Control of building ventilation systems

Abstract

A building ventilation system for extracting stale air from within the building 1 and introduces fresh ambient air from outside the building. The ventilation system is operable to remove a predetermin

Description

TECHNICAL FIELD OF THE INVENTION This invention relates to ventilation systems for buildings. BACKGROUND The main function of modern house ventilation systems is to remove excess moisture from the building. The amount of ventilation which is required varies depending on the life style, number of occupants and outside air conditions. Current building regulations aim to provide adequate ventilation in new houses, but the requirements are somewhat arbitrary and, in the majority of cases, result in more extraction than is actually needed to solve the problem. Mechanical Ventilation with Heat Recovery (MVHR) is an increasingly popular form of ventilation system that includes a network of ducting connected to various rooms in the building. MVHR systems are used to remove excess moisture and stale air from the building whilst at the same time introducing fresh, clean and filtered air back into the home via a heat recovery unit. It should be noted, however, that since water tends to migrate naturally an extensive network of ducting is not required in many situations. The heat recovery unit is generally located inside the building, e.g. in a loft space, cupboard or plant room, and works by continuously extracting air from within the building, most commonly the wet rooms of the property (bathrooms, kitchens, en suites, utility etc), whilst at the same time drawing in a fresh supply of air from the outside. Heat is recovered from the extracted stale air via a heat exchanger incorporated in the unit and is used to warm the clean, fresh filtered air supply for the other rooms such as living rooms, bedrooms etc. Under ideal conditions up to 95% of the heat may be be recovered, although this figure depends upon the temperature difference between inside and outside, the size of the heat exchanger, and the air flow. MVHR systems often have "Summer By-Pass" functions which allow the property to continue to be ventilated and receive fresh, clean filtered air, but the heat recovery process is intermittently bypassed to maintain the internal air temperature at a comfortable level. Thus, when the inside temperatures are high and the outside air is cooler, the home is cooled. The heat recovery unit usually runs continuously at a low level (m3 / hour), but in some installations the rate of ventilation can be manually boosted by means of an on / off switch when higher rates of ventilation and extraction are required such as when showering or cooking. In other systems automatic boosting may be provided by means of a humidistat which increases the amount of ventilation when the relative humidity rises. Relative humidity (RH) is expressed as a percentage. It is the ratio of the absolute humidity at a given time to the highest possible humidity (100%), which depends on current air temperature. Relative humidity does not indicate how much water vapor is in the air, but what percentage of the maximum vapor pressure has been reached. Using a humidistat which aims for a particular RH is not an effective and energy-efficient way of preventing moisture problems. The reason is that the optimum RH will change depending on the temperature inside the home. For example, in winter the optimum RH could be below 40%, but when it is warmer outside the optimum could be above 50%. SUMMARY OF THE INVENTION The present invention proposes a ventilation system which works to remove a set amount of water rather than aiming for a particular RH. This is achieved by varying the run time or the rate of air extraction, or both, depending on the difference between the absolute moisture content of the air in the building, and the absolute moisture content of the air entering the building. Warm air can hold more water than colder air. Absolute humidity is the mass of water vapor divided by the mass of dry air in a given volume of air. Absolute humidity is a measure of the actual amount of water vapor or moisture in the air, regardless of temperature, and may be expressed as grams of moisture per cubic meter of dry air (g / m3). The maximum absolute humidity of warm air at 30 °C is approximately 30 g / m3 whereas the maximum absolute humidity of cold air at 0 °C is approximately 5g / m3. This is roughly the difference between the humidity in the summer months and the humidity in the winter months. The warmer air holds more water, whereas the colder air holds less water. The aim is to remove a predetermined amount of moisture every day, e.g. 1 litre. This is done by calculating how long the extraction should run to achieve this, taking account of the difference in the water content of the air going in and the air coming out. For maximum efficiency this can be done automatically by measuring the absolute moisture content of the air being extracted and the ambient air entering the building. One way of measuring the moisture content is to measure this directly using the principles of a hygrometer as in the detailed embodiments described below. Indirect methods of obtaining the absolute moisture content can also be used. For example, the temperature and relative humidity of the air can be measured, from which the absolute moisture content can be obtained, e.g. from a look-up table. A less convenient method is to enter the inside and outside temperature and relative humidity manually using the current or expected conditions inside and outside the building. The volume of water to be extracted over the required period can likewise be preset, or manually set, but it is preferred to adjust the extraction volume depending on the prevailing conditions, e.g. increasing the extraction volume as the outside air temperature reduces. BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a schematic diagram of a MVHR type building ventilation system; Figure 2 is a schematic diagram of a control unit used in the ventilation system. DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Fig. 1, a MVHR type building ventilation system is shown installed in a typical two-storey house 1. The system includes a heat recovery unit 2 which in this example is installed in the loft. An extraction duct 3 draws stale damp air from various wet locations within the building and, after passing through the heat recovery unit 2, conducts the extracted air to the outside of the building through a discharge duct 4. At the same time, clean fresh air is drawn into the building through a supply duct 5, again passing through the heat recovery unit 2 to be admitted to various locations in the building through a distribution duct 6. The heat recovery unit 2 contains a passive air-to-air heat exchanger 7 which recovers heat from the extracted stale air and transfers it to the incoming clean air. The unit 2 also contains two (or more) fans 8a and 8b, one in the extracted air duct and one in the incoming air duct to produce the necessary two-way air flow, along with an air filtration unit 9 for the incoming ambient air, and an electronic control unit 10. The control unit 10, shown in Fig. 2, performs various functions. In the embodiment shown, the control unit receives input from two moisture sensors 11 and 12. (Each moisture sensor could alternatively be replaced by temperature and RH sensors if the indirect method is used, as explained above.) The first moisture sensor 11 is arranged to monitor the moisture level in the air being extracted from the building, e.g. within the extraction duct 3 close to, or within, the heat recovery unit 2. The second moisture sensor 12 is arranged to monitor the moisture level in the external ambient air being drawn into the building, for example within the supply duct 5 near to, or within, the heat recovery unit 2. The absolute moisture level readings are either obtained directly from moisture sensors 11 and 12 or from a look up table using the measured temperature and RH values. The absolute moisture levels may typically be expressed in grams of water per kilogram of dry air or grams of water per cubic metre of dry air (g / m3). The indirect method of obtaining the absolute moisture content of the air using temperature and relative humidity has already been explained. In cases where the moisture sensors 11, 12 measure absolute humidity this can be done using the principle of a hygrometer. There are several types of hygrometer which could be used. One type measures humidity based on resistance or capacitance using electronic sensors in the air stream. A device which measures capacitance does so by utilizing two metal plates with space between them and measures the amount of moisture in the air by measuring the static electric charge between the two plates. Another type of hygrometer uses an electrical current passing through a ceramic material (or another moisture-absorbent substrate) to test the moisture in the air. When the ceramic component comes into contact with moist air the electrical current changes due to the resistance provided by the water, and the measured resistance correlates with the amount of moisture in the air. The control unit 10 works out the difference between the absolute moisture content of the extracted air and the incoming air, e.g. in grams of water per kilogram of dry air. The control unit then calculates how long the extraction process needs to run and varies the extraction time based on a predetermined amount of water to be removed, e.g. 1 litre per day, and the fan delivery e.g. in kilograms of dry air per hour. In general, the amount of air extracted will be varied in an inverse relationship to the difference between the two measurements. In other words, a large difference in moisture content will require a short running time whereas a relatively small difference will require a significantly greater extraction time to remove the required amount of water. In some embodiments the control unit 10 may control the speed of the fans 8. For example, the fans can be operated at two speeds - a relatively low idle speed which maintains a base level of ventilation, and a higher boost speed when it is required to actively lower the humidity within the building. In such a system, changes in the rate of air delivery are taken into account in determining the required running time. In another embodiment the moisture sensors 11 and 12 may be replaced by a menu which allows the user to manually select the inside and outside air conditions. In a preferred embodiment the amount of water to be extracted can be changed to take account of the prevailing conditions. Although it would be possible to manually set the extraction volume, this can be achieved automatically, for example by monitoring the outside air temperature, e.g. by means of a temperature sensor 13 located in the incoming air duct 5 (assuming that such a sensor is not already provided for determining the absolute humidity). The target extraction volume can thus be increased as the outside air temperature falls. With the proposed ventilation system the humidity level within the building is better matched to the prevailing conditions. Furthermore, energy consumption is substantially reduced whilst, at the same time, achieving better regulation of the internal moisture levels. Whilst the above description places emphasis on the areas which are believed to be new and addresses specific problems which have been identified, it is intended that the features disclosed herein may be used in any combination which is capable of providing a new and useful advance in the art.

Claims

1. A building ventilation system configured to extract stale air from within the building and introduce fresh ambient air from outside the building, wherein the ventilation system operates to remove a predetermined amount of moisture taking account of the difference in moisture content between the air extracted and the air introduced.

2. A building ventilation system according to claim 1 which is arranged to determine the moisture content of the air being extracted and the moisture content of the ambient air being introduced using the temperature and relative humidity of the respective air flows.

3. A building ventilation system according to claim 1 or 2 which is arranged to measure the absolute humidity of the air being extracted and the air introduced.

4. A building ventilation system according to any preceding claim wherein the moisture content is measured using hygrometers disposed in the air being extracted and the air introduced.

5. A building ventilation system according to any preceding claim wherein the moisture content is measured by sensing the static electric charge between two plates.

6. A building ventilation system according to any preceding claim which measures the moisture content by sensing the resistance to an electrical current passing through a moisture-absorbent substrate.

7. A building ventilation system according to any preceding claim wherein the amount of air extracted from the building is varied in an inverse relationship with the difference in moisture content between the air being extracted and the air introduced.

8. A building ventilation system according to any preceding claim which varies the extraction time to remove the required amount of moisture.

9. A building ventilation system according to any preceding claim which varies the rate of extraction to remove the required amount of moisture.

10. A building ventilation system according to any preceding claim wherein the predetermined amount of moisture to be removed can be varied.

11. A building ventilation system according to any preceding claim wherein the predetermined amount of moisture to be removed can be manually set.A building ventilation system according to any precedingclaim wherein the predetermined amount of moisture to be removed is varied as a function of external air temperature.

13. A method of operating a building ventilation system to extract stale air from within the building and introduce fresh ambient air from outside the building, wherein the ventilation system operates to remove a predetermined amount of moisture taking account of the difference in moisture content between the air extracted and the air introduced.

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