Plant Room – MVHR


The Mechanical Ventilation with Heat Recovery (MVHR) system generally needs little attention. These notes explain how the system is configured and what maintenance is required.

Configuration Notes

The MVHR unit is a PAUL Novus 450 installed in the Plant Room. It is managed using the touch-screen controller mounted in the cupboard in the Utility Room (where any warning messages are slightly more likely to be noticed).

Normally the unit should be left in ‘Auto’ mode which means that the fan speed is controlled by the Home Automation system (the ‘MVHR Fan Boost’ setting):

  • A boost level of 0 corresponds to Fan Speed 1
  • A boot level of 50 corresponds to Fan Speed 2
  • A boost level of 100 corresponds to Fan Speed 3

The Building Regulations requirements for ventilation in each bathroom are met at Fan Speed 2 but unless all bathrooms are in use this delivers excessive ventilation and Fan Speed 1 is often adequate, although CO2 levels in bedrooms will exceed 1000 ppm by the end of the night at this setting. If required, a short boost at a higher speed will help to clear steam or smells.

When the unit was new it was fitted with a ‘standard’ heat exchanger but this was replaced with an ‘enthalpy’ heat exchanger in July 2018:

  • The ‘standard’ heat exchanger allows heat to flow from the incoming to the outgoing air but it blocks any transfer of moisture. As a result, when the incoming air is dry (such as in the winter) the ventilation will tend to result in low humidity inside the house – especially if there are only a few occupants.
  • The ‘enthalpy’ heat exchanger allows the transfer of both heat and moisture, so it slightly helps to maintain healthy levels of humidity inside the house.

The original heat exchanger is in a box in the Plant Room and can be reinstated if the humidity preservation by the ‘enthalpy’ heat exchanger becomes a problem.

Regular Mainternance

Filter Change

The main maintenance task is the filter change, which needs to be done about every 6 months. 1st April and 1st October works well; harvest time can be dusty so it’s best to change the filters relatively soon after harvest has been completed. A quick vacuum clean of the filters at the half-way point doesn’t hurt.

There is an F7-grade filter on the air intake from outside and an F4-grade filter on the air extract from the house (which keeps dust off the heat exchanger). These are available as a paired set. While there is a smaller (Novus 300) and larger (Novus 450) model in the range, they all take the same-sized filters.

After the filters have been changed the control panel needs be used to reset the filter change indicator. (TODO: Add notes / photos)

The best place to buy replacement filters is “21 Degrees” (what used to be the Green Building Store): Filter for Paul Novus MVHR and the cheapest option is to buy sets of 10 filters of each grade – i.e. 5 years’ worth.

Heat Exchanger Cleaning

While not strictly necessary if filters have been changed on a regular basis, the heat exchanger should be washed every five years or so.

Miscellaneous Notes

Fan Bearing Replacement

Some of the fan bearings failed in autumn 2022 and were all (2 bearings on each of 2 fans) replaced. While only the bearings on the intake fan had actually failed (being more exposed to cold and damp outdoor air) it always pays to replace them all at the same time. More details on this page.

The replacement bearings have rubber seals and should be slightly better at resisting corrosion than the originals. Maybe they’ll last 8 years, rather than 6 – so expect to replace again in about 2030?

Control Panel Cabling

The wired connection between the MVHR unit and its touch-screen control panel uses standard ‘Ethernet’ cabling but this is not a TCP/IP network connection. It uses a proprietary protocol (based on RS-485?). This does mean the control panel can be located anywhere there is a nearby network socket – as long as the socket near the MVHR unit (Patch Port 118) is cross-patched to the location of the control panel, using the patch panel in the Comms Room. Purple Ethernet leads are used to distinguish the cables from others.

Note that the MVHR unit must be power-cycled to re-pair with the control panel if ever this is disconnected.

0-10V Control Input

In ‘Auto’ mode, the fan speed varies between Level 1 and Level 3 depending on the voltage present on the 0-10V input (0V = Level 1; 10V = Level 3). This control voltage is generated by an Osram-branded DALI to 0-10V converter mounted in the trunking near the alarm panel. The output from this is connected to the MVHR unit with a green (‘KNX’) cable but this is not a KNX signal, just an analogue voltage.

CC BY-SA 4.0 Plant Room – MVHR by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.