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    • CommentAuthorjohnuready
    • CommentTimeApr 21st 2017
     
    Today I checked my temperatures on MVHR, whats the formula for checking efficiency?

    My output temperature to Atmosphere looks to be high?

    Actual numbers:

    Outside = 16.11c

    From Dwelling = 22.31c
    To Dwelling = 21.12c
    From Atmosphere = 16.88c ( picks up heat going through roof and utility room )
    To Atmosphere = 21.56c

    Am I running ok?
    • CommentAuthorjohnuready
    • CommentTimeApr 21st 2017
     
    forgot to add:

    inside temp set to 21c

    summer bypass is showing on
    • CommentAuthorjohnuready
    • CommentTimeApr 21st 2017 edited
     
    What should we set the outside and inside temperature on the MVHR?
    • CommentAuthorDarylP
    • CommentTimeApr 21st 2017
     
    summer bypass is 'short circuiting' the HX....?
    • CommentAuthorEd Davies
    • CommentTimeApr 21st 2017 edited
     
    http://www.engineeringtoolbox.com/heat-recovery-efficiency-d_201.html

    t1 = inlet air (from atmosphere) = 16.88
    t2 = supply air (to dwelling) = 21.12
    t3 = extract air (from dwelling) = 22.31

    μt = (t2 - t1) / (t3 - t1) = (21.12 - 16.88) / (22.31 - 16.88) = 78.1%.

    The exhaust (“to atmosphere”) temperature doesn't come into the formula though, as you say, yours does look rather high. The incoming air is warmed by 4.24 K whereas the outgoing air is only cooled by 0.75 K. Four possibilities spring to mind:

    1) The airflows are very unbalanced.
    2) There's a lot of condensation going on.
    3) The incoming air is being warmed from the house somehow (beyond that in the inlet pipe you already mentioned and accounted for).
    4) The temperature measurements are wrong.

    [Edit to add: Hmmph, that was a waste of time if the summer bypass actually is on. Strange if it is with the somewhat low outside temperature, though.]
    •  
      CommentAuthordjh
    • CommentTimeApr 21st 2017
     
    Posted By: Ed Daviesthe summer bypass actually is on. Strange if it is with the somewhat low outside temperature, though.

    The outside temperature isn't the main determinant of whether the summer bypass is open. On mine it opens when the extract air temperature from the house is above a set point as long as it's cooler outside than inside. The external temperature is only relevant as a limit that prevents the bypass opening if the external temperature is below 10°C, presumably to prevent excessively cold air being delivered to habitable rooms. If you want cold drafts, you have to open windows.
    •  
      CommentAuthordjh
    • CommentTimeApr 21st 2017
     
    Posted By: johnureadyWhat should we set the outside and inside temperature on the MVHR?

    Not sure what you mean here. Those temperatures aren't something you set, they're something you observe.
    • CommentAuthorEd Davies
    • CommentTimeApr 21st 2017
     
    Posted By: djhThe outside temperature isn't the main determinant of whether the summer bypass is open. On mine it opens when the extract air temperature from the house is above a set point as long as it's cooler outside than inside.
    Ah, right, thanks. So basically, it's on if the house needs cooling (but won't be cooled too much)?

    Is 22.31 °C a bit low for that to trip in, though? Presumably, particularly at this time of year, you'd want to store some heat in the house fabric while there's an opportunity to do so. E.g., ready for a cold weekend with 0 °C forecast overnight Sunday for CM1.
    •  
      CommentAuthordjh
    • CommentTimeApr 21st 2017
     
    Posted By: Ed DaviesIs 22.31 °C a bit low for that to trip in, though? Presumably, particularly at this time of year, you'd want to store some heat in the house fabric while there's an opportunity to do so. E.g., ready for a cold weekend with 0 °C forecast overnight Sunday for CM1.

    I have mine set to open at 23°C and close at 21.5°C. It was open last week, but has closed this week. There's no real preference either way at this time of year, it's just a question of what the weather actually does. During winter I had the setpoints 1°C higher, although I don't know whether it made any difference.
    • CommentAuthorjohnuready
    • CommentTimeApr 21st 2017
     
    Ed - thanks for the formula, easy to follow. Are you using a Ventaxia?

    Anybody using Ventaxia as the inside and outside temperatures have programmable set points.

    Anybody set these values?
  1.  
    When calculating the efficiency of HRV allow for 5% leakage within the heat exchanger between the flow side and the return side, this is quite common with plate heat exchangers as there's quite a pressure difference through the heat exchangers with one fan sucking and the other blowing.
    Also allow for the heat generated by fans, more efficient fans will have lower heat-loss.
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2017
     
    Viking House: “Also allow for the heat generated by fans, more efficient fans will have lower heat-loss.”

    Good point. SAP Q requires a specific fan power (SPF) of < 2 W/(l/s) [¹], i.e., 2 joules/litre.

    The volumetric heat capacity of air is about 1.3 J/(l·K) so you might reasonably expect a rise in temperature of each of the two streams of something approaching 0.8 °C. That's fairly easy to neglect when the outdoor temperatures are around freezing but significant when they're only a few degrees below indoor temperatures.

    Obviously less if the MHRV's SPF is better than required by SAP but maybe worse if the fans are working harder than they should for some reason.

    [¹] http://www.partf.co.uk/sapq.htm
    • CommentAuthorcjard
    • CommentTimeMay 4th 2017 edited
     
    Posted By: johnureadyEd - thanks for the formula, easy to follow. Are you using a Ventaxia?

    Anybody using Ventaxia as the inside and outside temperatures have programmable set points.

    Anybody set these values?


    Curiously enough, I've just made a post about ventaxia's summer bypass behaviour.. My values are still on the VA defaults but if you set your indoor temp to 21 and the unit is measuring 22.4, the SBp will open so long as the world is above the outdoor temp setting

    Don't expect this to be anything like aircon; the manual even says that the unit cannot cool a house, it's just intended to help a bit. The summer bypass flaps in mine seem to provide an additional route for air rather than blocking off the hx entirely. Maybe I've not seen another flap that does block it off...
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