Five Things You Should Know Before Ducting Your MVHR System

Llyr Davies, Residential Business Development Manager

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Today’s market offers the self-builder multiple options for each renewable product they’re looking to install in their dream home, and this certainly applies to Mechanical Ventilation with Heat Recovery (MVHR) which is the most popular method of low-energy whole home ventilation for new build properties. A key part of any MVHR system is the ducting that is used, and as always, there are several options to consider and regulations to meet. There are many measures outlined in the building regulations that need to be met to ensure your installation is compliant, ranging from the size of the ducting used, insulation thickness and whether it’s rigid or flexible.

1. Insulation

One key measure is the need for the ducting located in a cold area (e.g. loft space) to be insulated. The insulation needs to be the equivalent of 25mm thick and have a thermal conductivity of 0.04W/(m.k) or less. This ensures that condensation doesn’t form due to the difference in air temperature inside the duct when compared to the air temperature in the cold space. If you don’t have a loft space there is still a need for insulated ducting on the intake and exhaust legs, as they will be carrying cold air in a warm space. The level of insulation can be achieved by wrapping PVC duct in a layer of rockwool for example, or by using a one-piece, insulated duct such as Nuaire’s thermal ducting.

2. Size of Duct

The two most common sizes used are 125mm diameter circular and 204mm x 60mm rectangular duct. You may choose between these two sizes to suit your build, whether you have a shallow service void where the rectangular duct would be best, or if you’re constructing using webbed joists where the circular duct may be most suitable. However, it is also important to remember that if you have a large volume of air being distributed around your home, a larger size duct may be utilized to keep in-duct air velocities as low as possible. This helps keeps the pressure resistance lower and your MVHR unit running more quietly.

3. Type of Duct

The majority of fully installed systems in the UK would comprise of PVC ducting with a certain amount of insulated wrapping. But there are several new products on the market that make the humble PVC ducting even easier to install and more efficient overall: products such as self-sealing connectors, which negate the need to manually tape and seal each connection whilst also achieving a very good level of air tightness.

4. Branched or Radial

A branched ducting system would have a main duct leg with T-pieces located along its length to branch off to each room that requires ventilation. This is a widely used method of supplying and extracting the air around a house. You can incorporate different sizes of ducting in the system as well as having silencers installed where desired. A radial ducting system would involve having two manifold boxes connected back to the MVHR unit, one for extract and one for supply. From these manifolds, a semi-rigid length of 75mm circular duct is extended to each room to provide adequate ventilation. This method is becoming more popular as it allows a certain amount of flexibility when it comes to the install stage.

5. Steer Clear of Flexible Duct

Building regulations will only allow a maximum length of 300mm of flexible ducting to be used to overcome obstacles. Although it may seem a quick and easy way of running the ducting throughout your build, there are many drawbacks, not least its non-compliance. Flexible ducting will increase the resistive pressure applied to your MVHR unit when compared with rigid duct. This in turn can have a negative effect on the unit’s performance by increasing the fan speed and noise, whilst lowering its heat recovery efficiency.

 

If you're building your own home and would benefit from MVHR and ducting advice tailored to your build, please contact Nuaire directly on 02920 858500 or check out our MVHR Design Service here.

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