Two Rooms - One Fan?

A question that we are often asked is whether it is possible to use an inline fan to extract across more than one room? Inline fans are absolutely suitable for this but there are a number of points that are worth noting.

Firstly, sharing an inline fan across multiple rooms will affect the extraction rate from each space. Essentially, you will be dividing the total airflow by the number of extraction points attached to the ducting. As an example, If you were to install the Tornado TT100PROT across two extraction points, instead of a single 300m3/h extraction rate, each of the two extraction points will be pulling at around 150m3/h. 

Again, this is an approximation and it is worth noting that differing ducting distances can favour the shorter run, giving a weaker extraction rate from the extraction point situated further away from the fan unit. 

You can combat the extraction drop by installing a larger fan - for example, if we swap out the TT100PROT (a 4”/100mm unit) for a TT150PROT (a 6”/150mm unit) the 300m3/h will jump to 576m3/h which will give 288m3/h across both extraction points. However, you will need to bear in mind that the exhaust side of the fan will require 6”/150mm ducting and exit grille, and although you can still use 4”/100mm ducting and internal grilles from the spaces you are extracting from, when these two runs are combined the ducting will need to return to 6”/150mm to avoid putting the fan unit under undue pressure of pulling a larger volume of air through a restricted ducting diameter. (Fig. 2 in the image below)

This leads us to the biggest question - the wiring!

DISCLAIMER - We are not in a position to give detailed advice on wiring from a legal and health & safety position and always suggest that any installation requiring electrical work is undertaken by a qualified electrician.

However, we can explain the principles to help you make an informed decision on both the feasibility of your installation as well as the equipment and process needed so that you can make an informed decision as well as finding the correct items to order.

***The images used in the following text are for illustration purposes only and are not intended to be used as installation or wiring instructions but simply as a visual aid to help explain the principles being put forward.***

The issue is simple. If you have a single inline fan in your loft servicing multiple rooms - which room will trigger the fan? And if both rooms can trigger the fan but you want control via the light switches - how do you wire the rooms so that you are not affecting the lights in both rooms at the same time from one light switch?

For the example below we will use an installation for a basic fan without a timer and therefore utilising only a switched live connection, but the principle is the same for a fan that also requires a constant live.
 

Looking at a traditional single room inline fan installation such as in the image above, when the light switch is activated, power is sent to both the light and the fan simultaneously on the same circuit. 

If you now include a second room and attach the light switch and circuit from this room as well (see above), then regardless of which light switch is turned on, the lights will also turn on in the second room as well. Note in the image above that the light switch in the room on the right is still in the off position, but because the wiring to the fan is shared, this circuit is now also live and will turn the light on in this room as well regardless.

Ideally, we would want the lights to be independent but both switches still operate the fan. To do this, your electrician will need to install double pole light switches. A double pole switch allows you to control two independent circuits at the same time - allowing each light switch to send independent power to the lights in the individual rooms as well as power to the fan unit.

This now allows for the lights in each room to be independent of the other:

While the lights are independent, it is worth noting that the fan will run and extract from both rooms, regardless of which light is on, or if both are on, and will not turn off until both lights are turned off.

If you require double pole switches we have options for a pull cord and standard wall mounted switches on site and you can find T and Y duct connectors for both 4”/100mm and 6”/150mm ducting in the accessories sections.

There is another option you can look at for a two-room one fan installation which bypasses all the complex switch wiring. Continuous running MEV units can duct from multiple rooms and choosing a model with a built-in humidistat will allow the unit to boost automatically when required. You can still choose to install a master boost or purge switch (some models support wireless boost options) if you want to be able to manually boost at any point, but with the continuous trickle ventilation and control of humidity levels, MEVs are a far simpler and less invasive installation option.

So, armed with this new information hopefully this will help you decide if one of these methods will suit your installation needs.