MVHR Vs Positive Input

[TrialsLife]

Hi All,

 

First post on here and I am looking for some help.

 

I live in a pre 1920's, end of terrace, Victorian solid walled home. The house is not draughty, but far from air tight! We have been having issues since moving in with damp/condensation on the windows.

 

I was considering fitting a positive input ventilation unit to help the situation,  however I have managed to get hold of a Vent Axia HR250 MVHR unit free of charge! 

 

Whilst I understand that MVHRs work best in an air tight environment, and my environment is far from it, will I still see a benefit over installing a PIV unit? 

 

Cheers for any help.

 

 

[Stones]

I think so.  I've had MVHR fitted to builds that I have done so the early 2000's.  Back then building regs only mandated 100mm of glass wool in the walls and air tightness wasn't even considered.  Compared to a house with, night and day.  You may not get much in the way of heat recovery / energy savings, but I'm sure you'll notice the difference in air quality.  

[ragg987]

As I understand PIV (no experience of them), these simply bring external air into the house and rely on existing gaps in the fabric to vent out. So every gap becomes a vent out.

 

The MVHR usually pushes air into a dry room (e.g. bedroom) and draws out stale air from a damp room. It  relies on an air-flow between rooms, e.g. through open doors or gaps below them to move and diffuse the fresh air.

 

My concern here would be that an MVHR in a leaky house would result in the air from the dry room exiting the house through leaks in the dry room and not making it to the damp room. I.e. the air-flow would follow the path of last resistance. And extract air from the damp room might get replaced by cold and damp air from outside e.g. through gaps in the windows etc.

 

This would not be the end of the world - you lose some of the benefit of the heat exchanger in the MVHR, and you could also change the relative flows of incoming vs outgoing air to achieve a small positive pressure inside the house, which would reduce the effect of cold air coming in through gaps in windows etc and improve comfort.

[MikeGrahamT21]

The more airtight the building, the better MVHR will perform without doubt.

 

A few months back I read a university paper from Leeds Met (i think) Uni, which fitted MVHR to a number of properties and Standard Extractors to a number. They gathered information on all before they were fitted, and performed basic airtight measures on the MVHR properties (hardboard on top of floor boards, and mastic wherever possible), bringing the ACH down from around 20 to 11 or 12 ACH (so still quite high), and they performed very well, obviously fixed all of the issues with condensation, but also dramatically lowered CO2 concentrations in the air, and reduced consumption of gas for heating. So it just goes to show, that even in a building which is very leaky, they do still work. Obviously the less leaky, the more savings potential, and after installation, get a smoke pen, and work your way round sealing all the gaps, its a long hard job, and some will be more sealable than others, but you will sure make an improvement. The MVHR properties outperformed the Standard Extractor fan ones by a fair margin in terms of energy costs, and humidity/CO2 reduction.