Heat Pump with Underfloor Heating or Radiators

One of the most common questions we deal with when planning a heat pump is simple: "I have radiators at home - can this even be combined with a heat pump, or do I have to install underfloor heating?" The answer isn't black and white. A heat pump can work with both systems, but with significantly different efficiency, running costs, and sometimes a need to modify the heating system. In this article we explain why this matters at all, what the real differences in running costs are, and how underfloor heating and radiators can safely be combined in a single house.

Why the temperature of the heating water is key

A heat pump is not a gas boiler. While a condensing boiler can heat water to 70 or 80 °C without much loss of efficiency, a heat pump works on a completely different principle - it draws heat from the outdoor air (or from the ground, in the ground-water version) and, using a compressor and refrigerant, "pushes" it up to a higher temperature level. The bigger the difference between the outdoor temperature and the temperature the pump needs to heat the water to, the more electricity the compressor consumes per unit of heat produced.

This efficiency is expressed as the coefficient of performance, COP - the ratio between heat produced and electricity consumed. A COP of 4.0 means that from 1 kWh of electricity, the pump produces 4 kWh of heat. The temperature of the flow water into the heating system (not the outdoor temperature, which is dictated by the weather and can't be influenced) is the one system parameter we can actually choose ourselves at the design stage - and it has a huge impact on what running the pump will actually cost.

As a rule of thumb, for every degree Celsius we lower the required flow water temperature, electricity consumption for heating drops by roughly 2 to 2.5%. A small number at first glance, but over an entire heating season it adds up to hundreds of kilowatt-hours of difference.

Typical COP values by flow temperature

Specific values vary from model to model and also depend on the outdoor temperature at any given moment, but as an indicative frame of reference for a typical mid-range air-water heat pump at an outdoor temperature of around 2 to 7 °C (the typical average during the Slovak heating season), these approximate values can be expected:

• Flow temperature 35 °C (typical underfloor heating) - COP approximately 4.0 to 4.5
• Flow temperature 45 °C (low-temperature radiators, larger surfaces) - COP approximately 3.3 to 3.8
• Flow temperature 55 °C (older radiators, partly oversized) - COP approximately 2.6 to 3.0
• Flow temperature 65 °C (original radiators sized for a boiler, unmodified) - COP approximately 2.0 to 2.3

The difference between 35 °C and 65 °C therefore means roughly double the electricity consumption to heat the same house. This is why every heat pump installation is advised to lower the design temperature of the heating system as much as possible - and why underfloor heating is the natural partner for a heat pump.

Underfloor heating - why it's the "ideal" partner

Underfloor heating works on the principle of a large radiant surface with a small temperature difference from the room. While a radiator typically has a surface area of 1 to 2.5 m² and must be hot enough to release all the needed output through such a small area, the floor in a room has an area on the order of 15 to 25 m². Thanks to this, the floor surface only needs to be 3 to 5 °C warmer than the room air, which means the water in the underfloor circuit generally only needs a temperature of 28 to 35 °C.

It is precisely this low flow temperature that is exactly the environment in which a heat pump operates most efficiently. That's why new builds almost always design the combination of heat pump plus underfloor heating - it isn't a marketing trend, but a physical match between what the pump can do efficiently and what underfloor heating needs.

Another advantage is thermal inertia. A floor with screed has a large thermal mass - once heated up, it holds its temperature for a long time even after the pump switches off, which lets the heat pump run in longer, less interrupted cycles. This reduces the number of compressor starts, which is good both for the unit's lifespan and for temperature stability in the room.

What to watch out for with underfloor heating and a heat pump

• Floor thickness and buildup - overly thick tiling or an unsuitable screed can slow down heat transfer, which needs to be factored into output sizing.
• Pipe spacing - closer spacing (e.g. 15 cm instead of 25 cm) allows working with a lower flow temperature for the same output, which is desirable with a heat pump.
• Room-by-room control - room thermostats for individual circuits prevent overheating rooms that have their own heat gain (e.g. from south-facing windows).
• Furniture without legs placed directly on the floor - large pieces of furniture placed directly on the floor (e.g. built-in wardrobes) can locally restrict heat output and need to be factored into circuit design.

Radiators with a heat pump - when it works and when it doesn't

The large majority of older houses in Slovakia have radiator heating originally designed for a gas or solid-fuel boiler, typically for a flow/return temperature of 75/65 °C or 90/70 °C. If a heat pump is simply installed in such a house without any modification to the heating system, and set to reach the original design temperature, the result is a working, but significantly inefficient, system - with a COP of around 2.0 to 2.3, only slightly better than a direct electric heating element.

The good news is that in the vast majority of cases, radiators are not designed for maximum frost throughout the whole season - the design temperature (e.g. -11 °C or -15 °C depending on location) actually occurs only a few days a year. For most of the heating season, the radiators only need a lower temperature than they were originally designed for, because it isn't cold enough outside to require full output. This is exactly why a well-set heat pump with weather compensation control (flow temperature changes with outdoor temperature) can run at more reasonable temperatures around 45 to 50 °C for most of the season, rising higher only on the coldest days.

When radiators need to be enlarged or replaced

If we want the heat pump to run permanently at a lower temperature (e.g. 45 °C even in the hardest frost), the original radiator designed for 75 °C simply won't be enough - at a lower temperature difference it releases less heat. As a rule of thumb, lowering the flow temperature from 75 °C to 45 °C requires increasing the radiator's heating surface to roughly 2 to 2.3 times its original size to deliver the same output. In practice this means one of the following options:

• Replacement with a larger panel radiator or a radiator with more panels/fins (type 33 instead of type 11, i.e. a triple panel with two rows of fins instead of a single flat panel).
• Adding an extra radiator to the same room, if there isn't room for a bigger unit.
• Keeping the original radiators and accepting that the pump will run at a higher temperature on the coldest days (lower COP for just a few weeks a year, with decent efficiency for the rest of the season thanks to the weather compensation curve).
• Combining with a bivalent source - the original boiler stays as a backup source for the coldest days, with the heat pump covering the rest of the season.

Which option is right depends on the budget, how well insulated the house is, and whether the owner plans to insulate the facade or replace windows within the next few years (after insulation, both the required output and the required radiator temperature drop, so it's worth waiting to replace the radiators until after the insulation work).

Combining underfloor heating and radiators in one house

A very common situation is a house where the ground floor (living room, kitchen) is heated with underfloor heating, but the first floor (bedrooms, children's rooms) has radiators for various reasons - either because underfloor heating was never planned there, or because the floor build-up height on that level was too low to fit an underfloor heating system. The good news is that this kind of combination can be implemented with a heat pump without any problem, but the hydraulics need to be designed correctly.

How this is solved technically

The most common solution is splitting the heating system into two separate circuits with different temperatures:

1. Direct (unmixed) circuit for underfloor heating - connected directly to the heat pump output, or via a manifold with a circulation pump, running at a lower temperature (e.g. 32-35 °C).
2. Mixing circuit for radiators - has its own three-way or four-way mixing valve with a circulation pump that adds hotter water from the main circuit up to the higher temperature radiators need (e.g. 45-50 °C), controlled by its own weather compensation curve independent of the underfloor circuit.

Thanks to this, the heat pump only ever produces water at the lowest temperature needed (the one for underfloor heating), and the radiator circuit raises the temperature locally only where it's needed - so the pump doesn't have to raise the temperature for the whole house, including the underfloor heating downstairs, just because of a few radiators upstairs. This solution requires somewhat more complex controls (two temperature sensors, two weather compensation curves, a control unit capable of managing it), but it is a proven and commonly used principle here too.

In most installations with a combined system, a buffer (balancing) tank between the heat pump and the distribution system is also recommended. It serves two purposes: it ensures a sufficient volume of water in the system so the pump doesn't have to start and stop too often (especially if the installed output is somewhat higher than the house's current need), and it also decouples heat production from heat consumption, which also helps during the outdoor unit's defrost cycles in frost.

Running costs - a concrete example

To show the real-world impact, let's give a simplified but realistic example. Imagine a family house with an annual heating demand of 12,000 kWh (a typical value for a moderately insulated house of around 120-140 m²). At a COP of 4.3 (underfloor heating, 35 °C), electricity consumption for heating would be approximately 2,790 kWh per season. At a COP of 2.1 (original radiators, unmodified, 65 °C), the same house with the same heat demand would consume approximately 5,710 kWh of electricity - more than double.

At the same annual heat demand of 12,000 kWh, a house with original, unmodified radiators (COP 2.1) consumes roughly double the electricity of a house with underfloor heating (COP 4.3) - a difference of almost 3,000 kWh per heating season.

In a combined system, where underfloor heating covers most of the house and a smaller part (e.g. two rooms upstairs) has appropriately oversized radiators running at 45-50 °C, the real average system efficiency over a season usually falls somewhere between these two extremes, closer to the better one - because most of the heat goes through the more efficient underfloor circuit.

These figures are indicative and serve only to illustrate the principle - the exact consumption of a specific house depends on heat losses, climate zone, the specific pump model, and how it is set up and controlled. Nevertheless, the principle holds universally: the lower the heating water temperature, the lower the electricity consumption for the same warmth in the rooms.

How to proceed if you're planning a heat pump for a house with radiators

1. Get a heat loss calculation done - a designer or experienced installer will calculate the house's actual heat losses room by room and compare them with the output of the existing radiators at a lower temperature.
2. Identify the "problem" rooms - usually not all radiators in the house need replacing, just the ones already at the limit of their output (corner rooms, bathrooms, rooms with large windows).
3. Consider a partial replacement - instead of replacing all radiators across the board, it's often worth replacing only the critical units with larger ones or a higher-output type.
4. Plan the controls - a weather compensation curve set up for the specific house is just as important as the hardware itself; a poorly set curve can ruin the efficiency of an otherwise well-designed system.
5. Consider the order of steps - if you're also planning insulation or window replacement, it's smarter to do this before or at the same time as installing the pump, not afterwards, since it lowers both the required output and the required temperature.

Common mistakes worth avoiding

• Leaving the original weather compensation curve "at maximum" just to make sure the house is "definitely warm" - this is the most common cause of unnecessarily high consumption after installing a pump.
• Underestimating hydraulic balancing - if some radiators get too much water and others too little, the system as a whole needs a higher temperature than would otherwise be sufficient.
• Too small a buffer tank, or none at all, in a system with multiple circuits - leads to frequent compressor cycling and unnecessary wear.
• Forgetting about summer cooling - if the pump also has a cooling function, underfloor heating can be used for mild cooling too; radiators aren't suitable for this, so it needs to be factored in at the design stage if there is interest in this feature.

Frequently Asked Questions

Can a heat pump be connected directly to old cast-iron radiators?

Yes, technically it can - old cast-iron radiators generally have a larger heating surface than modern panel radiators of the same "size", so they sometimes handle a lower flow temperature surprisingly well. However, it's always necessary to verify by calculation whether, at the chosen lower temperature (e.g. 50 °C), they can cover the heat loss of the given room at the design outdoor temperature.

Is it worth replacing all the radiators in the house at once?

Not always. It's often enough to replace only the radiators that are already undersized or are in rooms with higher heat loss. The remaining radiators can stay, and the system is set up to run at a somewhat higher temperature only temporarily, on the coldest days.

Is underfloor heating always better than radiators for a heat pump?

In terms of efficiency, yes - a lower flow temperature means a higher COP. However, underfloor heating isn't suitable or possible everywhere (e.g. renovations without lifting the floors, rooms with low ceiling clearance), in which case a well-designed low-temperature radiator system is a sensible alternative.

How much does efficiency drop if I leave the original radiators unmodified?

As a rule of thumb, if the pump has to permanently reach around 65 °C instead of 35-45 °C, the coefficient of performance can drop to roughly half that of underfloor heating - meaning double the electricity consumption for the same amount of heat. The real-world impact depends on how many days a year that high a temperature is actually needed.

Can underfloor heating on the ground floor and radiators upstairs be combined in one house?

Yes, this is a common solution. It requires two separate circuits with different temperatures - a direct low-temperature circuit for underfloor heating and a mixing circuit with its own valve for the radiators, controlled by independent weather compensation curves.

Does the type of radiator (panel vs. column) affect heat pump efficiency?

Yes, indirectly. A radiator with a larger heating surface (more panels, more fins, bigger size) can deliver the same output at a lower water temperature. That's why, when switching to a heat pump, a type 33 radiator (two panels with two rows of fins) is often recommended instead of the original type 11 (a single flat panel), even keeping the same dimensions.

Related topics

• How to choose a heat pump
• What heat pump output do I need
• Heat pump air-water vs. ground-water
• How much a heat pump saves compared to gas and electricity
• Heat pump installation - what you need to know

You can find the full range of heat pumps in the category Heat pumps.

Have a question on this topic?

Not sure what to decide, or dealing with a specific situation in your home - combining underfloor heating and radiators, old radiators and a heat pump? Write to us - we're happy to help.

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