AIR SOURCE HEAT PUMP FAQS
Is renewable tech takes warmth from the air and repurposes it for space heating. But is it the right option for your project? Our expert Q&A goes beyond the basics to answer the in-depth questions you really want to ask
What should you look for to make sure your ASHP is specified correctly?
Robin Adderley: Always appoint an installer who is registered with the Micro Generation Certification Scheme (MCS). This will ensure you get the best advice and gives you access to the Renewable Heat Incentive (RHI).
Phil Birchenough: Your installer should always look at the efficiency of the house and what you need from your heating solution in order to offer you the best solution. As with any system, upgrading insulation in existing homes is important. You should also be confident your engineer fully understands the product they are fitting.
Andrew Mclauchlan: The unit should be specifically sized to account for local temperatures, too. It needs to be able to provide all the warmth a property demands, right down to this design temperature. If an ASHP isn’t providing enough heat, then it’s been sized incorrectly.
In 2013, the Energy Saving Trust (EST) reported that most ASHP installations weren’t performing as well as expected. Has much changed since then?
David Hilton: The EST report looked at the System Performance Factor, taking into account the heat losses of the whole house – it wasn’t just measured across the inlet and outlet of the heat pump itself. So one of the key things to come out of it is that we should always compare like-with-like when we’re looking at things like the Seasonal Coefficient of Performance (SCOP). The other major finding concerned installation quality. At the time, ASHPS were often being fitted as if they were boilers, rather than taking into account the tech’s unique performance criteria. We’ve moved on quite a bit, and there are now more installers with specific heat pump experience.
Why do some suppliers still list the COP (coefficient of performance) rather than the SCOP. Isn’t the latter a better reflection of the appliance’s efficiency?
David Hilton: Suppliers will work with the data that the manufacturers give them. As a consumer, what’s important is that you make sure you’re comparing the same performance factors when looking at different models. And remember that, while the SCOP is more detailed, it’s still a benchmark that’s been developed under controlled conditions rather than a real-life scenario. Andrew Mclauchlan: If you buy a car, you’re told its top speed to show its maximum performance. But you also look at the miles per gallon to get an idea of how it’ll perform in the real world. We provide both the COP and the SCOP on our data sheets to make sure customers have all the information they need to make an informed decision.
One of the criticisms of air source heat pumps has been that they’re noisy. Has anything been done to improve this?
David Hill: If you go back 10 years, you’d have had to position your ASHP round the back of the garage – they were so noisy you wouldn’t have wanted them anywhere near the house. The good manufacturers have looked very hard at this aspect, introducing clever fan technology with new shapes and blades. They’ve also focused on how the air is drawn in to the unit and how it comes back out. It’s important to check exactly what claims are being made: is that 45DBA rating for when it’s running at full power, or does it shoot up to 60DBA? The former is around the level of a washing machine or loud fridge – but at the 50-60DBA range, it will be a lot noisier. An Mcs-approved installer should give you a calculation to show how your appliance rates – if they don’t provide this, it’s a bit of a red flag.
Is it ever a good idea to fit an air source heat pump to an old boiler-fuelled radiator and pipework system?
Phil Birchenough: This depends on the operating temperature of the system. Older emitters are designed to work with a conventional appliance (ie a boiler) that runs at a higher flow temperature. Connecting these to a heat pump will reduce their Delta T (the difference between the room temperature and that of the water within the central heating system). Consequently, the warmth produced might not be sufficient to heat the entire house. So if you’re connecting an ASHP to an old pipework and radiator setup, an additional source of energy would be needed. The most efficient approach is to fit a modern emitter system, such as underfloor heating or radiators designed to work at a lower flow temperature.
What impact does the heat transfer fluid have on ASHP sustainability?
David Hilton: Because there are so many different metals inside a heat pump and it’s a closed loop system, the transfer fluid usually includes some anti-corrosion components. Vegetable-based eco versions are available, but in my view it’s better to go with a solution that protects the system and gives it longevity – an important part of sustainability. Some transfer fluids contain glycol and similar products that can be potential contaminants. The Environment Agency won’t want to encourage any leaking of these. However, there are in-between solutions that biodegrade and still contain the anti-corrosion elements needed. Your installer should specify an option that’s compatible with the manufacturer’s instructions and local Environment Agency guidelines.
How much guidance do installers give homeowners on getting the best performance from their heat pump?
Phil Birchenough: Post-installation, an engineer should do a thorough handover and advise the homeowner on how to make the most of their system. They will have designed the setup to deliver maximum efficiency and output for the requirements of that property and should show you what settings are best for your needs. Normally, they’ll tailor it to your preferences and give a full run-through of how you can change the operational settings. Ideally, they should also take you through some basic troubleshooting tips.
Can you use a heat pump to provide hot water – doesn’t the higher temperature needed just slash the COP?
David Hill: Different models offer different efficiency levels for supplying domestic hot water (DHW). The first step is to pair the appliance with a special cylinder that works on a low flow temperature. This is fitted with a larger-than-usual heat exchange coil to transfer enough energy from the ASHP (the highest output temperature is 55°C) to get the water in the tank up to 50°C. Some installers might plug the ASHP into your existing cylinder, which almost certainly won’t work.
A new, well-insulated house might need around 20,000 kwh of heat per year for space heating, but only 3,000 for DHW. So it’s a small proportion of your overall usage. The appliance will only run at its highest temperature for about two hours per day to provide this hot water. It’s automated, so a sensor keeps tabs on the tank all day and boosts the temperature if it drops below 45°C, before switching back to space heating only. High quality versions will feature two sensors in the tank. This is important, as it reduces the risk of overheating the cylinder and wasting energy.
There’s a lot of talk about hybrid heating systems, combining boilers with ASHPS – is that an admission that heat pumps on their own don’t work?
David Hilton: No, because hybrid systems are designed for larger or harder-to-treat dwellings. For example, you might have a stone barn where an ASHP would adequately heat it until external temperatures got down to maybe 5°C. Every time it goes below that level, which might only be 10 or 20 days a year, the boiler kicks in. So we’re using the renewable tech in order to cut down oil or LPG usage, for instance. There are also some ASHPS intended to work in slightly lower temperatures. In that case, you might have a hybrid setup where the heat pump would take care of the space heating and the boiler powers the hot water supply. Robin Adderley: Hybrid systems have their place. However, in a new build dwelling that is sufficiently well insulated, an ASHP alone should provide all the space heating and hot water requirements. In the case of retrofit systems, possibly in older properties that have a higher heat loss, combining the pump with an additional heat source may become more attractive in terms of capital cost.