Don’t overlook these three things when specifying MVHR systems

19 July 2023 by Jason Quinn

We’re having some robust conversations about MVHR specification and installation as more homes seek Passive House certification and some designers focus on keeping costs as low as possible. 

It’s an interesting time right now because we have more data and anecdotal insights. The rate of design and construction of Passive House projects is on a steep upward trajectory in Aotearoa and there are more mechanical ventilation options compared to five and 10 years ago. We’re also learning from multi-occupancy Passive House apartments. The years of Covid have heightened the general public’s awareness of indoor air quality and CO2 levels, in part as a proxy for assessing the risk of viral infection.

The cost of keeping a space well-ventilated and at a comfortable temperature is, in most instances in New Zealand, horrendously high. That’s why people generally don’t enjoy both of these necessities, even well-off people who can pay hundreds or thousands of dollars a month on central heating in winter. 

A MVHR in a Passive House building makes it affordable to do both. However ensuring CO2 levels remain below 1000 ppm will have an impact on heating demand and cost more than prioritising only energy efficiency. 

How people live in the house (and most crucially, how many people live there) will have a material effect but I expect in almost all cases higher flow rates will be required than the PHPP ventilation defaults to stay below optimal CO2 upper limits.

To give an idea of flowrates, about 15 L/s per person is required to achieve 800 ppm and about  10 L/s/person to stay below 1000 ppm. This assumes no ventilation from an open door etc. The figure below graphs average room CO2 levels at various fresh air supply rates in an example office with 20 people. (Note how without supply air, CO2 literally goes off the chart in two hours.)

Graph: CIBSE

In a residential setting, two adults and a large dog sleeping in a bedroom with the door closed might need ~25 L/s (90m3/hr) to stay around 1000 ppm overnight.

Until recently, certified Passive House homes in Aotearoa New Zealand have had large footprints and a small number of people living in them; low occupancy rates provide a fat buffer for CO2 levels. More care is needed when modelling small apartments and social housing, where occupancy rates are likely to be considerably higher (officially or unofficially).

Skimping on MVHR can be costly

This leads to my second point, about the implications of cost-cutting on ventilation systems. Yes, it’s important to reduce the extra costs involved in building to Passive House standard, so all the benefits they offer are available to more people and especially those most vulnerable to fuel poverty and respiratory illnesses.

In a couple of instances, Passive House designers have tried to save money on ventilation by going with a non-packaged system, typically buying a unit from a new entrant and flex duct off the shelf. In some instances I have talked people out of going down this route by presenting the information that follows. In other cases, I had to advise that their project would fail certification. In all cases, the designer upgraded their MVHR specification.

Cobbling together components is not an apples-to-apples comparison with a certified MVHR system that drops in as a complete assembly and uses semi-rigid ductwork. These two things are not equal. As certifiers, we require a designer to demonstrate that their lower cost solution hits the same quality metrics. That includes energy efficiency of the motor, durability and acoustic levels. Beware the unintended consequences that take the shine off the initial cost savings.

A. Ducting

Flex duct is, to put it bluntly, a problem. In our experience, it doesn’t last long, you can’t clean it and it moves around during testing making commissioning challenging. Commissioning involves measuring the air flow at each value (with the internal doors shut) and making any necessary adjustments to balance supply and extract air. Flex duct is prone to moving around as the flow rate is adjusted, changing the volume inside the ducting. You can bend the proper semi-rigid stuff and the volume stays the same; the radius is quite smooth. If you bend flex duct the pressure drop will be high, which means the amount of air that flows through the duct will be drastically reduced.

I heard about a case in Australia where it took two engineers a day and a half to commission a system that used flex duct. At their hourly rates, I think the initial cost savings evaporated before the system was even running. Flex duct systems also tend to be quite leaky; and when some amount of air is or maybe isn’t leaking from the ducting, it’s difficult to impossible to calculate what rooms are getting what amount of fresh air. Duct leakage testing is a whole industry in the US so when folks use flex duct and don’t do duct leakage testing, that’s a red flag for us as certifiers.

B. Bypass function

Next, does the cheaper unit actually include a genuine bypass? This is a mechanical valve that bypasses the heat exchange core in the summer to reduce overheating. Cheaper ones just turn off the supply fan, potentially creating issues with combustion devices and carbon monoxide safety. Outside air will be pulled in through the building fabric at points of least resistance so it’s unlikely all rooms will have fresh air.

C. Energy efficiency

You want to be sure your client gets what they paid for. Does the specified system provide measured efficiency data by a third party, including electrical efficiency data? Many MVHR (even ones made overseas) have no measured data. They just use the core heat exchanger data and guesstimate which means that the performance can be MUCH lower than what it says on the marketing literature.

D. Installation

The details of how a MVHR system is installed are critical—a BRANZ study back in 2020 found only 40% efficiency of a local system in part due to poor install details. If the transfer ducts and heat exchange unit aren’t wholly contained within the thermal envelope, heat losses can be substantial. One easy fix they identified was reducing the number of bends in the flex duct, which reduced pressure drop and increased efficiency.

E. Other requirements

Passive House certification sets an upper maximum decibel reading at each extract vent. These need to be measured at commissioning. We expect certified systems that have been competently installed to easily meet this requirement.

Install and clean cone filters on extract vents

Thirdly, cone filters should also be fitted on every extract vent. Zehnder, to its credit, supplies these as standard with its systems, but I’ve walked into finished Passive House homes to find the cone filters still boxed on top of the MVHR unit. More education of installers and homeowners is needed. Other suppliers don’t even have cone filters available for sale; this needs to change.

Pictures are worth a thousand words and I’m hoping this will convince those Passive House designers who seem sceptical about the value of filters or who laugh when I tell them they have to be cleaned and replaced regularly. Here’s a cone filter from an extract vent in a certified Passive House home, not checked for a year.

Photo: Sustainable Engineering Ltd

The good news is these filters can be emptied, washed and reused—more frequently than this one pictured. Monthly is a good idea. Weekly for the first month in a brand new Passive House building, as lots of construction dust is pulled into the system. 

The main filters in the units themselves need to be changed on schedule; in the photo below we have a brand new filter next to the six-month-old one it is replacing.

Photo: Sustainable Engineering Ltd

Best practice in a just-completed building would see the filter inspected weekly. Tap out any large particles; it can also be vacuumed. Construction dust will end its lifespan early, count on replacing the filter at the three-month mark. It depends on the level of dust at the time the MVHR was turned on.