Vented & Unvented Hot Water Cylinders

Before 1989, it was much easier to decide which hot water cylinders to install. vented cylinders were legally allowed for domestic use. But since then, unvented cylinders have been introduced and people now have the choice of vented and unvented hot water cylinders. Each has their benefits and drawbacks to consider.

The hot water cylinder you choose also depends on whether your mains water is fed directly or indirectly into the system. This also determines how water is heated, either through a heat exchanger or electric immersion. You will find that many homes take advantage of both methods by installing a combination of both, one for daily consumption and another for backup or storage.

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Vented Hot Water Cylinders

Even after the introduction of unvented water cylinders, vented systems are still more commonly used. In this system, the mains water is stored in a cold water storage tank. Since the water supply uses gravity to distribute water around the house, this tank is usually located in the loft. Water is fed indirectly and a heat exchanger is needed to produce hot water.

Benefits of vented hot water systems

Vented hot water systems are much less complicated to install. For this reason, they are more affordable and easier to maintain. With this system, water is heated when it passes a series of coils – the heat exchanger – which is located within the unit. This makes vented hot water cylinders very economical to have.

The fact that the mains water is fed into a storage tank makes it handy when the water mains is cut off. You will still have some backup hot water supply, despite not having water running through the mains.

Drawbacks of vented hot water systems

On the other hand, the use of a tank means a fair amount of space is required to install a vented hot water system. And, with the heat exchanger producing all the hot water, you risk being left with no hot water should the part breakdown.

Another drawback with using a cold water storage tank is low water pressure. Because the water pressure is driven by gravity, pressure can drop when more than one tap is being used at the same time. To resolve this, a pump needs to be added to guarantee good flow of water around the house at all times.

Hot water cylinder installation cost

The installation cost of vented hot water cylinders depends on the complexity of the project and also on the location that you are based in the UK. However, there are ways to reduce the installation cost when it comes to replacement cylinders.

Plumbers usually quote a day’s rate for this kind of project when the job is not very complicated and the cylinder can be removed easily. Installing a standard vented hot water cylinder would cost between £500-£600, if you leave the plumber handle everything. The price includes the water tank (£200), the thermostat (£20), the labour (£250-£350), the cost of scrapping the tank and any waste created plus any secondary materials such as washers, fitting and PTFE tape.

You can also reduce the installation cost by:

Making all the connections as easy as possible to access by positioning them all on one face.
When replacing an old cylinder, ensure that the connections are in the exact same position as in the new one.

Unvented Hot Water Cylinders

The newer unvented hot water systems have the mains water directly fed into them. There is no need for a cold water storage tank. Hot water is basically produced similarly to vented systems but instead of a heat exchanger, it uses an immersion heater which runs on electricity. Unvented hot water systems constantly provide hot water at mains pressure.

Benefits of unvented hot water cylinders

Those who have unvented hot water cylinders installed in their homes can always rely on the pressure from the mains. This means that the system can be installed almost anywhere you like, without having to depend on gravity for good water flow. You won’t have to worry about pressure getting a little too high either as many have in-built safety devices to cope with this, as well as the issue of water expansion. As with the immersion element, this makes unvented systems more reliable and even if it does breakdown, they are easy to find and replace.

Disadvantages of using unvented hot water cylinders

As you may have guessed, unvented hot water systems are more complex than vented cylinders. Installing one will require a specialist and this means added cost. There is also an issue with unvented systems not being compatible with some power showers and mixers. The use of an electric immersion heater also increases its running costs. Electricity is three times pricier than gas, which is what most vented systems run on.

Other than vented and unvented hot water cylinder systems, there are other boilers that can supply you with hot water without the need for cylinders. If you choose to install hot water cylinders, keep in mind that most are made from thin copper. When not insulated properly, your thin copper cylinders can suffer from a lot of heat loss. A good coat of foamed polyurethane will ensure that your cylinder system is always running at its most efficient.

Costs for replacing an unvented hot water cylinder

If you would like to upgrade your hot water cylinder from vented to a system the cost should be around £700 in total for both labour and the cylinder as this job usually takes usually two days for a plumber to install a stainless steel, unvented, insulated and pressurised hot water cylinder. However, prices can vary depending on where you live in the UK.

Gas Gafety Guidelines (PDF)

Watch the following video for more useful advice

What type of cylinder is best for me?

There are some important factors you should consider while making a choice between vented and unvented cylinders. These include:

The size of your house;
The type of heating system and the amount of pressure from the mains supply; and
The cost of installation and routine maintenance.

What Existing Heating System and Mains Pressure are you working with?

Vented hot water cylinders and regular heating systems are the traditional systems that are most common in older homes. It is important to choose the right cylinder type to avoid any compatibility issues. If an unvented water cylinder is installed on a regular heating system, it may cause an increased pressure far above what the old network pipes and radiators can handle.

On the other hand, if the pressure from the mains is relatively low, installing an unvented system will not lead to any complications. However, it is important to assess all the possible options because an unvented water cylinder will maintain the current low pressure while supplying water directly from the mains. A possible work around would be to install an additional pump that will amplify the pressure of the flow.

How big is your House?

You should not consider installing a vented cylinder if you do not have plenty of space preferable with high elevation. A vented water cylinder system needs to be placed in an elevated position to store the cold water because the system uses gravity-enhanced delivery.

Do you have two or more bathrooms?

If yes, then a vented cylinder is the best option for your home because the cold-water tank supplies sufficient water concurrently without dropping the water pressure.

On the other hand, an unvented tank relies heavily on the flow from the mains. If the mains are divided into several outlets, which are used simultaneously, the pressure will become weaker. However, this will not be an issue for any home that has less than three bathrooms.

What Size of Hot Water Cylinder Do I Need?

Choosing the right size of cylinder is a very important factor as it will ensure that it can meet the water needs of your home without causing severe shortage or energy-wasting by heating more than the required amount of water.

The capacity of a cylinder is calculated in liters, however many homeowners do not have an idea about the quantity of water they use per day. To calculate the right capacity, you need to look beyond taking a bath, washing dishes and watering the lawn. Proactively think about the morning rush periods when everyone will be in the bathroom and the busy weekends when you are watering the lawn while operating the dishwasher and washing simultaneously.

However, the easiest way to estimate the required size is to make an estimation based on the number of bedrooms, bathrooms, and showers. The required capacity does vary slightly depending on the water-heating method (i.e. direct or indirect heating system) because the cylinders that are heated indirectly through boilers usually reheat quicker.

FREQUENTLY ASKED QUESTIONS

Typical Problems with an Unvented Hot Water Cylinder and Practical solutions

If you are having problems with your unvented hot water cylinder but don’t know what to do about it, then you are just at the right place, but firstly we need to give you a brief history about the unvented hot water cylinder and then we will provide answers to some of the frequently asked questions that we get asked.

Are unvented hot water cylinders dangerous to work on?

Unvented hot water cylinders can be dangerous to work on. In particular for homeowners that do not have any necessary qualifications. To handle this kind of jobs, a plumber needs to have the necessary qualification and also hold an unvented certificate to be able to handle different types of boilers. If not, he can be liable to pay a big fine or maybe even jail time.

Why I am not getting hot water from my unvented hot water cylinder?

This is most likely due to a faulty immersion heater and may mean that you have to replace it or just its stat. With an economy 7, it can become tricky to diagnose it, but can be easier when a qualified engineer makes use of separate power sources to do it. Ensure to find out if the heating engineer has an extension lead and a plug.

Why am I not getting water at all since I have an unvented hot water cylinder?

If you are not getting either hot or cold water through the hot water tap, then this is probably because the tap’s mesh or filter is blocked.

What about when my water is too hot on my unvented cylinder, why is this?

This is probably because the stat of the cylinder has been set too high, higher than the usual 60 to 65° or maybe that the stat has is no longer functioning properly and so needs replacing.

What about when my water is too hot on my unvented cylinder, why is this?

This is probably because the stat of the cylinder has been set too high, higher than the usual 60 to 65° or maybe that the stat has is no longer functioning properly and so needs replacing.

What about when there is water dripping from the plastic pipe, is there a problem with that?

If there is water dripping from the D1 to D2 pipe through the Tundish, It is usually caused by an overflowing pipe that runs from the unvented cylinder to the water waste and then to the outside.

Then there is a problem, most times, illegal fitters might think that it is the PRV that is faulty but that’s wrong.

There is water pouring through the pipe of my unvented hot water cylinder, what should I do?

Whenever there is water flowing through the Tundish of the plastic overflow pipe, firstly, turn the electric off and then turn off the main water afterwards. Make sure you do not try to fix it yourself, always make sure to call a qualified unvented fitter specialist.

Do I have to get my unvented cylinder serviced every 12-months?

This is not compulsory but it is advisable. Getting the unvented cylinder serviced every 12-months is not mandatory but is recommended, at least if for nothing else, so as to enjoy the warranties that are available to you on the unvented hot water cylinder.

What does the expansion vessel do on the unvented hot water cylinder?

The expansion vessel has the capability to hold in water and then expand. Ever wondered what is in the expansion vessel? It contains air that is enriched partly with water and partly with Nitrogen.

The hotter the water becomes, the more the pressure within the vessel. When the vessel is taken for service, the unvented engineer will use a tool that will find out the pressure point. Also by using this tool, he can also find out whether the vessel has collapsed or not.

I’m not getting enough water through from my tap and I have an unvented cylinder, why?

If you are not getting a sufficient amount of water or maybe no water at all, this means that the mains water stopcock is isolated or that the filter has been blocked. In situations like this, the only thing you can do is to call an unvented fitting specialist.

What is the Tundish for on the cylinder?

The Tundish enables the customer to see where water is leaking through the overflow so that you can call the unvented hot water cylinder specialist.

Who do I call if I have a problem with my unvented hot water cylinder?

If you are having issues with you unvented hot water cylinder and you are unsure, you should call an emergency plumbers asp.

What should I do if my unvented boiler isn’t being used?

There isn’t any problem if your unvented boiler isn’t being frequently used. However, it will need cleaning and disinfecting after 40-days or more if it hasn’t been used. This is done by turning on the heat on the central heating boiler of the indirect unvented hot water cylinder to a temperature of about 70°.

This will help kill all the bacteria in the water. It is however noteworthy that if it is hard water and there is a possibility of scales being in unvented hot water cylinder. This causes bacteria to be trapped and can lead to Legionnaires disease.

Call a specialist to help you clear the scale within the unvented cylinder; he will place the chemicals inside the cylinder to help you get rid of the scale. Once this is done, heat up the cylinder to kill the after which you run the hot water off.

If I have poor water pressure, can I have an unvented cylinder?

Poor water pressure implies that you will not be able to have an unvented cylinder unless you are able to find a way to get better water pressure. To be able to ascertain if your water pressure and flow is good, you will need instruments like a Weir gauge and a pressure gauge.

To have an unvented hot water cylinder it will require at least 1.5-bar water pressure and also nothing less than 20-litres every minute.

Which is the better cylinder, a vented or an unvented hot water cylinder?

Having to choose the better option from either a central heating system or a hot water system is really a matter of opinion. One may not necessarily be better than the other, it all depends on its usage.

For a vented cylinder, the water is usually pressure fed from its property within, but with an unvented cylinder, you usually rely on pressure from the main water pressure and flow.

Why do we have so many pipes on an unvented cylinder?

The main reason for having the extra pipes is because usually, so much is going on at the same time. Safety pipes fitted into unvented hot water cylinders makes them less dangerous. Just as with a system boiler, you still get the same flow and return, likewise the hot water outlet and feed into the direct or indirect cylinder.

You may find an extra pipe (depending on where you have the expansion vessel and you will also find an overflow (discharge) pipe also known as; the D1 and the D2 discharge. The first pipe that comes from the safety device is and this is followed by the Tundish, after which is the D2 pipe which takes the water to its exit point.

What should the pressure be on the unvented cylinder?

As explained above under the sub head ‘If I have poor water pressure, can still have an unvented cylinder‘, the water pressure should read a minimum of 1.5 whereby the main water pressure should be at least 3.0-bar or ideally 3.5-bar.

Why do we have so many pipes on an unvented cylinder?

The main reason for having the extra pipes is because usually, so much is going on at the same time. Safety pipes fitted into unvented hot water cylinders makes them less dangerous.

Just as with a system boiler, you still get the same flow and return, likewise the hot water outlet and feed into the direct or indirect cylinder, you may find an extra pipe (depending on where you have the expansion vessel and you will also find an overflow (discharge) pipe also known as; the D1 and the D2 discharge. The first pipe that comes from the safety device is and this is followed by the Tundish, after which is the D2 pipe which takes the water to its exit point.

What should the pressure be on the unvented cylinder?

How do I go about becoming a professional unvented boiler fitter?

To become a professional unvented cylinder specialist, you will need to get the necessary qualification G3 which you can obtain by attending a 1-day course and writing the exam all on the same day.

Minimum qualification required is the NVQ certification in plumbing, level 2 or other proof that you are a gas engineer. These are the main qualification that you will need, though there are others.

Why is my unvented cylinder leaking?

There are several reasons why this may occur but most times this would be because the boiler is over pressurised. Most times, the water will tend to come out through the overflow then out through the Tundish.

It is also known that some unvented hot water cylinders have weak spots and so whenever there is immense pressure, the cylinder pops thereby causing a water leak.

Why has my unvented cylinder exploded?

As is stated in the above paragraph 'why is my unvented cylinder leaking?'. Weak spots usually cause explosions but sometimes the word explosion is also used to express situations whereby water just surrounds the unvented hot water boiler.

Real explosions normally happen when there are no safety devices and the cylinder is constantly being heated up without turning it off. Likewise, this too can be as a result of using non-qualified unvented hot water cylinder specialist.

What should be the temperature on the immersion heater of my unvented hot water cylinder?

The default temperature setting is usually at 60 to 65° and this is the recommended temperature unless some maintenance work is being carried out by a competent engineer.

If the temperature rises beyond this normal, there is usually a safety device that stops the water from getting too hot such as an efficient working stat will cut off the power to the cylinder if it gets too hot.

Does an unvented cylinder heat up via direct cylinder or does it need a system boiler to heat up?

Due to the fact that an unvented hot water cylinder can rely sole on electricity, it does not need a system boiler to heat up water through immersion heaters. A system boiler major advantage is because it is cheaper option when compared to gas and electric.

How would I know if your plumber is a qualified unvented hot water cylinders plumber?

Your emergency plumber should have his G3 markings and should always carry his I.D to show that he /she is qualified to work on the unvented hot water cylinder.

They can also carry additional cards like the gas safe card and have his qualifications printed on it.

Ensure to check the date of his qualification as validity of qualification is only for 5-years after which he/she shall retake the qualifying exams again.