Unvented vs combi with storage tank

[Dynst]

Hi, I am replacing my boiler at home and had pretty much decided that we would go for an unvented system. It is a large 4 bedroom house with 4 people living here.

We currently have one main bathroom with bath and shower and an ensuite with shower. Looking to add a 3rd shower in the future, all mixers. We are going to replace the showers we have with mixers (currently electric) and are looking forward to a decent powerful shower.

We have really good water pressure so thought that this would be the way forward, 300 litre tank and a WB boiler.

Now it has been suggested that we go for a Viessmann Vitodens 222F with 130 litre storage tank as this will do the job.

Our only real specification is being able to run more than one shower or bath and shower at the same time and still maintain, heat and power in the showers. Also my daughters tend to stay in the shower for about 30 minutes.

Can anyone advise as I don't want to make an expensive mistake.

thanks in advance

 

[Mike Jackson]

The 222F is a nice boiler and will provide 20 l/min of hot water at 60C for 10 minutes. After that their is a drop in performance.

I personally can't understand why someone coming in to look at a job where the customer has requested an unvented cylinder would look to downgrade to a combi unless there are other factors. I've done it once when the incoming flow rate wasn't high enough to justify the extra expense of going the unvented route but I clearly explained to the customer why this was the case.

If you've got a good enough incoming flow rate go for the unvented set up, probably not going to cost a lot more than using the 222 either as that is an expensive beast.

 

[AWheating]

Go for unvented if you flow and pressure are good like mike suggested.

 

[Killy Bing]

As per mike.
get the installer to give their reasoning and ask to see their g3 card (unvented)

You may find they do not have this so wish to push you to a combi so they can do the job.

a 222 is a very nice but premium boiler
if you have the correct flow rates and pressure for an unvented then i couldn't justify installing one especially when you're already happy to have an unvented!

 

[Chalked]

Unvented every time?

 

[IDCHAPPY]

30 minute shower?? i could have 5 showers in that time :lol:

 

[Linkyplumb]

Unvented, unless flow rate is an issue.
How much flow have you got? Flow is the most important factor, not pressure.
Time how long it takes to fill a jug/milk bottle, to see what flow you have.

 

[Dynst]

Hi, firstly thank you very much for your responses and I agree what my daughters manage in one showering session would cover me for a week!


i have just filled a half litre glass in about a second and a 4 pint milk bottle in around 5 seconds from the kitchen tap. Does this sound like a good flow rate?


At first, the mention of an unvented system seemed fine until I asked him to up the tank in the attic above the boiler, he asked me if I knew how heavy they were and then he came up with the 222 idea.


The 222 was described to me as a combi and unvented system in one with everything in the box so no fussing with other bits, which seemed okay, but I really didn't want to go for a combi and the more I read, this is really just a big combi.


After what you have all said it would seem that he possibly just doesn't fancy the extra work of getting the tank up into the attic and the additional work that this would add.

 

[townfanjon]

I can only agree with the lads above, fit one, you will not be disappointed.
It may be worth considering fitting a twin coil (solar) now, in case you ever decide to fit solar the future.

 

[lame plumber]

combi and stored water are a waste of time, isnt achieving anything

 

[Ermintrude]

Storage combi is good for first fie mins then rubbish. And after 5 years leaky and dead after 7 I would recommend an ACV SLME cylinder. It's the mutts nuts. It will out perform anything else. 300l will suffice for a 3hr monsoon!

The cylinder stores water at 85c and blends down to 65 on exit. So it gives you 25% more hot water than run of the mill unvented, plus is solar compatible and will recover in 15mins so before it's empty it's full again!

 

[Dynst]

Hi thanks again for everyone's help.


i spoke to the engineer tonight and he says that this is an unvented system with a combi. It would be more efficient than heating a 300 litre tank and keeping it hot and we would empty a tank if we have long showers or showers at the same time where as this will maintain hot water and not run out because the combi element is always filling the tank.


i asked about the 20 litres a minute at 60 degrees for 10 minutes as the system says but he said that no system was likely to provide that and we shower at around 40 degrees so it would run longer than 10 minutes anyway at 20 litres a minute.

 

[Ermintrude]

Your engineer is a half whit who only fits combi boilers . A 300l cylinder had a standing loss - heat loss when just left full of hot water of about £0.50 a day! U couple a 300l SLME and a decent boiler and it will run rings round the tat he's peddling. Sounds like he's got no clue! An ACV will exceed the spec and is very likley to do it.

 

[Whyme]

not run out because the combi element is always filling the tank..

Did he mean the hw drawoff feeding the cylinder. i like everybody else says unvented. there is another alternative. money dependent. a thermal store with external plates..

 

[Ermintrude]

Thermal store ...... Bag of poop!

 

[Whyme]

with external plate heat ex ,haha have to agree to disagree.

 

[Ermintrude]

External plate bit better but still not as good as a nice cylinder tank in tank by the three letter wizz

 

[Dynst]

Right, I am now looking at a Worcester Bosch 30ri and a 300 litre tank, the other option is a 24i and a 180 litre tank. There is about £600 difference between the two so I was thinking to go for the first option?

i have asked that the tanks are twin coil (?) to future proof in case we decide to go for solar further down the line.

I struggled to find costs for the acv but it looks like a 300 litre tank was coming at about £2000 which I just can't stretch to, although it sounds fantastic.

 

[Dynst]

I have found a 210 litre ACV smartline multi energy unvented cylinder for £1100 which is roughly the same price as a 300 litre Worcester. Because it so much more efficient could I get away with a smaller tank and if so would it work with the 30ri Worcestor boiler or should I reduce the size of the boiler too?

 

[AWheating]

You need to size the cylinder to your hot water demands. Then size your boiler to the demand of the entire heating and hot water system. Allow 3 to5 kW for the cylinder if the manufacturer does not suggest a figure.