This post has been updated since originally posted. (update 23 Sept 2018)
Going back to first principles of a passive house, two of the five standards to achieve certification relate to the temperature and quality of air inside the house:
- Airtightness of a maximum of 0.6 air changes per hour at 50 Pascals pressure (as verified with an onsite pressure test in both pressurised and depressurised states)
- Thermal comfort must be met for all living areas year-round with not more than 10% of the hours in any given year over 25 degrees C
Passive houses typically use a mechanical heat recovery and ventilation system to achieve this. (This system is abbreviated to either MHVR or MVHR – the terms are interchangeable). The MVHR is a sophisticated piece of kit, comprising ducted piping into most rooms, an external input vent, an external output vent, and a big heat exchanger transfer unit that does the heavy lifting and resides inside a cupboard in the house. The ducted piping varies in purpose, with some pipes taking air in (these are sited in the heat generating areas like the kitchen, laundry and bathrooms), and some distributing air out (located in the living rooms and bedrooms). The ducting ends in the ceiling of each room with a discrete vent/inlet.
[diagram source: International Passive House Association]
In summary the MVHR gathers warmed air from the kitchen, laundry and bathrooms, and takes it back to the heat exchange unit. Simultaneously it brings fresh air in from the outside which it passes through a filter to remove insects, dust and pollens/pollutants. The MVHR then passes the incoming air against the warm air that has been gathered from within the house, passing 80-90% of the heat from the outgoing air to the incoming air. It then releases the “old” air to the outside.
The MVHR is quite sophisticated – it monitors internal and external temperatures and transfers only the amount of heat needed to maintain the internal comfort levels within the target range of 20-22 degrees, and it only carries out as many air changes as are needed to do this.
The ducting is pretty special too as it is made so that there is no audible noise coming from the air movement which is taking place constantly, and there is minimal heat loss or accumulation.
We chose to install the unit offered by Wolf, a German manufacturer. The choice of MVHR is informed by the data from the PHPP analysis, and this analysis also determines the settings to be used once the system is up and running, which will be specific to the volume of air held by the house and the way the house is oriented, shaded and other design aspects. (All of these factors are used in PHPP to determine if the house will achieve passive certification)
The ducting is installed within the thermal envelope in the services cavity between the insulated panels and the internal Gib lining. We chose ceiling inlet/vents, but it is possible to have these in the wall with a decorative cover. Each of our bathrooms, the WC, the laundry and the kitchen have inlets to gather the warm air. We have outlet vents in the bedrooms and living rooms. The Wolf unit is located in a cupboard on the ground floor and our external fresh air vent was initially on the West elevation, and the exhaust vent originally on the South. We would probably advise against having the fresh air vent on the North or West as in the heat of the summer, these elevations are hotter than the others and as heat rises our fresh air vent brings in air that is probably a few degrees warmer than if we were collecting it on the South elevation. It’s only a few degrees, but on the very hot days this has had an impact. Furthermore, the vent on the west elevation is located directly above the corrugated iron roof of the kitchen, and so the air in that area gets quite hot in summer.
In September 2018 we decided to switch these vents around. It was a simple process of swapping over the ducting on the top of the unit. One advantage to this switch was the ability to remove the U-bend in the ducting that had been present since the beginning. The left hand photo below shows the ducting as it was originally installed, and on the right shows it as it is now. Tight bends in ducting can lead to friction within the duct and therefore can cause air flow noise.
The Wolf system has a display control, which we have in the living area, which displays the internal temperature and provides alerts when filters need to be cleaned or changed or if any other maintenance is required. The control panel also allows settings to be changed. Upstairs we have a simple dial with three settings – we run the system on the middle setting most days, but if we want to have it run on boost (i.e. it moves a bigger volume, so changes the air more often) we move it to setting #3, and if we are away we move it to setting #1 and it simply ticks over. Reasons you might want to run “boost” is if internal heat levels are rising for example if you are having a party and have lots of people creating body/breath heat, or your teenager is spending a particularly long time in the shower.
We used the boost setting overnight during the hottest part of the summer, and in this way the system “dumped” internal heat outside by not transferring the heat to the incoming, cooler, fresh air.
The air circulation provided by the MVHR has an added bonus of removing excess humidity. This is particularly useful for bathrooms where we don’t have any form of extraction fan and the MVHR system takes the steam away under the standard #2 setting for all but the hottest and longest showers and baths! (Of course the heat from this steam is recovered and transferred to the incoming air for circulation into the rest of the house.) The removal of moisture is important in a home that is very well insulated and airtight as otherwise the house would retain both heat and moisture from human breath and cooking and clothes drying, and with nowhere to go this moisture would cause mould. The MVHR in combination with the triple glazing means that the windows do not get condensation on the inside either.
We did choose to include a 1kW heat bar in the machine itself and this has been triggered by the Wolf unit if it detected that a small heat boost was needed. We used this at the very start of living in the house when it was the end of winter and the house itself had not achieved ambient temperature over 20 degrees. It is important to remember that a passive house retains its temperature by ensuring that every single object inside the house (walls, ceilings, floors, furniture, fittings etc etc) are all at a minimum temperature, and so for the first few months after moving in everything is still being brought up to that temperature.
We also used the heater during the winter but found the results to be very disappointing; very little impact on heat levels and huge impact on power bills!! When we swapped the in/out vent ducting we took the opportunity to remove this unit. The photos below show what it is like:
Positive benefits we have found with the system other than the temperature control aspect include a reduction in hay fever symptoms for all family members and odour removal. In fact, we often vacuum the filters (we will change them annually) and we are constantly surprised by the extent of dust, pollution and insects that have been prevented from coming into the house! We have a smoker barbecue and it is located on the west side of the house about 10m from the original position of the inlet vent. One time we forgot to turn off the MHRV system when we used it, and after 10 hours of wood smoke burning nearby the filters looked like this (however, the unit did it’s job as we had no odour or smoke inside the house!):
When we did the vent switch in September 2018 we also removed the heater unit and this meant that we reconfigured the ducting within the cupboard. The photo above already shows how more efficiently the ducting now is, but one thing that did surprise us, in a disappointing way, was the discovery of installation damage in one of the ducts. The damage resulted in a compression of the ductwork that restricted the width of the duct by 2/3! No wonder we could hear some air flow noise in some rooms, and no doubt this had impacted on performance. Let this be a lesson to anyone looking to install one of these to check that the installation has been done well!