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Posted By: ringiWhen the indoor RH is higher, the MVHR removes more humidity per second. When the indoor RH is lower, more humidity is given off by any exposed liquid water etc including cooking etc.
Buffering is more likely to be in all your soft furniture etc, unless you have unpainted walls, or have used spacial paint on new walls.
Also your RH meter may not be telling the truth....
Posted By: Ed DaviesWhat's your ventilation rate (m3/h), Dave?
Posted By: djhIt's clay paint on lime rendered straw bales.
Posted By: ringiIs the lime render still drying out?
That wall makeup must be doing some buffering, however I expect your RH is control by the MVHR well enough that there is very little buffering going on over the time scales of a few hours.
Posted By: Ed Davies20 °C, 45% RH: about 8 g/kg (water vapour vs dry air) [¹]
2 °C, 100% RH, about 4 g/kg.
So each kg of air exchanged with the outside would get rid of 4 g of water.
180 kg/h (roughly 150 m³/h) would therefore be getting rid of 720 g/h or 17.28 kg/day. That does, indeed, sound like an awful lot.
Apart from anything, evaporating water at that rate would take nearly 500 W.
BTW, my humidity meter in not drastically different conditions is showing about 40% now. I don't know how accurate it is but it at least seems cromulant with humidistat on my dehumidifier (not running now, of course).
Posted By: djhAre you sure about the 8 g/kg? It looks more like 7 g/kg to me.It looked like a tad over 7.5 so I rounded to 8. The other one might be a fraction over 4 g/kg, too. So overall, maybe 3.5 g/kg loss rather than 4.
Err but that's dry bulb; wet bulb is much higher. How do you read the chart?Start at 20 °C on the bottom axis - it's the “dry bulb†temperature which is what you've quoted, presumably. Up the green axis to half way between the red 40% and 50% lines. Horizontally right to the right axis. That's awkward to do but you can zoom in a lot then hold the mouse (almost) still and scroll sideways to get reasonable accuracy. Looks to me to be just above half way from 0.005 to 0.010 groats of water vapour per groat of dry air.
Does that suggest a different explanation?I don't know. This house has shiny wallpaper and multiple layers of paint everywhere (including on the carpets in places
) so it's not going to behave like your strawbale and clay paint but on the other hand it might also be drying out a bit with me here with the heating on much of the time. One could speculate that humidity buffering is about shorter-term changes rather than anything seasonal which might be what this is about.
Posted By: Ed DaviesStart at 20 °C on the bottom axis - it's the “dry bulb†temperature which is what you've quoted, presumably. Up the green axis to half way between the red 40% and 50% lines. Horizontally right to the right axis.
One could speculate that humidity buffering is about shorter-term changes rather than anything seasonal which might be what this is about.
Posted By: ringiDo you cook with gas and/or have a open gas fire?
Posted By: djhThat's what I was doing but it appears to me to be well below the midpoint of the 0.005-0.010 groats/groat interval.You're right - I was following the wrong red lines down for 40 and 50% RH so effectively measuring for 55% rather than 45%. Even when I traced the red lines with the mouse pointer I couldn't quite believe I'd done that: must be some sort of optical delusion.
Posted By: WillInAberdeenIn contrast AIUI any porous materials that bridge across the construction will absorb water from high RH areas (outside) and desorb it in low RH areas (inside) so act to pump water into the house, against the prevailing vapour pressure gradient,…On first reading that sounds awfully like one side of a perpetual motion machine but it's also acting as a pretty effective heat pipe in the opposite direction: releasing heat on the outside as the vapour is adsorbed then absorbing heat on the inside as it's evaporated again.
Posted By: WillInAberdeenDo many folks with mhrv typically adjust the ventilation rate in summer/winter?Good reminder - why does DJH have such a high ventilation rate? He mentioned boosting it at night to allow the duct heater to work better but would cutting it significantly during the day (to allow the RH to rise a bit) make a saving?
Posted By: SteamyTeaNow I have no idea of the make up of these straw bale shacks with lime render, but is there something in there that controls the RH i.e. weed killer, anti-fungal or just plain potassium carbonate.But Ringi and I are also seeing fairly stable RHs in very different houses.
For more about the difficulties of using an RH sensor: http://www.vaisala.com/Vaisala%20Documents/White%20Papers/lsh-Trouble-with-Humidity.pdfBit of FUD and misdirection in there:
Relative humidity is simply the measure of the amount of water vapor in the air compared to how much it can possibly hold at that temperature.No it isn't. I wish people would stop using this misleading short hand.
For example, a sensor rated to measure 10 to 90% RH from -40 to 70C must perform in humidityBut RH sensors measure RH, not ppb.
conditions ranging from 10,000 parts per billion to 200,000,000 parts per billion. The dynamic range this represents is 20,000:1,…
I still fail to see what this is all about, your RH is your RH, and short of a small water leak, there is probably little that can be done about it.Since, apart from cost, water in the building is the biggest constraint on keeping energy in the house I'd say that having a good understanding of this is right at the middle of what GBF is about.
