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Energy Assessment: What ventilation rule to use with MVHR

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  1.  
    • CommentAuthorwookey
    • CommentTimeJan 11th 2012
     
    What is the normal rule of thumb for calculating heat losses when MVHR is installed?

    My existing spreadsheet uses Room_Vol * Temp_Diff * Air_changes * Specific_heat_capacity_air, which I think came from SAP.

    Do we just do the same, but add a factor of 1-MVHR unit efficiency?
    • CommentAuthorEd Davies
    • CommentTimeJan 11th 2012
     
    Shouldn't that be volumetric heat capacity (kJ·m⁻³·K⁻¹) rather than specific heat capacity (kJ·kg⁻¹·K⁻¹), not that there's a huge difference with room air having more-or-less unit density (1.2 to 1.3 kg·m⁻³)?
  3.  
    I assume Air_changes above refers to the MVHR ventilation rate. Are you separately allowing for fabric air leakage?

    David
    • CommentAuthorwookey
    • CommentTimeJan 11th 2012
     
    Ed, er yes. And it probably is (I've got '0.33' in this 'ere spreadsheet, with no units recorded :-( Spreadsheets are the swork of the devil due to this sort of thing.)

    David: That's just 'recommended air changes (per hour?)' = i.e 1 for bedrooms for, 2 for kitchens. For calculating heat loss due to fabric leakage in typical houses. But I guess with MVHR the air changes stays much the same, only the corresponding heat loss changes, and one can hopefully assume the fabric losses are much lower. But no value is given for this sum - it's just 'normal/average'.

    Do people normally allow separately for fabric leakage?
    •  
      CommentAuthordjh
    • CommentTimeJan 11th 2012
     
    Posted By: wookeyThat's just 'recommended air changes (per hour?)' = i.e 1 for bedrooms for, 2 for kitchens. For calculating heat loss due to fabric leakage in typical houses. But I guess with MVHR the air changes stays much the same, only the corresponding heat loss changes, and one can hopefully assume the fabric losses are much lower.

    I'm just following this thread, hoping to learn something, and haven't done any checking but off the top of my head something doesn't seem to add up. My memory says that building regs requires an ACH of 0.44 and that that is higher than what you might get via PHPP (0.25 - 0.33 ish). IIRC, there's a thread on AECB somewhere where I jokingly suggested that MVHR systems should have a 'building regs test compliance' button. So whole numbers like 1 or 2 seem big. Though nowadays I do think that more ventilation is better.

    Do people normally allow separately for fabric leakage?

    Again, off the top of my head (OTTOMH?) I think the rule of thumb is to divide the airtightness test result by twenty to estimate the actual infiltration.
  6.  
    Posted By: wookeyDavid: That's just 'recommended air changes (per hour?)' = i.e 1 for bedrooms for, 2 for kitchens. For calculating heat loss due to fabric leakage in typical houses. But I guess with MVHR the air changes stays much the same, only the corresponding heat loss changes, and one can hopefully assume the fabric losses are much lower?
    This depends upon the MVHR air change rate, MVHR efficiency & the fabric air leakage. Hence all the discussion about getting the air leakage down to a particular level before fitting MVHR.

    Posted By: wookeyDo people normally allow separately for fabric leakage?
    I'm assuming that the typical fabric air leakage will be around one third of the air leakage measured at 50Pa. My figures show losses at design temperature of 122W due to MVHR inefficiency and 405W due to fabric air leakage. This may be pessimistic, but I can't find the reference at the moment.

    David
    • CommentAuthorEd Davies
    • CommentTimeJan 12th 2012
     
    Posted By: wookeyEd, er yes. And it probably is (I've got '0.33' in this 'ere spreadsheet, with no units recorded :-( Spreadsheets are the swork of the devil due to this sort of thing.)


    Yes, difficult to guess what that is. SHC of dry air is quoted as 1.005 kJ/kg·K so VHC would be something like 0.8 kJ/m³·K. Neither looks like 0.33. Wh/kg·K would be slightly smaller, around 0.28, and Wh/m³·K would be around 0.22.

    (During the late 1970s, while a research student, I had a go at designing a programming language with proper use of units. Some papers around at the time dismissed the idea, “baby-talk dimensional analysis” is one phrase which stuck in my mind, but I still think there'd be a lot to be said for it. JPL & Lockheed Martin might have avoided a bit of embarrassment in the vicinity of Mars, for example.)
  8.  
    Posted By: Ed DaviesI had a go at designing a programming language with proper use of units. Some papers around at the time dismissed the idea, “baby-talk dimensional analysis” is one phrase which stuck in my mind, but I still think there'd be a lot to be said for it. JPL & Lockheed Martin might have avoided a bit of embarrassment in the vicinity of Mars, for example.)


    ADA already has units, the problem with the Mars lander is they mixed up imperial and metric.

    Paul in Montreal.

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