Heat pumps - from the fundamentals to the future
With heat pumps predicted to play a central role in decarbonising heat in new homes as set out in the Government’s Future Home Standard, how can these systems be designed now and, in the years to come, to make the most of their environmental and energy-saving credentials? Steve Keaton of Vaillant looks at the factors designers and installers need to understand.
Heat pumps are now in the limelight thanks to their recognition in government-backed schemes such as the Green Homes Grant and the Renewable Heat Incentive. With zero emissions at point of use, there is no doubt that they are perfectly placed to help drive down carbon emissions from domestic heating. However, it is essential to design a heating system with ground and air-source heat pumps correctly to enable them to deliver the carbon and energy savings promised. What’s more, it is important to understand how these systems can be taken one step further so that their carbon-reduction potential can be maximised for future residential developments.
Many crucial aspects of heat pump design are related to the low and medium flow temperatures of these systems, such as sizing and controls. Unlike traditional solutions which operate at anywhere between 60 to 85 degrees, any flaws in the design of a heat pump system will result in more noticeable consequences, such as higher electricity bills and lower levels of comfort.
This is why extra attention is needed when calculating pipe sizing and flow rates. Lower flow temperatures and lower temperature differentials for heat pump heating systems mean that more water going through the pipes is needed to ensure sufficient heat transfer. Correctly calculating the system will ensure that the pipework and heat emitters such as radiators will be appropriately sized so that a space is warmed to the desired temperature.
A balancing act
Because water takes the route of least resistance, it wants to flow to the first radiator and not the last in the hydraulic circuit. This is why it is crucial to correctly carry out hydraulic balancing at the commissioning stage so that all heat emitters in a property get the required amount of heat, including the last. Unbalanced systems can lead to higher system return water temperatures, reducing the operational efficiency of the heating system and can also impact on the customer’s comfort.
In essence, hydraulic balancing ensures the flow of water is balanced across all radiators in the system, so that each one receives the right amount of water and therefore heat. It is the final piece of the puzzle in delivering a well-designed, installed and commissioned heating system.
Stay in control
With a traditional high temperature system, the boiler fires up to heat a building to the required temperature quickly. Heating solutions designed to run with lower flow temperatures need to be on for longer, which is why setting up the controls in the correct way is paramount to enable the system to perform efficiently. We recommend using ‘comfort’ and ‘set back’ temperatures. The former is usually around 20-21oC, the latter for night time or when occupants are out of their home or building, at around 16-17oC. Using controls with load or weather compensation will also ensure that a heating system is only on for long enough to maintain the right temperature. So, if only a small increase in energy is required to meet the set temperature, the heating system will work at a reduced output to use the minimum amount of energy needed.
More hot water
Most heat pumps in the market operate with lower temperatures, which means that a back-up heater (direct electric immersion heater) is required in the hot water cylinder to store hot water above 60oC – the point at which legionella bacteria is killed. However, the recently launched aroTHERM plus uses natural refrigerants which can produce higher system temperatures. This means that the flow temperature into the cylinder, directly from the heat pump, can be up to 75oC. As a result, the use of a direct electric immersion heater to protect occupants from legionella is not necessary, and users can take advantage of a more effective system which can deliver more useable hot water at point of use.
For apartment blocks, heat pumps are being increasingly used as a centralised heat source as part of a low-temperature district heat network or ambient loop. These systems often include a buffer storage tank which acts as a ‘heat battery’, with heat supplied by the central heat pump. Individual apartments can draw heat from subsidiary circuits which are linked to the buffer tank. The use of ambient loops is growing in popularity, where a large heat pump heats a loop to 20oC for example. Smaller heat pumps installed in individual apartments draw their heat energy from the loop and generate the additional heat required within each dwelling.
The benefits of this system are that the central, larger heat pump can work more efficiently with a lower temperature. The individual heat pumps can deliver significant energy savings too by using a higher source temperature. Heat is only being generated where it is needed, avoiding heat loss in the common areas and lowering the cost of insulating the distribution pipework.
Looking forward to the future, these heat networks could interact with energy prices in real-time via application programming interfaces so that bill payers can take advantage of low electricity prices. This way, building owners can generate most of the heat required when electricity is cheaper. When electricity prices are high, energy is only used to maintain the system temperature.
Fundamentally, designing and installing a heating system with a heat pump doesn’t differ a great deal from a boiler installation. Everything you should do for the latter – getting flow rates, sizing and water quality right - you have to do with the former. Systems with heat pumps may be less forgiving to faults in their design and installation, but with greater potential to deliver heat with zero emissions at point of use to many more people, they are paving the way towards decarbonising heat in our homes of the future.