ENERGY SAVING
Water quality is key: this is a particular problem in retrofit systems. With hot water systems, you will often have a scaling issue in the pipes. In a heat
loop above that temperature (which is not always practical) or install a three-way valve to help ‘nurse’ the system up to temperature, as typically seen installed on condenser water loops between chillers and cooling towers. Three-way valves in the system side (and source side of WSHPs) are well worth the investment. Also, reversible machines may need different flows in their different modes, with different duties and design delta-Ts and duties in each mode. You are asking the machine to operate in a different part of its operating envelope depending on whether it is in heating or cooling mode. The duty of the machine can therefore change dramatically between modes and the water flow may also have to change. So, close co-ordination is required with pump, valve and pipe sizing. Bear in mind, too, that heat pumps may be noisier at night and heat pump power draw may not be maximum at the full load design point; be sure to design for that scenario, either with the requisite infrastructure or the necessary limiting devices. Night time is typically defined by local authorities as
from 11pm to 7am. Although most people will have left offices by this time, so commercial heating is not affected by this, domestic users won’t want to turn their heating or hot water off between these times. Most people want their house warm or a tank of hot water available for the morning, so the heat pump will potentially be working its hardest at, say, 5am. Aside from these design issues, vocabulary is also important. So, for example, when is a four-pipe unit not a four-pipe unit? What does ‘polyvalent’ mean? What does ‘hybrid’ mean?
The industry is still developing its understanding of definitions associated with heat pumps. So, for example, a chiller with heat recovery will have four pipes so could rightly be called a four-pipe system. People are now using ‘four-pipe’ and ‘polyvalent’ interchangeably. So, be careful
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pump system, any level of scaling or water quality problems are exacerbated by the fact that the
with your phrasing and accurate with your descriptions. In short, make sure when you are designing or specifying a job that your definition is the same as that of the rest of the HVAC team to avoid misunderstandings. Remember too that some compressor technologies are better than others in heat pump applications. For example, there can be oil recovery issues on large screws, or part-load efficiencies of variable speed devices could be compromised by fixed outlet / delivery temperatures. As for sizing of polyvalent units / systems, it can be tempting to size for the maximum of the two (heating and cooling) loads, but it is best to size for only the concurrent load, followed by sizing for the lowest of the heating or cooling peaks: sizing for the maximum of the peaks is bad practice because it is expensive, inefficient, and detrimental to reliability. It will also shorten the life of the plant. The best way to overcome this problem is to size the machine at the lowest of the two peaks (heating and cooling) and make up the difference with a dedicated machine. Finally, on larger projects, sequencing of multiple machines is key to maximise efficiency in any given scenario, especially in combined heating and cooling systems with disparate load peaks.
temperature difference between the delivered hot water and the control temperature are typically not as great as with a boiler.
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