Articles
Hot Water Cylinders - The Heart of a Heating System
Feb
2012
A hot water cylinder can be simply a cylinder for storing hot water or it can be a good deal more. And modern heating systems need the cylinder to do more than merely store water. The problem arises in trying to work out just what is needed for a given situation.
The Vented Cylinder
This is the standard copper cylinder that used to be found in every airing cupboard. It was sited there as its poor insulation kept the airing cupboard warm. Priced at £150 upwards it was a cheap, easy option. It needs a header tank in the loft and hot water is gravity fed to taps and showers. Water pressure at the tap is therefore entirely dependent on the distance between the tap and the header tank.
Vented cylinders can be directly or indirectly heated. An indirect hot water system has a primary circuit flowing through a heat exchanger coil which is heated by the boiler. Water in the taps runs from the mains through the cylinder and to the taps. The tap water never goes through the boiler. In a direct system, the water in the cylinder is heated directly by the boiler or immersion heater and feeds then the taps.
The Unvented Cylinder
The problem with the vented cylinder is that the hot water pressure is dictated by the height of the header tank. Typically priced upwards of £500, the unvented cylinder overcomes this by pressurising the system, usually with an external pressure vessel. The Megaflow is a well know brand of unvented cylinder. What this does is provide constant and predictable hot water pressure at all outlets without the need for a header tank. Because an unvented cylinder is pressurised it brings a few constraints. The cylinder must have annual maintenance by a competent person to ensure it is safe and they cannot be used with solid fuel. Solid fuel cannot be switched off when the cylinder is up to temperature so there is a risk of overheating.
Thermal store
The option to vented or unvented cylinders is the thermal store. It overcomes most of the associated problems and provides a means of integrating multiple heat sources into a single heating system. The water in the cylinder is used as a battery for heat and as such never goes anywhere, certainly not to the taps. The water in the store and the mains water never mix but are separated by a heat exchanger that transfer the heat from the boiler to the mains water.
Thermal stores are usually stratified (hot water at the top and cooler at the bottom) which makes them particularly suited to modern heating system. They allow intermittent renewable energy sources such as wood burning stoves and solar thermal to contribute fully to a system that has a boiler or heat pump as the primary heat source.
Integrating Renewable Energy Effectively
For a solar thermal system to function properly the collectors must be warmer than the water you wish to heat. Because of this, solar coils are typically located in the bottom (coldest) part of a hot water cylinder so that the collectors can input useful heat during days of low sunshine. Thermal stores also allow the system to be organised so that it is not heated to capacity by the primary heat source in the morning but allows the solar thermal array to contribute throughout the day.
A well-insulated house can produce small, frequent heat demands to keep rooms up to temperature resulting in boiler or heat pump cycling (frequent switching on/off). The thermal store eliminates this by meeting the small, frequent demands and acting as a buffer to translate them into larger, less frequent calls on the heat source.
Achieving an efficient heating system is about 2 things: balance and control. Balancing the heat sources so that the cheapest is used most and controlling where and when heating is delivered. Compared to a simple copper cylinder a thermal store is very expensive, £2000 or more, but the opportunity they present to balance a multi-source heating system and the high level of control then possible make them well worth the extra investment.
Keep Control
As important as the heat store is the control system. It is a waste of energy and money to heat a room everyday that is only used intermittently. A good zone control system will allow each room to be separately controlled for time and temperature.