Air Source Heat Pump Installation Guide
A heat pump only performs if the whole system is designed for it — the heat loss, the emitters and the flow temperature all have to line up. Get the design right and commissioning is the easy part.
An air source heat pump (ASHP) is not a boiler swap. It delivers a lot of heat at a low flow temperature, which means the design of the whole system — heat loss, emitter sizing and pipework — matters far more than it does on a gas job. Fit one like a boiler and you get an unhappy customer and a poor efficiency figure.
This guide walks through the install the way an MCS-minded installer approaches it: room-by-room heat loss, siting and noise, designing for low flow temperatures, then commissioning and the Benchmark and MCS handover paperwork that unlocks the warranty and the grant.
Key takeaways
- Start with a room-by-room heat loss calculation — the whole design flows from it, not from the old boiler's size.
- Heat pumps run best at low flow temperatures, so emitters and pipework often need upsizing to suit.
- Site the unit for free airflow and low noise; permitted development has noise conditions to meet.
- The electrical supply and DNO notification must be sorted — see the electrical requirements for a heat pump.
- Commission properly and issue the Benchmark and MCS handover certificates; MCS is what unlocks the BUS grant.
Start with heat loss, not the old boiler
The single biggest mistake is sizing a heat pump from the boiler it replaces. Boilers are routinely oversized; a heat pump must be matched to the property's actual heat loss so it runs long and steady at a low flow temperature rather than cycling. That means a proper room-by-room heat loss calculation to a recognised method before you specify anything.
The heat loss figure sets the heat pump output, the emitter sizes and the flow temperature you design to. Skip it and every downstream decision is a guess — which is why MCS and the grant schemes require it.
Emitters, flow temperature and pipework
Heat pumps are most efficient at low flow temperatures — think in the region of 35–45°C rather than the 70°C-plus a boiler runs. A radiator sized for 70°C water will not put out enough heat at 45°C, so existing emitters often need upsizing, and underfloor heating is an excellent partner because it works happily at low temperatures.
Pipework and the system's flow rate have to carry the heat at that lower temperature, so runs may need upsizing too. This is where radiator sizing done properly earns its keep — the numbers decide whether a room is warm or the customer is disappointed.
Design low, run efficient
Every degree you can drop the flow temperature improves the seasonal efficiency (SCOP). Upsizing emitters to allow a lower flow temperature is not a cost to avoid — it is the design working.
Siting, noise and the electrical supply
The outdoor unit needs clear airflow and space for the condensate to drain and disperse, away from where it will ice a path or annoy a neighbour. Installing under permitted development carries noise conditions, so position the unit and check the assessment to stay within them.
On the electrical side, the heat pump needs a suitable dedicated supply and, in many cases, notification to the DNO because of the load. Treat that as part of the job from the start rather than a surprise at commissioning — the electrical requirements for a heat pump cover what the circuit and notification involve.
Commissioning and handover paperwork
Commission the system to the manufacturer's instructions: set the flow temperature and weather compensation, balance the emitters, confirm flow rates, and check the refrigerant and controls. A heat pump that is installed but not properly commissioned will underperform and generate callbacks.
Complete the heat pump Benchmark commissioning record and the MCS handover certificate, and hand them over with the design documents. MCS certification is what makes the customer eligible for grant support such as the Boiler Upgrade Scheme, so the paperwork is not admin — it is part of the value you deliver.
Commission and hand over the same day
TradePlanr generates the heat pump Benchmark and MCS handover certificates as PDFs, filled and signed on site and stored against the install — ready for the customer, the warranty and the grant application.
Frequently asked questions
Can I just swap a boiler for a heat pump?
No. A heat pump must be designed to the property's heat loss and run at a low flow temperature, which usually means upsizing emitters and sometimes pipework. Sizing it from the old boiler leads to poor efficiency and an underheated home.
What flow temperature should a heat pump run at?
Heat pumps are most efficient at low flow temperatures, typically in the region of 35–45°C rather than a boiler's 70°C-plus. Emitters and underfloor heating must be sized to deliver enough heat at that lower temperature.
Do I need to notify the DNO?
In many cases yes, because of the electrical load. The heat pump needs a suitable dedicated supply and DNO notification is often required — sort it early rather than at commissioning.
Why does MCS matter?
MCS certification of the installation is what makes the customer eligible for grant support such as the Boiler Upgrade Scheme, and it underpins the warranty. Commission properly and issue the Benchmark and MCS handover paperwork.
From guidance to action
Related guides
Underfloor Heating Installation Guide
Wet vs electric UFH, screed and pipe spacing, commissioning and handover documentation.
Radiator Sizing Guide (Heat Loss & BTU)
Room heat loss, BTU output and delta-T corrections — how to size radiators that actually heat the room.
Heat Pump Electrical Requirements
The electrical work behind an air-source heat pump — supply capacity, dedicated circuits and isolation.
