Unvented Hot Water Cylinders

Unvented hot water cylinders, like Megaflo, need checking to maintain their safety. We'll be dealing with that on this page. Working with unvented cylinders also requires a specific qualification. That's important too!

If you're in our area and have problems with your unvented hot water cylinder and can't find the information you need on this page, please ring Paul.... 01297 678 517

Unvented hot water cylinders are very safe to use, provided that they are correctly installed and maintained. If they are badly installed or not maintained, they can become dangerous. That's why working with unvented cylinders (with a capacity of more than 15 litres) requires a separate qualification.

Traditionally, hot water cylinders worked at atmospheric pressure and were open-vented cylinders. They were fed from a cold water storage cistern (tank) above them, which was usually in the loft. They had a safety vent pipe which went up from the cylinder, over the top of the cold water stored in the tank which supplied them. In a fault situation (e.g. if an immersion heater failed to shut off and the water boiled) excess pressure was relieved by water being pushed back up into the cold water storage tank. This could be either via the cold feed pipe between the cold tank and the cylinder or via the vent pipe from the top of the cylinder, or both.

Nearly all new hot water systems are sealed systems. These are either combination boiler systems (which do not have a hot water cylinder at all) or systems incorporating unvented hot water storage cylinders. Most combi boilers do not have a hot water storage capacity greater than 15 litres, so they do not come under the same set of rules that apply to unvented hot water cylinders.
Provided that the incoming cold mains water pressure and flow rate are sufficiently high, unvented hot water cylinders are able to supply high flow rates of hot water for showers and baths.

Unvented hot water cylinder operation

Above 4°C water expands when it is heated. (Unlike most liquids, water does unusual things below 4°C, but that's another story.)
Cold water is taken into the cylinder and heated up, causing it to expand. Because the hot water system is a sealed system, the expansion must be allowed for or it would cause a very rapid rise in pressure which would burst the cylinder or pipework.

Unvented cylinders use one of two different methods to cope with the water expansion. Megaflo, Oso, Kingspan Tribune, Kingspan Albion and one or two other manufacturers, use an internal air pocket (sometimes called a bubble-top). We'll look at that below. Most manufacturers use an external expansion vessel, which is either spherical (like an oversized football) or cylindrical (with domed ends). It usually sits above the cylinder. Expansion vessels for unvented hot water cylinders tend to last for several years (maybe 5 to 10 years) but they will need repressurising every 2 or 3 years. These expansion vessels are for potable (drinkable) water and are usually coloured white but may also be grey or blue, or stainless steel.

The expansion vessel has a "rubber" diaphragm or bladder inside it, which separates the internal space into two zones, the water side and the "air" side. The form this rubber separator takes doesn't really matter. It just needs to be really flexible and to keep the water and air sides separate from each other. We'll call it a diaphragm. The air side may be filled with nitrogen or air; it doesn't make a significant difference (air is about 80% nitrogen anyway). There is a water connection at one end of the expansion vessel and an air valve (car tyre valve) at the opposite end.

When the expansion vessel is first installed, it comes with the air side pre-pressurised, typically to 3 bar. This stretches the diaphragm so that the air pocket takes up the whole of the expansion vessel. When the water side is initially connected to the water in the cylinder, no water is likely to enter the expansion vessel. This is because the incoming cold water pressure is limited to 3 bar and is often less than that. With 3 bar pressure on the air side, and a maximum of 3 bar pressure on the water side, the water cannot push the diaphragm back from its pre-charged position.

However, as the water is heated up, it expands. The cylinder and pipework do not stretch very much at all, so the expansion of the water (as it is heated) is accommodated by water being forced into the water side of the expansion vessel. Remember, the rubber diaphragm keeps the water and the air separate.

The water may come in from the cold mains at 10°C and should never be heated above 90°C. Heated from 10°C to 90°C, the water expands by about 3.5% of its volume, so the water in a 200 litre cylinder would expand by about 7 litres (3.5% of 200). 7 litres of water will be forced into the expansion vessel, because the air in the expansion vessel can be compressed.

Typically, a 200 litre cylinder is fitted with a 24 litre expansion vessel. It starts off with the 24 litres of air in it, pressurised to 3 bar. If 7 litres of water is forced into the expansion vessel, the air molecules on the air side are compressed from 24 litres into 17 litres (about 71% of the original volume available). The pressure inside the air pocket rises from 3 bar to about 4.2 bar. This is still a safe working pressure for the cylinder.

The diaphragm inside the expansion vessel is semi-permeable, meaning that air molecules (mainly oxygen and nitrogen) can slowly pass across it. Air molecules are incredibly small and there are thousands of billions of them in the expansion vessel, but over time they pass across the diaphragm and are dissolved in the water. This is a very slow process but, gradually, the expansion vessel loses its air pocket pressure and the air pocket reduces in volume. If the air pocket has been reduced to 10 litres, and 7 litres more water was to be forced into the vessel as the water was heated, the initial 10 litres would be compressed to 3 litres. The pressure on each side of the diaphragm would go up from 3 bar to 10 bar. This would risk the cylinder rupturing.

To prevent the cylinder rupturing, pressure relief safety valves are fitted; these dump water to waste to protect the integrity of the cylinder. More on that below.

Megaflo, Oso, Kingspan Tribune and Kingspan Albion unvented hot water cylinders

Megaflo, Oso and Kingspan Tribune unvented hot water cylinders don't use an external expansion vessel; they have an internal air pocket designed to cope with expansion of the water as it is heated from cold to hot. They are sometimes called bubble-top cylinders. (The red expansion vessel which may be fitted near these cylinders is likely to be a central heating expansion vessel.)

At first glance, doing without an external expansion vessel seems like a good idea. External expansion vessels have extra production and installation costs and may need replacing every 5 to 10 years. They also take up more airing cupboard space. An internal air pocket only requires the hot water outlet pipe from the cylinder to have a dip tube built in to the cylinder. This is done in manufacturing and costs very little. Without an external expansion vessel, installation is simpler and cheaper too. However, using an internal air expansion pocket comes with a significant problem and we see this as a design fault.
One manufacturer, Fabdec Excelsior, incorporate venturi technology which they claim automatically replenishes the internal air pocket. More on that, below.

The problem with an internal expansion air pocket is that it is in direct contact with the water; the air relatively quickly dissolves into the water and the air pocket reduces. To try to overcome this problem, Megaflo use a floating plastic baffle to limit the contact area between the air and the water. In our experience, the air pocket almost completely dissolves away in well under a year; in some cases it dissolves away in less than 6 months. This means that, for much of the year, the air pocket is not large enough to accommodate the expansion of the water. Water is forced out of the expansion pressure relief valve, dripping into the tun-dish whenever cold water is being heated.

Megaflo cylinders come with instructions for restoring the air expansion pocket. These can be found in the User Instructions and on a label on the side of the cylinder. If the Megaflo cylinder is fitted at least one floor above the lowest hot water tap, these instruction may work well and the householder may be able to successfully top up the air pocket. If, however, your Megaflo cylinder is fitted on the ground floor, these instructions do not work because you are very unlikely to have a hot water tap at a lower level.

Besides noting whether cold water is being passed through the tun-dish, there is another way we test whether the air pocket in your Megaflo cylinder has largely disappeared.
While no hot taps are running, we turn off the cold water feed to the Megaflo cylinder. We then open a hot water tap (preferably on the same floor as the cylinder). When the tap is opened, the air in the expansion pocket is able to expand again, pushing water out through the hot tap. When the air pocket has been newly restored, this will push more than 30 litres of water out of the hot tap before it stops running. When the air in the expansion air pocket has been dissolved away, only a jug full of hot water will run from the tap. Remember to turn the tap off before opening the cold feed to the cylinder again.

We see the air expansion pocket as integral to the safety of an unvented cylinder. If the air expansion pocket has been lost, the cylinder is subjected to much higher pressures than would be the case if the air pocket was complete. If the air pocket has been lost, the cylinder pressure in a Megaflo will reach 8 bar every time the water is heated up from cold. With a newly restored air pocket, the pressure is unlikely to go above 5 bar.
For us, the best thing to do with an unvented hot water cylinder that uses an internal expansion air pocket, would be to fit an external expansion vessel.
Surprisingly, the majority of homeowners with unvented hot water cylinders are unaware that the ability to accommodate water expansion really matters.

We're planning an article detailing how we recharge the air expansion pocket in a Megaflo unvented hot water cylinder. The method Megaflo outline works well enough when the cylinder is upstairs, with a hot water tap on a floor level below it. It doesn't work when the Megaflo cylinder is on the ground floor. We'll describe how we recharge the air expansion pocket (bubble) in those cases. When we've written the article we'll put it on our Free Heating Advice website and we'll leave a link to it here.

It should be possible to test whether the Fabdec Excelsior cylinder really does maintain its own internal air pocket successfully. I haven't come across one of these cylinders but, if I do, I will check to see whether the air pocket is providing adequate expansion capability. If it is, we should not see water passing out through the tun-dish when fresh cold water in the cylinder is being heated.
If anyone has one of these cylinders, I'd be interested to hear how well the air replenishment technology works!

Unvented hot water cylinder safety

Unvented hot water cylinders store water at above atmospheric pressure and relatively close to boiling point. In a worst-case scenario, the water temperature could go a long way above its atmospheric boiling point while the water was still being kept in the liquid state because it is stored above atmospheric pressure. If the cylinder were to rupture at that point, the water would almost instantly flash over into steam which would have about 1600 times the volume of the water. To prevent this risk, unvented cylinders have many layers of safety built-in. Provided that these safety systems are not compromised, unvented hot water cylinders are remarkably safe.

To prevent an unvented hot water cylinder rupturing if the expansion pocket fails, the cylinder is fitted with two pressure relief valves. One valve is a simple, spring loaded pressure relief valve, often called an expansion safety valve. It is set to open at maybe 6 bar or 8 bar. If the pressure in the cylinder goes too high, the valve is forced open and water is dumped to waste, passing out through the tun-dish so that it can be seen. As water is dumped, the pressure in the hot water system is prevented from increasing further even if the water is still expanding; this limits the risk of the cylinder rupturing. Generally, pressure relief valves do not need to be replaced if the valve has operated.

The second pressure relief valve is a combined Temperature and Pressure Relief valve (TPR valve). The pressure section of this valve is forced open at about 10 bar, if the first expansion pressure relief valve has failed. Again, water would be dumped to waste, via the tun-dish. The TPR valve also has a temperature relief section. This opens a pathway for water to be dumped to waste if the temperature of the water becomes high enough to approach boiling point. If the temperature section of the valve has opened, it should close naturally as it cools. It does not normally need to be replaced.

Besides the pressure relief valve and the temperature and pressure relief valve, the cylinder is fitted with an inlet pressure reducing valve which limits incoming pressure to 3 bar.
Heat sources are required two have 2 thermostats in each case, so the electric immersion heater(s) have a main thermostat and a safety limit thermostat. There is a thermostat which closes off the supply of heating water coming from the gas or oil boiler and there is a thermal cut-out as a backup. These are in addition to the gas or oil boilers' own thermostats and limit stats. If all these thermostats should fail, the temperature relief section of the TPR valve is designed to dump water if the temperature reaches 90°C. This would prevent the water reaching it's atmospheric boiling point (100°C), thus preventing it from converting to steam even if the cylinder ruptured.

Components fitted to unvented hot water cylinders are designed and tested by the cylinder manufacturers. If any of these components need to be changed, including the two-port motorised valve fitted to the primary coil, they must only be replaced with components specified by the manufacturers of the unvented cylinder.

Which would we choose? External expansion vessel or internal air expansion pocket?

We think the external expansion vessel wins every time.
Internal expansion pockets are dissolved away far too quickly. When the air pocket has been dissolved away, the cylinder is subjected to much higher pressures (8 bar) whenever water is being heated.
If the Fabdec Excelsior cylinder is able to automatically maintain its air expansion pocket it might prove to be the best option. If it was fitted with a pressure gauge, it would be easy to see whether the maximum pressure was consistently kept below 5 bar.

Systems with external expansion vessels are much less frequently subjected to such high pressures. It's true that external expansion vessels lose their charge pressure but this is a slow process.
If a 24 litre expansion vessel loses one third of its charge pressure, and has to accommodate the 7 litres maximum expansion of a 200 litre cylinder, the system pressure will still only rise to less than 5½ bar. This means that a 6 bar safety valve could be used and the system would never be subjected to pressures above 6 bar.

Re-pressurising external potable expansion vessels is relatively easy. The incoming mains cold water to the cylinder must be turned off and a hot tap opened. The "air side" of the expansion vessel can then be pumped up to its original charge pressure (usually 3 bar) using a car tyre pump. The initial charge pressure should be marked on the expansion vessel.
The hot tap would then be closed again and the incoming cold water to the cylinder turned back on.
External expansion vessels may eventually fail but they are relatively easy to replace.

Fitting an external expansion vessel to an unvented hot water cylinder

For us, the best thing to do with an unvented cylinder that uses an internal air expansion pocket would be to fit an auxiliary external expansion vessel.
Megaflo, Oso and Kingspan Tribune unvented cylinders are all likely to lose their internal expansion pocket much too quickly, often in less than 6 months. This means that for much or most of the time, they are subject to higher pressures and are dumping water every time it is heated up.

An auxiliary expansion vessel would need to be fitted as close as possible to the cylinder, and downstream of (after) the 3 bar pressure reducing valve. A 24 litre potable water expansion vessel with a pre-charge pressure of 3 bar should be fine.
To limit the risk of water stagnating in the leg of pipe connecting the expansion vessel, the pipe leg should be kept as short as possible.
However, the water in this leg of pipe would not really be static. Each time the cylinder was heated up, water would be forced into the expansion vessel, and each time a hot tap was opened, that water would be pushed back out of the expansion vessel, along the pipe. This would have the effect of refreshing the water in the pipe.

If a 10 bar pressure gauge was fitted close to the auxiliary expansion vessel, the householder could occasionally check the pressure. With a 24 litre expansion vessel which had a pre-charge pressure of 3 bar, the water pressure would rise to a maximum of 4¼ bar when heated from cold.
Over two or three years, the expansion vessel would gradually lose charge pressure. When it had lost a third of its charge pressure, the water pressure might reach about 5½ bar when heated from cold. This would alert the householder to the need to have the expansion vessel charge pressure raised again.

This article may be amended from time to time...


(07/05/2022) Two years on I've now stopped routinely wearing a mask when I'm working. However, if you would prefer me to wear a mask when I'm in your home, please let me know and I'll be happy to do so. Jacky and I are fully vaccinated and will use lateral flow tests if we show any symptoms or if we are exposed to anyone known to have Covid.