Today, air condi­tioning in Germany is respon­sible for around 35 percent of energy consump­tion and around 120 million tons of the green­house gas CO₂ per year. These emis­sions have to be reduced by more than 40 percent to achieve climate protec­tion targets by 2030 and to become less depen­dent on energy imports. At the same time, it is impor­tant to balance energy and health require­ments. Even with optimum insu­la­tion and hermet­i­cally sealed building envelopes, an air exchange must be ensured in order to prevent damage to the building’s struc­ture, such as mold forma­tion, and an impact on the air quality. Push-pull systems for decen­tral­ized resi­den­tial venti­lation are a prac­tical solu­tion for this. They ensure a demand-based, controlled supply of fresh air while keeping energy consump­tion low.

Push-pull system for decen­tral­ized resi­den­tial venti­lation with a ceramic heat exchanger. (Photo | ebm-papst)

No matter which direc­tion it spins in, the airflow remains the same. The AxiRev 126 stands out thanks to its distinc­tive appear­ance: its symmet­rical blade design enables iden­tical perfor­mance curves in both direc­tions of rota­tion. (Graphic | ebm-papst)

Heat recovery during venti­lation

Push-pull venti­lation units convey used air from the living space to the outside (push) for a defined period of time, usually around one minute. The fan installed in the venti­lation unit then changes its direc­tion of rota­tion, i.e. it reverses (pull). During push oper­a­tion, an inte­grated heat accu­mu­lator stores the heat energy from the exhaust air. In the pull phase, it is trans­ferred to the fresh outside air so that it flows into the building preheated. A filter also clears dust and pollen from the air coming in from outside. Two units per room or zone are required to ensure balanced venti­lation. In some cases, several rooms are venti­lated using one pair of units.

The units for decen­tral­ized resi­den­tial venti­lation are easy to install; they are easily inte­grated into the building façade. However, this poses a chal­lenge for the fans used: even with changing wind forces, they must deliver an air flow that is as constant as possible in both direc­tions of rota­tion. However, that is no problem for the new AxiRev reversing fan, which ebm-papst devel­oped espe­cially for the require­ments in this type of push-pull appli­ca­tion.

Steep char­ac­ter­istic curve, high pres­sure increase

The EC motor and fan geom­etry have been selected so that the fan is designed precisely for the oper­ating points commonly used in push-pull systems. The char­ac­ter­istic curve for the pressure/air flow is very steep, which means that, even in stormy weather condi­tions, there are only slight fluc­tu­a­tions in the air flow. In typical appli­ca­tions, S2 clas­si­fi­ca­tion as per DIN EN 13141-8 for wind pres­sure suscep­ti­bility is achiev­able. This means that wind and storms have little influ­ence on the effi­ciency and func­tion of the decen­tral­ized resi­den­tial venti­lation unit. The blade design is almost symmet­rical and ensures that the char­ac­ter­istic curve and so too the volume of air conveyed are iden­tical in both direc­tions of rota­tion. In a push-pull appli­ca­tion, the fan then typi­cally changes the direc­tion every 60 to 70 s, meaning that resi­den­tial venti­lation can work effec­tively in a paired oper­a­tion.

This is based on a wide range of design and aero­dy­namic details. These include, for example, the “flat fitted” blades that enable higher pres­sures. The patented blade design with striking blade tips and the open­ings at the blade ends mini­mizes the noises induced by tip gap vortices, which reduces the noise emis­sions. Matching the number of blades and struts also plays a role in this context. The thir­teen struts with an aero­dy­namic profile also reduce turbu­lent trails, which ensures a very good psychoa­coustic noise quality. This means that the fan is not only quiet but its oper­ating noise is also perceived as pleasant. This also applies to both direc­tions of rota­tion and, of course, the three-phase EC motor devel­oped specif­i­cally for this area of appli­ca­tion without irri­tating commu­ta­tion noise, and switching between the direc­tions of rota­tion cannot be heard.

Energy-effi­cient and suit­able for retro­fits

Thanks to the motor and aero­dy­namic opti­miza­tions, the fan only consumes a small amount of energy during oper­a­tion. With a typical flow quan­tity of 42 m³/h per push-pull unit, a stan­dard device is less than 2 W. As EC motors work at high effi­ciency even in partial-load oper­a­tion, this also applies if the venti­lation is only in minimal oper­a­tion, for example because no-one is present in the rooms. The large speed range from 4,200 to below 500 rpm also covers this oper­ating mode. The fan can be infi­nitely adjusted, e.g. in terms of humidity, temper­a­ture and CO₂ content of the ambient air. The wide speed control range enables perfect adap­ta­tion to each appli­ca­tion.

With a diam­eter of 126 mm and a height of 26 mm, the new fan is very compact. It is supplied as a complete system that can be plug and play and is also suit­able for retro­fitting, as the foot­print and dimen­sions are the same as the previous model. There­fore, the fans are easy to exchange without having to change the design of the device. The user then bene­fits from a signif­i­cantly increased air flow, increased effi­ciency (7 percentage points per single fan) and a 5 dB(A) decrease in noise level. The air flow, effi­ciency and noise advan­tages provided are in compar­ison with the previous version 4412F in a typical appli­ca­tion.

“The air volume is iden­tical in both direc­tions of rota­tion”

The AxiRev 126 is the new stan­dard for effi­cient push-pull appli­ca­tions in resi­den­tial venti­lation. Tobias Sieger, respon­sible for the venti­lation design and flow simu­la­tion, explains how the reversing fan achieves this.

Mr. Sieger, why is the AxiRev needed?

With increasing require­ments for envi­ron­mental protec­tion and energy effi­ciency, more and more build­ings are being insu­lated and rooms are being sealed airtight. However, to ensure a healthy, pleasant indoor climate, a different form of air exchange is required. Decen­tral­ized resi­den­tial venti­lation units are being increas­ingly used for this purpose because they can be easily installed in existing building facades. However, the fans used face several chal­lenges, which the AxiRev solves in an optimal way.

What are these chal­lenges?

Firstly, the fans have to deliver a constant air flow in both direc­tions of rota­tion, even when the wind force changes. Secondly, they should operate very quietly while consuming as little elec­tricity as possible.

And the AxiRev does all of that?

Yes, we have devel­oped the new AxiRev reverse fan specif­i­cally for these push-pull appli­ca­tions. In this type of appli­ca­tion, the fan changes its direc­tion of rota­tion every minute. We equipped it with new low-noise motor tech­nology and an inno­v­a­tive aero­dy­namic design. The result is a uniform air flow, pleasant noise level, and maximum energy effi­ciency.

How did you make it wind­proof?

The char­ac­ter­istic curve is very steep, which means that, even in stormy weather condi­tions, there are only slight fluc­tu­a­tions in the air flow. This means that wind and storms have little influ­ence on the effi­ciency and func­tioning of the decen­tral­ized resi­den­tial venti­lation unit. The almost symmet­rical blade design ensures that the char­ac­ter­istic curve and thus also the volume of air conveyed are iden­tical in both direc­tions of rota­tion.

How did you reduce the noise?

This is based on a wide range of design and aero­dy­namic details. The striking blade tips and the open­ings at the ends of the blades mini­mize noise induced by the tip gap vortex, which in turn reduces the noise emis­sions. The thir­teen struts with an aero­dy­namic profile also reduce turbu­lent trails, which ensures a very good psychoa­coustic noise quality.

This means that the fan is not only quiet but its oper­ating noise is also perceived as pleasant. The highly effi­cient EC motor devel­oped espe­cially for this appli­ca­tion also works without annoying commu­ta­tion or switching noises, even at extremely low speeds — and flow design and motor tech­nology also enable very low energy consump­tion.