12 Common Heat Pump Installation Mistakes and How to Avoid Them

Most heat pump problems trace back to a small number of design and setup mistakes. This guide explains the 12 that cause the most trouble, from oversizing and high flow temperatures to poor controls, why each one matters, and how to avoid it. It is written from published installation standards and guidance, not from a survey of installers.

The UK heat pump installation industry has grown quickly (see MCS data). That growth is good for competition and availability, but it also means some installers are relatively new to heat pump technology. The mistakes below are the ones most often discussed in installation standards and consumer guidance.

Whether you are about to commission an installation or already have a heat pump that is not performing as expected, this guide will help you understand what good looks like — and what to watch out for. For the complete installation process, see our installation guide.

Mistake 1: Oversizing the Heat Pump

Oversizing is one of the most common mistakes, for example fitting a 12 kW unit where a correctly calculated 8 kW unit would do. It comes from the boiler habit of treating bigger as safer. With heat pumps, bigger is usually worse.

Oversizing causes short-cycling, where the heat pump repeatedly starts and stops because it reaches temperature too quickly. This wastes electricity (the compressor draws the most power during startup), increases wear on components, and can actually make the home less comfortable due to temperature swings.

The fix is simple: insist on a proper room-by-room heat loss calculation, and question any installer who suggests a size without doing one. Our interactive sizing tool gives you a reference point to check against installer recommendations.

Mistake 2: Running Flow Temperatures Too High

A common cause of poor performance is simply a flow temperature set too high, at 55°C or even 60°C, on a system that could heat the home at a much lower temperature.

Heat pumps are most efficient at low flow temperatures — typically 35-45°C. Every degree you increase the flow temperature reduces the COP (efficiency). At 35°C, a heat pump might achieve a COP of 4.0. At 55°C, it might drop to 2.5. That difference alone can add £300-£500 per year to running costs.

Many installers set the flow temperature high "to be safe" or because the homeowner complains that radiators do not feel as hot as they did with the boiler. The correct approach is to design the system around low flow temperatures — using weather compensation and, where necessary, upgrading a few radiators. Our radiator guide explains this in detail.

Mistake 3: Skipping the Heat Loss Calculation

MCS requires a heat loss calculation; it is not optional. Problems arise when it is done carelessly, or the installer sizes the system by eye instead.

A proper heat loss calculation takes 1-3 hours and considers every room's dimensions, construction, insulation, and glazing. It is the foundation of the entire installation design — without it, the heat pump size, flow temperature, and radiator requirements are all guesswork.

If an installer provides a quote without surveying your home or shows a heat loss calculation that looks like a single page with round numbers, that is a red flag. Ask to see the room-by-room breakdown.

Mistake 4: Poor Weather Compensation Setup

Weather compensation is central to heat pump efficiency: it adjusts the flow temperature to the outside temperature. If it is not activated, or set to the wrong curve, the heat pump runs hotter than it needs to most of the time.

Weather compensation uses an outdoor temperature sensor to automatically reduce the flow temperature when it is mild outside. On a 12°C day, the flow temperature might be 30°C. On a -2°C day, it might be 45°C. This dynamic adjustment is what gives heat pumps their high seasonal efficiency.

Without weather compensation, the heat pump runs at a fixed flow temperature — usually one that is higher than needed most of the time. This can increase annual electricity consumption by 15-25%.

Mistake 5: Not Flushing the Existing System

Older heating systems often contain sludge, rust and debris. Connecting a new heat pump to a dirty system risks reduced efficiency, blocked heat exchangers and early failures.

A proper powerflush or chemical flush of the existing pipework and radiators is essential before connecting a heat pump. Heat pumps operate at lower flow rates than boilers, making them more susceptible to blockages from system debris. The cost of a thorough flush (£300-£500) is trivial compared to the cost of a blocked heat exchanger repair.

Mistake 6: Wrong Cylinder Choice

The hot water cylinder matters more than many people expect. A standard cylinder designed for a boiler has a small coil, so a heat pump takes much longer to reheat it. A heat pump cylinder has a larger coil, so it reheats faster and the heat pump runs more efficiently.

Heat pump-specific cylinders have larger internal coils (2-3 m² surface area vs 1-1.5 m² in standard cylinders) to maximise heat transfer at the lower flow temperatures heat pumps use. Using a standard cylinder forces the heat pump to run longer or at higher temperatures to heat the water — both of which increase energy consumption.

Mistake 7: Bad Positioning of the Outdoor Unit

Position matters. A heat pump tucked into a corner between two walls has restricted airflow and can recirculate its own cold exhaust air, so it works harder, runs less efficiently and may ice up more often.

The outdoor unit needs clear airflow — at least 300mm clearance on the sides and 500mm at the front where the fan exhausts cold air. Avoid corners, recessed positions, or locations where exhaust air can recirculate back to the intake. Also consider noise impact on neighbours and proximity to bedroom windows. Our heat pump noise guide covers positioning for noise reduction.

Mistake 8: Ignoring Radiator Sizing

Being told that no radiators need changing can sound attractive in a quote, but if the heat loss calculation does not support it, rooms may struggle to reach temperature in the coldest weeks.

Heat pumps run at lower flow temperatures than boilers, which means radiators deliver less heat output. The heat loss calculation should identify which radiators are adequate and which need upgrading. Being honest about this upfront — even if it adds to the quote — produces happier customers and better-performing systems. See our comprehensive guide on radiators and heat pumps.

Mistake 9: Not Educating the Homeowner

A heat pump is not a boiler. It runs differently, sounds different and heats differently. If nobody explains this, homeowners can assume something is wrong when the system is working as designed.

Heat pumps run continuously at lower intensity rather than blasting heat in short bursts like a boiler. Radiators feel warm rather than hot. The system needs to run for longer periods. These are features, not faults — but homeowners need to understand this before installation to set correct expectations.

Mistake 10: Treating It Like a Boiler Swap

A broad underlying mistake is treating a heat pump installation like a boiler replacement. It is a different technology with a different design approach and different commissioning, and simply swapping one box for another tends to produce poor results.

A heat pump installation is a system design project, not a product swap. It requires consideration of the building fabric, the heating distribution system, the hot water strategy, the electrical supply, the control strategy, and the homeowner's lifestyle. Installers who approach it with this mindset consistently produce better results.

Mistake 11: Skimping on Controls

Controls are not an optional extra. Weather compensation, sensible room temperature control and scheduling that suits how you use the home all affect running costs.

Advanced controls allow the heat pump to respond to actual conditions rather than operating on fixed schedules. Smart thermostats that learn occupancy patterns, zone valves that heat only occupied rooms, and weather-compensated controllers that adjust flow temperatures automatically all contribute to efficiency. If you are also considering solar panels, integrated controls can prioritise using solar electricity for heat pump operation.

Mistake 12: Not Considering Future Insulation Improvements

If you plan to add wall insulation soon, the heat pump should be sized with that in mind. Otherwise the system may end up oversized once the insulation goes in.

A good installer asks about your future improvement plans and factors them into the system design. If you plan to improve insulation after the heat pump installation, discuss this during the survey so the system can be sized appropriately. Our cost guide discusses the economics of insulation-first approaches.

Frequently Asked Questions

What is the most common heat pump installation mistake?

Incorrect sizing — usually oversizing — is the most common mistake. Many installers instinctively choose a larger unit than needed, leading to short-cycling and higher bills. A proper room-by-room heat loss calculation prevents this.

How do I know if my heat pump was installed correctly?

Key signs include: the installer completed a room-by-room heat loss calculation, the system runs continuously at low flow temperatures rather than cycling frequently, your home reaches comfortable temperatures without backup heating, and all MCS documentation has been registered and provided.

Can a bad installation be fixed?

In most cases, yes. Common fixes include adjusting flow temperature settings, reprogramming weather compensation, and rebalancing radiator flow rates. Contact the original installer first, or seek a second opinion from another MCS-certified installer.

How many heat pump installations go wrong in the UK?

We have not found a reliable published figure. Many problems are minor settings issues that can be fixed. Choosing an experienced MCS-certified installer who completes a proper heat loss calculation reduces the risk.

What should I look for when choosing a heat pump installer?

MCS certification, at least 2-3 years of heat pump experience, willingness to show the heat loss calculation, customer references, and a detailed written quote. Avoid anyone who quotes without surveying your home. Use our suitability checker as a starting point.