7: Water heating – part 2 of 2
Method of heating & Storage
These two functions are deeply interwoven so I’ll deal with them together.
As solar hot water panels are to be used for the bulk of the domestic hot water heating there will be a requirement for hot water storage. Also, the system is to be oversized for this task so as to contribute to space heating hence a much larger than normal hot water storage facility is needed. This will need to be super insulated so as to minimise the losses. Instead of a 100 to 150 litre (20 to 30 gallon) storage capacity the tank will be up to 1,000 litres (200 gallons).
Such naturally sourced heating is inherently more difficult to control as its input is from a highly variable source. This means that, in the summer months, hot water temperatures could get well over the 40oC design temperature, whereas in the dark days of winter there will be practically zero input from solar and all heating will come from the gas boiler.
A large storage tank like this also needs some clever internal design. If both heat sourced inputs (solar and gas) were fed into the base then, when the solar input is small, but adequate, the general circulation of convection currents will tend to make a large, generally warm, tank whereas what is needed is a smaller amount of hot water concentrated at the top. This would cause the gas boiler to kick in unnecessarily.
Tanks of this capacity and type can be used to supply hot water in two ways, either by drawing off the water directly or by using the stored heat to provide instant heating for supplied fresh water.
We have yet to resolve the decision between supplying stored or instantly heated hot water to the point of use.
Before we get down to detailed calculations the general principles are…
- 1,000 litre storage (limited by space availability)
- Solar panels to provide main heat source
- Gas boiler to back up when solar radiation is low
- Maximum storage temperature is 60oC to 70oC
- Thermostatic temperature control on output set to 40oC
- Stratified tank construction to maximise solar benefit.
Pressure of supply
The general practice in British dwellings has been to supply mains pressure water to the kitchen and to a roof tank which supplies all other hot and cold water demands. Sometimes the mains cold is supplied to other locations instead of the low pressure feed. This arrangement minimises many safety considerations but can cause problems with flow rates in low buildings (such as bungalows!) so making it difficult for thermostatic mixers to operate correctly and to get decent flow rates for showers.
Making all the water supply at mains pressure is of particular advantage when using instant water heating, so the requirements relating to this high pressure water supply will need some investigation.