Jarek Kurnitski, director of the Department of Civil Engineering and Architecture at Tallinn University of Technology (TalTech), noted that air-to-water heat pumps have become particularly popular in new buildings. In older homes, outdated stoves or boilers are also increasingly being replaced with heat pumps. "The best solution is a ground-source heat pump, which performs better in our cold climate. However, it's clear that not all properties are suitable for it and that's why so many people opt for air-to-water heat pumps instead," Kurnitski said.

A heat pump always performs better in an airtight and insulated house, the professor emphasized. "Sometimes people might do things in the wrong order. First, they install the heat pump, only to realize their electricity bills are still high. Only then do they start insulating and installing a ventilation system with heat recovery. This does happen," Kurnitski acknowledged.

Older residential buildings typically use stove heating or boilers fueled with pellets, for example. Kurnitski pointed out that in such homes, residents often choose a heat pump because they're tired of maintaining a stove. "The question usually arises when the boiler is old and needs to be replaced. In most cases, people opt for the more convenient solution," he said.

"That said, the house should be reasonably well insulated and airtight. In the latter case, a new ventilation unit is also needed. A heat pump works more efficiently the lower the heating curve — that is, the less the heating water temperature needs to be raised. There are quite a few factors that need to align for a heat pump to be a good fit for a house," he stressed.

Don't buy blind

According to Jarek Kurnitski, one source of frustration for consumers may be how air-to-water heat pumps are marketed. "You have to take the coefficient of performance into account," he explained. "With ground-source heat pumps, the actual coefficient is typically around four — meaning one kilowatt-hour (kWh) of electricity produces four kWh of heat. For air-to-water heat pumps, this factor is barely three." In other words, a ground-source heat pump is a highly cost-effective heating solution in virtually any building, while air-to-water systems are somewhat less efficient.

Kurnitski pointed out that technical specifications for air-to-water heat pumps often cite their best possible coefficient of performance. "These figures are based on outdoor temperatures of 7 degrees Celsius and a low flow temperature of 35 degrees for the heating water. Under such ideal conditions, the best coefficients can even exceed five, and ground-source heat pumps can go over six. But those conditions only occur for a few days at the end of the heating season," he explained.

In Estonia's cold winters, heat pumps operate under very different circumstances, the professor added. "When heating domestic hot water, the coefficient is around two. For space heating, you might get up to three, but the annual average tends to fall below three. Still, the heat pump is two to three times more efficient than direct electric heating," he said.

To avoid unrealistic expectations, people need to understand that the performance figures offered by sales reps often don't reflect real-world conditions — actual coefficients can be half of what's advertised. "Professional installers always provide a forecast or energy calculation, showing how much the heating demand in kilowatt-hours will change with the heat pump. People should know in advance what kind of result to expect," the professor added.

Kurnitski also noted that in homes heated with stoves, the existing energy consumption may not be known, which makes forecasting more complicated. "But generally speaking, you shouldn't buy a pig in a poke — you should know what you're getting," he said.

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