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Data and monitoring: Condensation and fog (and MHRV)

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      CommentAuthorDamonHD
    • CommentTimeNov 20th 2011
     
    Hi,

    Our MHRV has not yet been installed, and we are experiencing the expected condensation on windows now that outside temperatures are in the region on 4C overnight.

    (On the inside of older windows, and the outside of high-spec 2G!)

    Normally we shut the doors and open the windows of bedrooms in the morning, with rads off, to let the moisture out.

    On a day like today with fog, ie supersaturated externally, that isn't going to work!

    So as a data point I'm hereby noting that I'm putting on a small dehumidifier in one bedroom with all doors and windows closed and rad off to help keep on top of things.

    I'll probably be deploying this dehumdifier to the smallest room to help dry the room fabric once the plumber has replaced our toilet that decided to leak from the inlet pipe into our electrics yesterday morning...

    Rgds

    Damon
    • CommentAuthorSteveZ
    • CommentTimeNov 20th 2011
     
    We have Argon-filled e-glass warm edge double glazing and we get condensation on the outside of the glass these damp mornings - what's that all about?
    •  
      CommentAuthorDamonHD
    • CommentTimeNov 20th 2011
     
    Because they are preventing heat getting outside from the room their outer face cools off fast and drops below the dew point... Thus you are seeing dew on the outside of the good units.

    Rgds

    Damon
  4.  
    The outside pane of the window can "see" the night sky & so will exchange radiation with outer space. As outer space has a radiative temperature of around -270oC, the outside pane can drop below outside air temperature & the dew point, causing it to collect moisture in the same way as a car left outside overnight.

    http://en.wikipedia.org/wiki/Radiative_cooling

    A low performing window will leak enough heat to keep the outside pane above the outside air temperature & the dew point. A high performing window will not leak as much heat & will thereby allow the outside pane to reach a lower temperature & collect more moisture.

    David
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 20th 2011
     
    My front porch window gets condensation on the outside, shall have a look at the data loggers/weather station data next time I notice it and see what it all means.
  6.  
    If the porch is unheated then the glazing doesn't need to be that good to get condensation on the outside.

    It probably just means that the outside air is close to 100% humidity.

    David
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 21st 2011
     
    Was 99% humidity at 7:30AM today (and then rained as I was walking). Did not look at porch window.
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011
     
    MHRV going in right now, holes being drilled, teeth on edge, etc!

    Rgds

    Damon
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 21st 2011
     
    Strange that, I am going to have a hole drilled in my tooth soon :sad:
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011
     
    A bit smaller than the one in my wall!

    Rgds

    Damon
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 21st 2011
     
    I will probably disagree at the time, but will give me something to think about as they get the tunnelling machine out.:devil:
  12.  
    Reference water content in air- just reading a report on waste to energy plant specifying 47,000 cubic metres of water vapour created per hour, wondering how to establish actual water content. I understand 30g/cubic metre but this appears low. Thanks in anticipation.
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011
     
    Can we try to keep this thread to my domestic MHRV install please Brian?

    Rgds

    Damon
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 21st 2011
     
    So this unit, is it governed by humidity, temperature or time? Does it have a 'flap' on it to stop air blowing in?
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011 edited
     
    Humidity to go into boost mode. There are other options, such as PIR and pullcord.

    Else runs continuously (2W) which should keep out all but the stiffest of breezes. I intend to turn it off entirely in summer and let it ventilate naturally.

    Rgds

    Damon

    PS. Starting a write-up here: http://www.earth.org.uk/MHRV-mechanical-heat-recovery-ventilation.html
    • CommentAuthorEd Davies
    • CommentTimeDec 21st 2011
     
    Posted By: Brianwilson: “I understand 30g/cubic metre but this appears low.”

    Dry air has a density of about 1.3 kg/m³ so 30g/m³ would be about 2.3% by mass which is about right for “saturated” air at 25°C:

    http://en.wikipedia.org/wiki/Water_vapor#Water_vapor_density

    Note on that graph, though, how dependent on temperature the equilibrium water vapour density is.
  17.  
    Many thanks Ed- will not elaborate as apparently I am instructed to only discuss Damon's domestic adventures or his teeth.
    Season's greetings
    Brian
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011 edited
     
    Install done. Took about 4 hrs.

    Because so humid today outside (~75%RH) and had all the windows open had to tweak down the setting at which the fan boosts to 'high'.

    On current low setting I cannot hear it in the next room, though it may be slightly audible at night.

    Rgds

    Damon

    PS. Pics available later...
    • CommentAuthorborpin
    • CommentTimeDec 21st 2011
     
    Damon, from your write up "As usual we find out something about the house's construction every time we work on it. In this case, going in the small gap above the bathroom window from the outside the construction was:"
    Is the house TF by any chance? I think you have made a dirty great hole through the lintel above your window!
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011 edited
     
    It is timber frame.

    Yes, we have made a hole in a lintel I believe, though this is several inches above the top of the window frame.

    No we don't think the house will fall down; we gave it some consideration.

    Rgds

    Damon
    • CommentAuthorborpin
    • CommentTimeDec 21st 2011
     
    I was warned, on pain of death, by my last architect to never put any holes in the wooden lintels! A small window is probably OK as long as there is no joist above and he was extremely cautious. YMMV though :bigsmile:
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011
     
    Have adjusted the cut-in for 'boost' mode up to about 85% as started running with a couple of guests, excited kids and a little cooking downstairs. Will make sure that it does come on boost before bathroom walls dripping though!

    Rgds

    Damon
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011
     
    With kids in the bath the RH peaked at ~93%, but has dropped back to about 90%, and the temperature is rising slowly. A little condensation on window and the mirror under it, but not elsewhere. So, from the pathroom point of view it seems to be working...

    Rgds

    Damon
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 21st 2011 edited
     
    Mmmm. Either the fan's gone *very* quiet or my fan has cut out after a few hours' service!

    Will have to investigate more in the morning to avoid waking sprogs...

    Rgds

    Damon
  25.  
    93% RH seems very high, I would not like it to get any higher than 60%RH.
    If its foggy at 4 degrees outside its still drier than 60% RH 20 degree indoor air.
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 22nd 2011
     
    VH: surely only if we were actually heating the house at that point?

    Rgds

    Damon
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 22nd 2011
     
    Posted By: DamonHDVH: surely only if we were actually heating the house at that point?


    My house, even when not heated, seems to be warmer than the ambient and the RH seems lower (tis RH rather than just H).
    So it is quite possible that the RH in a house is lower than ambient. Though that does open the question about 'what is heating', people, fridges, TVs (not here, so may be the hot air on R4) and possibly even light bulbs.

    Is the fan whizzing away this morning?
  28.  
    Sorry Damon, I'd assumed it was heated. I'd be interested to know the return air temperatures when its freezing outside.
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 22nd 2011
     
    VH/ST: well, since yesterday was so much warmer than the previous few, and we had windows open all morning while works were in progress, it quite likely was warmer outside than in, or nearly as warm...

    BTW, I wiggled with wires in the pattress looking for loose connections, and didn't find any, but mysteriously the fan did start operating again. So I'll have a grown-up electrician come and check it over. Fan isn't bust anyhow, it seems.

    Rgds

    Damon

    PS. VS, I will attempt to monitor return temps. While the kids were having a bath last night humidity stayed much lower than usual and the airflow back into the room was not noticeably colder though the delta-T between inside and outside was probably only 8K...
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 23rd 2011
     
    First night with fan on low all night as planned and only a little condensation on bedroom windows rather than 100s of ml.

    But, external temp quite high at 11C and humidity also at over 80%RH, so not a real test yet.

    Rgds

    Damon

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