Heat pumps are in hot demand. Some 2.2 million were sold in Europe in 2021 alone. That's 34% more than the previous year, according to data from the European Heat Pump Association. The industry group estimates that by 2030, five times that figure will be installed annually.

Most new buildings already have heat pumps installed. But an increasing number of homeowners living in older buildings are considering a switch to the more environmentally friendly heating technology. 

Heinrich Pfennings is one of them. He wants to stop using gas for heating. His single-family home in Overath, on the outskirts of Cologne in western Germany, was built in 1990. His gas heating system is over 30 years old.

"We used to have to pay around €1,350 ($1,400) for gas per year. But gas has gotten more expensive. We now pay €1,600 per year," he said. And prices are set to rise even more, he added.

Pfennings currently needs 21,000 kilowatt-hours (kWh) of gas to heat about 170 square meters (1830 square feet) of house as well as water for the three people there. That's 123 kWh per square meter — an even older unrenovated house would need double that.

At the time Pfenning's property was considered well insulated. But new builds are much better and energy demand is often less than 50 kWh per square meter.

Heat pumps for older buildings without floor heating?

So what's the best option for Pfennings's house? A water, air or ground source heat pump? Possibly with the option of turning on gas during especially cold winter nights? Knowing what to get and then finding the right company to install the appropriate pump is not uncomplicated.

Few heating engineers in Germany have firsthand experience with heat pumps. As a result, Pfennings was told different things by different engineers about whether it was worth installing one in his house.

"Some said, 'Forget about getting a heat pump if you don't have underfloor heating. And others said, 'Sure, it could work."

Pfennings, a computer scientist by trade, read up on the technical parts and talked to neighbors who already had heat pumps installed. 

The less a heat pump or any heating system has to work to keep a space at a comfortable temperature, the better it functions. So if the flow temperature — the temperature of the water flowing through the radiators — is low, the heat pump will be more efficient.

Heat pump efficiency increases by 3.5%, for every single degree Celsius it doesn't need to heat, says Werner Schenk, an expert on the technology and professor at Munich's University of Applied Sciences.  

Pfennings put his house to the test last winter and lowered the water temperature in his heating system.

"I checked how the house was going to react to lower flow temperature. My controls didn't allow for a temperature below 40 degrees Celsius. And it worked with this lower flow temperature of 40 degrees even on cold days," he said. 

How efficient are heat pumps when added later?

Using heat pumps usually works in even older buildings as well, says Marek Miara, who works for the Fraunhofer Institute for Solar Energy Systems ISE. He and his colleagues have looked at 60 different heat pump systems in Germany.

"Heat pumps using air as a heating source produce on average three kilowatt hours heat from one kilowatt hour of electricity in older buildings," he said. Heat pumps using groundwater or soil are a bit more efficient, as they generate about 3.9 times as much heat, he added.

With an average temperature of 2 C in winter, the Pfennings's hometown, Overath, belongs to the warmer places in Germany. He opted for an air source heat pump and hopes it can be installed quickly. 

Pfennings's choice is a bit cheaper than a ground source heat pump because that would require drilling into the earth. Getting it installed will cost him a little more than €31,000. The German government will subsidize about 30% of that.

How to turn up a heat pump's efficiency?

Pfennings was advised to get a buffer tank so the heat pump can run for longer periods of time. It basically acts as a battery and saves energy and money, because the pump won't need to be turned on and off constantly.

It's also important to keep the valve in radiators open at all times. That enables the heating to run at lower flow temperatures, making it more efficient. 

Solar power makes heat pumps even cheaper

The heat pump Pfennigs settled on is going to produce about 4.2 kWh of heat with one kWh of electricity. That's according to a calculator run by the association of Germany's heat pump manufacturers. If he needs 21,000 kWh of heat per year, he is going to need about 5000 kWh of electricity to run his pump.

Pfennings has solar panels installed on the roof of his house and those will generate about 40% of the required electricity. They should cover the heat pump's energy needs from spring to fall. But in the winter months from November to January, it will need a bump from the power grid. 

Still, Pfennings is going to cut down on costs with the new system. His heating bill should drop to €2,000 a year from €2,500. And it's not only his cash he'll be saving. He's also avoiding the three tons of CO2 emissions that would have otherwise been blown into the atmosphere annually.

This article was originally published in German