Due in part to the climate in which the UK finds itself, there is an almost universal requirement for heating in buildings whether that be for residential, commercial, industrial, leisure, education or healthcare purposes. A source of heat can be required for comfort space heating, environmental control, Domestic Hot Water heating or for industrial processes. Traditionally, there have been three main temperature ranges (in the UK) used for the design of heating systems as follows:
- High Temperature (or Pressure) Hot Water Systems (HTHW, >120DegC, dT 45-65DegC)
- Medium Temperature (or Pressure) Hot Water Systems (MTHW, 90 - 120 DegC, dT 25-35DegC)
- Low Temperature (or Pressure) Hot Water Systems (LTHW, <90DegC, dT 5-20DegC)
Each temperature range has its advantages when used in particular applications but outwith specialist process industries, LTHW is the most common system in use. Traditionally, a flow temperature of 82DegC and a return temperature of 71DegC (a differential of 11DegC) have been used for the design of heating systems. These temperatures correspond to 180 and 160 DegF (a differential of 20DegF) respectively. It seems likely that the round numbers were originally chosen when Fahrenheit was the common unit in use and the less round numbers are as a result of the direct conversion to Centigrade. The flow temperature of 82DegC provided a safe flash margin and a return temperature of 71DegC was sufficiently above the dewpoint of the flue gases which was particularly important in the days when fuels had a much higher sulphur content. This design "rule" was implemented for a very long period of time and has only changed in recent years, particularly driven by the desire to incorporate renewable energy sources into heating systems. A typical heating system today would be designed for a flow temperature of 80DegC and a return of 60DegC widening the differential to 20DegC. The widening of the dT has a proportional reduction in required water flow rates which in turn reduces pipe sizes and pump duties. Emitters tend to be slightly larger to reflect the reduction in Log Mean Temperature Difference (LMTD) between the water temperature and the temperature of the heated medium. Where renewable sources of energy are designed as the main or lead source with a fossil fuel boiler often used only as a backup, the temperatures can be reduced further to 50DegC and 30DegC respectively. Temperatures lower than the dewpoint of the flue gases permit condensing of the water content of the combustion products thus releasing some of the stored latent heat in the steam. The efficiency of condensing boilers is improved significantly over their non-condensing counterparts.
In a traditional design, the heating water would be pumped around the hydraulic circuit at a constant volume flow rate regardless of the load. The adjustment at the emitter to reflect demand would be achieved with the use of a 3-port control valve which diverts the unused portion of the flow from the supply back to the return without travelling through the emitter. The emitter is therefore subject to variable flow even if the main circuit is configured for constant flow. This results in increasing return temperatures and reducing dTs as the load decreases but with a constant pumping cost. In contrast, variable flow systems, as the name suggests, incorporate a varying flow rate to match the demand. This is achieved by using 2-port control valves at the emitters so that the varying flow rate through the emitter is reflected in the main hydraulic circuits. Back at the circulating pump, by sensing the rise in differential pressure of the circuit as the control valves close, the pump speed can be reduced automatically by the use of an inverter thus reducing pumping energy by the reciprocal of the flow rate reduction cubed. For example, if the flow rate halves then the pumping energy will reduce to 12.5% (1/2^3) of that at full load. Therefore, not only are variable volume flow designs lower in capital cost to install, they can also realise significant annual savings in pumping costs. Of course, variable flow systems are not a panacea and can have their difficulties if not designed correctly but when correctly implemented can have significant cost benefits over traditional constant flow systems.