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MVHR vs leaky houses?

Discussion in 'Plumbing and Central Heating' started by cjard, 22 Sep 2014.

  1. cjard


    6 Sep 2008
    Thanks Received:
    United Kingdom
    I'm starting on a new build, aiming for high levels of air tightness and insulation and I've a question about MVHR and the traditional method of engineering the structure to be slightly leaky..

    Is MVHR any better for heat retention and heating running costs than a trickle vented house? Let's say there's MVHR system that recovers 90% of the heat in the exhaust air and heats the inlet air with it... This house loses 10% of its heat continuously, right?

    Now, if the house leaked a little air, that would be warm air, being replaced by cooler outside air which will need heating.. So conceptually we might need to put the same amount of extra heat into our leaky house as we put into an MVHR controlled house, so what's the point of MVHR (other than you can more rapidly freshen all the air in the house without losing all the heat) ?
    If the MVHR system cycles 100 cubic metres of air in the house per hour, then is that the same as having a house that leaks 10 cubic metres of warm air per hour?

    And, if 10 cubic metres change per hour was sufficient for regs, is there any point to installing an MVHR? Is a house where the air change rate is 10x higher a more healthy place to live? Essentially, just wondering what the compelling arguments for MVHR are?
  2. dishman


    8 Jul 2008
    Thanks Received:
    West Yorkshire
    United Kingdom
    I looked into this a while back.

    From what I remember ,to make a MVHR system worthwhile I think you have to aim for an airtighness well beyond the regs as they are just minimum standards.

    Basically the more airtight the house, the better. You should aim for being as close to Passivehous standard as you can get.

    This interesting publication relates to retrfitting modern systems to existing housing stock and the air tightness levels (among other things) required to make the retrofit worth while. The principle and conclusions would still apply for a new build though.


    With regard to the MVHR, they calculate the airtighness required to make the system worthwhile. Furthermore they highlight how well planned and installed the system has to be for it to be effective.
  3. dhutch


    12 Oct 2011
    Thanks Received:
    United Kingdom
    I will have to have a read of that report.

    My parents house was build for them in the late 80's and included a early Villavent system from new. I expect the air tightness of the building is better than most of its age, but well below the current minimum requirements.

    However the now called MHRV system makes a huge difference to the house compared to similar aged houses I have lived in as a student, and to my earlier ex-council 1940s build.

    There is not issue with damp or condensation anywhere ever, you dont have to open windows when cooking, or showering, or drying clothes, or coming in with wet jumps on, or when you have extra people round in winter. Its great.

    Its costs something in electricity to run, but is quite reasonable, especially considering the house is electrically underfloor heater, due to not being on mains gas and not wanting oil at the time it was built. The unit will now be around 245 years old, and in the last year one of the bearings of thw two fans has started showing some wear (giving noise) but its had nothing other than the odd filter clean since installation, including surviving a minor house fire.

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