R290 Monobloc Heat Pumps for Old House Boiler Replacement

Introduction: Replacing an old boiler with a heat pump in a European house depends on whether the existing radiator circuit can be satisfied at a realistic flow temperature, and a 75°C R290 monobloc unit is typically evaluated around that system-fit question.

Replacing a boiler in an old house usually comes down to one question: can the heat pump deliver 75°C flow through the radiators already on the wall? European retrofit work rarely starts with a clean sheet. The house has radiator circuits sized decades ago, a boiler in the corner, and a homeowner who does not want floors opened. The first issue is whether a replacement heat pump can produce water hot enough, at low enough outdoor temperatures, to keep rooms warm. On a 75°C R290 monobloc heat pump, the decision comes down to three things: the flow temperature the existing radiators actually need, how the unit connects to the circuit without indoor refrigerant work, and how winter operation and hot water demand affect sizing.

Why Old Houses Need High Flow Temperatures from a Heat Pump

A house built around a gas or oil boiler was designed around a flow temperature of roughly 70 to 80°C. The radiators, pipe sizes, and pump curves assume that. When a heat pump runs at 45 or 55°C instead, the same radiator gives off far less heat—often well under what the room loses on a design day. That is the single biggest reason a boiler replacement project stalls: the emitter circuit cannot release enough heat at the temperature the new unit prefers. Insulation upgrades shift the picture the other way. A house re-roofed and re-glazed since the radiators went in loses less heat than the original schedule assumed, which works in the heat pump’s favour. So the first job on site is not choosing a model; it is establishing what the building actually needs. That means room-by-room heat loss, the type and size of every radiator, the design outdoor temperature for the location, and the flow temperature required to cover the load at that condition. Run that calculation before comparing units. If the radiators cover the load at 70°C flow, a 75°C-capable unit fits with margin. If they only cover it at 80°C, adding emitters or resizing one or two rooms is the honest answer. Two heat pump manufacturers can quote the same nominal size and still deliver different output on a cold day, so ask for the design-condition figure rather than the headline rating.

How R290 Monobloc Units Connect to Existing Radiator Circuits

A monobloc heat pump keeps the entire refrigerant circuit inside the outdoor casing. A monobloc layout uses a water-only indoor connection: the connection into the house is water flow and water return, plus power and controls. Nothing on the indoor side carries R290. On an old-house job, that changes the labour profile completely: no brazing, no vacuum pump, and no refrigerant handling inside a finished room where a leak would be hard to trace.

Water connection. Two pipes join the indoor heating circuit, flow and return, sized for the design flow rate. The unit covers a 10°C to 75°C outlet range, so one model can serve low-temperature underfloor loops and high-temperature radiators from the same circuit, at whatever temperature the emitters need.
No indoor refrigerant piping. The propane charge never leaves the factory-sealed outdoor unit, so indoor work stays on the wet side. That removes indoor refrigerant certification and leak checking from the schedule and keeps the homeowner’s rooms out of the works.
Service access. Internal water-side parts use clip-type connections. A technician can open a fitting and replace a component without draining and rebuilding the whole circuit, which matters when the nearest engineer is a long drive from the property.
Hydraulic conditions. Old circuits often carry seized thermostatic valves, single-pipe loops, undersized pipework, or a bypass that was never fitted. Flow rate and pressure drop must be checked and balanced across all emitters before commissioning; otherwise the furthest radiator stays cold while the nearest one is already at temperature.

When comparing monobloc heat pump suppliers, ask how the indoor water connection is arranged and how service access works on an old-house circuit.

How Low Ambient Operation and DHW Demand Shape Retrofit Selection

R290 is a good match for cold-weather work. Propane boils at around -42°C at atmospheric pressure and carries high latent heat, so the circuit keeps moving heat even when outdoor air is well below freezing. The rated -25°C to 45°C operating range on a monobloc DC inverter unit is wide enough for many winter design conditions in northern and central Europe. Output and efficiency still fall as outdoor temperature drops—that is true of every air-source machine. Sizing therefore has to use the design-day condition, not the nominal kW. The range runs from 6 kW to 16 kW on single phase, with a 16 kW three-phase option and a nominal A+++ energy class. Request output at the design-day condition from the selected manufacturer, not just the nominal rating. Domestic hot water is the second load, and it is easy to underestimate. A cylinder needs a stored temperature high enough to keep the household supplied and to run a periodic pasteurisation cycle. The 75°C maximum outlet makes that straightforward: the heat pump can support cylinder heating without relying on an immersion element for the bulk of the temperature lift. But DHW demand adds to the design load, and a household with heavy morning draw will push the sizing up a step. The platform integrates heating, cooling, and DHW, but in this retrofit setting the DHW load is often the part that changes sizing. Where the existing boiler stays on site, hybrid operation lets the heat pump cover most of the year while the boiler takes the coldest days or a fast cylinder recovery.

Conclusion

Old-house boiler replacement is a system-fit decision, not a catalogue decision. A 75°C R290 monobloc heat pump can hold the flow temperature boiler-era radiators were sized for, connect on the water side only, and keep working down to -25°C ambient. What decides the project is the survey: heat loss per room, the radiator schedule and its output at the required flow temperature, hydraulic balance, hot water demand, and the electrical supply feeding the unit. Green Power Heat Pump is a heat pump supplier with R290 monobloc DC inverter units from 6 kW to 16 kW. For contractors managing repeat retrofit work, consistent monobloc supply can reduce ordering friction, but sizing remains project-specific. Send the radiator schedule, heat loss estimate, and cylinder size to receive a model recommendation and the system conditions to confirm on site.

FAQ

Q：Can a 75°C heat pump replace a gas boiler without changing existing radiators?

A：Often yes, but the radiators decide. If the existing emitters can cover the room heat loss at 70°C flow or lower, they can stay in place. If they only cover the load at 80°C or above, some emitters need adding or resizing. Measure the radiator schedule against the heat loss before committing, and remember that radiator output falls faster than flow temperature.

Q：How does an R290 monobloc heat pump handle -25°C winter operation?

A：The unit is rated to run from -25°C to 45°C ambient, so heating continues through the design winter condition. Propane’s low boiling point and high latent heat keep the refrigerant circuit working in cold air. Output and COP do drop as it gets colder, which is why sizing has to be checked against the design-day heat loss rather than the nominal rating.

Q：What should contractors confirm before selecting a monobloc heat pump for an old house retrofit?

A：Five things decide the model: room-by-room heat loss, the radiator schedule and its output at the required flow temperature, hydraulic balance and pipe sizing on the existing circuit, the DHW cylinder volume and hot water draw profile, and the available electrical supply—single phase or three phase. Those items set both the unit size and the flow temperature the system can actually run at.