As someone that's deeply involved in the design and installation of heating systems, I want to share essential advice for consumers considering an air source heat pump. My business partner, Luke, and I run Optimus Heating, and we’ve achieved recognition in the industry, winning Vaillant’s heat pump challenge last year and being one of 10 chosen projects in Viessmann’s Top of the SCOP’s competition. We specialise in creating highly efficient heating systems, often achieving efficiencies over 500%.

This guide is not a pitch for business but a genuine attempt to help consumers navigate the misinformation surrounding heat pump installations. If you’re looking for a system with long-lasting efficiency, low running costs and minimal noise, here are the six red flags to avoid when considering a heat pump installation.

Red Flag 1: Design Flow Temperature

If you’re being sold a heat pump with a flow temperature of 50C, be cautious. A higher flow temperature can limit the efficiency of your heat pump and shorten its lifespan. Some manufacturers offer a 7-year warranty, which is far shorter than the potential lifespan of a heat pump installed with the correct parameters.

The reason for this limited warranty is that manufacturers have no control over how the heat pump is installed, particularly concerning flow temperature. Many mass-market installations are designed for 50C flow temperatures, which can significantly reduce the lifespan of the compressor and increase noise levels. Additionally, for every degree increase in flow temperature, you may experience a 3% loss in efficiency. A heat pump designed with a 35C flow temperature can be up to 45% more efficient than one designed for 50C.

Lowering the flow temperature naturally enhances efficiency, reduces running costs, extends the lifespan of your heat pump and minimises noise. Always question any company promoting flow temperatures above 45C.

Red Flag 2: Rooms Designed at Different Temperatures

CIBSE design guides recommend different temperatures for different rooms, such as 18C for kitchens and 21C for living rooms. However, designing emitters for varying temperatures in adjacent rooms can lead to inefficiencies.

According to thermodynamics, heat will always move from hot to cold areas. This means that the warmer room will lose heat to the cooler room, causing your system to become unbalanced and your heat pump to work harder than necessary. It’s nearly impossible to maintain different temperatures in different rooms unless they’re vacuum-sealed.

Your home should be designed with a single consistent temperature, with a setback temperature for cooler bedrooms overnight. Always question any company that doesn’t use one consistent design temperature across your property.

Red Flag 3: Third Party Controls, Zoning & TRVs

Imagine a simple two-up, two-down house, where each room has a 2.5kW heat loss/load, totalling 10kW for the entire house. If you install a 10kW heat pump and then zone the house into upstairs and downstairs, shutting off the upstairs zone during the day, you’ve essentially halved the water volume in the system. This reduction in water volume can cause your heat pump to cycle on and off, reducing efficiency and potentially shortening its lifespan.

Heat pumps operate most efficiently when they can modulate their power to maintain a steady temperature. However, third party ‘smart’ heating controls often act like on/off switches, which can cause the compressor to cycle inefficiently. These controls were originally designed for combustion heating (boilers) and are not ideal for vapour compression heating (heat pumps).

The best approach is to use weather compensation controls, which allow your heat pump to adjust its power output based on outdoor temperatures, maintaining a consistent indoor temperature without cycling the compressor. Always question any company that suggests using third party or ambient controls.

Red Flag 4: Buffers, Hydronic Separation & Fixed Speed Circulation Pumps

Buffers can be useful in some cases, but they’re often unnecessary for the vast majority of domestic UK housing stock. A buffer is essentially a cylinder of water connected to both the heat pump and the central heating system. In many cases, the use of a buffer with a fixed speed circulation pump can lead to inefficiencies, particularly when the pump doesn’t modulate according to the outdoor temperature.

Most heat pumps are designed to operate with a temperature differential (ΔT) of 5C between the flow and return. However, when a buffer is used, the differential can become distorted, leading to the heat pump having to work harder to maintain the desired temperature. This can result in inefficiencies and increased wear and tear on the system.

Buffers are often used by installers to simplify the installation process, but this can lead to a lazy and inefficient system. Instead, consider a volumiser, which can add system volume without the need for hydronic separation, allowing the heat pump to operate more efficiently.

Red Flag 5: Central Heating Pipe Sizing

Proper pipe sizing is crucial for the efficient operation of your heat pump. The designer should always be able to provide calculations to ensure that your house’s pipework is capable of transporting the full power output of the heat pump. If the pipework isn’t large enough, upsizing radiators will be pointless, as the energy won’t be able to reach them efficiently.

A good designer will consider this and prove their design mathematically, leaving nothing to guesswork.

Red Flag 6: Power Output Size of the Heat Pump

The heat pump you choose should be matched to the specific heat loss of your home. If the heat pump is oversized, it will cycle on and off, leading to inefficiencies and potential damage. On the other hand, if it’s undersized, it will overwork and struggle to maintain the desired temperature, especially in colder weather.

As a crude example, if your heat loss is 3.8kW and you are being sold a 6.9kW heat pump, this heat pump is probably oversized and will potentially cycle.

Proper sizing is essential to ensure that the heat pump operates efficiently and lasts as long as possible.

Closing Thoughts

These are the six red flags to watch out for when purchasing a heat pump. Luke and I are passionate about sharing our knowledge to push the industry in the right direction. We want consumers to be empowered to ask the right questions and choose a system that works efficiently, doesn’t cost a fortune to run and lasts a long time.

Recently, I discussed these points on a Facebook thread about a heat pump installed by a large nationwide company. After someone from that company viewed my LinkedIn profile, the post was removed. I don’t think that’s fair to consumers, so I’m sharing this content here, unaffiliated with anyone.

The marketplace is full of tempting offers, clever marketing and glossy adverts, but many big players are selling you a lie. By asking these red flag questions, you can uncover the truth about what you’re being sold. And if you're still unsure, leave a comment below or on the Renewable Heating Hub Forums and we'll try to guide you in the right direction.

Also, please feel free to share this information widely and use it to hold salespeople and designers accountable. Remember, buying a cheap system might cost you more in the long run, with higher energy costs and potential failures from overworked heat pumps.