Strictly speaking, propane has only advan­tages over synthetic refrigerants—Jonathan Frei, devel­oper at Heim AG, is convinced of this. And he should know. The heating system manu­fac­turer from Aadorf, Switzer­land, is one of Europe’s leading compa­nies when it comes to the use of the non-fluo­ri­nated refrig­erant propane—also known as R290—in heat pumps. The effi­ciency and perfor­mance of these devices depend crucially on the choice of refrig­erant, as it absorbs heat from the ambient air and trans­fers it to the heating system. And this is where propane performs partic­u­larly well compared to other chem­ical substances, as Frei explains: “R290 has excel­lent ther­mo­dy­namic prop­er­ties, allows oper­a­tion at outside temper­a­tures of minus 20 degrees Celsius, enables flow temper­a­tures of 75 degrees Celsius and has a compar­a­tively very low global warming poten­tial.”

This last prop­erty is becoming partic­u­larly impor­tant. For decades, fluo­ri­nated gases were the medium of choice in refrig­er­a­tion. However, their high global warming poten­tial (GWP) is cata­strophic. By way of compar­ison, the widely used refrig­erant R410 A has a GWP of 2,088, while propane has a GWP of three. In the Euro­pean Union and Switzer­land, the use of fluo­ri­nated gases is there­fore to be grad­u­ally restricted. But why haven’t manu­fac­turers switched to propane earlier, given its outstanding prop­er­ties? The answer is simple: “R290 belongs to the safety class of flam­mable gases,” says Frei.

For the devel­oper, however, a suit­able safety concept ensures risk is manage­able. Heim AG has already gained exten­sive expe­ri­ence with the non-fluo­ri­nated refrigerant—both in outdoor heat pumps and in an indoor brine-water heat pump. What was still missing from the port­folio: an indoor air-to-water heat pump for single-family homes and smaller apart­ment build­ings. “With the LWKMi pro, as the new heat pump is called, we wanted to develop a highly effi­cient and future-proof device that would fit easily into any base­ment,” says Frei, summa­rizing his task.

A compar­ison of refrig­er­ants

Measured by their global warming poten­tial (GWP)

Perfect for reno­va­tions

Public discus­sion tends to focus on outdoor heat pumps. However, their indoor coun­ter­parts offer several advan­tages: no space is required around the perimeter of the house, neigh­bors cannot hear any oper­ating noise and they are ideal for reno­va­tions. This is because there is space for a heat pump where the old gas or oil heating system used to heat the water.

Public discus­sion tends to focus on outdoor heat pumps. However, their indoor coun­ter­parts offer several advan­tages: no space is required around the perimeter of the house, neigh­bors cannot hear any oper­ating noise and they are ideal for reno­va­tions. This is because there is space for a heat pump where the old gas or oil heating system used to heat the water.

The higher flow temper­a­ture offers another advan­tage: no addi­tional elec­tric heating is required to meet the hygiene stan­dards for hot water prepa­ra­tion. This is because synthetic refrig­er­ants can only achieve temper­a­tures of 55 to 60 degrees Celsius. “Propane thus ensures signif­i­cantly more effi­cient oper­a­tion,” says Frei.

However, much stricter regu­la­tions apply to the use of propane indoors than outdoors. “The topic is still rela­tively new in the market. The chal­lenge is to first find out which stan­dards apply and then to develop a safety concept in order to bring a safe product to market.” On top of that, the new heat pump has to be effi­cient and quiet.

The tricky busi­ness of pres­sure

A key lever for increasing the effi­ciency of a heat pump is the fan. Its task is to deliver exactly the amount of air to the heat exchanger that is required for optimal and effi­cient oper­a­tion. This is more tech­ni­cally chal­lenging for indoor heat pumps than for outdoor ones. This is because the air is conveyed into the building and to the unit via a duct. The length of the duct, every bend and even the heat exchanger itself slow down the air. This leads to pres­sure losses that the fan has to over­come.

In prin­ciple, this is not a problem—but the instal­la­tion space in the heat pump is limited: “It must be small enough to still fit through the door and into the base­ment,” says Frei. A small fan can only cope with the high pres­sures if it rotates faster. But then it consumes more elec­tricity and is louder. Frei had to strike a balance: find a fan that takes up as little space as possible and moves enough air through the system at a compar­a­tively low speed.

In addi­tion, Frei wanted to make the fan an impor­tant part of his safety concept: When the sensors detect a propane leak, the impeller should rotate as quickly as possible to safely trans­port the gas outside. To do this, the fan needs to be approved, as the elec­tric motor is a poten­tial source of igni­tion. Who could offer Frei a fan that meets all these require­ments?

A joint devel­op­ment project

The devel­oper there­fore turned to Samuel Schlit­tler, a sales repre­sen­ta­tive at ebm-papst in Switzer­land. The two met at a trade fair, and this was their first oppor­tu­nity to work together. “With the AxiEco plug-in and the AxiTone, we have fans for heat pumps in our range that are already approved for non-fluo­ri­nated refrigerants—but these were ruled out from the begin­ning,” says Schlit­tler, explaining the chal­lenge. This is because axial fans are designed for open systems such as outdoor heat pumps and cannot deliver the high pres­sures required. There­fore, only a centrifugal fan can be consid­ered. The RadiPac C, which ebm-papst devel­oped for high volume flows with high counter pres­sure, is predes­tined for the task. “With the 500 and 630 millimeter impeller vari­ants, the required pres­sure can be gener­ated at compar­a­tively low speeds and thus very quietly,” explains Schlit­tler.

But Frei was preoc­cu­pied with another ques­tion: How could he save even more space to make the device more compact? How close could he place the fan to the heat exchanger without compro­mising effi­ciency and acoustics? “There is a rule of thumb for how big the distance should be, but we didn’t know what would happen if we fell short of that,” says Frei. Schlit­tler there­fore called in his colleagues from the devel­op­ment depart­ment in Mulfingen. The request thus turned into a joint devel­op­ment project. Frei provided the flow special­ists with the housing data so that they could deter­mine the optimum distance.

One key improve­ment sugges­tion was to use baffles to adjust the flowed so that the air flowed through the heat exchanger in the best way. Frei was thrilled: “What really impressed us was how ebm-papst worked with us and supported us during the devel­op­ment process. We were able to inte­grate the fan perfectly into our system so that it ulti­mately func­tions opti­mally.” This also included details such as the elec­trical connec­tion: To elim­i­nate the need for a space-consuming connec­tion box, ebm-papst pre-assem­bled the cables. This allowed the RadiPac C to be connected directly to the device elec­tronics: “This saved us a few more centime­ters in instal­la­tion depth.”

Ready in time for the heating season

Now every­thing seemed perfect. Only one thing was still missing: the fan’s approval for oper­a­tion with propane. The motor already met the strict safety require­ments, but in combi­na­tion with the elec­tronics, it lacked approval. “The ques­tion was not whether we would get it, but whether the approval would come in time for the desired market entry,” says Schlit­tler. But in this case as well, there is a happy ending. Just in time for the 2025–26 heating season, the expected certifi­cate arrived.

The LWKMi pro is avail­able in three perfor­mance classes for a heating output range of 1 to 26 kilo­watts. The first heat pumps have already been installed, and Heim AG is plan­ning to produce several thou­sand units. After all, demand is high: more and more oil and gas heating systems are wearing out, and sustain­able solu­tions such as heat pumps are more in demand than ever. Thanks to propane, Heim AG’s heat pump is future-proof. “We already thought about this when devel­oping the device,” says Frei with a wink.