Unvented cylinder in house with low water pressure

13 Mar 2008
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United Kingdom
We've just bought a big house in Dorset that needs a LOT of work to bring it up to modern standards. It currently has a typical gravity-fed hot water & heating system with a small cylinder (for size of house) run off a Stanley (AGA-type) stove/boiler. The radiators are museum pieces, connected to a single-pipe heating circuit that winds its way around the house.

As part of other works I'm having the old heating pipes and rads replaced with a modern 2-pipe circuit and new rads. In time we will replace the old Stanley with a modern boiler but it will stay for now.

I'm scratching my head as what's best to do about the hot water system. In my old house we had good water pressure and a big Santon unvented cylinder in the loft giving mains pressure H&C water (central heating was also pressurised). Here the pressure is a lot lower (planning to get proper pressure & flow-rate test done but kitchen tap suggests results will be unimpressive), the cylinder is on the first floor and there is a very noisy shower pump in the airing cupboard to pump water up to a loft bathroom (conversion done in 2003) and first floor shower.

Ideally I'd get rid of the old vented cylinder, shower pump and header tanks (in the roofspace) and install another big Santon in the airing cupbaord in its place (with 2 expansion vessels for water & heating circuit). However I'm concerned about the pressure and flowrate I'll get in the shower and the loft bathroom.

My questions are :

a) Is there any way of "boosting" the pressure of an unvented system (eg. with a pump either before or after the cylinder) ?

b) If not, how do people go about getting a good shower in a house with an unvented cylinder but limited pressure / flow-rate (eg. a standalone pumped shower heater run off a separate cold feed ?)

c) Does it make any difference at all to the performance of an unvented system whether the cylinder itself is installed on the ground, first or second floors ?

d) If I have an unvented water cylinder, must the central heating circuit also be converted to run unvented ?

Any advice much appreciated !
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Before I can answer your questions you need to measure your static water pressure using a gauge. Most plumbers merchants stock one, about £20.

If the static pressure is good, an accumulator will resolve the poor flow. if the static pressure is poor, we would fit a TCWS Charger and Maxiboost system. This is a pumped accumulator.
OK, I'll try to get one and measure the static pressure when I'm next down there then post again, thanks !

Out of interest, what are the pressure & flow-rate thresholds below which an accumulator or pumped accumulator are usually recommended ?
Dorset has a good pressure, typically 3bar plus so the mains may be damaged or small.

You could install an large sectional storage tank in an outbuilding/garage or cellar, an pump set and unvented cylinder all in the same area.
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About 1.25Bar, but it depends on how many floors and what appliances/performance criteria you have.
I think the main issue with the pressure is that the house is on the very top of a hill (a local plumber mentioned that pressure at his house in the valley below is excellent !).

The whole family likes a good "power shower" but we've long been used to making sure only 1 person has one at a time (although pressure was good at our old house, flow-rate was limited by a smallish supply pipe up the drive) so we're not looking for Niagara Falls. Ideally though we'd like to install a shower up in the loft bathroom ... !?

After SimonD's initial reply I tried to search (without success) for TCWS Charger & Maxiboost. However I came across another company called Dualstream (http://www.gah.co.uk/GAH_Dualstream/frameset.htm) that sells a combo unvented cylinder & accumulator system that appears (on the face of it) designed for this sort of situation. Have you come across them before and are there other equivalents that you'd recommend ?

Will get back to you on the subject of static pressure after the weekend !
Hi guys, the static pressure at the outside tap (at ground level) as measured by the little pressure gauge I bought is just over 2 bar. Flowrate with the kitchen tap fully open is about 12 litres per minute, so I guess an accumulator will be needed to give decent showering performance ... ?

By my reckoning I should just about be able to squeeze a 300 litre UV cylinder and 300 litre accumulator (based on GAH Dualstream's dimensions) into the large airing cupboard on the first floor landing. This would then supply 1 bath and 2 showers on the first floor plus another bath (possibly to be changed to a shower in time) on the second floor. There are 5 bedrooms.

Upgrading the water main to the house isn't really feasible as the only place Wessex Water were able to prove the pipe when they installed a meter the other day was right outside the kitchen window where it enters the house. It then runs (via an unknown route, possibly passing under an extension built in the 60's) to a point in the lane about 50m away.

a) Is the GAH Dualstream system my best bet would you say ?

b) Are there any things to beware / be careful of when installing an accumulator (over and above the normal UV cylinder requirements) ? I'm surprised that GAH quotes 10 year warranty on the UV cylinder but only 2 years on the accumulator : are they inherently unreliable ?

c) Is it OK to leave the primary circuit from the boiler to the UV cylinder and the radiators as a vented system (keeping the small central heating header tank in the roofspace) or must this be pressurised too ?

Any help & advice much appreciated !

Steve T
The GAH product is the right one for the job. We usually fit a 500 litre acc with a 300 unvented, in fact I have this setup at home.

If you are intending to store this amount of water in an upstairs airing cupboard, make sure you have allowed clearances for maintenance and that the house can support the weight.

The warranty is what it is; we have not had a failed accumulator yet but I am sure it is not unknown.

Primary circuits are unaffected other than cutting in the normal unvented protection valve.

I think you have got to the stage where you need someone to visit and talk over the options, i am sure the manufacturer can suggest someone local.

Accumulator systems are going to become more accepted now that the Gas Installer (CORGI's in house magazine) has run a 3 page article on them this month. Other people on this forum may even start fitting them :eek:
Thanks Simon, that's very helpful. Happily the airing cupboard has double doors and the cupboard itself is about 1200 wide by 600 deep (so I don't think I can fit in a 500 litre accumulator but I hope a 300 litre one will work well enough so long as the bathrooms aren't all in use at once). The cupboard sits above a load-bearing masonry wall below so the weight should be OK but I'll get the joists checked and doubled up if needed.

I've started putting feelers out already to find someone in / near Poole able to fit them but I'll certainly talk to GAH as you suggest. What would you reckon to be a "fair" price for installation (assuming no other complications) ?

One other question : the main shower currently has a wall-mounted pump unit that draws hot & cold water to give a "power shower" (and empty the small HW cylinder in no time). I presume I'll need to replace this at the same time with a standard mixer shower (by choice I'd leave it until I'm ready to refurb the whole room but I guess it won't be designed for use at mains pressure) ?

Thanks again for your help and I'll let you know how I get on.


Steve T

ps. I don't suppose your firm fancies a trip down from Surrey to the seaside ... ?!
Hello Steve

You will need to replace the pumped shower with a thermostatic one designed for mains pressure.

I cannot give you any meaningful idea of price because everyone's house is different. If they were all the same I, and other installers on this forum, could save a fortune in time and money by doing quotes remotely. GAH will tell you the price of the equipment or you can contact your local merchant.

Poole is a lovely place but the unproductive time driving there and the distance should warranty or servicing being required make it, I'm afraid, uneconomic for both of us!
Upgrading the water main to the house isn't really feasible as the only place Wessex Water were able to prove the pipe when they installed a meter the other day was right outside the kitchen window where it enters the house. It then runs (via an unknown route, possibly passing under an extension built in the 60's) to a point in the lane about 50m away.

Shucks they didn't try very hard, want me to bring me coat hangers next time I go the the seaside :LOL:

If you want an good Corgi guy on the doorstep send me an Pm and I'll pass it on.
Doitall, tried to send you a PM but it says I must first be your Friend ??? Can you invite me or am I missing something
Best have an accumulator. Explanation:

"The flow of water to taps from a mains fed system is limited by the size of pipes and fittings that the water has to flow through. Where you have no restrictions, and large pipes, it is possible to get a large flow, even with a low pressure.

The problems start when pipes are smaller that desired, and act as a restriction. Old mains supply pipes into properties are typically 1/2 inch and even with a good pressure the water simply cannot flow through the pipe in great quantities.

To the user this is seen as a severe drop in flow rate from a tap when a second tap is opened simultaneously. There is not enough water coming in to do both, so it gets shared.

An accumulator is a simple fix. It is a water vessel containing a balloon of pressurised air. The mains water squashed the air as it fills the vessel, until the air pressure increases to match the mains supply pressure. Now, when a second tap opens up, the additional water required can be supplied from the vessel, at full mains pressure, without the restriction there is on the mains supply.

When all the water charge in the vessel has gone, the system returns to normal flow rates, so it is important to estimate the size you will need based upon how much water you may need before all taps shut and the vessel can recharge.

Installation could not get any easier, with one connection anywhere into the cold mains pipework. A non-return valve is also needed on the incoming mains to prevent the charge from disappearing back into the mains supply should the local pressure drop temporarily.

They are maintenance free and DIY installable, and with typical prices around the £200 mark, you are unlikely to find a cheaper way to improve your flow rates to taps."


http://www.rwc.co.uk/Product.aspx?page=CAT6 Go to No. 3

It stores only cold water. They are simple, very simple and highly effective. No need for silly tanks and noisy pumps and it does hot and cold too, so mixer taps all around without problems. It can go anywhere in the house, or in the attic of a garage. All it needs is a 22mmm pipe to the hot and cold water system. It could be fitted in an insulated shed at the bottom of the garden, way out of the way - just run one underground plastic pipe to it, that is all. It is the same cost of just a power shower pump to buy.

Avoid Dual Stream as they are "expensive", for something which is just plain simple.

You have oil? Yes? No gas? If only oil, try an oil combi with the accumulator. Oil combis generally have good flowrates - very cost effective. After that try a heat bank, no more expensive than an unvented, much superior and can do the CH as well having an integral CH buffer too. All rads can have TRVs and no wall room stat when using a Smart pump.
HI this is my first time of using this forum and I've had bad experiences from local Plumbers mainly through lack of understanding un vented high pressure systems. I have lost trust in my local plumbers ability to rectify my reduced flow rates / pressure.

3 years ago I had a twin story extention built on to the house during this time I asked for the water mains feed pipe to be replaced in std22mm piping and feed directly to the loft space.

At the same time I moved the boiler and installed an un vented high pressurized tank system along side in the loft giving lots of servicing space and keep in things out of the way.

So all operating parts are located in the loft of the extention ground level water pressure is high 4.2Bar at the out side tap with all taps, valves closed.

The system is complete with twin accumulators one on the rising main set to 3.5 bar (air side) and one on the heating system set to 2 bar( on the air side). I cant get this much higher for the rising main as I have tried to equal the raising main pressure I have suspicions that the accumulator is under sized.

The problem that I have, and have had from new is that only one shower or bath can be used / filled at any one time because of dramatically reduced flow./ pressure rates pls assist how I can investigate rectify this drop of.

Yours MCH :?:

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