A heat pump is only as good as its installation. The same unit, correctly specified and commissioned, can deliver the efficiency on its datasheet or a fraction of it, and the difference rarely shows up as an obvious fault. It shows up as bills higher than expected and rooms that never quite reach temperature, with nobody able to say why.
This guide covers what proper installation actually involves, the decisions that determine whether the system performs, and how to judge whether the work done on your property was done well.
The Decisions That Matter Most Come Before Installation
Most articles about installation begin with mounting the unit. By that point the outcome is largely determined, because the decisions with the greatest consequence were made earlier.
The heat loss calculation
Capacity should be matched to the building’s calculated heat load, not estimated from floor area. Insulation quality, glazing, ceiling height, orientation, and hot water demand all change the answer substantially, and two houses of identical size can differ by several kilowatts.
Both sizing errors carry real consequences. An undersized unit runs continuously and still fails to reach temperature on the coldest days. An oversized unit reaches the set point quickly then shuts down, cycling repeatedly rather than modulating steadily, which reduces seasonal efficiency and wears the compressor.
The figure that catches people out. When sizing, the minimum modulated output matters as much as the maximum. A unit that cannot throttle below 6.5 kW will cycle badly in a house whose mild-weather demand is 3 kW, and mild weather makes up most of the heating season. Oversizing is a genuine error rather than a safe margin, which is why “a bit bigger to be sure” is poor advice with modulating equipment.
The emitter question
This is the decision that most often separates a satisfying installation from a disappointing one, and it is frequently never discussed with the owner at all.
Every emitter needs water at a particular temperature to deliver its rated output. Underfloor heating operates at 30 to 45°C. Radiators sized for a gas boiler expect 70 to 80°C. A heat pump becomes markedly more efficient the lower the flow temperature it must produce.
A unit producing 35°C water achieves a COP around 4.5. The same unit producing 45°C achieves around 3.6. That is roughly 25% more heat from the same electricity, from an identical machine, determined entirely by what it is feeding.
Connect a heat pump to radiators demanding 75°C and the system will work while costing far more to run than the specification suggested. Nothing is faulty. The mismatch is the problem, and it must be addressed before installation rather than discovered after.
Insulation first
Improving the building envelope reduces heat demand permanently, which means the equipment can be smaller, cheaper to buy, and cheaper to run. It also lowers the flow temperature the emitters require, which directly improves efficiency.
Doing this after installation leaves a system sized for a building that no longer exists, oversized and cycling inefficiently for the rest of its service life.
Siting the Outdoor Unit
Where the unit sits affects both performance and how neighbours feel about it.
Airflow clearance on all sides. The unit extracts heat from air passing across the heat exchanger, so restricting that airflow directly reduces output and efficiency. Follow the manufacturer’s clearance specification rather than fitting it into the tightest available space.
Avoid recirculation. A unit in an enclosed corner or narrow passage can draw in the air it has just cooled, which progressively lowers the source temperature it is working from. The symptom is a system that underperforms in cold weather for no apparent reason.
Noise and placement. The fan and compressor produce sound. Avoid positions directly beneath bedroom windows, and avoid corners where sound reflects off two surfaces and amplifies. Check local requirements on noise at boundaries, since some jurisdictions specify limits.
A stable base. The unit needs a level, solid foundation capable of carrying its weight, with vibration isolation between unit and base. A unit on an inadequate base transmits vibration into the building structure.
Condensate drainage. In heating mode the outdoor unit produces condensate, and in freezing conditions that water can form ice around the base. Provide a drainage route rather than allowing it to accumulate where it will refreeze.
The Hydraulic Side
For an air-to-water system, everything downstream of the unit determines whether the heat produced actually reaches the rooms.
Pipe sizing. Connection size steps up with capacity because flow rate does. Reducing the pipe at the unit introduces a restriction exactly where flow matters most, and undersized pipework throughout starves the system.
Flow rate. Each unit requires a minimum flow rate to operate correctly, and inadequate flow causes the unit to cut out on protection or run inefficiently.
Expansion vessel. Water expands as it heats, and in a closed system that expansion needs somewhere to go. Check whether one is integral to the unit or must be specified separately, since not all models include it.
System cleanliness. On a retrofit, the existing pipework may contain years of sludge and debris. Flushing the system before connecting new equipment protects the heat exchanger, and fitting a magnetic filter keeps circulating debris away from it thereafter.
Oxygen barrier pipe. Floor heating pipe without an oxygen barrier allows oxygen to diffuse through the pipe wall into the water, corroding pumps, valves, and heat exchangers elsewhere in the system. The pipe itself is unaffected, so the damage appears years later in components far from its cause.
Refrigerant Work Is Certified Work
This deserves stating plainly, because it defines what an installer must be qualified to do.
Handling fluorinated refrigerants requires certification in most jurisdictions: F-Gas certification in the European Union, EPA Section 608 in the United States, with equivalent schemes elsewhere. Deliberately releasing refrigerant to the atmosphere is an offence carrying substantial penalties.
Monoblock units, where the entire refrigerant circuit is sealed within the outdoor unit and only water pipes enter the building, avoid refrigerant work on site entirely. Split systems, where refrigerant lines run between outdoor and indoor units, require a certified technician to make and charge those connections.
The technical reason behind the regulation is real. A refrigerant circuit must be evacuated with a vacuum pump before charging, and the vacuum held to confirm integrity. Moisture left inside reacts to form acids that corrode the compressor from within over the following years, and the owner never connects the eventual failure to the installation that caused it.
On refrigerant choice
Older systems used R-22, which is now phased out in most markets, and R-410A, which is itself under progressive quota restriction because of its global warming potential of 2088. Equipment charged with a restricted refrigerant becomes harder and more expensive to service over time, independently of its mechanical condition.
Current equipment uses lower-GWP alternatives. Legom heat pumps use R32, with a GWP of 675 and a smaller required charge for the same output, which avoids that trajectory.
Electrical Requirements
Confirm before installation rather than discovering during commissioning.
Larger units draw substantially more current than residential models and may require or benefit from three-phase supply. Verify the available supply capacity at the property, and on commercial projects confirm whether three phase is present before committing to a capacity.
Electrical work of this kind is regulated in most jurisdictions and requires a qualified electrician. The unit also needs correct isolation and protection, and the control wiring must connect the thermostats, base station, and heat source so that the system operates as a coordinated whole rather than as independent parts.
Commissioning: The Stage Most Often Rushed
Installation is not complete when the unit runs. Commissioning is what turns installed equipment into a functioning system, and skipping it leaves performance on the table permanently.
Set the flow temperature correctly. To the lowest figure the emitters can work with, since every degree lower improves efficiency. Weather compensation, where flow temperature adjusts to outdoor conditions, improves this further.
Balance the circuits. Loops and radiators differ in length and resistance, so without deliberate adjustment the nearest circuit takes a disproportionate share of the flow while distant ones are starved. The result is the familiar complaint of some rooms warm and others never quite reaching temperature, with the system apparently working throughout.
Vent thoroughly. Air blocks circulation as effectively as a closed valve, and it needs venting again after a few days as dissolved air works its way out.
Configure the controls. Confirm each thermostat operates its own zone, set the pump to an appropriate control mode rather than maximum fixed speed, and set any bypass correctly.
Record everything. Flow temperature, balanced flow rates, and control settings. Without a commissioning record there is no reference against which future drift can be measured, and this absence is why so much degradation goes undetected for years.
How to Tell Whether Your Installation Was Done Well
Most owners cannot assess the technical work directly, but several indicators are visible without expertise.
| Check | What good looks like |
|---|---|
| Heat loss calculation | You were shown one, room by room |
| Flow temperature | You were told what it runs at and why |
| Emitter discussion | Radiator sizing was assessed, not assumed |
| Commissioning record | You were given documented settings |
| Outdoor unit clearance | Clear space on all sides per the manual |
| Balancing | Every room reaches temperature at similar rates |
| Certification | Refrigerant handling qualification confirmed |
| Handover | Controls explained, not just switched on |
If the answer to several of these is no, the installation may still work but is unlikely to deliver what the equipment is capable of. A commissioning visit from a competent installer can often recover much of it.
Signs of a Poor Installation
Bills higher than expected despite the system apparently working, which usually points to flow temperature set too high or a mismatch with the emitters.
Some rooms never reach temperature while others are fine, which points to balancing rather than equipment capacity.
The unit cycles frequently in mild weather, indicating it is oversized for the load and cannot modulate low enough.
Noise complaints from the outdoor unit, which usually reflects placement rather than the equipment.
No commissioning documentation, which means nobody recorded what the system was set to and nothing can be measured against it later.
After Installation
A few habits protect the investment.
Keep the outdoor unit clear of leaves, debris, and encroaching vegetation, since restricted airflow reduces output and efficiency measurably. Check system pressure periodically and investigate if it falls repeatedly rather than simply topping it up. Verify flow temperature annually against the commissioning record, since a mixing valve that has drifted will change performance without any obvious symptom. And have the system serviced before the heating season rather than during it, when availability is worst.
Operate it as a heat pump rather than as a boiler. It works best running steadily at a lower temperature than firing hard in short bursts, and owners accustomed to switching heating on when they feel cold sometimes find the adjustment unfamiliar. Operating it like a boiler produces both worse comfort and worse efficiency.
“The installations that disappoint almost never fail because of the equipment. They disappoint because nobody asked what flow temperature the emitters needed before the unit was chosen, or because nobody balanced the system afterwards. Both are decisions that cost very little at the right moment and are expensive to correct later. If I could give a homeowner one question to ask an installer, it would be what temperature the system will run at and why. An installer who can answer that clearly has thought about your building. One who cannot has quoted a product.”
— Maggie Shen, Director of Legom
Components That Support a Good Installation
Legom manufactures the complete hydronic chain at its facility in Jiaxing, Zhejiang Province: air-to-water heat pumps from 5 kW to 16 kW using R32 with full DC inverter compressors and rated to -35°C, alongside manifolds, floor heating pipe with oxygen barrier protection, thermal actuators, room thermostats, base stations, and HVAC valves.
Because the components are produced together, compatibility is designed in rather than assumed, which removes one category of installation problem entirely. For the cost side of a heat pump project, see our guide to heat pump installation cost. OEM and ODM services are available across the range.
Frequently Asked Questions
What makes a heat pump installation proper?
Three things above all. A heat loss calculation establishing the actual demand rather than estimating from floor area. An assessment of what flow temperature the emitters require, since that determines the efficiency the system will achieve. And proper commissioning afterwards, meaning the flow temperature set as low as the emitters allow, the circuits balanced, the system vented, and the settings documented. Mounting the unit correctly matters too, but these three decide whether the installation delivers what the equipment is capable of.
Can I install a heat pump myself?
Parts of the work are restricted by law rather than by difficulty. Handling refrigerant requires certification such as F-Gas in the European Union or EPA Section 608 in the United States, and in many markets purchasing refrigerant is also restricted to certified persons. Electrical connection is regulated work in most jurisdictions. A monoblock unit avoids refrigerant work on site, since the circuit is sealed within the outdoor unit, but the hydraulic and electrical work still requires competence and in most cases qualification.
How do I know if my installation was done well?
Several indicators are visible without technical knowledge. Were you shown a room-by-room heat loss calculation? Were you told what flow temperature the system runs at and why? Was radiator sizing assessed rather than assumed? Were you given documented commissioning settings? Does every room reach temperature at a similar rate? Does the outdoor unit have clear space on all sides? If the answer to several is no, the system may work but is unlikely to be performing as well as it could.
Why is flow temperature so important?
Because it determines efficiency more than any other operating variable. A heat pump producing 35°C water achieves a COP around 4.5, while the same unit producing 45°C achieves around 3.6, roughly 25% less heat for the same electricity. Since flow temperature is dictated by what the emitters require, the emitter assessment effectively decides the efficiency you will achieve. This is why connecting a heat pump to radiators sized for a gas boiler disappoints, and why underfloor heating pairs so well with it.
Where should the outdoor unit be placed?
Somewhere with unobstructed airflow on all sides per the manufacturer’s clearance specification, on a level and solid base with vibration isolation, with a route for condensate drainage. Avoid enclosed corners and narrow passages, where the unit can draw back in the air it has just cooled and progressively lower the source temperature it works from. Avoid positions beneath bedroom windows and check any local noise limits at boundaries.
What is commissioning and why does it matter?
Commissioning is the process of setting the system up correctly after installation: setting flow temperature as low as the emitters allow, balancing the circuits so each receives its designed flow, venting thoroughly, configuring the controls, and documenting all of it. It is frequently rushed or skipped, and the consequence is a system that works but underperforms permanently. The documentation matters as much as the process, since without a record there is nothing to measure future drift against.
Do I need to replace my radiators?
It depends on what flow temperature they require. Radiators sized for a gas boiler expect 70 to 80°C, which pushes a heat pump into its least efficient range. You do not necessarily need underfloor heating, but the radiators generally need to work at 45 to 55°C, which usually means oversizing them. Improving insulation reduces the demand and can achieve the same result. Get this assessed before the heat pump is chosen, since it affects both the sizing and the efficiency you will achieve.
How long does installation take?
It varies considerably with scope. Replacing a boiler with a heat pump where existing pipework and emitters are suitable is measured in days. A project involving new distribution, emitter replacement, or a hot water cylinder where none existed takes considerably longer. What should not be compressed is the commissioning at the end, since that is where an installed system becomes a working one, and it is the stage most often shortened when a project runs late.
Reviewed by Maggie Shen, Director at Legom, on September 5, 2026. This guide to heat pump installation was reviewed for technical and regulatory accuracy, including refrigerant handling certification requirements, current refrigerant phase-down status, and commissioning procedure.