[0001] The invention relates to a method at a ventilation unit with heat recovery, the method comprising leading outside air via the inlet side of a heat exchanger into a building as supply air flow and guiding exhaust air via the exhaust side of the same heat exchanger out of the building as extract air, whereby heat contained in the exhaust air is transferred in the heat exchanger to heat the supply air, and the method comprising monitoring the freezing of the heat exchanger by means of an electromagnetic radiator apparatus and initiating antifreezing functions, if necessary. The invention further relates to a ventilation unit with heat recovery.

[0002] The above methods and ventilation units are solutions known in the art. [0003] As a heat exchanger, ventilation units provided with heat recovery comprise a structure that is composed of a plurality of narrow passages forming the exhaust side and, respectively, a plurality of narrow passages forming the inlet side. Exhaust air, i.e. air to be removed from a building, is guided via the exhaust side out of the building as extract air, and outside air to be supplied into the building is guided via the inlet side into the building as supply air. Heat contained in the exhaust air is transferred in the heat exchanger to the supply air.

[0004] The passages on the inlet and exhaust sides of the heat exchanger may be arranged in various ways. For instance, the heat exchanger may be a cross-flow heat exchanger, in which the inlet side passages are substantially at an angle of 90 degrees to one another, for example. As another example, a counter-flow heat exchanger may be mentioned, where the passages of the inlet and exhaust sides are arranged in such a manner that, on the inlet side, air flows inside the heat exchanger substantially in a direction opposite to the air flow of the exhaust side. As additional examples, heat exchangers operating on the parallel-flow principle and, furthermore, heat exchanger solutions based on a rotating cell may be mentioned. Heat contained in the air flowing on the exhaust side is transferred through the walls between the passages of the inlet and exhaust sides to the colder air flow of the inlet side passages. The walls between the exhaust side passages and the inlet side passages are naturally formed into thin, highly heat-conductive structures. [0005] Air is led to the inlet side and the exhaust side and further through the heat exchanger by means of blowers and appropriate ducts and flow controllers. The ventilation unit naturally also comprises control and regulation means controlling the operation of the ventilation unit to enable and pro- vide desired ventilation.

[0006] If the outside air temperature falls below 0 degrees and the moisture content of exhaust air is sufficiently high, humidity in the exhaust air condenses on the walls of the exhaust side passages of the heat exchanger, and if the temperature drops further, the condensed humidity freezes and fi- nally completely blocks the exhaust side passages.

[0007] Previously known solutions attempted to prevent the freezing in different ways. An example of known antifreezing solutions is the use of a thermostat. A thermostat is disposed at the coldest location after the heat exchanger. The thermostat may measure the temperature of exhaust air or sup- ply air. The thermostat may switch off the outside air blower, slow down the rotational speed of the outside air blower, guide the outside air flow past the heat exchanger by means of a damper structure, choke the outside air flow, or increase the exhaust air flow. The thermostat may also control a pre-heating radiator for outside air, by which the temperature of outside air arriving at the heat exchanger is maintained above the freezing temperature.

[0008] Thermostatically controlled antifreezing is sufficiently reliable only if the operation is adapted to high exhaust air humidity, for example to the frequency how often a sauna of the apartment is used. However, in winter the moisture content of indoor air is low for the most of the time and it is unneces- sary to use antifreezing that reduces the efficiency of heat recovery. In addition, the undesirable pressure difference between inside and outside air caused by unnecessarily switching off the outside air blower complicates the ignition phase of the furnaces.

[0009] As another example, the monitoring of pressure losses on the exhaust side of the heat exchanger may be mentioned. Freezing is allowed to develop to an extent that a pressure switch senses the pressure-difference increase caused by freezing and initiates the defrosting, as was described above in connection with a thermostat.

[0010] A disadvantage of the pressure-controlled antifreezing is a relatively big pressure difference required by the pressure switch. Blowers cannot be operated at low rotational speeds. Also, using the blower at different powers makes the use of the pressure switch inaccurate.

[0011] As a third prior art example, the use of an electromagnetic radiator apparatus for sensing the freezing may be mentioned. Such a solution is described in Fl patent publication 103535. In the aforementioned known solution the electromagnetic radiator apparatus comprises an electromagnetic radiator and a receiver detecting electromagnetic radiation. The radiator and the receiver are arranged to co-operate in such a manner that the receiver senses the radiation coming from the radiator. In the known solution, the radia- tor is disposed in the exhaust side passage inside the heat exchanger and the receiver is placed outside the heat exchanger. Freezing of the heat exchanger prevents radiation from propagating from the radiator to the receiver, whereby the equipment initiates the antifreezing, as was described in connection with the previous examples. [0012] A drawback of the solution according to Fl patent publication

103535 is that the electromagnetic radiator apparatus is disposed inside the heat exchanger and is thus in contact with the heat exchanger. Such a solution complicates the service of the heat exchanger. Namely, the heat exchanger must be taken out of the equipment every once in a while for cleaning. The electromagnetic radiator apparatus must be detached from the heat exchanger for the time of cleaning, if the cleaning is to be carried out well and with little effort. If the electromagnetic radiator apparatus is not detached, it may lead to a poor cleaning result. The electromagnetic radiator apparatus may also be damaged during detachment. [0013] The purpose of the invention is to provide a solution, by which the prior art disadvantages can be eliminated. This is achieved by a method and a ventilation unit in accordance with the invention. The method of the invention is characterized by guiding electromagnetic radiation from the outside of the heat exchanger through the heat exchanger and further out of the heat exchanger. The ventilation unit of the invention, in turn, is characterized in that the emitter emitting electromagnetic radiation and the receiver detecting electromagnetic radiation are disposed outside the heat exchanger on its opposite sides and that electromagnetic radiation is arranged to propagate from the emitter to the receiver through the heat exchanger. [0014] An advantage of the invention is its simplicity and that the solution is particularly advantageous in terms of service and maintenance. For example, cleaning and testing of the heat exchanger become substantially easier with respect to the prior art, because both the emitter emitting electromagnetic radiation and the receiver detecting electromagnetic radiation are disposed apart from the heat exchanger outside thereof. [0015] The invention will be explained in the following in more detail by means of the example shown in the attached drawing, in which

Figure 1 shows a principle view of a ventilation unit with heat recovery, and

Figure 2 shows a principle view of a heat exchanger of the ventila- tion unit with heat recovery, provided with a solution according to the basic idea of the invention.

[0016] Figure 1 shows the principle of a ventilation unit with heat recovery. The main parts of the ventilation unit are denoted in Figure 1 with reference numerals. Reference numeral 1 refers to a supply air duct, reference numeral 2 to an exhaust air duct, reference numeral 3 to an outside air duct, and reference numeral 4 to an extract air duct. In Figure 1 , reference numeral 5 represents a recirculation air duct. When the apparatus is mounted, these ducts are connected to the piping of the building, by which air is led to and from the ventilation unit. [0017] In the figure, reference numeral 6 refers to a heat recovery cell, i.e. a heat exchanger, by which heat contained in the exhaust air is transferred to the supply air to be supplied into the building. Supply air is taken from the outside and led to the heat exchanger and, accordingly, exhaust air is led out of the heat exchanger 6 as extract air. [0018] In Figure 1 , reference numeral 7 refers to an exhaust air blower and reference numeral 8 to a supply air blower, respectively. Reference numeral 9 refers to a post-heating resistor and reference numerals 10, 11 and 12 to filters. Reference numeral 13 refers to a by-pass damper for heat recovery, which in Figure 1 is in 'summer' position. In Figure 1 , reference numeral 14 represents a condensation water hose.

[0019] Only the essential main parts are marked with reference numerals in the attached Figure 1. It is obvious that, in practice, the ventilation unit also comprises a plurality of other details, such as control means, by which the operation and use of the unit is controlled, etc. For a person skilled in the art, all details provided in Figure 1 with reference numerals and the operation thereof as well as the entire structure and operation of the ventilation unit are fully conventional, wherefore they are not explained in greater detail herein. The example of Figure 1 shows ILTO 430, a ventilation unit with heat recovery produced by Swegon ILTO Oy. The purpose of Figure 1 is not to restrict the invention to any specific apparatus or apparatus type in any way, but in this con- text to generally and simply describe which kind of technique and apparatuses the invention concerns.

[0020] Figure 2 shows the basic idea of the invention in principle. The same reference numerals are used in Figure 2 as in Figure 1 to refer to the corresponding parts. Figure 1 also uses reference symbols T (supply air), P (exhaust air), U (outside air), J (extract air), and K (recirculation air). For the sake of clarity, Figure 1 only shows a heat exchanger 6. The heat exchanger 6 is shown in Figure 2 only in principle. The heat exchanger of Figure 2 is in principle a counter-flow heat exchanger but it is obvious that the invention is not restricted thereto, but the heat exchanger may be of any known type, such as a cross-flow heat exchanger. For a person skilled in the art, different heat exchanger types are fully conventional technique, wherefore they are not explained in greater detail herein.

[0021] Exhaust air flows along the exhaust side passages of the heat exchanger 6 of the ventilation unit out of the building and further as ex- tract air to the outside air. Accordingly, the cold outside air is brought along the adjacent passages of the inlet side as supply air into the building. The exhaust side passages and the inlet side passages are arranged alternately in the heat exchanger so that there is an inlet passage adjacent to each exhaust passage, etc. Such a structure is accomplished, for instance, by forming the heat ex- changer 6 from rather thin plates so that narrow gaps remain between the plates, providing the exhaust side passages and the inlet side passages. Heat is transferred from a higher temperature towards a lower temperature, which means that heat contained in the warm exhaust air is transferred through the wall to the colder air flowing in the adjacent, colder inlet side passage. In the exhaust side passage, the flowing air naturally cools down accordingly.

[0022] Exhaust air to be removed from a building, such as an apartment, often contains humidity, which condenses into water in the exhaust side passages of the heat exchanger 6. If the outside air temperature is sufficiently low, the water condensed on the walls of the exhaust side passages of the heat exchanger freezes and eventually blocks the narrow exhaust passages. [0023] In the invention, an electromagnetic radiator apparatus is used for monitoring the freezing of the heat exchanger. The electromagnetic radiator apparatus comprises an emitter 15 emitting electromagnetic radiation and a receiver 16 detecting radiation coming from the emitter 15. Electromag- netic radiation may be any suitable radiation, such as infrared radiation, visible light, etc. The invention is based on the ability of electromagnetic radiation to permeate through the open passage of the heat exchanger 6. Upon freezing, the exhaust side passage of the heat exchanger is usually blocked. Electromagnetic radiation cannot thus pass through the exhaust side passage. Ac- cording to the essential idea of the invention, electromagnetic radiation is guided from the outside of the heat exchanger 6 through the heat exchanger and further out of the heat exchanger. Most preferably the radiation is thus guided to pass through the exhaust side of the heat exchanger, but the invention is nevertheless restricted thereto, but it is obvious that the blocking of the inlet side of the heat exchanger may also be monitored by means of the invention. The emitter 15 emitting electromagnetic radiation and the receiver 16 detecting electromagnetic radiation are thus disposed outside the heat exchanger 6 on its opposite sides so that the electromagnetic radiation is able to propagate from the emitter 15 to the receiver 16 along the exhaust side passage of the heat exchanger.

[0024] Hence, the emitter 15 emitting electromagnetic radiation and the receiver 16 detecting electromagnetic radiation are placed apart from the heat exchanger 6 as was described above and also in Figure 2, which means that the emitter 15 and the receiver are not in contact with the heat exchanger 6. This provides an essential advantage over the prior art solutions, as was already stated earlier. There are advantages to service and maintenance, for example, because the heat exchanger 6 must be taken out of the unit from time to time and the devices fixed to the heat exchanger complicate the service/cleaning and, also, the devices may become damaged during detachment and attachment.

[0025] The electromagnetic radiator apparatus arranged in the above manner is used in the invention for controlling the defrosting automatics of the heat exchanger. The arrangement functions so that the emitter 15 emits infrared radiation, for example, from the outside of the heat exchanger 6 through the exhaust side passage and the receiver 16 receives it outside the heat exchanger 6, as illustrated in Figure 2, for example. If the exhaust side passage freezes, electromagnetic radiation can no longer pass through the passage and reach the receiver. In this case, the arrangement initiates the defrosting function in which, for instance, the supply air blower is switched off and the heat of the exhaust air melts the ice. A corresponding function is also achieved by guiding the outside air past the heat exchanger or by activating the heating arrangement, such as heating resistors, by which the outside air is heated before supplying it to the heat exchanger. The aforementioned defrosting arrangement may also comprise combinations of aforementioned implementations known per se. [0026] The defrosting period may be adjusted suitably, it may be, for instance, 10 minutes, 20 minutes, half an hour or something else. The calculation of the defrosting period begins when the ice has melted so much that a radiation connection from the emitter to the receiver has been re-established.

[0027] The above example of the invention is not intended to limit the invention in any way, but the invention may be varied entirely freely within the scope of the claims. Consequently, it is obvious that different details of the invention may also differ from the examples shown in the figures.