VENTILATION, HUMIDITY AND NOISE Last month we looked at Heat Recovery Ventilation, one of the benefits of which is said to be lower humidity levels in the home. But as we shall see later, all ventilation systems result in lower humidity levels, not just those based on a heat recovery unit albeit heat recovery may be desirable in itself. Relative Humidity and psychrometric charts An understanding of the relationship between humidity and temperature can be gained from a psychrometric chart, a simple version of which is given below. (Avoid the common mistake of using the word, psychometric. That word is derived from the Greek psyche soul, whereas our word comes from psychros cold.) On a psychrometric chart, temperature is shown along the horizontal axis, and moisture content (also called absolute humidity) along the vertical axis. Air is composed of gases, chiefly nitrogen and oxygen, and a very small amount of (invisible) water vapour. The maximum amount of water vapour that can be contained in the air is very dependent on the temperature, and is given in the chart by the curved saturation line. If saturated air is cooled, then its moisture content must diminish and the excess water vapour condenses out as water droplets forming mist or dew (outside) or condensation (inside, as on single glazing in cold weather). In practice, Relative Humidity is a more common measure than moisture content. At any given temperature, the Relative Humidity is the percentage of water vapour present compared to the maximum possible (given by the saturation line in the chart). Psychrometric chart (Temperatures and humidities within the yellow box are comfortable.) Plotted on the chart are two lines, corresponding to Relative Humidities of 40% and 60%. These lines are easily derived from the saturation line at any given VENTILATION, HUMIDITY AND NOISE 1 APRIL 2007.
temperature they equal 40% and 60% of the saturated moisture content. In a dwelling it is reckoned that, for comfort and health, air temperature should be in the range 18ºC 22ºC, and Relative Humidity 40% 60%. The area coloured yellow on the chart corresponds to these optimum conditions. Condensation and latent heat Let s now examine what happens when moist air is extracted via a heat exchanger. Let us imagine that the air is being extracted from a humid bathroom, and is initially at 20ºC with Relative Humidity 80%. From the chart we can see that the saturation moisture content at 20ºC is 15 grams of water per kg of dry air. So for RH=80%, the moisture content is 12 grams of water per kg of dry air point A on the chart. (12 = 0.8 x 15.) In the heat exchanger, the extracted air is cooled. For the first few degrees of cooling, this is straightforward as the air cools to the point B, corresponding to a temperature of 17ºC. At this temperature, the air is saturated with water vapour, so as it is cooled further, water condenses out until the air is finally expelled at a temperature of say 5ºC, corresponding to point C. If we consider a kilogram of air undergoing this cooling, how much heat can potentially be recovered in the heat exchanger? The specific heat of air is about 1,000 Joules / kg.ºc. The temperature drop is 15ºC, so 15,000 Joules are given up by our kilogram of air in cooling. The more interesting consideration is the amount of heat given up by the condensing water vapour. The chart shows that 6g of water condense out in the process. (6g = 12g 6g.) Since the latent heat of water is about 2,500 Joules/gram, we can see that about 15,000 Joules of heat are released by the condensing water vapour. (15,000 = 6 x 2,500.) So in this particular case, as much heat is released as latent heat in the condensation process as is given up as specific heat in the cooling of the air. This heat from condensation is a sort of bonus, and can be used to warm up incoming air from sub-zero temperatures or be the source of heat for a heat pump to create warm air heating. The main point of the above exercise is to show the importance of the latent heat released by the condensation of water vapour. (In more complex psychrometric charts, the heat content, or enthalpy, of the air is shown the sum of the sensible and latent heats.) As I mentioned last year in my article about Irish selfbuild (in the December issue), air source heat pumps can be particularly effective in the humid climate of the British Isles because of the amount of latent heat given up when water vapour condenses. (In midwinter, the average Relative Humidity in Britain is something like 90%.) Using the psychrometric chart Now let s have a look at another scenario illustrated on the chart. Consider ventilation on a midwinter s day with outside temperature at 0ºC and RH = 100%, the conditions for freezing fog. If this fresh air is taken into the house and warmed up to 20ºC, its moisture content stays the same. This process is shown by the line FG on the chart. Although the moisture content stays the same, the Relative Humidity drops from 100% (at F) to about 25% (at G). In theory, a Relative Humidity of 25% is too dry for comfort and for health, and this gives the rationale for the rotating disc heat exchanger described last month. In practice, though, there are probably enough VENTILATION, HUMIDITY AND NOISE 2 APRIL 2007.
sources of water vapour within the house to raise the Relative Humidity in the house to above 40%. Note that this drop in Relative Humidity occurs by whichever means the temperature of the fresh air is raised. The advertising for Heat Recovery Ventilation gives the impression that to achieve a lowering of the humidity levels in a house you need an HRV system. But that is not the case. By whatever means the cold, fresh air enters the house, whether via a heat exchanger, a trickle vent, or whatever, as the temperature of the air is raised so its Relative Humidity drops. We don t have much sensory awareness of humidity except in extreme conditions. If you want to become more conscious of how humidity varies in your home, you could purchase a hygrometer. The traditional versions work by the cooling effect of wet muslin wrapped round a thermometer bulb, but the digital instruments now available are more convenient. (See Further Info.) NOISE AND VENTILATION The theory of ventilation is complex enough in itself, but there are even more complexities when the interaction of ventilation and noise is considered. Sometimes a ventilation system creates noise, sometimes a ventilation system allows the easy transmission of external noise and/or internal noise. Noise created by ventilation systems Individual extract fans Modern fans are quieter than the fans of old. But fit in-line fans in a floor/ceiling void to decrease the noise even further. I particularly dislike extract fans with shutters that clack in the wind. This happens in the cheaper models with flaps that are opened by the pressure of the air stream. Unfortunately the flaps are also opened by gusts of wind blowing by sucking out warm air unnecessarily and also causing a clacking sound as the flaps snap shut when the gust passes on. Go for shutters which are opened and closed by direct electro-mechanical means. Discuss this with your electrician before accepting a quote otherwise you ll probably find that he fits the cheapest fan available. [Also, on a similar theme, discuss the brand of electrical fittings light switches, consumer unit, etc that your electrician intends to use in second fix. The market leader for quality is the MK brand, but the chances are that your electrician will fit a cheaper brand unless prompted otherwise.] Centralised systems Centralised systems, HVR or MEV, still suffer from a reputation for being noisy, though some manufacturers are stressing how quiet their units are now. Check this out before you make a purchase. And take care with the mounting of the unit so that it is acoustically de-coupled from the structure supporting it. Most of the time the unit should run on the lowest power setting and be virtually silent. External noise For a house built in a noisy environment, some consideration needs to be given to the effects of ventilation on sound insulation. VENTILATION, HUMIDITY AND NOISE 3 APRIL 2007.
Trickle vents allow in external noise, as do extract fans. An extra advantage of the supply air windows, described last month, is that they are better sound insulators than windows with sealed units and trickle vents. Another approach is to fit vents through the wall rather than through the window head. Some wall vents have acoustic damping. (Wall vents have other advantages over the simple vents fitted in window heads. They can incorporate filters to keep out pollen, etc, and they can be fitted with check valves to prevent high winds blowing through. Continental trickle vents for windows are more sophisticated than British ones perhaps they will come to Britain soon.) But the best way of keeping out external noise is to fit an HRV system, perhaps with some noise attenuation ducting. Internal noise The under-door gap The theory of house ventilation is: Air is extracted from wet rooms, and fresh air enters dry rooms. The implication is that air travels from dry rooms to wet rooms. And/or: Air enters through trickle vents into rooms on the windward side of a house, and leaves through trickle vents on the leeward side. And/or: Air enters through trickle vents in the lower storey and leaves through vents in the upper storey, under the stack effect. But how does air move from one room to another if the internal doors are shut? There is an assumption in ventilation theory that there is a gap (5mm 8mm) at the bottom of internal doors to allow air movement. But a gap at the bottom of a door greatly reduces the sound insulation. As far as I know, there is no way of reconciling these two conflicting requirements. To put it bluntly, effective ventilation results in ineffective internal sound insulation. [As an aside, may I recommend that you fit hardwood threshold strips at internal doorways. One of the advantages of doing this is that the door can be fitted to give a precise under-door gap; otherwise the gap is dependent on whatever floor covering happens to be used.] Ductwork Ductwork acts as a channel for sound transmission. The ductwork of centralised ventilation systems (HRV and MEV) and Passive Stack Ventilation systems (PSV) can allow noise to be transferred from one room to another. This might be especially noticeable when a duct from the ground storey passes through a room in the upper storey. If the duct can be boxed in with sound insulation or contained within a cupboard, so much the better. VENTILATION, HUMIDITY AND NOISE 4 APRIL 2007.
And just as ductwork passing through a floor reduces the effectiveness of the floor as a noise barrier, so it also reduces the effectiveness of the floor as a fire barrier. If this concerns you, a fire damper can be fitted in the ductwork. FURTHER INFO: Thermometers Direct Thermometers and hygrometers. www.thermometersdirect.co.uk. For the curious reader who wants to do his/her own ventilation sums, a figure for the density of air may be useful: 1.2 grams/litre. Words: 1885. Copyright article by Robert Matthews in SelfBuild & Design magazine, April, 2007. VENTILATION, HUMIDITY AND NOISE 5 APRIL 2007.