There is no doubt that hot water cylinders are enjoying something of a revival. This is largely driven by the rise in popularity of renewable energy systems, which require hot water storage to properly harvest potentially intermittent or low in power output heat sources. In addition, households, who need a system that will supply multi-outlet use at much higher flow rates than combination boilers are capable of, are upgrading their heating and hot water systems to include a hot water store.
The rule of thumb for usage is 35-45l per person at a temperature of 60°C. At a 60°C cylinder storage temperature, a 100l bath at 40°C will require 60l of hot water and 40l of cold, and a standard shower about half that. Another caveat would be the type of shower, certain types can use up to 20l a minute, in which case a five minute shower would use as much stored hot water as a bath. If consumption is as high as this, a more accurate measurement would be 50-70l per person.
When sizing requirements, engineers must try to establish household usage patterns. The right questions will reveal maximum demand patterns – maybe two showers are always taken in the morning at the same time or perhaps the kids take their baths just before bedtime. Always allow for this maximum demand.
Fuel types
The fastest way to re-heat a hot water cylinder is using a high output appliance, such as a gas or oil boiler. Whatever the future of these technologies, they do have the ability to put a lot of energy rapidly into the water, giving typical re-heat times of 20 minutes for a 150l cylinder using a 20kW boiler.
Heat pumps are rising in popularity for a number of reasons, however they are all generally lower in power output than gas or oil boilers, typically between from 4-15kW.
If we presume a mid-output of this range, say 8kW, then expect to see a re-heat of around 60 minutes for a 150l cylinder. There is a further consideration here – a heat pump must have a heat pump cylinder fitted to optimise the re-heat time. The HWA technical guide on heat pump cylinders will assist engineers to specify the right one, as will the HWA cylinder sizing calculator. Both are available at www.hotwater.org.uk.
Some systems use more than one type of input, perhaps a heat pump and a solar thermal input. In these cases, look at the worst case scenario. In other words, treat the solar volume as a bonus and calculate hot water storage on the heat pump capacity only. There are more than a few days in the UK where solar gain is minimal.
Some systems use wood burners or cooking appliances for inputs. Again, these are best treated as ancillary inputs. Look at the kW ratings to establish the effectiveness of the source and remember that there will be periods in the year when the wood burner will not be lit.
Demand changes
If demand changes in the household, it is not always necessary to increase the size of the cylinder. For example, extra demand from grown-up children returning home could be met by increasing the temperature of water in the cylinder, blending it with cold water via a tempering valve to subsequently achieve a lower water temperature at the taps, thus producing more usable hot water from the same size cylinder.
Most cylinders store water at approximately 60-65°C. If this was increased to 70°C, you get more water delivered at 55-60°C as the valve mixes both hot and cold water, which increases the cylinder’s water capacity. This means that less hot water is drawn from the cylinder, therefore boosting its capacity and the amount of useable hot water for the home.
As a rule at 70°C storage, the amount of usable hot water that you can generate is approximately 20% more than at a storage temperature of 60°C. Of course this is dependent on the primary flow being capable of reaching a higher temperature to heat the stored water to 70°C. Electric water heaters or solar PV are ideally suited for this.
The Distribution Tempering Control Valve (DTC) scheme by NSF operates as a third-party approval scheme to verify that tempering valves comply with the performance criteria of both, Part G of the Building Regulations and the European Standard EN 15092:2008.
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