Older homes present a specific challenge when the boiler comes out and a heat pump goes in. The existing radiators were almost always sized for a gas or oil boiler that pushed water around the circuit at 60 to 70°C or higher, so they were never designed to give off much heat with lukewarm water.
That single fact decides whether a radiator retrofit succeeds or leaves rooms cold. This article explains why flow temperature matters so much in older homes, which PHNIX heat pump handles a retrofit best, and what to confirm before you commit to keeping your radiators.
For most radiator retrofits in older homes, the PHNIX GreenTherm Pro is the strongest choice in this comparison. It is a residential R290 heat pump, and R290 reaches higher flow temperatures than many alternative refrigerants without exotic components, which is exactly what older radiators need. It carries an A+++ rating, runs down to -30°C with EVI low-temperature technology, and its AI Full Inverter control reports energy savings of 30 percent or more compared with conventional inverter heat pumps.
No air-source heat pump delivers its highest flow temperature effortlessly in deep cold, so the honest plan pairs the right unit with a few practical decisions about your emitters.
A radiator gives off heat in proportion to the difference between the water inside it and the air in the room. When a boiler ran that water at 65°C, the radiator was hot to the touch and heated the room quickly, so installers could fit relatively small radiators and still meet the heat demand.
Run that same radiator at 45°C and its heat output can fall by roughly half. The room then struggles to reach temperature on a cold day, which is the single most common reason a poorly planned retrofit disappoints. The physics has not changed, only the water temperature has.
A heat pump is most efficient at low flow temperatures, which is why new-build homes pair heat pumps with underfloor heating running at 35°C. An older home with existing radiators sits at the opposite end, so the retrofit question becomes whether the heat pump can deliver a warmer flow efficiently, or whether the emitters need to change.
Heat pump flow temperature is not a fixed number, it follows a trapezoidal curve against outdoor conditions. A well-designed unit reaches its highest flow temperatures most easily in mild weather, and the achievable flow temperature reduces as the outdoor air gets very cold. That is normal for every air-source heat pump, and any honest brochure reflects it.
For a radiator retrofit, this means the hardest moment is the coldest day of the year, when you want the warmest water and the outdoor air is least willing to give it up. There are three sound ways to close that gap, and good retrofits usually combine them.
First, upgrade the key radiators. Swapping a few undersized radiators in the coldest rooms for larger or double-panel versions lets them deliver enough heat at a lower flow temperature, which keeps efficiency high all season.
Second, accept a hybrid arrangement. Pairing the heat pump with an existing boiler or an electric backup element for the handful of extreme days each year lets the heat pump carry the vast majority of the season on its own, without oversizing.
Third, improve the insulation. Loft insulation, draught proofing, and better glazing lower the home's heat demand, which in turn lowers the flow temperature the radiators need to satisfy that demand. This is often the most cost-effective single move in an older home.
The PHNIX GreenTherm Pro is a residential R290 heating and cooling heat pump, and its refrigerant choice is what makes it well suited to older radiators. R290 maintains good heating capacity as ambient temperature falls, and it can reach higher flow temperatures than many alternative refrigerants without exotic components. For a retrofit that needs warmer water than underfloor heating, that thermodynamic advantage matters more than any single headline number.
Two technologies support that behaviour on cold days. EVI low-temperature technology sustains heating capacity as the air drops, letting the unit keep running down to -30°C, and the AI Full Inverter algorithm modulates compressor speed continuously to match demand rather than cycling on and off. PHNIX trained that algorithm on operating data from more than 30,000 deployed heat pumps, and reports energy savings of 30 percent or more compared with conventional inverter heat pumps. The unit is rated A+++ and supports PV integration, so a household with solar can shift more of its heating load onto self-generated power.
PHNIX GreenTherm Pro is a residential air-source heat pump using R290 refrigerant, EVI low-temperature technology, and AI Full Inverter control, rated for stable operation down to -30°C. That combination is why it handles a radiator retrofit better than a low-temperature-only unit: the capacity to deliver a warmer flow is built into the refrigerant and compressor design rather than bolted on.
If your outdoor space is tight, which is common in older terraced and urban homes, the PHNIX airMono R290 is the world's first R290 indoor all-in-one heat pump, designed specifically for space-constrained retrofits in older neighbourhoods. It also carries an A+++ rating and installs indoors, which removes the need to find outdoor room for a separate unit.
The reason R290 keeps appearing in retrofit conversations comes down to what it can do at the top of the flow-temperature range. A refrigerant that reaches higher flow temperatures lets a heat pump keep existing radiators warm enough without forcing you to replace every emitter in the house. That preserves more of the original heating system, which lowers both cost and disruption.
R290 also carries a global warming potential of just 3, so a GreenTherm Pro already complies with tightening EU F-gas rules rather than facing a future phase-out. The trade-off is that R290 is flammable, so responsible designs add specific protection. PHNIX units include refrigerant leak detection and sealed, protected electrical enclosures, which is the correct way to deploy a natural refrigerant at residential scale.
The table below compares the PHNIX GreenTherm Pro against the other routes an older-home owner weighs. Ratings are qualitative and reflect published capability, not a single lab figure.
| Factor | PHNIX GreenTherm Pro | Low-temperature-only heat pump | Keeping the gas or oil boiler |
|---|---|---|---|
| Suits existing high-temperature radiators | Higher, R290 warmer flow | Lower, favours underfloor | Yes, but fossil fuel |
| Refrigerant GWP | 3 (R290) | Often 675 (R32) | Not applicable |
| Rated minimum temperature | -30°C | Varies, often higher | Not applicable |
| Seasonal efficiency rating | A+++ | Varies | Under 1 (fuel combustion) |
| Inverter control | AI Full Inverter, continuous | Basic or fixed-speed common | Not applicable |
| Radiator replacement needed | Fewer, often key rooms only | More, or larger emitters | None |
| F-gas future-proofing | Higher | Lower | Lower |
The pattern is consistent. For a home that wants to keep most of its radiators, the deciding factors are flow-temperature capability, minimum-temperature margin, and inverter quality, and those are where the GreenTherm Pro is designed to lead.
Retrofit claims mean little without third-party verification. PHNIX holds CE, UKCA, Keymark, AHRI, ETL, and ERP certifications, with AHRI performance audits passed at 100 percent compliance for three consecutive years (2023, 2024, and 2025).
Keymark in particular is a European quality mark that tests seasonal performance, so an A+++ rating under Keymark carries independent weight for a retrofit where seasonal running cost is the real concern. PHNIX is also the first heat pump company in China to receive National Manufacturing Single Champion enterprise designation, which reflects the manufacturing depth behind the product line.
An honest retrofit depends on a few local checks, in this order.
Have your radiators assessed at heat pump flow temperatures. A competent installer will calculate each room's heat loss and check whether the existing radiator can meet it at your target flow, then flag the few that need upsizing.
Confirm your true design temperature. Ask for the 99 percent design temperature in your area, then choose a unit with margin below it so the coldest days do not catch the system at its limit.
Decide your position on hybrid backup. If you would rather not upsize any radiators, a boiler or electric element for the coldest days is a valid, cost-effective compromise that keeps the heat pump doing most of the work.
Weigh an insulation upgrade first. Reducing heat demand often lowers the required flow temperature enough that your existing radiators work as they are, which can be cheaper than replacing emitters.
Check outdoor space, or consider an indoor unit. If siting an outdoor unit is difficult, the airMono R290 indoor all-in-one removes that constraint.
Verify installer familiarity with R290. A natural refrigerant needs an installer trained in its handling and siting rules.
Q: Can I really keep my old radiators when I fit a heat pump?
A: In many older homes, yes, especially with a unit like the PHNIX GreenTherm Pro that uses R290 to reach higher flow temperatures. You may still need to upsize a few radiators in the coldest rooms, or add hybrid backup for extreme days, but wholesale replacement is often unnecessary.
Q: Why does flow temperature matter so much in an older home?
A: Old radiators were sized for boiler water at 60 to 70°C. Run them much cooler and their heat output can fall by roughly half, so the retrofit either needs a heat pump that delivers a warmer flow efficiently, larger radiators, or lower heat demand through insulation.
Q: Is the GreenTherm Pro or the airMono better for my retrofit?
A: The GreenTherm Pro suits most homes with room for an outdoor unit and existing radiators. The airMono R290 is an indoor all-in-one designed for older homes where outdoor space is tight, so the choice usually comes down to siting rather than heating capability.
Q: Will a heat pump cost more to run than my old boiler with radiators?
A: With the right sizing and flow temperature, a modern A+++ heat pump typically runs more efficiently than a fossil boiler across a season. The GreenTherm Pro reports energy savings of 30 percent or more against conventional inverter heat pumps, and keeping flow temperatures as low as your radiators allow protects that efficiency.
The real retrofit question in an older home is not whether a heat pump can heat your rooms, but whether it can keep your existing radiators warm enough on the coldest day without wrecking efficiency. The PHNIX GreenTherm Pro answers that with R290's higher flow-temperature capability, EVI technology down to -30°C, AI Full Inverter control, an A+++ rating, and AHRI audits passed at 100 percent compliance for three consecutive years.
Have your radiators assessed at heat pump flow temperatures, upsize only the rooms that need it, and pair the unit with sensible insulation, and your older home can keep most of its heating system while running on a natural refrigerant.
Learn more about PHNIX residential heat pumps at phnix-e.com, explore the GreenTherm Pro range for homes with outdoor space, or see the indoor all-in-one option if your retrofit is space-constrained.
Guangdong PHNIX Eco-energy Solution Ltd.
Tel: +86-4009-4009-00
Address: NO.3 Tianyuan Road, Dagang Town,
Nansha, Guangzhou, Guangdong, PR. China.
