Automated Home 2.0 – #15 The Mechanical Ventilation Heat Recovery (MVHR) System


Our mechanical first fix is almost complete so it’s a good time to look at our MVHR system.

Drimaster PIV

When spec’ing our first self-build in the late 1990’s we added this Drimaster PIV unit (Positive Input Ventilation).

This sent a low pressure feed from a vent in our hall ceiling, driving moist air out of the house and doing away with the requirement for trickle vents on all our windows and extractor fans in the wet rooms. Although it worked well, the down side was that as it pushed out our warm heated air, it drew in cold air from outside to replace it.

We knew we wanted a more efficient system in the new house and so we looked around for a company with expertise in low energy heating. We were lucky to find an expert right on our doorstep and back in February of 2017 we had our first meeting with Daly Renewables.

Daly’s describe themselves as “an engineer-led company committed to designing low energy efficient renewable energy systems”. We liked the fact that they are, as they put it, “a Single Point Responsibility”. We weren’t talking to a sales team either, but the engineer that would be designing our system.

An MVHR is effective in homes with an airtightness rating of around a 5 or lower but what exactly is it? Over to Ryan for more of an explanation…

Old School

The minimum form of ventilation currently required to satisfy Building Control is a combination of trickle vents and extract fans. A trickle vent is the most basic form of ventilation and consists of a slot opening in a window which can be manually opened and closed to let fresh air straight in from outside. This is uncontrollable and the problem with these arise during a cold windy night when you get a cold draught from the vent entering the room, which affects the comfort of the occupants. Combined with extract fans in bathrooms, it begs the question – why insulate your home so well and leave what are effectively holes in your walls and windows?

When we tighten a house to reduce energy consumption, we also risk making the building envelope vulnerable to moisture problems, including mould (a major health issue) and rot. Excess indoor humidity can also increase dust mites and other allergens, damage electronic equipment and accelerate the outgassing of formaldehyde from common building materials and furnishings. If the house is in an area prone to ground radon, then radon accumulations can also increase in a tight house that is not properly ventilated (though separate radon mitigation is more effective at reducing this risk).  A tight enclosure with mechanical ventilation will almost always provide better indoor air quality with lower energy bills and more comfort than a leaky house depending on the house to “breathe”.  Houses don’t breathe – people breathe and they do it 24/7 year around. An important aspect of thermal comfort is humidity levels and directly related air quality.

Quite simply put- the better insulated and sealed a house is, the more important a controlled form of ventilation becomes.  

Ryan Daly

A Better Way

Vent-Axia High Flow MVHR Unit

The most efficient and effective type of ventilation is mechanical ventilation with heat recovery. In this system a pre-calculated flow of air is extracted from wet rooms, removing moisture, odours and VOC (found in paints, adhesives etc).

A supply of air is accurately placed into habitable rooms, ie each and every bedroom and living areas. A high efficiency heat exchanger recovers up to 90% of the heat energy in the extracted air into the incoming filtered fresh air.

We have fitted this Vent-Axia Sentinel Kinetic High Flow unit into the new Automated Home. Once it’s setup it requires little or no adjustment with an automatic summer bypass and a built-in humidity sensor that boosts the fan speed when it detects an increase in moisture (for example when taking a shower).

Vent-Axia MVHR in the Automated Home

To it we have connected the Hybalans Ducting System and this routes the supply, as well as the discharge of ventilating air with the highest possible energy saving and maximum convenience.

Early planning is really beneficial here, so ensuring your Architect, Mechanical Expert and Builder all know the site for the MVHR and the routes your ducting will take.

It consists of 2 manifolds, 1 for Supply and 1 for Extract. The ducting is 92mm Semi Rigid Plastic which runs to each individual room. The main advantage of this system is that it is very easy to balance, especially compared to a traditional teed off system which can be very hard to get the air flows correct on. The air quantities per air valve are simply centrally adjusted in the air distribution boxes. Adjustment of the air valves is unnecessary.

MVHR Ducting

Due to low resistance losses, the working point and fan motor rpm of the HRV unit are reduced. This creates an extra reduction of the system noise.

We carefully select the MHRV unit to ensure that the operating capacity of the System is in or around 50%, or less where possible.

For several years we have been installing HRV into an increasing number of homes, with this system having several benefits over others:

  • Negates the need for trickle vents and extractor fans.
  • System sized to provide 0.75 Ach (Air Changes per Hour).
  • Double skinned steel unit with 25mm Insulation surrounding- ultra quiet in operation.
  • Counterflow Heat Exchanger – up to 90% heat recovery.
  • Hybalans Radial ducting system designed to negate crosstalk between adjoining rooms.
  • Automatic Summer Bypass.
  • Energy saving EC fans (electronic commutation technology).
  • Controller should provide real time adjustment to fan speeds, required air temperature and a pre programmable boost facility.
  • Automatic boost on unit with built in humidity sensor.

Next Time

In a future post Ryan will take us through the heating system we’ve chosen. In the meantime you can read all the blog posts on the Automated Home 2.0 and follow along on our Instagram. : Listen to this Podcast on MVHR Design

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