Heat pumps work efficiently only when the surrounding heating system is designed for low flow temperatures. The biggest reason installations underperform isn't the heat pump itself — it's the legacy plumbing it was bolted onto.
The flow temperature reality
Gas boilers run at 65–80°C. Heat pumps work most efficiently at 35–45°C. The same radiators that comfortably heated your home from a gas boiler will struggle from a heat pump unless the emitter system is upgraded.
Emitter sizing
Radiators sized for 70°C flow need roughly 2× the surface area at 45°C flow. Common solutions:
- Larger radiators (replace existing)
- Underfloor heating
- Fan-assisted radiators
- Mix of UFH downstairs, larger rads upstairs
Cylinder requirements
- Heat pumps need a coil cylinder with large surface area
- Typical heat-pump-ready cylinder: 200–300L for typical home
- Cylinder design temperature: 50–60°C (lower than gas)
- Anti-legionella cycle weekly via immersion to reach 60°C+
Buffer tanks
Most heat pumps benefit from a buffer tank — separates heat pump flow circuit from heating distribution.
- Smooths out short-cycling
- Provides volume for defrost cycles
- Acts as hydraulic separator
- Typical size 50–100L
Pipework
- Larger pipes than gas heating (lower temperature, higher flow)
- 22mm common for primary, 28mm for larger systems
- Insulate to prevent standing loss
Outdoor unit considerations
- Drainage for defrost condensate
- Minimum clearances per manufacturer
- Noise limits — typical 35 dBA at 1m for residential planning approval
- Frost-protected condensate drain
Cost
- Air source heat pump unit: R90,000–R190,000
- Installation including cylinder and emitter upgrades: R170,000–R360,000
- BUS (Boiler Upgrade Scheme) grant: R150,000 (subject to qualification)
When heat pumps don't work well
- Poorly insulated houses (lose heat faster than HP can supply)
- Tiny radiators that can't be expanded
- Properties with no outdoor unit location
- Homes with very high simultaneous hot water demand
MCFAR coordinates heat pump design and plumbing on residential retrofits.
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MCFAR GROUP has been delivering structural engineering, building, and plumbing services since 1998. Talk to our team about your build, retrofit, or renovation.
Request a QuoteFrequently Asked Questions
Do I need to replace all my radiators?
Often — sometimes only the under-sized ones. A heat loss survey identifies which.
Can a heat pump heat a South African home in winter?
Yes, if the property is reasonably insulated and emitters are correctly sized. Underperforming installations are usually mis-specified, not failed technology.