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Posted By: fostertomThanks VikingTo add to that, when water vapour is squirting through that pinhole in a VCL from inside to outside, if it finds something cold to condense on after squirting through to outside, then water is continually taken out of the vapour phase there, as it becomes liquid. This has the effect of keeping the water vapour concentration low, outside, relative to the inside concentration, so the PVP differential is maintained and never equalises, so the supply of water vapour to the condensation surface keeps coming, and liquid water keeps on condensing out.This is how a pinhole functions as a very active one-way pump of water vapour, and can pass great quantitie, that create much liquid water.
Posted By: Peter Clarkif that cold something is natural and IS hygroscopic etc, then that water will be dispersed throughout that element and will travel to the outside (and perhaps the inside) by ... capillarity
Posted By: Peter Clarktravel to the outside (and perhaps the inside) by vapour permeability
Posted By: Peter Clarktravel to the ... inside
Posted By: Peter Clarkhygroscopic and vapour permeable and capillary open
Posted By: fostertomSounds like you're "keeping up" well! And I'm no expert, just ivory-tower extrapolation from A-level Physics 43yrs ago!All you say is right, but to be pedantic ...Posted By: Peter Clarkif that cold something is natural and IS hygroscopic etc, then that water will be dispersed throughout that element and will travel to the outside (and perhaps the inside) by ... capillarityby capillarity Yes, by vapour permeability No. Only if conditions become right (the summer after?) and that dispersed water evaporates will itPosted By: Peter Clarktravel to the outside (and perhaps the inside) by vapour permeabilityAnd if there's a strong inboard VCL (regardless of pinholes) that water won'tPosted By: Peter Clarktravel to the ... insideAnd once you're usingPosted By: Peter Clarkhygroscopic and vapour permeable and capillary openmaterials, a strong inboard VCL is unnecessary anyway, provided you organise them into a breatheable sequence, obeying the 5:1 ratio of inboard to outboard water vapour resistivities.
Posted By: fostertomPVP is proportional not to the difference in RHs, but to the difference in absolute concentration of water vapour molecules, in the 'from' and 'to' zones. Whether that's concentration by volume, by mass or by moles or something, my A-level Chemistry is too rusty. That concentration, for given RH at given temp, can be looked up in psychrometric tables/charts.However, your MHRV is basically trying to fill your interior with the same air as outside, albeit at raised temp. So the incoming air will have exactly the same concentration of water vapour molecules as outside. But when it arrives, there's almost always more water vapour being added, from interior sources. So I'd say that your humidistat can run the MHRV for as long as it likes, but internal concentration will always be higher than outside, so there will always be a PVP gradient from inside to outside.When a lot is being generated inside e.g. while showering, then that gradient will be v high locally, so any pinholes in the bathroom wall's VCL will be pumping masses of water vapour into the insulation - which the puny MHRV airflow will take a long time to dilute away.
Posted By: fostertom"So the incoming air will have exactly the same concentration of water vapour molecules as outside"
Posted By: Viking HouseCan somebody explain the Relative in Relative Humidity? is it relative to the temperature or relative to the maximum vapour the air can hold at that temperature?
Posted By: Viking HouseWe have a humidity monitor on our FiWi HRVhttp://www.viking-house.ie/fine-wire-hrv.html" >http://www.viking-house.ie/fine-wire-hrv.htmlwhich keeps ventilating til the internal RH% drops to 40% @ 20 degrees
Posted By: Viking HouseI was told (now I'm a builder not a scientist) that if you lift the temperature of 10 degree 70% RH external air by 1 degree then the RH drops to 63%. Now this is the bit I don't understand, what if I rise the temperature of the 70%RH external air by 10 degrees, does it then become to 0% RH air?So if what I am told is true then 40%RH 20 degree internal air should balence 85% RH 10 degree external temperature air as the PVP would be balenced!
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