This month, heat pump specialist Bob Long looks at choosing the most effective emitter for the job
Having completed an accurate heat load analysis of your project, and selected a suitable heat pump, the next step would be the choice of emitter, influenced by considering a number of factors.
The most common type of emitter is the panel radiator and would generally be reliant on water flow temperatures anywhere between 65 and 80°C.
This temperature range is not readily achievable by heat pump technology, and although some manufacturers are offering high temperature units, the operational economics will be compromised by the higher temperatures.
Clearly, a retrofit scenario that reduces the operating temperature of the radiator will not deliver the required amount of energy to heat the space, without increasing the size of panel radiator. In some installations this can be achieved but wall space is often the limiting factor.
Further consideration, when retrofitting an existing system, should be given to the water flow rate. A defined volume of water, transported around the system at 70°C, will contain more energy than a similar volume of water at 40°C.
Similarly, the circulating pump capacity must be increased to ensure delivery of an adequate amount of energy. When increasing pump capacity, further consideration should be given to pressure drop which will increase proportionately with higher velocity of water flow.
When deciding if a retrofit is the best way forward, all these factors should to be considered, before embarking on a complicated solution.
For a new installation, there are basically three emitter type-options available, fan/coil unit, underfloor pipe matrix, or panel radiators.
Fan/coil units are designed to be very compact yet have large capacity. It is important when selecting a suitable unit that the sales literature clearly states the output for the intended water temperature.
Fan/coil units can be wall mounted, where the exterior finish often benefits from a range of visually pleasing styles, or the fan coil units can be located in ducts, which direct and deliver the warm air to various strategic outlets.
The fan coil unit method of energy delivery in perhaps the most versatile of all heat emitting systems available. A suitably sized fan/coil unit can deliver at least the same amount of energy as the most efficient underfloor system, and is suitable for new-build and retro-fit installations.
Underfloor heating is often an attractive choice in new-build projects as it provides good heat emission at relatively low water temperatures, and the actual heat exchanger is out of site. The only visible items are probably going to be the supply and return manifolds, but these are usually located in a cupboard.
The heat load of the particular room plays a great role in the selection process, as does the physical size of the room.
The total heat load of quite a large room could probably be accommodated with a single fan/coil unit, but would not promote an even air pattern, and maybe leave one part of the room at a lower temperature, so it is important for the selected unit to not only match the heat load required, but to dissipate the energy evenly. This may require two or three fan coil units, each supplying one third of the required energy, but producing a much more even temperature, than a single emission point.
There are further considerations when choosing emitters, such as water flow rates, pipe diameters, pressure drop, pump selection etc. But a most important consideration not yet mentioned is, of course, your client’s lifestyle requirements, and the influence this has on the heating system’s overall design.