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Green Building Bible, Fourth Edition
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Heating and cooling: MVHR versus extra heating? ASHP/GSHP

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    I was pondering over this last night:

    Seen as a MVHR extract fan uses energy to run and recovers let's say 60% of heat as no doubt the maximum figures are never reached in real terms (just a guess?) would it not be cheaper/more efficient to just use a regular non heat recovery low energy fan and heat the air more taking advantage of the COP of a heat pump?

    Can anyone quantify this using figures/a formula?

    This would be in a non-air tight house

    I would also need to factor in the cost of a MVHR versus a cheap inline fan?
    • CommentAuthorRobL
    • CommentTimeJan 31st 2015
     
    Well, the cop of an mvhr can be worked out:
    A good one (sap Q) should have a specific fan power of 0.5W/l/s, ie. 0.5J/litre.
    At 60 % efficient for some reason, and say 20DegC inside and 10 out, and air having a specific heat capacity of 1.3J/litre/degC:
    Energy reclaimed per litre is 10*1.3=13J
    Energy used per litre is 0.5J
    COP=13/0.5 =26
    Impressive high COP that!

    As for the house airtightness - do you open vents / windows in winter to keep RH down to 50%? If not, it's not airtight enough yet. Mvhr just ventilates efficiently, and it's in addition to the normal inefficient ventilation. You have to need extra ventilation to fit it.
    • CommentAuthortony
    • CommentTimeFeb 1st 2015
     
    I think that what you are asking is "should I throw away heat and add a bit more to make up for the heat lost in that way.

    the answer is no! use MVHR and seal up air leakages and as we need ventilation you will be better off with controlled ventilation and the heat revovery is the most efficient thing that you will ever do apart from collecting solar energy through the windows.
    • CommentAuthorEd Davies
    • CommentTimeFeb 1st 2015
     
    Given that the fans are typically on the house side of the heat exchanger (presumably) most of the energy used to drive them will go into the house. Substantially all the heat from the supply fan goes to the house. Most of the heat from the exhaust fan will be recovered by the exchanger so the fans are nearly as effective as resistance heating. If the MHRV is 60% efficient, as you suggest, then it'll effectively be 80% efficient, less the kinetic energy in the exhaust air.
  5.  
    Thanks for classifying that for me folks, I was trying to work out out bit couldn't grasp the figures involved.
  6.  
    It's the best thing since sliced bread so don't do it on a shoestring! De-prioritise something else instead.
    • CommentAuthorringi
    • CommentTimeFeb 1st 2015
     
    Not having cold drafts and no smell of damp is great!

    Think of the MVHR as a preheater for the incoming air, so yo don't feel it.
  8.  
    I'm thinking of getting one of the mitsubishi units in all honesty or even one of the £150 eBay specials.

    If all it is is a fan and a hat exchanger then I can't see the reason in paying over the odds for a fan basically.
    • CommentAuthortony
    • CommentTimeFeb 1st 2015
     
    Should be two fans in a box with a heat exchanger, possibly filters and a condensate drain.
  10.  
    https://www.youtube.com/watch?v=akkhDjxwIu0

    Make a Heat Recovery Ventilator from Coroplast.....
    • CommentAuthorcjard
    • CommentTimeFeb 1st 2015 edited
     
    Posted By: VictorianecoI'm thinking of getting one of the mitsubishi units in all honesty or even one of the £150 eBay specials.

    If all it is is a fan and a hat exchanger then I can't see the reason in paying over the odds for a fan basically.


    Read this thread - people go into some interesting detail about the Mitsubishi ones:

    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=12026" >http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=12026
    • CommentAuthorcjard
    • CommentTimeFeb 1st 2015
     
    ps; while one of the £150 ones might well do to start with, by the time youre done making your house tight and warm, you're going to want something accurately specified for the job..

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