and the three showers are unaffected.

You need to measure your dynamic flow rate and pressure, this means running a tap and then measuring the pressure and flow from another tap at the same time ideally separately feed and as close to the incoming water main as possible (no point in using the outside tap and kitchen sink tap if they are feed off the same 10/15mm pipe). You then need to check this against the minimum specs of the cylinder you are going to buy this is usually around 20l/min and 3 bar. I am lucky and have 5 bar static/4 bar dynamic pressure with a flow rate of 30l plus (hard to measure the two taps at the same time I think it's closer to 40l a min). I have a 32mm incoming water main and the main pipe runs are 22mm through out the house, if you have a 15mm incoming main (this sounds pretty small) or internal plumbing this may need upgraded as the cylinder needs a 22mm water feed.

You may need to take into account your internal plumbing as you will have to balance your cold supply if you incoming pressure is over 3.5bar to match the hot water which is reduced to 3.5bar at the cylinder. If you don't do this you will have different cold and hot water pressures. You ideally want to allow the full mains pressure to the cylinder and reduce the cold water pressure without affecting the cylinder feed. This was tricky in my house due to the cylinder being at the end of the cold water pipe run which meant I needed 4 pressure reducers valves to balance my cold feed to every outlet. If I had reduced the cold water pressure at the incoming water main I would be sharing 3.5 bar through the house but with the additional reducer valves I have 5 bar to the cylinder and each reducer valve.

£3k sounds pretty cheap, my system was 4k and another £160 for the additional pressure reducing valves that I fitted myself. As an added bonus it reduced our gas bill by approx. 40% over our very old storage combi boiler.

Mike1983 · 13 Dec 2015 at 15:57

The only downside I have found with my unvented system is how long it takes for the hot water to come out of the tap (not how long it takes to heat up..)

In other words, even though the hot water cylinder can be full, it still takes a bit of time (more than my previous mavity fed system) for the hot water to travel to the outlet (tap / shower).

I think this is purely down to the length of run of the master inlet feed as it travels a long distance from opposite sides of the house (downstairs corner room, up to the loft and across to the opposite side of the house)

Pressure very good and multiple outlets on is no problem.

Mike

Dangerous Dave · 14 Dec 2015 at 15:19

Depending on your pipe layout with some modifications you could add a circulation pump on the hot water supply, our house already had this on our old system and it pumps the hot water around the main hot water pipes meaning you get instant hot water at the outlets.

This works very well, I only run the circulation pump for a few minutes when the cylinder is being reheated. The pipes are heavily insulated so it doesn't appear to make any noticeable difference to our gas usage with it off or on but we do use a ridiculous amount of hot water as my wife prefers baths rather than showers.

You would have the same problem with a combi boiler anyway, I think you would be mad to pick a combi boiler over an unvented system in a house or flat with more than 1 bathroom.

Participant · 29 Feb 2016 at 22:21

Update guys. We're going with an unvented installation in the garage and work is underway:

Specs:

- Worcester Bosch 30CDI system boiler
- Worcester Bosch 300L unvented cylinder with immersion, twin coil, and secondary return connections
- 32mm MDPE pipe to upgrade the water supply pipe
- Secondary return circuit
- Worcester Wave controls

They spent most of the afternoon trying to get through the concrete garage floor for the supply pipe! Can't wait for a decent system now.

panthro · 1 Mar 2016 at 10:17

Nice bike

Looks good though, I'm thinking of getting an unvented system once our extension is built, but moving the cylinder to the loft.

Jez · 1 Mar 2016 at 10:34

I have an unvented system (a lot smaller than that thing looks, though!

) - basically speaking the pressure out has to equal the pressure in. In other words the hot system mirrors the cold system - if you currently have directly fed cold taps then whatever they are doing is roughly how the hot system will behave too. Open a few taps together and see what they feel like.

Sadly ours isnt really good enough for multiple showers at the same time, they get very slow so although we use separate bathrooms we tend to run one after the other.

#Chri5# · 1 Mar 2016 at 11:30

This is something I've just started looking into at our new house. Downstairs shower room has an electric shower which I think has a 40a breaker so around 9.5kw (according to the manual I found). That's a bit of a dribbler but serviceable.

Upstairs bathroom has a hose on the taps but it's dire, probably as the hot water tank is next door, so no mavity assistance. A pump should help there but doesn't (AFAIK) give us any options for improving downstairs...

panthro · 1 Mar 2016 at 12:01

Jez said:
I have an unvented system (a lot smaller than that thing looks, though! ) - basically speaking the pressure out has to equal the pressure in. In other words the hot system mirrors the cold system - if you currently have directly fed cold taps then whatever they are doing is roughly how the hot system will behave too. Open a few taps together and see what they feel like.

Sadly ours isnt really good enough for multiple showers at the same time, they get very slow so although we use separate bathrooms we tend to run one after the other.

Did you change the diameter of your cold water inlet? My brother (plumber) said that as we're getting an extension built, we should ask the builder to dig up the main feed into the house and change it to 22mm as it's most likely 15mm and currently the pressure is very low.

Jez · 1 Mar 2016 at 12:22

panthro said:
Did you change the diameter of your cold water inlet? My brother (plumber) said that as we're getting an extension built, we should ask the builder to dig up the main feed into the house and change it to 22mm as it's most likely 15mm and currently the pressure is very low.

My house is old with a 21mm (it's imperial I guess but it measures 21mm externally) incoming main. We did a lot of building work too, perhaps I should have looked at getting it changed for a much bigger run from the street. Digging the old one out would have been a no-go as it runs under the internal floor slab, but I guess we could have abandoned it and gone new. I'd want larger than 22mm if I were you though?

panthro · 1 Mar 2016 at 12:50

Jez said:
My house is old with a 21mm (it's imperial I guess but it measures 21mm externally) incoming main. We did a lot of building work too, perhaps I should have looked at getting it changed for a much bigger run from the street. Digging the old one out would have been a no-go as it runs under the internal floor slab, but I guess we could have abandoned it and gone new. I'd want larger than 22mm if I were you though?

Yeah we'll get that done (depending on how much it costs, can't imagine it would be much more labour) if it means getting better pressure in the house. We're increasing the bathroom from 1 to 3 plus a utility room so I think we'll need as much pressure as we can get. Apparently 22mm is preferred for an unvented system.

Edit - sorry to go OT.

Jez · 1 Mar 2016 at 13:15

BTW its not actually pressure we are talking about, its flow (Think volts and amps, all houses have 240V which is the "pressure" but you may have a larger or smaller supply)

Yeah i cant think that a wider pipe would cost any extra really, the pipe itself will be very cheap.

Participant · 8 Mar 2016 at 20:58

All done:

It's pretty good, but the full potential of the system will be realised when this 32mm MDPE pipe is connected to the external stopcock by Severn Trent Water in a couple of weeks:

Currently the system is served by approx. 50m of 15mm copper running from the pavement to the other side of the house, then back to the cylinder in the garage again. This delivers ~16 L/m.

When the new pipe is connected, there'll be approx. 6m of 32mm pipe from main to cylinder :cool:

. I'm expecting flow rates of roughly 50 litres per minute.

Mickie · 8 Mar 2016 at 23:32

50 LPM is amazing, does the house have 22mm hot pipes too ?

hotdog · 9 Mar 2016 at 00:06

Hi
Very interesting thread, who do you contact to get the stopcock upgraded to a 32mm pipe?
Also what is the damage from the water company - im going to have to go this route but with a high flow combi...

Participant · 9 Mar 2016 at 06:47

I'm dealing with a very helpful guy face to face, rather than the muppets on the phones.

I requested a lead assessment and someone came round and told me immediately that I didn't have lead pipes. I then engaged him in conversation about my project and he gave me his mobile number and said that if I get the supply to within 1m of the stopcock, he'll arrange the job and sort the connection for free.

32mm incurs a higher standing charge in my area, but that's a price worth paying in my book.

fezster · 9 Mar 2016 at 09:22

Dangerous Dave said:
This was tricky in my house due to the cylinder being at the end of the cold water pipe run which meant I needed 4 pressure reducers valves to balance my cold feed to every outlet. If I had reduced the cold water pressure at the incoming water main I would be sharing 3.5 bar through the house but with the additional reducer valves I have 5 bar to the cylinder and each reducer valve.

Your post intrigued me. Is there a difference in the incoming water main being reduced *before* feeding your cylinder and the house, or having reducing valves just before the cylinder and each outlet?

I ask because I have a megaflo unvented which is balanced at the incoming main source. I have 22mm pipes throughout the house for both hot and cold. These reduce down to 15mm just before entering shower valves. I get great pressure and flow rate on every shower. However, there is definitely a noticeable drop when you turn a second (or third) shower on, even though performance is still great.

My shower heads are pura 200mm/250mm - and I believe these can provide over 40l/min flow rate, quite high, so more likely to be affected by multiple outlets being used.

My feeling is that 28mm pipe runs may have been needed as the main trunk through the house, but your post about reducing valves before outlets got me thinking.

EDIT - worth adding, my incoming main is 32mm MDPE and I get 50l+/min.

Dangerous Dave · 9 Mar 2016 at 16:32

fezster said:
Your post intrigued me. Is there a difference in the incoming water main being reduced *before* feeding your cylinder and the house, or having reducing valves just before the cylinder and each outlet?

I ask because I have a megaflo unvented which is balanced at the incoming main source. I have 22mm pipes throughout the house for both hot and cold. These reduce down to 15mm just before entering shower valves. I get great pressure and flow rate on every shower. However, there is definitely a noticeable drop when you turn a second (or third) shower on, even though performance is still great.

My shower heads are pura 200mm/250mm - and I believe these can provide over 40l/min flow rate, quite high, so more likely to be affected by multiple outlets being used.

My feeling is that 28mm pipe runs may have been needed as the main trunk through the house, but your post about reducing valves before outlets got me thinking.

EDIT - worth adding, my incoming main is 32mm MDPE and I get 50l+/min.

You need to be careful where you balance your cold and hot water pressure, the cylinder usually has one built in the pressure relief valve, if you put a pressure reducer value on the incoming mains to balance them you will be reducing your pressure for the whole system to 3.5 bar even if you have 4-6 bar on the incoming main. Ideally it should be split so that the full mains pressure gets to the cylinder PRV and the cold water PRV so you can get the maximum benefits of the incoming pressure. It is also good to have the outside tap directly to the mains so you get no pressure drop when using it with a long hose etc. This only really applies if you have a mains pressure that exceeds 3.5 bar which a lot of properties do not have.

The cylinder will be reduced to 3.5 bar anyway which is only going to be enough to supply three showers, I know I can run three with no drop in performance but if I open up a hot bath tap it affects the pressure to the showers but opening a cold tap has no affect. I have 32mm MDPE incoming mains with a short run of 28mm copper up to where it splits off to the cylinder/cold water feeds, rest of the system is in 22mm, I don't think I could get any better performance unless I had an accumulator installed.

To the OP system looks good however I am fairly certain to comply with current building regulations the outside pressure relief pipes need to be covered with a cage.

Participant · 10 Mar 2016 at 07:17

Dangerous Dave said:
To the OP system looks good however I am fairly certain to comply with current building regulations the outside pressure relief pipes need to be covered with a cage.

I'll be issued with a building regs certificate as I believe the plumber just has to send off for one and it's issued. However I will mention this when they come back to replace the temp cold water supply with the perm one.

I do think it's possible to get a bit silly with all this stuff and from what I've seen, building regs are more likely to say 'safely discharged' and 'visible place', rather than 'must use a cage'?

The one on the left is the boiler and the one on the right is the unvented, if that makes any difference to 'safe discharge'.