Most people think of heat pumps as a low-carbon replacement for a boiler. They provide heating in winter, hot water year-round, and can help reduce energy bills.
What many people don’t realise is that a growing number of air-to-water heat pumps can also operate in reverse and provide cooling during the summer.
This summer, we’re increasingly seeing people switch their heat pumps to cooling mode to help keep them more comfortable. Over the summer, we’ve switched many systems from heating mode into cooling mode and in the right scenario the results can be surprisingly effective.
However, cooling a building with a heat pump isn’t as simple as turning the temperature down. The performance depends heavily on the building itself, the type of emitters installed and the risk of condensation.
Air Conditioning is a relatively mature technology, but is there a big cooling opportunity with the Air-to-Water heat pumps increasingly getting installed in homes?
How does a heat pump cool a building?
In cooling mode, the heat pump simply reverses the process used for heating. Instead of extracting heat from the outside air and bringing it indoors, it removes heat from inside the building and rejects it outside.
The challenge then becomes how to actually remove the heat from the rooms and then put it outside.
Underfloor Heating and Cooling
Underfloor heating often works surprisingly well as a cooling system and as often it’s getting installed to get a high-efficiency heating installation it also lends itself well to doing the same when cooling.
Rather than creating cold draughts, the floor becomes slightly cooler than the room and gently absorbs heat from the space above. This creates a very comfortable environment because the cooling is spread across a large area.
A typical cooling floor might operate at around 18-22°C rather than the much colder temperatures found in air conditioning systems.
The advantages include:
- Very quiet operation
- No visible equipment
- Uniform temperatures throughout the room
- Good comfort levels
Although it provides cooling across a really large area, the limitation is cooling power before condensation starts to accumulate across the floor.
A floor can only absorb so much heat before its temperature approaches the room temperature. This means underfloor cooling works best in well-insulated buildings with modest solar gains rather than highly glazed properties with large overheating issues.
Can Radiators Be Used for Cooling?
In theory, yes. A standard radiator supplied with cool water will absorb heat from the room.
The problem is that radiators usually have relatively small surface areas and have been designed for heating so the hot water rises in them. While they work well with 35°C+ water during heating mode, they become far less effective when supplied with 18-20°C cooling water and really prone to condensation.
The cooling output is often only a fraction of the heating output.
This means conventional radiators can provide some background cooling but are rarely capable of removing enough heat to control temperatures during a severe heatwave. Vertical radiators can work better as can access the warmer air higher in the room but still generally struggle to perform as well as underfloor cooling.
Fan-Assisted Radiators
Fan-assisted radiators (sometimes called low temperature radiators or fan convectors) bridge the gap between standard radiators and full air conditioning.
A small fan moves room air across the heat exchanger, dramatically increasing the amount of heat that can be absorbed.
Compared with standard radiators they offer:
- Higher cooling output
- Faster response times
- Better room temperature control
- Create more noise
- Smaller physical sizes
For existing heat pump systems, fan-assisted emitters can often provide a significant improvement in summer comfort without requiring a completely separate cooling system. The issue is usually that in Britain these have been designed for heating and so don’t have a mechanism to handle condensation accumulating if the temperature is too low.
The Condensation Challenge
The biggest limitation with heat pump cooling is normally not cooling power. It’s condensation.
Everyone has seen a cold drink form water droplets on the outside of the glass. Exactly the same thing can happen on cooling pipes, radiators and floors.
When a surface becomes colder than the dew point temperature of the surrounding air, moisture condenses onto that surface.
This can lead to:
- Wet floors
- Condensation on pipework
- Damage to finishes
- Mould growth
- Potential long-term building issues
To avoid this, cooling systems need to carefully control water temperatures. In current British heatwaves dew point can be around 16-18°C depending on the weather.
This is why most air-to-water heat pumps operate with relatively modest cooling temperatures compared with traditional air conditioning systems.
Why Every Building Behaves Differently
“Will switching my heat pump into cooling mode make my house cool?”
Unfortunately, there is no simple answer.
The effectiveness depends on three key factors:
Solar Gains
Large areas of unshaded south or west-facing glazing can introduce significant amounts of heat into a building.
In many homes, solar gain remains the main cause of overheating. If sunlight is continuously adding heat faster than the cooling system can remove it, temperatures will continue to rise.
Internal Gains
People, appliances, lighting, cooking and hot water systems all generate heat.
Every unit of electricity used within a building eventually becomes heat somewhere indoors.
Building Fabric
Highly insulated buildings often perform surprisingly well with heat pump cooling because their cooling demand is relatively modest and temperatures change slowly.
In contrast, poorly insulated buildings let a lot of heat in which raises the amount of cooling required.
The building itself ultimately determines whether a heat pump cooling system feels transformational or merely a little helpful.
Could Fan Coil Units Be the Future?
As cooling demand increases, many designers are beginning to view heating and cooling as part of the same system.
One possible future as an alternative to air conditioning could be a heat pump connected to:
- Underfloor heating for winter comfort
- Domestic hot water production
- A cooling circuit with fan coil units for the summer
Fan coil units look similar to slim radiators but contain larger heat exchangers and fans that can deliver significantly more cooling capacity. The key difference is when they generate condensation it is collected and piped outside so the temperature can be run much lower.
This approach allows buildings to use one piece of equipment throughout the year, providing heating in winter and effective cooling during increasingly frequent summer heatwaves.
For many homes, especially those with large areas of glazing or home office spaces, this may become a more practical solution than installing a completely separate air conditioning system to help with the summer months.
How Woohoo Can Help
At Woohoo, we regularly look into overheating risk, cooling demand and heat pump performance as part of our building performance work.
Every building is different. The amount of cooling available from a heat pump depends on the building’s insulation level, glazing, ventilation strategy and internal heat gains.
By understanding where the heat is coming from and how quickly it can be removed, we can help identify what systems likely to provide the best result.
The goal is simple: how do we make great sustainable spaces that are comfortable in the future.