12 MCS Installers Share Their Biggest Heat Pump Mistakes

We spoke to 12 experienced MCS-certified heat pump installers from across the UK and asked them one simple question: what are the biggest mistakes you see in heat pump installations? Their answers — drawn from thousands of installations between them — reveal the errors that cause most problems, most complaints, and most wasted money. More importantly, they explain exactly how to avoid every single one.

The UK heat pump installation industry has grown rapidly, with the number of MCS-certified installers doubling in the past three years according to MCS data. This growth is excellent for competition and availability, but it also means many installers are relatively new to heat pump technology. The mistakes our experts describe are not theoretical — they are issues they encounter regularly when called to inspect or remediate other installers' work.

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

"The number one mistake, hands down, is oversizing. We see it constantly — installers putting in a 12 kW unit where an 8 kW would be perfect. It comes from the boiler mentality of 'bigger is safer.' With heat pumps, bigger is worse." — James, 8 years' experience, Yorkshire

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

"I've lost count of the systems I've been called to 'fix' where the only problem was the flow temperature was set to 55°C or even 60°C. Drop it to 40°C and suddenly the running costs halve." — Sarah, 6 years' experience, Somerset

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's not optional. Yet I still see installations where it clearly hasn't been done properly — or at all. The installer has just looked at the house and guessed." — David, 10 years' experience, Gloucestershire

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 what makes a heat pump efficient. It adjusts the flow temperature based on the outside temperature. But I find systems where it's either not activated or set to the wrong curve — meaning the heat pump runs too hot most of the time." — Mark, 7 years' experience, Norfolk

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

"Old heating systems are full of sludge, rust, and debris. If you connect a new heat pump to a dirty system, you're asking for problems — reduced efficiency, blocked heat exchangers, and premature failures." — Tony, 12 years' experience, Manchester

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 cylinder is more important than people think. A standard cylinder designed for a boiler has a small coil — it works, but the heat pump takes ages to reheat it. A heat pump cylinder has a massive coil, so it reheats much faster and the heat pump runs more efficiently." — Rachel, 5 years' experience, Kent

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 more than people realise. A heat pump tucked into a corner between two walls has restricted airflow and recirculates cold exhaust air. It has to work harder, runs less efficiently, and can ice up more frequently." — Chris, 9 years' experience, Devon

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 noise guide covers positioning for noise reduction.

Mistake 8: Ignoring Radiator Sizing

"Some installers tell customers they don't need to change any radiators — because it sounds better in the sales pitch. Then the customer calls back in January saying their living room won't get above 18°C." — Paul, 11 years' experience, Edinburgh

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, heats differently. If you don't explain this properly, the customer is going to think something is wrong when everything is actually working perfectly." — Laura, 6 years' experience, Bristol

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

"The biggest systemic mistake in the industry is treating heat pump installations like boiler replacements. Different technology, different design approach, different commissioning. Installers who just 'swap boxes' produce the worst outcomes." — Andrew, 15 years' experience, London

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

"Good controls can save 15-20% on running costs compared to basic ones. Room thermostats with weather compensation, zone controls for different areas, and smart scheduling based on occupancy patterns — these aren't luxuries, they're efficiency essentials." — Fiona, 7 years' experience, Glasgow

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 a customer tells me they're planning to add wall insulation next year, I size the heat pump for the insulated house, not the current one. Otherwise they'll end up with an oversized system after the insulation goes in." — Mike, 8 years' experience, Cardiff

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?

Approximately 5-8% result in a formal complaint, with serious problems affecting about 2-3%. Choosing an experienced MCS installer with good reviews significantly reduces 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.