The bathroom fan FAQ (frequently asked questions)
Ver 0.11 beta
- 22 April 2005
Geoff MerryweatherOdourVac ventilation systemsAuckland, New ZealandPh: 64-9-8375945, fax 64-9-8361096
CONTENTS
Version history:
- 0.1 - first release (1997)
- 0.11 - fixed error in noise ratings (22/4/05)
Introduction
The answers presented here are general by nature – ventilation and HVAC is a huge field that is far more complicated than I have presented here. There are many approximations and "rules of thumb" used which for most homeowners will be "good enough" as a guide. Code requirements vary from country to country and in many cases, from state to state. The Australian, New Zealand and US codes share common roots and have many common features, but there are still variation on the details. If it is important to meet any specific requirements, then better get specialist advice.
Most of the measurements in this document are metric. Sorry
to our US readers, but I have never been able to get my head around
the imperial system very well. A couple of approximate conversion
factors:
2 litre/ second approx = 1 CFM = 180m^3/hr
250 Pascals (Pa) = 1 inwg (inch water gauge)
Owing to the variation between cities, states and countries, I have to keep the advice fairly general, and you should check your local requirements. Similarly, I cannot really recommend specific brands or models, as what is available varies so widely, and distributors for specific markets repackage many of the fans.
Fans
Fans used for ventilation and air movement work by one of two different methods. The most common is the axial or propeller fan, similar to those used on stands for moving air in a room or used in computers for cooling power supplies. They can move large volumes of air at very low pressures.
The other type is a centrifugal unit, and these are similar to a centrifugal water pump. They have a plate with radial vanes and the air is forced out by the centrifugal force. These are either an in- line fan, where the air goes straight through parallel to the fan shaft, or the other type is the "squirrel cage" fan where air is drawn into the centre of the fan and exhausted through the side casing, perpendicular to the impeller axis. The latter is rarely used in domestic fans. They are often used on furnace blowers, and similar applications where bulk is not such a problem, and you need high pressures.
Axial fans are the most common, as they are the cheapest to produce and more compact. One disadvantage they have is that they cannot develop high pressures, with a limit around 60Pa-80pa for household sizes. Furthermore, if they are run with to little air, such as by using ducting that is undersized, then they become noisy as the air cavitates passing the blades.
Centrifugal fans with the same size inlets as axial fans can develop pressures up to 4 times that of an equivalent axial fan, however, they also usually cost a minimum of twice that of an equivalent axial fan. Fans can either be surface mounted (eg on a wall or window) or in-duct, where it is in the middle of a duct run.
Like all things, quality costs money. Good fans are quieter and better made, with ball bearings, and a tight fit between the fan blades and the housing (to prevent leakage past the blades). If you are using a window or wall fan with integral shutters, get one with silent opening. There is nothing more annoying than the loud CLACK of the shutters opening…
On a few fans, they will show a graph of airflow vs static pressure (back pressure, sort of). This means that if you have a certain pressure that the fan is pushing or pulling against, then you know the likely airflow rate. The problem is calculating this pressure, as installation can make a huge difference to the actual results. One thing that immediately springs to mind is the claimed airflow figures (moves up to XX l/s or YY CFM)…. They are taken in "free air" in a testing lab, and these results are very rarely seen in the field. Grilles, ducting, bends, kinks and constrictions all reduce the airflow down considerably. There are graphs of pressure losses through ducts and fittings, but these are usually copyright, so cannot be listed here.
In my experience of "ready to go" ducted fan kits, they very rarely exceed 2/3 of their rated maximum capacity, and often are less than half with a typical flexi-duct installation. The 100mm (4") axial fan kits used in many showers are a waste of time. With a maximum static pressure of ~20pa, they are lucky to be able to draw air through the grilles, let alone along the duct as well…
I have tested a leading brand 120mm axial fan with 2m of flexiduct, and while it had a claimed maximum of 36l/s, in these setup, which is typical of a household shower extraction, it only moved 11 l/s.
This depends on your building codes and any openable windows. If you have windows that open, this usually meets the legal requirements, so you are only interested in removing moisture. If you don’t have windows, check your code as to the minimum. The NZ and Australian codes say 25l/s per bath, shower or toilet in the room – this means 50l/s for a shower and toilet combo. This is a good starting point.
The other way of doing it is based on the room volume and is a useful double check. Measure the room volume (say 20 m^3) and the airflow of the fan (say 120 m^3/ hour). The number of times the room air is changed is 120/20 = 6 times per hour, which is about right in many cases. Choosing a fan that is to big means more noise and possibly drafts, but it is easier to fix than one that is to small, especially with undersized ducting as well.
Remember that air can’t get out unless it can get in. Undercut the bottom of the door by 15mm or so in order that the air can get into the room to replace what is taken out. You may even need a grille if you are taking a lot of air out.
An obvious point, but often overlooked when designing a house, is how to ventilate the internal rooms. Some building authorities still allow you to dump the air into the ceiling space, but this is gradually changing (and not before time). The moist air from your bathroom promotes rot, destroys the insulation and can warp your ceiling panels. In NZ the air has to be exhausted outside, and I urge you do the same, whether or not it is compulsory. This could go out through the roof via a pipe with an upside down U-bend or cowl on the top, or through the soffit or sidewall using a grille.
The exhaust point should not be close to a window or door where the air might be drawn inside again, nor should it blow over people passing by. Again, the building codes may have some minimum distances here. The Australian standard calls for 6m separataion from any inlets. This is not always possible in houses, so use your common sense. Also, check out Codecheck.com
I recommend that everyone use a run-on timer. This is a little box-o-tricks, often built into the fan, which lets the fan continue running for a set time after the power supply (eg the light switch) has been turned off. It should run long enough that the air in the room is changed at least once. The reason for doing this is that you finish your shower, dry off and rush out of the house to work, leaving the room closed up. Unless the air in the room is extracted, most of it will still be very moist, as the fan won’t have had a chance to clear the air before it was switched off.
The aim of locating the fan is to get the air to flow across the room, without taking a shortcut and leaving an area unventilated.. This means if there is a window, the air shouldn’t come in the window then straight to the fan without passing across the room first. It is really a matter of common sense. Put the intake as close to the problem as possible, which normally means over the top of the shower stall.
The usual materials for ducting are either a steel spiral sheetmetal rigid pipe or a flexible PVC or aluminium foil tube supported by spiral steel wire. The latter is easiest to install and is cheap, however it is very restrictive to airflow. The steel pipe requires special tools to be installed properly, which most homeowners or plumbers do not have.
The design, type and layout of the ducting chosen for the ventilation system has a large effect on the efficiency and noise of the system. The most common fault is to chose ducting which is undersized for the fan capacity, and this increases the noise of the system through the higher air velocity and also increases the friction in the system, which makes the fans work harder or require them to be larger than necessary.
The effect of grilles, bends, joints and size changes have to be taken into account when designing ducting systems. Standard packaged systems can vary greatly in performance when they are actually installed due to variations in technique. This is especially true of systems based on flexible ducting, where a single bend can effectively add 1-1.5 meters to the overall duct length.
PVC starts to become uneconomic on larger scale projects or above 150mm diameter sizes, and hence spiral steel or aluminium ducting is used. Recent developments include aluminium foil coated polystyrene, which is assembled and glued together on site. It’s main use is with air conditioning, as it is also makes a well insulated duct for carrying heated air, but it’s light weight, easy installation and low cost make it more likely to be used for general ventilation in larger projects.
The ducting should slope away from the fans, with no dips in the middle so condensation can build up.
A few things to watch when shopping for flexi-duct.
There are 2 main types available. The best sort is made from aluminium foil with the wire integrally moulded into the ducting. This is what the commercial air conditioning installers use. The other sort (the kind often used in household shower fan kits) is a lot cheaper, and consists of PVC film with the wire glued into place. There are two problems with this, the PVC is slightly permeable, which defeats the purpose of ducting the moisture outside, and more importantly, the glue fails over time. It usually takes 5 years or so, but there have been cases in where the glued ducting has had to be replaced in commercial air-conditioning systems after 5 years or so. It is probably less of a problem in household use, but be aware of it
Duct sizes should generally be kept as large as possible – a small difference in diameter makes a huge difference to the amount of restriction, as the back pressure from friction losses is inversely proportional to the power of 4. This means that halving the size of the duct increases the pressure lose by 32 times! A common fault is with heat and light combo fans, such as the IXL and HPM ones, is to reduce the duct from the 250mm spigot on the fan to a 150mm spigot.
Let us take the example of an inline fan in the ceiling:
If we assume that the peak pressure of the axial fans in a heat and light fan units is around 60Pa and a peak flow rate of 70l/s in free air with a more or less straight to circular curve between the points (see below). (or maybe from the manufacturer's data)
We can also make some other rule of thumb assumptions:
We have 2 bends, an exhaust grille, and an intake grille, as it is part of the fan and hence part of its quoted performance. The duct length is 3m
Our pressure loss is 15pa for each grille + ((3m+2*1.5m per bend)*2 Pa per metre)
= 30+ 12
=42Pa.
We know look up our fan curve for 42Pa static pressure and read off the amount of air being moved from the bottom axis. Now we all know that these assume that everything is installed perfectly and that that never happens. A reasonable figure would be around 80% of this calculated figure.
So what happens if we halve our duct size on the fan? If the intake is reduced as well, then less air gets to the tips of the fan blade, which is what does the work. Efficiency is reduced to around 60% or less, just by doing that.
On the exhaust side, halving the duct size means that our 12 Pa ducting losses become (1/0.5)^4*12 = 192Pa => no air is moved at all. In fact some air may move, but it is so small as to be irrelevant and ineffective
At the very minimum, the duct size should be at least a large as the size of the fan, i.e. a 150mm fan uses 150mm ducting. This is more important for axial fans, where it is the blade tips that do all the work. If the duct is smaller than the fan inlet, then or is not in line, then effectively the fan is unable to get any air to the fan tips, so it is only the inner section doing any work. If the exhaust duct is smaller than the fan outlet, then the air is likely to suffer from turbulence, and hence is less efficient.
Keep the use of flexible ducting to a minimum. It is very useful cheap and quick to install, but it is also a lot more restrictive than rigid pipe. For most home systems, use PVC pipe, as it is easier to install than steel and standard bends and fittings are easily available.
If you must use flexi-duct, then make sure you fully extend it and cut off any excess. If there is any excess left in the run, then it is likely to spring back, which reduces the inside diameter and hence increases the restriction. The manufacturers state that a 20% excess left in the duct run of flexiduct increases the air restriction by 3-7 times!
If the fan is in a wet area (eg shower), then you will need a 12v one with an isolating transformer.
Noise is a common problem with bathroom fans. You can have quiet, airflow or cheap – pick 2. Cheap fans are generally noisier than expensive ones, as well as moving less air, and the main difference is in the design of the fan blades and how much attention has been paid to their efficiency.
The noise comes directly from the fan as well as being radiated down the pipe. Rigid pipe, especially round sections, transmits noise more than flexi-duct. The flexiduct can vibrate and radiate the sound energy into the environment, such as the ceiling space. There are special duct materials that are made to muffle noise. They are usually fibreglass wrapped rigid or flexible ducting that works like a hot-rod’s glasspack muffler.
Sound power is measured in decibels, and an important thing to remember is that they are logarithmic, which means that they do not go up in a linear (straight-line) fashion, rather it is exponential. This means that a (more or less) 10db increase is twice as loud. A 2-3 dB difference is considered to be a just noticeable to most people, although this varies.
On most brochures, the makers include a noise level (say xx dB). There are a couple of things to watch if you want to compare different brands or models. The distance from the fan that the reading is taken makes a big difference. A reading taken at 1 metre is going to be louder than one at 3 metres. The quoted figures are measured in lab conditions and the surroundings make a big difference to the perceived level in your bathroom. Carpet on the floor and curtains help to reduce some of the reflections off the walls, and hence it appears quieter than a hard room.
In duct fans are usually quieter than wall or ceiling mounted fans, as the ceiling panel and insulation reduce the directly radiated noise, and the duct attenuates some of the noise (around 2dB per metre for typical household sizes). For this reason, if you have an in-duct fan, mount it towards the end of the duct, so it is far from the inlet over the shower to get the most effect. The fan should be no closer than 2 duct diameters from the end grille, in order to give the airflow a chance to straighten out and reduce turbulence losses.
Fan noise is largely dependent on the air velocity through the grilles, so most solutions involve reducing the air speed, and larger ducting and grilles, or multiple inlets usually do this.
Generally, the noise is also proportional to the amount of airflow and the fan speed, If you have problems with a noisy fan or are putting a system in:
1/ Mount the fan towards the end of the duct
2/ use larger ducting to reduce the air velocity, and hence the noise through fittings and grilles
3/ Use a larger fan but turn it slower. This can make a big difference, as the noise varies with the square of the fan tip speed – i.e. RPM. Most fans can be speed controlled, and the makers sell variable speed controllers for their fans.
4/ Use multiple intakes to reduce the air velocity through each grille
5/ If all else fails, try using "acoustic ducting" – you will have to try an air conditioning place for this. Try to find some where the inner core is not perforated into the fibreglass wool around it, so the moisture in the air cannot escape.