Heat pump technologies work more effectively when coupled with correctly designed central heating systems.
But, before discussing the system design for heat pumps, it is worth examining the common system types used for domestic wet systems.
The descriptions below are simplified and meant as a rough guide only.
Conventional systems became affordable to the masses in the 1950s. They were popular until the 1970s when combi boilers were introduced.
A conventional system uses a heat source such as a boiler to generate hot water. Hot water is pumped to both the radiators and to a cylinder to heat a store of hot water.
Radiators come in various designs and sizes, ranging from small flat panel radiators to huge triple or quadruple column radiators. However, their purpose remains the same – to emit heat into the rooms where they are situated.
There are variations in design, but the principles are much the same. These systems date back many years and even predate natural gas as a fuel source.
The cylinders in a conventional system were traditionally copper and stored in the airing cupboard. However, we saw a rise in high recovery unvented cylinders with further advances in technology. These cylinders heated and stored hot water more efficiently. With less room for expansion, molecules were packed tight, and water heated up quicker.
Combination Boiler Systems
A combination boiler system is the UK’s most common system. Combi boilers were first introduced in the 1970s.
Initially, people saw them as a flash in the pan, much like how people think of heat pumps today. However, the combi boiler eventually took over from the conventional system in the 1990s, becoming the most common central heating source.
Combi boilers replaced the cylinder and freed up space, producing instantaneous hot water and heating the radiators from a single unit. In addition, they were far more efficient and compact than conventional systems.
Combination boilers can produce high temperatures instantly, heating water from 10 to 45 degrees immediately. In addition, they can create flow temperatures of up to 80 degrees (into the radiators) heating rooms quickly.
Hot Water Storage
The flow temperatures produced by heat pumps are a lot lower than those achieved by a boiler (typically 55 degrees). The lower temperature means the design of the heating system is critical.
The move away from combination boilers and back to stored hot water might seem like a backward step, but the need for more efficient systems, using renewable energy sources, and the benefits surrounding this are crucial to achieving low carbon targets.
In addition, there is a need to re-educate homeowners and heating engineers on the design and usage of newer systems.
We have become conditioned into scheduling our heating systems to come on and off to suit our lifestyles. We have become used to the fact that our modern boiler will heat our houses instantly.
However, as heat pumps will not achieve the same flow temperatures as combi boilers, radiators must be correctly sized and balanced.
In some instances, we will have to replace radiators with larger ones. Still, there will be instances when engineers have over-specified the previous system, resulting in oversized radiators.
Pipework may also need replacing, but again not always. Most engineers would have fitted 22mm primary circuits and 15mm tails, which may be suitable, or require minimal alteration.
Finally, a heat pump should be coupled with a high recovery unvented hot water cylinder or, less commonly, with a standalone air source water heater.
When considering heat pumps, engineers need to be more than just a “fitter” as Low-Temperature Water System design is complex. Engineers must calculate heat loss for each room, considering the room size, construction, position and even the aspect of the room. Things like wall thickness, type of insulation, style and type of glazing will all affect the amount of heat lost from a room.
End users will need to alter how they use their system, even once when correctly designed for their home.
For example, we all tend to use the temperature of the radiators as a guide to how effectively our system is working; however, it’s the air temperature that matters. Therefore, in a low-temperature design, it is better to keep the room warm throughout the day, e.g., setting the room stat to 20 degrees, than to “surge heat” the space by demanding 23 degrees on demand.