Unvented Hot Water Systems
The typical traditional British hot water system comprises a large tank of cold water in the attic and a copper hot water cylinder in the airing cupboard. Water in the cylinder comes from the tank in the attic which in turn is kept topped up by a ball-valve (which, when it fails, results in an overflow from the tank to outside via a pipe sticking out of the eaves). Water in the cylinder is heated either directly by an electric immersion heater or indirectly from the central heating boiler via a coil of pipe in the cylinder: hot central-heating water from the boiler running through the coil heats the water in the cylinder.
The pressure of hot water supplied by this system is governed by the height of water in the tank above the draw-off point (tap, shower etc). In a typical system this gives reasonably good pressure at taps on the ground floor (e.g. the kitchen sink tap) but lower pressure on upper floors, and particularly low pressure if one tries to run a shower on the top floor, since the shower head may be barely a metre below the water level in the tank.
Note that the flow of water is not necessarily poor: a typical traditional setup can fill a bath quickly since the pipework is quite large-diameter and the taps are big and present little resistance to water when they are open. Problems are generally found when non-traditional fittings such as continental-style mixer taps - and indeed showers - are attached to traditional systems. For showers the usual solution is to fit some sort of pump: either built-into a shower mixer unit (often in a plastic box fitted to the wall of the bath or shower enclosure) or separate from the shower mixer itself (fitted in the airing cupboard, under the bath or in the attic, whichever is possible and convenient).
By comparison with the arrangement of a tank in the attic (which has to be adequately supported to hold 25 or 50 gallons of water, lagged to protect against freezing, covered to stop insects and rodents etc. getting in, and fitted with an overflow pipe) plus a cylinder in the airing cupboard, an unvented system comprises one single cylinder, fed from the mains water supply. It supplies water at mains pressure, not requiring pumps for showers or continental-type taps etc. In principle they are much simpler systems and it is hard to see why the traditional British system was ever employed. Unvented systems are not, however, as simple as they seem.
Mains pressure and flow
Since the pressure of the incoming mains is much higher than that from a tank in the roof an unvented cylinder needs to be made much stronger than a vented one. Many are made of stainless steel; where made of copper this is a lot thicker and heavier than for an unvented cylinder. Also, since mains pressure is not guaranteed, a pressure-limiting valve must be fitted to restrict the pressure which can be applied to the cylinder. And since pressure-limiting valves can fail, a pressure relief valve must also be fitted, as a second line of defence, which discharge to outside if pressure exceeds a safe value for the cylinder.
A corresponding problem (with all mains-pressure systems) is that the pressure available is limited to that of the incoming mains. If, for any reason, mains pressure is low (or absent - e.g. if a water main in the street bursts) hot water supply will suffer. In a tank-fed system the same (low) pressure will be available until the tank is empty. A related problem is that the flow available from the system is limited by the main supply. In older houses the main pipework from the street is only designed to fill the tank in the attic, at a modest rate, which limits the rate at which hot water can be supplied e.g. for filling a bath.
Water expands as it gets hotter. In a vented system water simply expands back up the pipe connecting it to the tank in the attic. In an unvented system water may be able to expand back into the mains but this is not guaranteed (devices like water meters or certain designs of stopcocks can prevent water going 'backwards'). To accommodate this expansion an unvented system must have some provision for water expansion. This can be in the form of an expansion vessel; a metal container with a synthetic rubber diaphragm dividing it into two parts: one part contains air (or Nitrogen gas) under pressure, the other part is connected to the cylinder. As the water expands it presses against the rubber diaphragn compressing the air on the other side slightly more: the pressure in the system rises, but not much. Some unvented cylinders are designed to be used without an expansion vessel: instead they contain a bubble of air trapped at the top of the cylinder which allows water to expand, compressing the bubble slightly more, without a great increase in system pressure. Of course either of these mechanisms can fail, so to protect against this another pressure-relief discharge valve (PRD) is fitted to discharge water to outside if pressure in the system rises excessively.
Water boils at 100°C, at sea level. At high altitudes such as the Tibetan plateau, the Andes and the Himalayas where air pressure is lower, it boils at a significantly lower temperature. At higher pressures - such as in a pressure cooker, or the boiler of a steam engine - it boils at higher temperatures, so one can have water that's still liquid at, say, 120°C. If the pressure in a pressure cooker or boiler containing water over 100°C is suddenly reduced (such as by the vessel rupturing) much of the water turns instantaneously to steam and, since steam occupies a lot more space than the same amount of water, it expands violently. In the heyday of steam ships, locomotives and boilers in factories etc, steam explosions were known and feared: a ship's boiler exploding in this way could destroy the ship. (A domestic pressure cooker is designed so that one cannot open it until the pressure inside is back to normal.) An unvented hot water cylinder - containing water at mains pressure (typically 2-3 times atmospheric pressure) - has the potential for this sort of problem if the water inside it ever gets hotter than 100°C. In North America where unvented cylinders are the norm these sorts of explosions do occur from time to time (for example: 1, 2, 3, 4). For this reason another set of safety devices must be fitted to unvented systems:
- heat sources (boiler, electric immersion, solar if fitted) must have an independent temperature cut-off. For example electric immersion heaters must have two thermostats: one that regulates the temperature, switching the element on and off, in normal operation; and another which cuts out the element and does not switch it back on again if the temperature exceeds a (higher) set level.
- in addition to a pressure-relief discharge valve, an over-temperature discharge valve must be fitted to let water out of the system if it gets too hot. This is usually combined with the PRD on the cylinder itself.
Presumably because of the potential hazards of unvented cylinders, their installation is notifiable (to the Local Authority Building Control department). A professional installer must hold a specific qualification for working on unvented systems (and can self-certify their work rather than having to involve Building Control directly). And it is recommended that an unvented system be serviced annually, to ensure the safety devices are working correctly and the system is generally in a good condition. Unfortunately there is no legal obligation on owners of such systems to have this work done and, inevitably when there are other calls on money and the effort of arranging servicing appointments, this is often not done, even when the householder is aware of the issues and requirements. Even where properties are let there is no requirement along the lines of Landlord's Gas Safety inspection, and letting agents do not seem to be demanding anything, even where they do ask for evidence that the property's electrical installation has been inspected for safety. As a consequence it seems probable that there will, at some time in the future, be incidents in which badly-maintained unvented systems do fail catastrophically. Maybe then regulations will be introduced to ensure systems' safety through proper maintenance and inspection. Until then it seems irresponsible to introduce more such systems to the housing stock. (Even when installed for a conscientious owner who will ensure their proper servicing, people move house and continued maintenance by future owners cannot be guaranteed.)
See also this article on the various types of domestic hot water systems.