How is the heat distributed around the building?
Heat pumps have become more and more viable over the last 10 years. This is in part because they have evolved and are also more energy-efficient, but mostly because the mains electricity that enables them to work has become ‘greener’.
This increase in viability has meant that they are now being installed into older buildings that hitherto were considered unsuitable.
The heat can be distributed to rooms in various ways e.g. directly to the air, but more commonly, heat is ‘piped’ to panel radiators or underfloor heating. This ‘wet’ heat-emitter system is most popular in countries where winters are relatively long.
Before we move on, it may be worth noting the effect that radiation has on comfort. If we heat the air in a room that has cold walls, we can ‘feel’ some discomfort, being surrounded by cold walls. Conversely, if we are surrounded by warm surfaces, we can feel ‘comfort’ even with lower air temperatures. This has been a selling point for underfloor heating for years. The point I am making is – don’t get only fixated by actual air temperature.
Also be mindful that the type of building has a dramatic effect on what is required from the heat emitter system. In modern low-energy buildings, almost any method should be suitable, but for older buildings, it is far more important to choose the best method and best design. So, hang on to that thought when reading this.
Whatever system is chosen, and in order to maintain high energy-efficiency, the heat-emitter system should be designed so that the circulating water temperature is as low as possible. For older buildings, this means that radiators may need to be sized considerably larger than standard. The pipework might also need to be slightly ‘fatter’ than that of a boiler system. That said, if the heat is ‘lower of longer’, then the sizing might be about the same.
See this new Heating simulator
It may be worth considering the time it takes to heat up a home. Traditionally, boilers have been big enough to raise the temperature of a room fairly quickly, thus responding quickly to user adjustments. Also, it did not cost much more to install a large-capacity boiler.
However, heat pumps achieve the best energy-efficiency if they are operated steadily at a relatively low output temperature. This encourages operating the heat pump system for longer periods, so smaller heat pumps become more cost-effective and viable.
This can take a lot of getting used to. Instead of simply adjusting thermostats on-demand, as needed, the user needs to think ahead. A heat pump should be able to automatically give the required comfort as the seasons change. Once the best settings and timings are found, they should be stuck to.
Many heat pump controllers have a holiday setting. This is more necessary with slow-response systems. If going away, it’s worth using this so that the house can warm up slowly over a day or two ready for when you return home.
You may see radiators with low mass that claim fast response. This can help (though debatable benefit) if you are operating in an intermittent way, as in traditional heating. However, in occupied-all-the-time housing, the advantage of this is minimal. In certain situations, old heavy cast-iron radiators could be an advantage, and act as a thermal ‘buffer’ in the system. Indeed, some Air Source systems need sufficient water volume to assist defrosting the outside unit. High water content radiators can be an advantage here.
Air distribution system
Air-to-air heat pumps can blow heated air around a room, but moving air is not always comfortable. That said, careful positioning of the ‘blower’ can improve matters.
Again, if the building is poorly insulated, the air flow may need to be high and uncomfortably draughty. But in a very well-insulated house, the airflow may be very low, so perfectly acceptable.
Air movement for cooling is another matter. We tend to like a draught when we are hot.
The simple system described in the previous paragraph simply re-circulates the air, and adds warmth to it. However, there is need for some fresh air, and some new-build houses have ducted systems that recover heat from exhaust air using a passive (no heat pump) system. Also known as MVHR (Mechanical Ventilation with Heat Recovery). Some may have a small heat pump added
For existing houses, these systems can be hard to install and set up efficiently. I am sure that we will see more and more products on the market. Some may be good, some less so.