There are two types of hot water cylinders found in homes today. The newer pressurised unvented hot water tanks and the older style vented hot water tanks.
Unvented hot water cylinders
Unvented hot water cylinders were only made legal in the UK in 1986, but have since grown rapidly in popularity. In an unvented system there is no cold water tank – instead, the sealed hot water cylinder is fed directly by the cold water mains. Since they are operating at mains pressure, they offer much better flow rates, meaning your shower and bath performance should be higher.
The other major benefit is that you don’t need to maintain a cold water tank in the loft (which vented systems require). This is good news since not only does it free up space, it also removes the potential freezing issue during our long cold winter periods.
In addition, since you aren’t relying on gravity to move the hot water around the home, the unvented cylinder can be located pretty much anywhere in your property.
Other advantages of installing an unvented system include reduced noise in the system (since there is no cold water filling of the water storage cistern), and since there is no water storage cistern and the system is essentially sealed, the cold water is not at risk from contamination.
Unvented water cylinders and the water expansion issue
Since water increases in volume as it gets warm, unvented cylinders need to include a mechanism that allows the expansion to take place, thereby keeping the cylinders operating at a safe pressure.
The major issue with unvented hot water cylinders is that since hot water flow depends on the cold water main pressure; if for any reason the mains water is turned off, your home will be without access to any hot water.
Since unvented hot water tanks operate at higher pressure than vented systems and have additional safety features installed, these cylinders need to be installed by boiler specialists who hold a qualification that complies with G3 of building regulations. This means they tend to be far more expensive to install than traditional vented hot water systems.
Vented hot water systems
Vented hot water tanks are still the most common type of hot water system found in the UK. Unlike newer unvented tanks, these copper tanks are fed by cold water from a header tank (normally located in the loft) and they use gravity to drive the hot water around the home. A vent pipe links the vented hot water cylinder and the cold water in the header tank.
As with the unvented system, expansion of warm water is still an issue, but in this case the expansion simply takes place via the vent pipe and in the header tank.
The hot water pressure tends to be governed by the height of the water tank above the tap or shower feed. This means that although on the ground floor of the home the pressure might be excellent, in rooms on upper floors the pressure will be lower. As a result, many showers in homes with vented hot water tanks use electric pumps to drive the hot water to the shower at increased pressure.
Vented hot water cylinders are far less complicated than the pressurised vented systems and for this reason they are much simpler to maintain and install. This makes them a far cheaper option when compared to the unvented system.
Indirect systems
Most hot water cylinders are heated via an external heat source such as a gas boiler or solar thermal. In this case the hot water is heated and then travels through a copper coil in the hot water tank. The heat is then transferred from the from the external heat source to the water inside the hot water tank.
Indirect cylinders tend to be fitted with a direct backup (such as a immersion heater). Even if the boiler is broken, you can still produce hot water as and when you need it.
You can get both vented and unvented indirect systems.
Direct systems
In a direct cylinder system, the hot water is heated directly by an internal element such as an immersion heater. The hot water tends to be more expensive to produce in direct systems. Some homes have no access to gas, for example a mid-level flat. In this case they are forced to go with a direct system for their hot water, so they may choose to take advantage of Economy 7, which will give them a cheaper electricity at night to heat the hot water with.
Normally this type of cylinder would be fitted with two different immersion heaters, one for the peak electricity and one for the off-peak electricity. If this is the case, you really need to make sure that the immersion heaters are set up on the timers correctly to ensure you are paying the least possible for the hot water. There is no point heating water via your peak immersion heater during the middle of the night.
You can get both vented and unvented direct systems.
Combi boilers
We have covered combi boilers in more detail here, however it is worth mentioning that they can produce hot water for your home without the need for a hot water cylinder. While you can use electric point of use hot water heaters, gas currently is about ¼ of the price of electricity. Therefore it makes sense to use gas to heat the water as well as to carry out your heating.
The issue with combi boilers is that since they operate at mains water pressure, as soon as there is a pressure drop (i.e. more than one tap is opened in the home) then the hot water pressure is split between the two outlets. They are however ideal for smaller properties since there is no need to store hot water which is not very energy efficient.
>>> Should I replace my conventional boiler with a combi? <<<
Installing an unvented hot water cylinder
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As a foreigner who has just purchased their first property in England, this page was very helpful to me. Thank you!
You haven’t included the newer vented heat stores, where the water to be heated is passed a coil immersed in heated water. Mains pressure water can be heated without the need for a pressure vessel.
Is that last paragraph really true – that combi boilers are ideal for smaller properties? I have a very small house, with 4.5kw of central heating, and the major suppliers do not make a combi boiler which can service such a small central heating requirement without significant loss of efficiency. Perhaps smaller properties might be better suited to a system boiler with an unvented hot water cylinder.
Deborah,
That is indeed a very small heat requirement, and probably the exception rather than rule. Still, in a very small property, the advantage of a combi is the extra space you gain from not having all those extra tanks around – that is a big advantage when space is at a premium.
Hi Deborah, Baxi do a system boiler that will modulate down to about 3Kw, team it up with a small unvented cylinder or thermal store. Think the boiler is a Eco Blue model.
Can an ‘unvented (indirect) hot water storage cylinder’ work with gravity fed hot water coming from a gas boiler?
How many different in-put power sources are possible with an invented cylinder? Eg could you have solar and ground source and oil? As you might have gathered we’re not plumbers! Thanks. Frances
Hi Frances, You can get dual or even triple coil tanks, it just depends how many hot water feeds you have – e.g. solar thermal and a boiler would require a dual coil. A boiler and a heat pump + a wood burner (that produces hot water too) would require 3 feeds. More coils, the more expensive the tank and the bigger it needs to be.
My unvented water cylinder has just been serviced and I was informed my immersion wasn’t working, but that it was probably an electrical fault. It is an indirect system powered by heat pump.
How would the immersion be operated in the event of a boiler breakdown?
Hi Emma
There can only be a couple of faults causing immersion not to work. Most probable cause is the immersion element has gone open circuit and would have to be replaced (you would have to drain the tank first) The thermostat may be faulty, but you can change that without draining. You should have a switch to switch on and off. Should be economical to have an electrician diagnose the cause, as it would be a 5 minute job to diagnose
Can anyone inform me if it is possible to install an unvented cylinder within a 600×600 x standard 2.1 high kitchen unit, I have an Architect who reckons he has seen this
In theory this is possible but the cylinder would have to be very small and hence very small capacity and in my opinion waste of money installing
If one were looking for getting advice over current boiler/hot water system and possible improvements to current set-up, would it be a heating engineer I would need? Is there any accreditation/qualifications I should be looking for? I’d like someone impartial so I don’t feel I’m being talked into spending money for minimal (or at worse) no gains!
Hi Katie, as long as they are gas safe they should know what they are talking about. The best way is to get several quotes (i.e. speak to lots of people) as some people prefer certain setups over others. If you want us to give our two pennies worth, give us a call.
I have a high capacity Worcester Combi Boiler 42Cdi. Worcester tell me that it cannot accept preheated water so I cannot fit a hot water storage tank to work in conjunction with my Solar system. Is there any kind of thermostatic valve on the market that can switch on when the water in the hot water storage tank is hot sot it feeds the house and the feed and output to/from the combi are closed. This would allow me to heat water via an emersion heater from the spare elect from my 4kw solar PV system rather than exporting it to the grid.
I would envisage that the valves in question would be electrically driven from a thermostat on the hot water storage tank. The operation would be similar to a zone valve in a central heating system.
Please note all letters in my email address are lower case.
Don, This is exactly the question I wanted to ask, so it would be great if we could get a response! I’m considering getting solar panels and it seemed to make perfect sense to pre-heat the water during the day in a pressurised tank to feed the combi boiler and greatly reduce the gas needed to heat the water. Having a diverter valve to use the water directly if it were hot enough makes sense to me (I am not a heating engineer!), and it would switch to going through the boiler as soon as the temp dropped below a threshold.
I would like to replace my hotwater tank with a split tank suitable for solar. We have solar panels installed and have no boiler as we are all electric. Currently heating hot water at night with economy 7 but would prefer to take advantage of heating our hot water directly instead of sending energy back to the grid. Tank is 4ft high and probably 18 inch circumference without the foam insulation. Where can I find a company in Sussex or Kent who would swap this tank over for me and what tank should I be looking for. Any advice would be welcome.
Our old pulsacoil unvented water cylinder has just begun to leak. It will need replacing, are there any schemes around that help with the cost of replacement? The cylinders seem to be quite pricey.
Hi Philip, there aren’t any schemes at present. If you are looking for an unvented cylinder try Joule – they are about half the price of Megaflo and offer the same sort of performance.
Would two 210ltr invented cylinders be more economic than one 400ltr
Would two 210ltr unvented cylinders be more economic than one 400ltr
We have a Valliant system boiler around 20 years old.Its a 4 bed detached property. Have a cylinder in loft with cold storage tank. Have been advised to change to unvented pressurised cylinder.Is there any advantage as cost 5K with new boiler, filter etc.
Very useful page, thank you. Regarding your comment about the combi boiler being at mains pressure and therefore susceptible to a reduction in pressure if another tap is turned on, does this same issue not also apply to an unvented (megaflo) system?
Hi there, we’ve just bought a rental property that has a very complex (apparently) hot water system. There is a Vaillant combi boiler in the kitchen. But there is also a header tank in the roof and a hot water cylinder with an immersion heater upstairs. There is an array of complex gear in the airing cupboard with a couple of pump type things, and things like springs around the pipes. What kind of system does this sound like to you please? I have some pictures. There is a timeclock in the airing cupboard too, along with the very high-tech looking weird gear!
We have a new home and have an unvented water cylinder system.One of the requirements to maintain the manufacturers warranty of 5 years is having the system service by an unvented registered engineer on an annual basis.How can I find out where I can get this service.we currently have the boiler serviced by British Gas but not the cylinder system
You need to find a gas safe engineer who is G3 approved. Give Megaflo a call – they will be able to help!
I am fitting a new shower, my existing hot water system comprises a cold water tank in the loft, a hot water storage tank at the same floor level as the shower. The height from the bottom of the water tank in the loft to the head of the shower is only 1 metre giving 0.1 bar of pressure, so a low pressure system, the pipe to the shower is 15 mm dia, what range of mixer showers are available to suit this low pressure system
Our unvented cylinder has been working fine throughout the winter despite the adjacent expansion vessel being apparently idle and showing nil pressure. I turn the re-pressurizing valve and nothing changes. Dead. But we still have hot water and central heating throughout the house. Even with the lodgers. Is this unusual?.
I’m converting to an unvented cylinder for hot water but am unsure whether to switch to a direct (immersion heater) cylinder or stick with indirect via my gas boiler. We have very little hot water use – no bath just an electric shower and effectively only two mixer taps requiring hot water, one in the bathroom the other in the kitchen. We run a cold fed dishwasher, which reduces use even more. The contractor advises a direct unvented cyclinder (125 litre) with two immersion heaters (top and bottom), saying I’d only need to use top one to heat c30 litres of water to meet my needs. So despite electricity being 3x more expensive than gas currently, I’d be no worse off and might even save some money. Is he right?
I have had all sizes of property, so also had all types of water cylinder, I currently have a combi boiler and it is fantastic, good pressure hot water is on tap, it is very economical too. In one property I had storage heaters, a water tank and electric shower, it cost me a fortune to heat that water every day which only got used for washing up.
Having experienced the flexibility of a Combi I would not go back, so for me the best sort of water cylinder is no cylinder at all!
I have been reading your article on hot water cylinders and the different ways to heat them up and thought you might be interested in the way I heat my water tank.
I have a conventional gas central heating system installed.
I also have a Charnwood multi stove fire installed around 1986, which is connected to the central heating system pipe work so if I want I can heat my water and radiators from the fire, this involves switching the gas boiler off using the insulating switch next to the gas boiler so not to use gas as well.
If I just want to heat the water using the fire the system allows me to heat the water through the gravity system without the need to run the central heating pump, saving electric, this also gives me heat from a gravity feed radiator at the top of the landing which warms the upstairs up as well.
When the weather is nice especially in the summer I then use the electric water emersion heater, 1 kw, which is low wattage but ideal from the free solar panels power switched on automatically from a clever little box which watches how much power the solar panels generate and when there is above 1 kw of free power switches the emersion heater on.
This system means I only pay £50 p/m for my energy and my supplier advises me I should be paying around £40 p/m.
I am looking at relocating an unvented indirect hot water cylinder with external expansion tank. The expansion tank is currently located above the cylinder. The total height exceeds the height available in the preferred new location. Is it possible that the expansion tank be installed adjacent to the cylinder rather than above the cylinder?
I am looking at getting an unvented cylinder to provide extra capacity to cope with 2 new showers in our loft conversion whilst the rest of our house runs off a combi. Is there an option to set up an indirect unvented cylinder just to run as a direct cylinder for now (i.e. off the immersion) as I know indirect cylinders have a back up immersion fitted, and then connect it to a new system boiler when our existing boiler dies (we still have 3 years left on the warranty so dont want to replace it now)? Thanks!
With a cylinder like this it is better to put high up ?
I’m now living in a semi retirement ground floor flat and so miss my gas combi boiler, but stuck here with just electricity. I got rid of the old Economy 7 heaters and had two electric radiators installed. I’m now on standard electricity rates but would like to change the present water cylinder to a smaller size. I don’t use much hot water, I only have a shower about 3 times per week and have no bath. I have an Albion maxistore direct – 144 litres but of course the economy is switched off. So, what cylinder do I need to buy? Direct? Vented or unvented? And smaller than what I have which is 1050 x 450 – 144lts.
Please help
You only having a shower 3 times a week I think is a bigger issue than the boiler mate.
How far above the indrect cylinder must the expansion tank be in a centreal heating bungalow
We have 120 liter hot water cylinder in our Flat. Lets say we have used half tank of hot water.
It will be filled by cold water from the mains, and in turn it will cool down the remaining half tank of hot water which was heated before right?
Hello
We have what I think is a vented hot water cylinder. We seem to lose hot water far quicker than we would expect to. The tank is on the first floor of a 4 storey town house (a cylinder house!). Could it be that hot water just naturally rises from the tank to the highest point in the house? Should there be a kink installed from the pipe coming from the cylinder to act as a false high spot? Hot water would then only be taken from the boiler when required by a hot tap or whatever.
Thanks.