If you ventilate a house with outside air at 5degC, and heat it up to indoor temperature, it can then absorb more than double the amount of water vapour than the same ventilation with outside air at 15degC. So ventilation in the winter is much more effective at controlling humidity than ventilation in the summer.

This suggests you should increase MHRV flow rates in summer to control humidity and decrease them in winter to avoid wasting heat.

But does anyone here actually do that? Do you use humidistat to do this or manumatically? Or do people stick at one flowrate all year, and supplement by opening windows in the summer?

Interested as am looking at a MHRV project and deciding how big to size the system.

I turn my off through the summer, as I go for natural ventilation then ( windows open) no condensation problems

Thanks all!

Noted the summer overheat issue some of you have down South... it's not such a consideration here!

I'm interested if anyone adjusts their ventilation specifically because of getting better drying effects with dry winter air than with muggy summer air?

I just leave ours alone the whole year. Auto summer bypass.

Cooling impact is limited - in summer we need open windows (and room fans at night when it's really hot)

Our MVHR goes to boost based on bathroom humidity sensors (amongst other things) - there are a few high humidity atmospheric considitions when it goes into boost permanently and i have a setting to over-ride for those few days a year.

Luckily, cold winter air is available exactly in winter :)

The dew point of air at 20°C and even 50% RH is only 9°C so it would require extraordinarily poor windows to suffer from internal condensation if there is a ventilation system running.

In our house, internal RH stays much the same throughout the year.

Posted By: WillInAberdeenThe picture we are getting is that most MHRV units are way oversized to cope with this (per DJH's 300m3 unit that runs at 50m3)

Sizing is determined by building regs required extract and supply rates. I don't remember ours offhand, but we nominally have four bedrooms of which one is normally occupied, so our normal ventilation rate is a long way below what we would use in a fully-occupied house. There is also a desire to somewhat oversize units, since that results not only in quieter operation but also more efficiency at lower fan speeds I think (I'm open to contrary information on the last point, but the powers at the various rates I have measured are 50 m³/hr -> 9 W, 125 m³/hr -> 29 W, 165 m³/hr -> 49 W, IIRC)

Posted By: WillInAberdeenSorry, that's what I meant by us 'not being driven by BRegs' - we are in Scotland.

I don't understand that. You have building regs, no? And they say to follow CIBSE Guide B and that is much like the English regs?

The system generally needs to be sized for the maximum designed occupancy - either for your own use at Christmas or perhaps a midsummer BBQ when it turns out to pour down, or for whoever owns the property after you. Clearly the same system needs to work all year round, so I'm not sure why the regs should take any account of the season? Extract and fresh air supply rates don't depend on the climate either.

I think the power requirements are a blend of increasing fan efficiency at higher speeds and increased duct losses at higher speed. But I'm not sure I've remembered my numbers correctly, now. In fact, I found my records and the numbers are 50 m³/hr -> 9 W, 125 m³/hr -> 21 W, 165 m³/hr -> 29 W

Posted By: WillInAberdeenNo, in Scotland there are Building Standards (not Regs).

Notice I didn't capitalise. I was not talking about the names of particular pieces of legislation. You still have regulations that govern building. Call them what you will.

I was responding to your claim that people oversize ventilation systems. You are now saying that in your case, you are not planning to provide ventilation by means of a mechanical ventilation system, but by a more complicated method of which a mechanical system forms but one portion. In that circumstance, clearly the sizing calculations appropriate fore a whole house ventilation system are not appropriate.

Posted By: WillInAberdeenSo we will look for 80m3/h of ventilation, from MHRV and leakage.

If you look at CIBSE B, you will see that the recommended fresh air supply rate is well above that. If you read other documents, notably Swedish ones, you will appreciate the importance of providing enough fresh air to people, especially children.

search for swedish iaq research

I think CIBSE are saying the focus should be on per person supply rates rather than per volume, or per floor area rates. That is generally sensible, IMHO. There is an assumption in regulations about the amount of space per person in domestic situations that simplifies domestic calculations (and reduces arguments from people trying to outguess the system). I believe it's more explicit in German law than English.

I would recommend sizing for an increased exchange rate, even if you don't use it most of the time. I'd also consider trying to increase humidity buffering, wherever possible, to avoid extremely low RH. It's also pretty easy to add humidity if required with a few plants or an upside-down earthenware pot standing in water, or even a humidifier :)

Excessively low RH is a well-known problem possibility in PH. Reducing ventilation rates can be a good way to ameliorate it. Increasing humidity supply is another: enthalpy exchanger in the ventilation system, humidity injection as mentioned above etc. It's not an insuperable problem.

Here are some file names from my disk that might point to other info:

blomqvist 2009 - Distribution of Ventilation Air and Heat by Buoyancy Forces Inside Buildings.pdf
crump et al 2009 - NHBC - Indoor air quality in highly energy efficient homes - a review.pdf
feist et al 2006 - Re-inventing air heating - Convenient and comfortable within the frame of the Passive House concept.pdf
grimsrud 2006 - Ventilation Standards and High Performance Buildings.pdf
koiv et al 2010 - Indoor climate and energy consumption in residential buildings in Estonian climatic conditions.pdf
kurnitski et al 2007 - Use of mechanical ventilation in Finnish houses.pdf
meisenberg & tschiersch 2011 - Thoron in indoor air: modeling for a better exposure estimate.pdf
nasa 1989 - Interior Landscape Plants for Indoor Air Pollution Abatement.pdf
siddall 2012 - Passivhaus ventilation: Its not a lot of hot air Indoor-Air-Quality-and-Ventilation-Mar-28_MJS_29-KdeS41.pdf
wargocki et al 2002 - Ventilation and health in non-industrial indoor environments: report from a European Multidisciplinary Scientific Consensus Meeting - EUROVEN.pdf
wolkoff & kjaergaard 2007 - The dichotomy of relative humidity on indoor air quality.pdf

I haven't actually looked at them again to see which might be helpful, I'm afraid. I just selected titles that looked like they might be relevant to you.

HTH, Dave

edit: PS I haven't read the paper you cited, but there was research in Sweden a while ago that indicated higher ventilation rates benefited school children, so maybe that has translated into a fashion for too high a ventilation arte in houses?