Following on from last month’s article discussing suitable choices for heat emitters, heat pump specialist Bob Long looks at the importance of lifestyles requirements and occupants’ specific heating needs
Consideration of lifestyle should be an integral part of system design, particularly when the heat source is to be a heat pump. Designing a heat pump system to optimise upon the economics of operation can be much more complex than designs employing combustible-fuelled counterpart.
Heating systems powered by combustible fuels such as natural gas or oil achieve much greater operational flexibility, able to match lifestyle needs much easier than a heat-pump powered system.
When referring to lifestyle, I am referring to the basic use of the property, and by whom.
No matter what you may have read about it being more economical to maintain a steady temperature in a home as opposed to allowing a dwelling to cool down and re-heat, it is absolutely wrong!
There is no economic merit in heating an empty home.
A conventional combustible-fuelled heating system has the advantage of being able to input large amounts of energy into a dwelling in a relatively short period of time, through the ability of the heat source to run high water temperatures, causing the emitters (radiators) to deliver large amounts of energy when needed.
This is not generally possible with a heat pump, unless the heat emitters are capable of delivering similar large amounts of energy, on demand.
In many instances, a high-temperature radiator panel will raise the air temperature in the room to a habitable level, before the temperature of the actual building fabric is satisfied.
Many heat pump system emitters, such as underfloor (UFH), or even retrofits employing existing panel radiators, cannot achieve this and so users find themselves running the heating system for many hours, sometimes unoccupied, to achieve the correct level of heating for the hours of occupation.
Heat pump systems designed for low occupancy homes should therefore differ from those designed for high levels of occupancy.
Assuming the dwelling may one day be sold, a low occupancy design will cater for all eventualities, but will necessitate the use of high output emitters wherever rapid heating results are required.
The type of high output emitter is largely confined to wall or duct mounted fan/coil units, as the output from panel radiators is limited by surface area and will, in almost all cases, be physically impractical.
UFH systems are also limited by water temperature and surface area.
When selecting fan/coil units, it is important to note the output at a specific working fluid temperature, and this can be quite difficult as manufacturers’ brochures are not generally too keen to display the relatively poor output achievable.
A further limiting factor is the amount of on-demand energy available from the heat pump.
In an example of a heating system designed to produce 10kWh of thermal energy, it would be impossible to supply the total quantity of energy required to heat a whole property in one hour, but it is possible to heat one or two priority areas in this short time period, and allow a longer time frame to heat low priority areas.
High and low priority areas should be defined by occupancy habits, suggesting that maybe the lounge and kitchen/diner are high priority, whereas bedrooms and hallways could be rated as low priority, and where an extended warming-up period is not going to impact too severely on living comfort.
Remember, in retrofit installations, your customer will have lived with the flexibility of a combustible fuelled boiler, and it is paramount that they are not disappointed by their green energy replacement technology.