MVHR zoned by floor for simpler retrofit?

[Clarkey]

Hi folks,

We're looking to retrofit MVHR into our 130m² (326m³) 30s semi, to improve air quality & introduce controlled ventilation, as other improvements gradually reduce air leakage.

 

I've built out a spreadsheet and various ducting models, and whilst a Vent Axia Econiq S, with radial semi-rigid ducts looks good on paper, wherever I site the HRU, the vertical ducting needs for a single system are creating blocking issues.

 

Reading about from others' successes in deploying multiple HRUs, it seems this need for vertical ducts in our two-story retrofit could be avoided by physically zoning the MVHR solution by floor, i.e.:

• Multi-room MVHR system per floor (85m² & 50m²), each with a dedicated HRU that has control inputs for... 
• Home Assistant to coordinate 'whole house' MVHR actions and/or balance between floor HRU systems, as needed.

 

I calculate needing MVHR units of 150m³/h (ground floor) & 120 m³/h (upper floor) for this application. A pair of Vent Axia Econiq S HRU seems overkill (and physically too deep to get into one of the lofts, which has a 650mm x 360mm access restriction). So, I've started researching apartment/flat installations and units such as the Zehnder 155 variants.

 

I'd welcome any recommendations/insights.

[JohnMo]

8 minutes ago, Clarkey said:

calculate needing MVHR units of 150m³/h (ground floor) & 120 m³/h (upper floor)

Are you sure?

 

If house volume is 130m² x 2.5 (for ventilation purposes) multiply by 0.3 ACH is total 97m³/h. Or am I missing something?

 

I would really investigate cascade systems for retrofit MVHR. This limits the ducts required hugely.

 

But if you are not nearly airtight MVHR is just waste of time, you actually increase ventilation heat losses. Look into demand controlled dMEV or MEV systems.

[JohnMo]

A cascade system for reference h can be done with any ventilation unit

 

https://www.brinkclimatesystems.nl/solutions/ventilation/multi-air-supply-system

[Clarkey]

Thanks @JohnMo for the response. 

 

I'm definitely not sure on any of my working assumptions or calculations - this is definitely all new to me! The figures I quoted for system capacities per floor were based on Passihaus supply recommendations that are higher than regs, include occupancy and factor the HRU at only 70% loading - so probably way over the top. Meanwhile, BPC calculated the house airflow rate based solely on UK regs and for the floor area they measured at 129m², as 38.7 l/s, which I believe is 139.32m3/h. Against this, the Vent Axia Econiq S that they recommended has 350m³/h capacity, so a 40% loading, (possibly because it's the smallest model in that range).  

 

Thanks too regarding cascade systems, I did investigate this after reading your comments in other threads. My understanding from some of the papers referenced suggested that this approach works where the path of least resistance can be forced to cascade through targeted rooms and not be short-circuited by easier routes from supply to the nearest extract, Unfortunately, both floors of our 'classic' 3-bedroom semi have all supply & extract rooms directly off the hall, stairs & landing area, so every potential cascade route would be shorted out. Our ducting challenges lie less with the horizontal aspect of getting to the room vents duct so much as the need for vertical 'risers' on the first floor. I did look at using floor vents for bedrooms to avoid the risers, but I'd worry that they'd get covered, draughty and/or with, no chance of Coanda effect, fail to diffuse, and just ventilate the route straight across the floor to the door. 

 

Regarding dMEV/MEV, I will look at this if I can't get MVHR to work, but I'd prefer to deliberately supply fresh, filtered & pre-warmed air to living rooms and bedrooms alongside extraction. I'm sure our house is miles from airtight but I take heart in the conclusion of the 2020 Passivhaus Case for MVHR paper that concludes with the following...

 

Quote

"There is no clear up-to-date evidence behind the rule of thumb that says MVHR is inappropriate for dwellings with air permeability above 3m³/m².hour @50Pa. This analysis has shown that MVHR systems result in improved ventilation and lower carbon emissions for all levels of airtightness. There is a compelling case for MVHR systems to be fitted in all new dwellings and to be strongly encouraged in retrofits where significant reductions in energy demand are being targeted."