Ventilation System



Few UK residential properties have anything that might be regarded as a proper ventilation system. Typically there is a reliance on extractor fans in bathrooms and kitchens coupled with trickle vents in window frames and an expectation that occupants will open windows if extra ventilation is necessary. In general – especially when there is little wind – this results in inadequate levels of ventilation.

For any building approaching Passivhaus levels of air tightness it’s necessary to have a robust ventilation solution and in most cases that means Mechanical Ventilation with Heat Recovery, generally abbreviated to MVHR. (In fact, the main reason for the high level of air tightness in a Passivhaus is to ensure the ventilation air goes through the heat recovery unit rather than simply leaking away.)

There was never any question that Marsh Flatts Farm would have an MVHR system but some different options were considered in relation to the ducting and the intake air defrosting solution.

Main Pros and Cons of MVHR Systems


  • Effective, controllable ventilation that isn’t reliant on weather conditions
  • Heat recovery means that little energy is required to ensure incoming air is as warm as outgoing air
  • No need to open windows for ventilation (especially beneficial in noisy locations)
  • Incoming air is filtered, removing insects and particulate pollutants such as pollen


  • Running costs are non-trivial (but more than offset by savings in heating costs)
  • Noise generated by the system can be an issue if not properly designed, installed and commissioned (but compare with the noise of standalone extractor fans)
  • Air must be allowed to flow from Supply rooms (bedrooms, living rooms) to Extract rooms (bathrooms, kitchens) which typically means having sizeable (15 mm) gaps under doors, which can lead to more sound transmission between rooms


MVHR System As-Installed


The MVHR unit is a PAUL Novus 450 supplied by the Green Building Store. This is a relatively high-throughput unit (the 450 refers to a maximum capacity of 450 m^3 of air per hour) and hence physically quite large.

PAUL is a well-respected brand in the MVHR industry and this is a high-end unit with a price tag to match however it’s both quiet and efficient – both in terms of energy consumption and heat-recovery efficiency.


The internal ductwork is mostly white plastic semi-rigid tubing, with ducts running back to manifolds in the Plant Room.

The ductwork was originally specified as circular galvanised steel ducting but during construction it was evident there was not enough space in the floor voids to accommodate the inter-room silencers so it had to be re-designed.


MVHR systems don’t need much fine-tuning and many experts recommend simply leaving them on Fan Speed 2 all the time but there are a couple of cases where adjustments might be beneficial:

  • Decreasing the ventilation rate when the property is unoccupied, to reduce running costs.
  • Increasing the ventilation rate to clear humidity or smells more quickly.

In keeping with the general theme of avoiding lots of different switch buttons for different systems all around the house I opted not to install any MVHR “boost” switches and there is just the one control panel, adjacent to the MVHR unit in the Plant Room. The PAUL Novus units don’t offer much scope for integration with other control systems and the only option is to give it a 0-10V control signal to adjust the fan settings. This is intended for use with CO2 or humidity monitors but I have it hooked up to a DALI – to – 0-10V adaptor which makes it controllable using the KNX control protocol via the lighting systems’ DALI – to – KNX bridge. Effectively this looks like a dimmable light, with 0% dimming equating to Fan Speed 1 and 100% dimming equating to Fan Speed 3.

Various automated control approaches are therefore possible: reducing to Fan Speed 1 when the property is unoccupied (e.g. the intruder alarm is set); boosting the ventilation for a set period when one of the bathroom PIR sensors is activated; simple timed schedules etc.


Somewhat annoyingly and despite them evidently having relatively sophisticated electronic control systems the PAUL Novus units are effectively “closed” from an operational monitoring standpoint, with seemingly no way to access the temperature and other data they display on their proprietary control panel. As a result, the only option is to install a separate set of monitoring probes. See this page for more details.

If anyone from PAUL is reading this – you need to follow what companies like NIBE and SolarEdge are doing and at least provide an option to upload data to the Internet.


Operational Experience

Running Costs

The ongoing costs are split about 50:50 between electricity and replacement filters.


At Fan Speed 2, the unit is using 65 kWh of electricity a month – i.e. about £10. Initially this seemed a lot – after being used to paying almost nothing for ventilation – but it equates to about 80 W which doesn’t seem bad for what it does.

(TODO: Check how this varies at other fan speeds.)


The filters need changing periodically. Exactly how often depends on how dirty the incoming air is and what volume of air is passing through the filters. After 4 months the filters appear very dirty but there’s no apparent increase in electricity consumption which would be the main indication of the fans having to work harder to maintain the specified airflow through blocked filters so they might survive longer – perhaps 6 months.


In general the system is very quiet and less intrusive than standalone extractor fans. In particular, there is no discernible noise from the room air terminals. There is quite a bit of noise in the Plant Room (something like 50 dB) which mostly seems to come from the bends in the ducts rather than from the unit itself. Some of this noise transmits to adjacent rooms and it’s just audible in Bedroom 2 (immediately below the Plant Room) at Fan Speed 2 or above.


None to date.


Hints and Tips

  1. Use an MVHR specialist to design and commission the ventilation installation
    • I used the Green Building Store and thoroughly recommend them
  2. Locate the MVHR unit as far away from bedrooms as possible, to reduce the impact of any noise from the unit itself
    • If installed in a loft space, aim to place it above a landing or bathroom rather than above a bedroom
  3. Install a sub-meter on the electricity supply to the MVHR unit
    • One of the small meters that looks a bit like an MCB and mounts on the DIN rail in a consumer unit is fine – these are only about £10
  4. Consider installing temperature sensors on the MVHR ducts to monitor how the system is performing
    • Makes it easy to check that the automated by-pass is working correctly, for example
  5. Consider locating the external air intake on an east-facing wall so the sun will help to pre-heat incoming air on cold mornings

CC BY-SA 4.0 Ventilation System by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.