How to Learn about Water Heating Systems? Technologies, storage and distribution requirements and controls

Water Heating Systems Fuel Types
Whilst the provision of domestic hot water is still largely
based on the use of fossil fuels such as solid fuel, electricity oil or gas in the majority of current EU housing stock. The EU has been progressively strengthening its measures to increase the use of high efficiency renewable technologies such as:
- Heat pumps (ground, water and air)
- Solid Biomass (logs, wood pellet and wood chip)
- Solar thermal systems

Design principles for successful renewable heat integration are:
1. Choose renewable technologies which are best suited to the householder demands, and which use the fuel or energy
most readily available.
2. Consider combining technologies, such that the advantages of one cover the weaknesses in the other (eg. solar thermal for DHW spring to autumn, plus biomass for space heating and winter DHW).
3. Incorporate fossil fuel back-up for 'worst case' situations, and for the short periods of maximum demand. This prevents unnecessary and expensive over-sizing, and gives the householder confidence in supply.

Heat Pump
Where heat pumps are fitted they are normally integrated into a low temperature heating system. For hot water storage, an auxiliary electric immersion heater or fossil fuel boiler is generally needed to provide a boost facility. It may be more economic to use the immersion or fossil fuel boiler to heat the stored water at temperatures above 45C, because the efficiency of the heat pump falls as the output temperature rises.

Solar Thermal Systems
A successful solar domestic hot water system requires the solar heat to be stored in a vessel to allow the heat to build-up slowly during the day. Because of the pattern of solar gain, it is very likely that a back-up heat source will also have to be integrated in to the system. Using a single cylinder, as shown in the picture opposite, is the most common solution adopted in the UK and Ireland which is where a cylinder is fitted with two indirect coils, the lower coil is heated by the solar, and the upper coil by a
boiler. There may also be an electric immersion heater in the storage cylinder.

Standardised Operation Conditions
During the design of a hot water heating system for a one family dwelling the following average values can be used for estimating the hot water consumption.
Depending on the appliances in the household, the following average consumption values per person can be calculated (45degC)
- Low Consumption 20-30 litres
- Average Consumption 30-50 litres
- High Consumption 50-70 litres

Water Heating Systems Losses
Primary Losses
The boiler should be positioned as close to the hot water storage cylinder as possible, so that heat losses from the primary flow and return pipes are reduced to a minimum.
- The best practice standard is that the primary pipe-work is insulated throughout its length.
- Fittings and valves should be insulated to provide the same insulation thickness as on the main pipe section (adhesives and jointing tapes are available to ensure a neat joint)
- Pre-insulated or district piping minimises the heat loss when running primary pipe-work underground from a shed / garage (biomass boiler) to the dwelling.

Storage Losses
For newly installed cylinders the best insulating medium is that applied at the factory as part of the manufacturing process, mould-injected rigid polyurethanefoam (PU), typically 50-75mm thick.

For existing cylinders an insulating jacket (at least 75mm thick) may be used, and fitted so that the are no parts of the cylinder wall is exposed. Fixing bands should not be over-tightened, as this will reduce insulation efficiency.

Distribution Losses
Hot water supply pipe-work should not be oversized but be adequate for the pressure and flow requirements:
- The best practice standard is that the hot water supply pipe-work is insulated throughout its length.
- Ideally the hot water storage cylinder should sited to minimise dead leg distances especially to the point of most frequent use.
Note: The kitchen tap is generally supposed to be the most susceptible to draw off losses since typically it is used for short bursts of hot water, compared to the bath tap where a long draw off is more common and an initial cold flow can still be utilised.

Water Heating System Controls
Water heating systems should be effectively controlled so as to ensure the efficient use of energy by limiting the provision of heat energy use to that required to satisfy user requirements.

Time Control - A 7-Day programmable timer allows the heating system to be set to match occupancy patterns on a daily and weekly basis. The separation of space heating and domestic hot water controls into zones allows each zone to operate for required periods only.
For example:
During the summer time the boiler can be set to switch on automatically before the householder wakes up in the morning to heat water for showers without turning on the central heating.

Temperature Control
A Cylinder Thermostat is a temperature sensing device used to control the temperature of the stored hot water within the cylinder whenever the hot water circuit is timed to be on. They should be fitted between a quarter and a third of the way up the cylinder. It is important to have a good, clean contact with the metal cylinder wall when fitting a strap-on thermostat. The recommended temperature for storage of domestic hot water is between 60degC and 65degC, being high enough to kill off the harmful bacteria in the water, yet low enough to deter the production of scale.

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