Low Temperature (or Pressure) Hot Water has been the most common form of heating within buildings for a very long time. Traditionally, a centralised boiler or group of boilers would heat water from a Return Temperature of 71DegC  up to 82DegC before distributing the heated water to the loads. The original flow and return temperatures are likely to have been direct conversions from those same temperatures expressed in terms of Degrees Fahrenheit (180/160). Nowadays, the introduction of heat sources other than fossil-fuel boilers has necessitated a change in design temperatures from both a practicality and energy saving point of view. Renewable sources of energy tend to operate best with wide dTs (Delta Ts or Differential Temperatures) and more importantly, lower return temperatures. The new standard dT used in the design of heating circuits is 20DegC. There are two new temperature ranges used in the design of modern heating systems and those are 80/60DegC and 50/30DegC for flow/return respectively.

The wider dTs mean that the LTHW flowrates are proportionately lower and therefore the capital cost of distribution pipework and pumps are commensurately lower. Conversely the final terminal device (radiator, AHU Coil, Heat Exchanger) requires to be slightly larger to compensate. However, the reduction in piping and pump costs more than offsets this slight increase.

The choice of heat sources has increased markedly in recent years with the introduction of Biomass Boilers, Air or Ground Source Heat Pumps, Solar Thermal and Solar PV (Photovoltaic) and Hydro Generation. Where the energy souce is electrical rather than heat, there are now options for Electric Boilers used to provide heat to LTHW systems. If there is no immediate demand for the electrical energy then this can be dumped into a Thermal Store to act as a preheat for either LTHW or Domestic Hot Water (DHW).

Callidus Design offer a number of services for Low Temperature Hot Water heating systems including design and specification of entire heating systems, system modifications and extensions, condition surveys, refurbishment and troubleshooting.