MVHR maintenance guide
At last a great guide to maintaining your MVHR from the excellent Passivhaus Trust
MVHR maintenance | Duct cleaning
My endoscope inspection showed up significant build up of dust in the bathroom extract ductwork (and to a lesser extent in the ensuite extract). The kitchen extract has a filter which stops the dust but allowed grease to clog up the extract fan in the unit (and may well have been why the fan failed).
The industry doesn’t seem to be set up for domestic ventilation cleaning, so I decided to give it a go myself. I discounted the systems where you have to screw multiple sections of rod together and opted for a Wohler Mini Viper S which has a thin flexible rod housed on a roll, and a 70mm ‘PEK’ brush (to suit the 63mm ID of my ducting).
You have to be quite rough with the brush, and push and pull it through the duct run several times, (so it would be best to avoid joints if possible) but afterwards the ducts look as good as new.
I rigged up my vacuum cleaner with a connector made from online ventilation parts (with double gaskets to provide an airtight connection to the plenum box in the ceiling) which worked perfectly and no mess!
For my system, the amount of dust build up in the ducts is very definitely affected by the dust filter, or lack of one, to the room terminals. There is far less dust build up to the ensuite extract where I fitted a filter than to the bathroom. It would seem sensible to fit the filter to extend the duct cleaning maintenance interval, but the downside is the filters at the terminals need vacuuming monthly, which few users would tolerate, so practically probably best to leave the filters off and then accept the need to clean the ducts regularly, in my case every 3 years.
Interestingly, the noise level at the bathroom terminal has definitely dropped now that it is no longer clogged with dust and this does beg the question why the system wasn’t specified with 90mm OD ducting rather than the 75mm to lower noise and extend the maintenance interval.
The total cost of the parts was around £250 (and quite a few hours research), but now I’m set up the cleaning is fairly quick, and I suspect this may have already paid for itself or will have done so after the next clean in 3 years’ time.
Overall, after a far bit of research and set up, the cleaning was easy, but it does suggest that the domestic ventilation industry, from designers through to installers must ensure that a cleaning strategy is implemented throughout. Without cleaning my system would likely have been unusable within around 10 years and had I not designed in cleaning, would have caused a real problem.
MVHR maintenance | PART 2 Borescope
The realisation of a disconnected duct prompted a full investigation into the system, starting with the easy to access visible room terminals using a Depstech borescope. The borescope clearly showed that all the connections at the terminals were good but was inconclusive in determining if there was any issue at the other end with the manifold.
However, the borescope did clearly show significant build up of dust on the EXTRACT ducts, but the SUPPLY ducts were clean and the kitchen extract was only slightly dusty (I assume due to the kithcen greese filter). The ensuite had far less dust build up than the bathroom and as both have a similar air flow rate, I suspect that as the ensuite always had a black fleece filter fitted suggests these filters at the terminal are effective (as one would expect).
So after 5 years my extract ducts definitely need cleaning and arguably should have been cleaned earlier, so a 3 year maintenance interval looks appropriate. It would be reasonable to assume that there will likely be a multitude of ventilation systems across the UK with clogged ductwork and form my limited experience there doesn’t seem to be an awareness of this issue. The question then is how to clean the ductwork. Zehnder have just the kit but at over £1,600 it’s not a DIY product and there doesn’t seem to be an option to hire the equipment or specialist contractors to undertake this work. (Here’s a video showing what’s involved – view from 36min 40 seconds)
MVHR maintenance
My Zehnder Q350 MVHR was installed as part of a (partial) deep retrofit in 2017 so it’s about 5 years old. Firstly, the MVHR system has been amazing. Trickle vents don’t work, and opening windows just wastes heat and leads to poor comfort, but the MVHR means year round excellent indoor air quality and comfort, energy savings, and added benefits such as beiog able to keep windows shut when the neighbour has a party (or the seagulls are on your roof!) and also clothes dry quickly, sometimes overnight.
That said, maintenance of the MVHR system has been more involved than expected and in my case living in a city, the INTAKE filters need changing every 6 months, but the EXTRACT (which can be vacuumed) can last up to 12 – 18 months, so a total cost of about £50/year. The kitchen grease filter needs flipping over every month or so and changing every couple of months, but these are cheap. I had fitted the black fleece filters to the extract terminals in the bathroom and ensuite to reduce the dust making it to the EXTRACT filter in the unit, but these clog up fast and need vacuuming (at the very least monthly), so maybe best left off and keep a closer eye on the filter in the unit itself. The Q350 controller indicates when the filters in the unit need attention but no such alarm for the kitchen grease filter, whcih gets clogged quick and I suspect there are many systems out there, even in Passivhaus buildings, with blocked kitchen extracts.
You’re supposed to maintain the unit itself every couple of years, but I didn’t do this as everything looked good, i.e. the unit was using a similar amount of energy as when installed (just ~21W) and noise was normal, but then the EXTRACT fan failed. A new fan was £460 plus fitting (so not cheap) but when removed it was clear that even with a well maintained grease filter in the kitchen it had got pretty grimy, (although this may or may not have been the cause of it’s failure), but I’ll be maintaining the unit every 2 years from now on (perhaps a further £100 per year if undertaken by a third party).
The heat exchanger wasn’t too bad, just need a vacuum and a degrease to remove the fat from the kitchen
Two of the sensors on the unit had failed and were swapped out FOC by Zehnder.
So overall the cost of running an MVHR could be seen as similar to a gas bolier?
The kitchen has a recirculating cooker hood with the MVHR terminal nearby (as per Passivhaus doctrine) but in future (retrofit) projects I’ll consider ducting the cooker hood directly to the outside as it’s much more effective for removing smells and should go a long way to reducing the build-up on the (expensive) MVHR unit.
The kicker is that when rechecking the flow rates to the terminals, we noticed that one or two of the extract terminals couldn’t reach the desired flow rates and on inspection found that at least one duct had come away. This is a shame as the system was very carefully designed and installed. More on the resolution to this issue next time.
Maintenance by Carl Fivash of Eco Ventilation info@cfecoventilation.co.uk
MVHR maintenance
MVHR maintenance is often overlooked and is more involved than most people realise and is critical to keeping the system low energy, quiet and long lasting. All installations really need a user guide, perhaps something like this (happy to forward file to anyone who wishes to tweek it for their own purposes)
Victorian ‘EnerPHit’ insulation to walls
A continuous 40mm semi rigid woodfibre batt thermally isolates the timber studwork from the (external) masonry party wall to the west with further woodfibre batts between and being water vapour open (and with the lime parge) should be a long term robust solution. The east party wall is more complicated due to the elevated risk of damp due to the neighbour’s chimney stack and the MVHR ducts so vermiculite was poured all the way down the chimney flues and continued up to insulate the wall in the loft. vermiculite doesn’t have quite as good thermal performance but it’s ability to flow into every area was critical and should stop flanking noise travelling up the chimney flues and negating efforts made with the floors!
Hove ‘Passivhaus’ is up!
The build continues a pace and is pretty much up. The timber and woodfibre insulation look fantastic in the sun. Some of the glazing panels are massive especially the two story panel to the atrium.
BSBM – TOUR sustainable renovation
I’m planning a tour of the latest stage in the renovation to my house, most notably a loft conversion which is in full swing and should be at an interesting stage for the BSBM meeting in JUNE on the 13th. So instead of meeting at the Trafalgar I’m suggesting meeting on site (92 Livingstone Road, Hove, BN3 3WL) at 7pm and then we can head to a local pub around 8ish. I’ll reconfirm nearer the time, but please let me know if you’re interested. Phase 1 of the renovation was completed in 2014 and included external wall insulation and triple glazing.
Victorian ‘EnerPHit’ – flues
The flues are now accessible showing the route for the ventilation ducting from the loft to the other rooms in the house
