Can a heat pump cool?

Step 3 of 11 · Explain

A valve turns the cycle round

In winter the model's air-to-water heat pump works like this: the coil behind the fans, the evaporator, takes about 6 kW of heat from 7 °C outdoor air. The plate heat exchanger beside the compressor, the condenser, gives about 7.4 kW to the water of the floor heating. The compressor adds the rest from about 1.4 kW of electricity.

A common explanation for cooling is that the compressor simply runs backwards. It doesn't: a compressor only pushes one way. Instead a four-way reversing valve sits on the compressor's outlet and decides where the hot gas goes. In heating mode it sends it to the plate exchanger, into the house. In cooling mode it sends it to the coil outside. The pipes stay the same; the refrigerant just flows round the loop the other way. Heat pumps already use the same valve in winter to defrost their coil.

Switched to cooling on a 32 °C day with 18 °C water for the floor, the model takes about 8.3 kW of heat out of the house for about 1.6 kW of electricity.

Step 4 of 11 · Explain

The two heat exchangers swap jobs

Because the refrigerant flows the other way, each heat exchanger gets the other's job. The plate exchanger, which heated the floor water in winter, is now the evaporator. Its refrigerant boils at about 14 °C. That is colder than the water coming back from the floor at about 24 °C, so heat flows from the water into the refrigerant, and the water leaves at 18 °C, cool enough to chill the floor.

The coil behind the fans, which took heat from the air in winter, is now the condenser. The compressor squeezes the vapour until it condenses at about 42 °C, ten degrees above the 32 °C garden air. Only because the refrigerant is hotter than the air can the heat flow out into it. In the 3D model the labels change with the valve: "Plate heat exchanger (now the evaporator)" and "Outdoor coil (now the condenser)".

Step 6 of 11 · Explain

The heat leaves with the fans' air

A widespread belief is that the outdoor unit of a cooling heat pump blows cold air into the garden, like the front of an air conditioner. It fails because the cold is not made outside: it goes into the floor water. What leaves the fans is the house's heat. In the model, 32 °C garden air comes out at about 37 °C, five degrees warmer. In winter it is the other way round, and the air leaves colder than it came in.

The energy balance shows where everything goes. About 8.3 kW of heat come out of the house, and the compressor's 1.3 kW of electricity end up as heat as well. Together about 9.6 kW leave through the coil into the outdoor air, and the fans' 0.3 kW warm the air on top. A heat pump cannot destroy heat; it can only move it. A fridge does the same, which is why the back of it warms the kitchen.

Step 8 of 11 · Try it

Hot days and cold water cost efficiency

In cooling, the efficiency figure counts the heat taken out of the house per unit of electricity. For cooling it is often called the EER (energy efficiency ratio); the model's chip still says COP. At 32 °C outside and 18 °C water the model reaches about 5.1: five units of cooling for one of electricity. On a mild 24 °C day it is about 7.

Two things lower it. The first is a hotter garden: the refrigerant has to get even hotter to shed its heat, so the compressor lifts the heat further. At 35 °C the model's COP falls to about 4.5 and the cooling from 8.3 to about 8.0 kW. The unit this model follows is rated to cool in air up to 45 °C. The second is colder water. With 10 °C water instead of 18 °C, the refrigerant must boil at about 6 °C instead of 14 °C, and the COP drops to about 3.7 while the cooling falls to about 6.4 kW. The manufacturer's data for the real unit show the same trend: an EER of 5.3 with 18 °C water and 3.3 with 7 °C water, both at 35 °C outside (Viessmann Vitocal 250-A). The model sits about 15 % below the datasheet at 18 °C.

That is why floors cool with water around 18 °C while fan coils, which blow air over a cold coil, work with about 7 °C.

Step 9 of 11 · Explain

Why floor cooling stays above 18 °C: the dew point

Warm summer air carries water vapour. Any surface colder than the air's dew point gets wet, the same way a cold glass of lemonade does on a hot day. Indoors in summer the dew point is often around 17 to 19 °C. The model uses one dew point of about 17 °C for the house.

Radiators fed with 12 °C water show what goes wrong: in the model they sweat, with droplets running down the panels. On a real floor that water would soak into the screed and the flooring and could cause mould. So floor cooling runs with water of 18 °C or more, and a dew-point sensor (a humidity sensor in the room or on the pipe) raises the water temperature or stops cooling when the air gets humid. Guides from heating manufacturers recommend about 19 to 21 °C for floors.

Step 10 of 11 · Explain

Passive cooling: the ground does the work

A ground-source heat pump can cool almost without the compressor. A hundred metres down, the ground stays near 10 °C all year, colder than the 18 °C floor water. So heat can flow from the floor water into the ground on its own. Real systems then switch the compressor off and only run the pumps that move the brine and the water. This is called passive cooling or natural cooling.

In the model the compressor keeps running, with almost nothing to lift, and the whole system draws about 0.3 kW instead of the 1.6 kW of active cooling with air. Manufacturers' guides describe passive cooling as gentler than active cooling and very cheap to run. Its bonus: the heat put into the ground over the summer is partly still there in the autumn. Passive cooling only works with a ground or groundwater source, never with an air heat pump, because summer air is warmer than the water you want.