Can I have an unvented system?

[Windy Ridge]

I'm having an extension built and need a new boiler etc. I'm keen on having an unvented system, but the builder's plumber sucked his teeth when I suggested it. He said it would be better pumped for a variety of reasons and I'd just like some opinions. The pressure is fine, but the flow isn't too good. The incoming main in the house is 15mm so can a larger one be tapped off upstream? (I think the main from the road is the original 1926 one. Iron?) His other reasons were that you have to ask the water board for permission, and that it'll be much more expensive than a conventional system. Oh............and he hasn't done the course yet.
I'd rather not step on the builder's toes before we've even started but this guy comes across as a bit of a numpty, and I've got to get my facts right before I get another plumber.
(By the way the house will be 4 bed, 2 bath/shower, 2 daughters and washing machine in 24 hour operation!)

 

[kevplumb]

by unvented you mean the hot water

 

[Agile]

You might have confused some people here because you are using words which merge an open vented pumped central heating system with an unvented hot water cylinder.

It is the latter you are asking about! To operate one of these successfully you need a static water pressure of at least 2 Bar and a good flow rate WHILST maintaining a reasonable pressure of say at least one Bar. This flow rate MUST be able to operate ALL water outlets and might be 50 litres per minute as an example.

In many cases this is not possible either because the distribution pressure is too low or the flow rate is to low.

The flow rate can be improved by replacing the mains supply pipe with a blue plastic pipe of say 25 mm or 35 mm. Either to your boundary only or ideally in conjunction with an increased pipe size on the water supply Co side. It usually costs £500 - £1500 ! This should be done by a plumber who holds the Water Regulations qualification!

An Unvented cylinder should only be installed by a plumber who has the UV qualification and needs to be notified to the Building Control department.

A new boiler should only be installed by a CORGI registered plumber and it must be condensing and also notified through CORGI to Building Control.

I think that you need to be advised by a properly qualified and experienced consultant on the best system for your needs.

Sometimes I feel overqualified when 90% of plumbing work seems to be done by people without the relevant qualifications!

Tony Glazier.

 

[Windy Ridge]

Sorry that I wasn't clear enough. I don't really mind how the central heating works,but I do appreciate that with an unvented system that it runs at higher pressure. I was thinking about the hot water and the use of an unvented system to provide higher pressure hot water for showers etc. The other option that I mentioned was to have a pump on the hot water system as with a shower pump that runs with a demand for hot water. I can see that when you guys say 'pumped system' that you mean the CH, but that's not what I meant. The flow rate is just on the low side but the pressure is fine, so am I right in thinking that the main between the road and my house is already going to be (or should be) of a diameter that will allow the required flow?
The problem is that I'm being advised by a friend who is u-v qualified (and would like the work), who I'm finding it hard to engage due to politics with the builder, and the builder's 'favoured' plumber who obviously advises against it because he's not qualified.

 

[Slugbabydotcom]

As a uv qualified plumber I can highly recommend unvented scylinders but your flow rate may be cause for concern. The minimum flow rate for most uv cylinders is around 20 litres per minute and 2 bar pressure as stated by agile usually provides an adequate flow rate.

The sharp intake of breathe is not unjustified as you may end up having to renew your water service for it to work satisfactorily.A 22mm min supply is required in most cases for silent operation as well as flow rate, so ask your water provider about the possibility of getting one and the suitability of the service they can provide to power an unvented cylinder.

In another post I saw that an ariston storage combi (non condensing) gave 21.5 l/min output @ 30º C temp rise but even this isnt as sustainable or as good as that which an unvented cylinder of suitable size can provide. It too would be restricted to your flow rate.

My opinion is that if you can afford a uv cyl then go for it.

Where this extra flow rate provided comes in useful is when filling a bath so if you have a large bath it could prove its worth. Also you get excellent performance with a thermostatic mixer shower exceeding most shower pumps and on a par with the best if your house has only a modest flow rate.

Make sure if you do decide on a unvented cyl that any showers are thermostatic and that a balanced supply is used for the cold supply to mixer showers and bidets. Basically as with any plumbing item follow the makers instructions to the letter and it will work.
Click on THIS Then click products then megaflow and unvented links for spec. There are other makes to consider but I just had that one handy

 

[keego]

Just as an aside, a lot of you are quoting sizes of pipe for the incoming supply. The only sizes of supply from the mains are as follows, 20mm, 25mm, 32mm,although the 20mm is very rarely used these days, after these there is 50mm & 63mm but you're talking large house with swimming pools etc.

 

[doitall]

And if you didn't know above 50mm has to be chlorinated after the installation is completed and before the supply can be put into service.

 

[kevplumb]

up here mate any council building you work in has to be done regardless

how did we ever survive with lead pipes years ago

 

[ChrisR]

Tale of caution.
I was recently asked to put in an unvented cylinder by a fairly experienced plumber, so did so.
It was a 250l Megaflo which was already on site. I enquired about pressure and flow. Neither had been measured but it was 4 bar with no flow, and I was told that the water main had been replaced, and not to worry.
Expensive mistake. It turned out the total flow from all mains taps, when fully open together, was 24 litres/min.
When this cold mains supply was connected to the down-service pipes which had been used from the cisterns (to be decommissioned)in the loft , the available flow was very poor. The rising main was 15mm which contributed to the resistance of the supply. Though the Megaflo is a type of unvented cylinder which has an air gap which stores some mains pressure (hot only), which helped, the result was nowhere near manageable. I rechecked the mains pressure, which was still 4 bar. It turned out that not ALL the mains pipe had been replaced.

The solution (with advice from DIA, hereby acknowledged) was to reinstate the (150 gallon) loft cisterns, and pump the supplies from them to the Megaflo and the CW taps, with a £400 pump. (Negative head variety, to keep the pressure up).

The original HW cylinder was pretty big and hadn't needed to be replaced - the water could have been pumped from it and the cisterns by a similarly priced pump.
The customer was persuaded that the problem was unforseeable(!), and paid for the pump.
The original plumber and I, with additional help, spent most of a day making the changes, all unpaid.

A few checks on pressure and flow up front, with no special knowledge, would have saved the cost of the Megaflo and a lot of work.
As Tony says, a surprising proportion of plumbers simply can't be bothered to work out basic things, and just blame someone else or the gods when they cock things up.

 

[doitall]

The solution (with advice from DIA, hereby acknowledged)

Does that mean it worked ok.

 

[ChrisR]

It seemed all OK. I used a Stuart Turner 2.3bar Monsoon "Extra". The Extra means more flow - something like 50 llitres a minute at 2 bar, which is enough for 3-4 taps at once. It wasn't tooo noisy - I put it on a couple of paving slabs. Big thing to find a home for once the flexy pipes supplied are on it, one vertical and one horizontal.
Single ended pump so Teed to supply Megalflo, and CW house supplies. No point using the "balanced" outlet from the unvented system cos the P reducing valve on the M is well over 2.3 bar so wouldn't do anything except resist the water.

WIth lots of taps on they'll empty their 3x50 gallon tanks in about 15 mins - could happen.

 

[COG]

As a side point i thourght that an uv cert was only required when the capacity was over a certain size and alot of eficiant uv cyl are ,this info comes after contacting a manafacturer regarding external controlls .

 

[ChrisR]

Yes but it's 15 litres - good for undersink ones. They are rarely installed right as far as I've seen. Hardly ever a pressure vessel or pressure reducing valve. Most people seem to think that the supply pipe accommodates expansion by increasing its volume!