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Split the difference


Decentralised ventilation systems, that is, ventilation that is split into multiple in-room fans, designed to deliver the same outcome as a traditional centralised system, are, for a number of reasons, an interesting solution to a number of problems.


There is no escaping from the fact that a centralised extract system or MVHR system presents some challenges, particularly in retrofit. Here are a few.

  • Ducts to and from fans have a cost. Requires some skill to install correctly and is often where mistakes are made.
  • Not all buildings are ideally suited to duct runs, and it can be a design and installation challenge to overcome sometimes.
  • Particularly in retrofit scenarios where routing through an existing home can be challenging.


In a retrofit, let’s face it; ventilation is one of the few activities where we have to get involved in the internal workings of someone’s home and the thought of running ducts down the back of cupboards or boxing ducts in when we could just drill a couple of holes and chuck a fan in, can seem a step too far.

There is also a dilemma in new homes, where we have accidentally ended up with a more airtight building than anticipated, and we now have to change from natural ventilation to something more robust.

Whether it is Continuous Mechanical Extract Ventilation (MEV) or Mechanical Ventilation with Heat Recovery (MVHR), the decentralised cousins of these approaches offer some attractive benefits.

But as with these things, we have to be careful. 


Whole House Ventilation Rates and noise


It doesn’t matter if you are in Scotland, England, Ireland or Wales. All regulations require a certain amount of background ventilation to deal with day-to-day activities and manage pollution in the home. These typically are calculated based on floor area and bedroom/occupancy profiles.

The reason for this is by taking the larger of the two answers to these calculations, you cover both the production of pollutants related to the surface area of the building and the activities of the people who reside in the home.


What needs to be understood is the impact this calculation can have on the requirements you place on a fan.

If I have a house and a requirement to deliver 30 Litres per second (L/s) of ventilation to meet the whole house ventilation rates. The load I’m asking a fan to cope with will depend on how many fans can share that load.

If I have one central fan, it needs to deliver 30 L/s. This is normally fine because big central fans are well capable of doing this quietly, especially tucked away somewhere like a cupboard or attic.

If I have a home with lots of wet rooms. Let’s say a Kitchen, Utility, Bathroom and En Suite and even a W.C. (this is common in new homes). Then I can split this 30 L/s by 5 times. That is a fairly modest 6 L/s per fan now.

Now the only thing I’m weighing up is the cost difference between centralised and decentralised if the product I choose can perform well at those levels and any boost rates I ask.

But it is also very common, particularly in retrofit,  to find just a couple of wet rooms. A bathroom and a kitchen.

I am now asking something very different of those fans. 15 L/s is a very different prospect for these types of systems.

That’s not impossible, but serious thought needs to be given to the performance we ask from these systems and what they are realistically capable of delivering.


Back pressure and how a fan behaves


Another interesting effect to consider is how decentralised fans react to back pressure and particularly variable back pressure. (wind)

Its not that one mode is better or worse than another, it’s just something that needs to be understood. If a fan is a constant volume fan. If it will adjust its speed to compensate for variable back pressure, then you can have an acoustic effect of the fan whirring up and down to compensate which can cause some irritation in some circumstances. If it’s a fixed fan speed then back pressure from wind will have an effect on performance.

The same is true in larger central fans as well, of course, but the impact is less felt because of the pressure they run at and the distance they are from the occupied spaces.

Another all to common occurrence, and nothing to do with the fan itself is a misunderstanding of what type of system it is, leading to a miss specification of things like air inlets in the same room as the extract causing short-circuiting.

Unfortunately, what often happens, is a specification of system type. As long as I say put that type of system in that type of room, everything else will fall into place.

Unless thought is given to the desired outcome, particularly noise and air performance, then what we see, unfortunately, is fans installed that never stood a chance of delivering what was required.

The same applies to Single Room Heat Recovery when used in a push-pull format. We ask fans to often move amounts of air that put them under stress or run them at noise levels that will cause a problem.

More so with these types of systems, sound levels are critical as they reside in quiet areas like bedrooms and have a variable sound profile which makes them more noticeable.

So when considering whether to centralise or decentralise, a keen focus on the desired outcome is required before deciding. And always just a case of choosing between the hassle of ducting or just drilling a couple of holes.


Author: Vince House

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