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Filter Clogging detection in HRV systems Disclosure Number: IPCOM000194671D
Publication Date: 2010-Apr-07
Document File: 3 page(s) / 2M

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Filter Clogging detection in HRV systems


In HRV systems, used in residential/small office applications, the air filters used in these units will be covered by dirt/dust through time. These filters need be changed in order to maintain the required air quality as well as the performance of the HRV units. As changing the filters are typically done by the maintenance people, there is a need for identifying the situation where a filter is indeed clogged (through gathered dust/dirt). An automatic detection of filter clogging is hence desirable.

Existing method

The current approach to detection of the filter clogging is based on the registration of the time where the HRV unit is started, normally is done by a switch which is activated by a side lid/cover of the HRV unit. When the lid is closed, a HW timer is started and after a predefined period (typically 6 months) an alarm signal, indicating the need for changing the filter, is activated.

The current method does not consider any environmental impact on the filter; A filter can be clogged much faster (or slower) than the predefined period.


We propose a SW based method based on the following concept:

Figure 1:  Pressure drop along the duct system

Different elements in the duct system introduce resistance to the air flow in the duct system. Hence these elements cause pressure drop along the duct system as it is shown on the figure. A fan acts as an air pump that adds an air pressure needed to compensate for these pressure drops while providing the required air flow. In our approach we use a sensor measuring the pressure difference over the fan, i.e. DP, and the control signal for the fan (that could be a PWM signal or a voltage/current signal).

The know facts are the following:

ÿ       The fan rotational speed is a function of the control signal, henceforth RPMref (in most case it is linear).

ÿ       The air flow through the fan, Q, is linearly proportional to the fan’s rotational speed.

ÿ       The relation between the air flow and the pressure difference over the fan is described by the following equation:
                                   Q =  K . sqrt(DP)                                                         (1)
where K is a constant and is HRV unit specific value. K is the same for HRV units of the same product series, i.e. it is obtained for one HRV unit and is then used for all HRV units of the same production series.

Considering the above-mentioned facts it is easy to conclude that, under nominal conditions, when a known control signal (RPMref) is given to the fan system, it will produce a certain air flow and hence pressure difference, henceforth called DPref. Hence, under nominal conditions and when the filter is clean, the set (RPMref, ...