Mechanical Ventilation

[Edislaw]

I am currently renovating my Victorian home and have discovered in a number of areas that decisions are needed which are usually made by tradesmen based on their knowledge of regulations and regularly ignore science. For example, I am currently considering ventilation, ignored the regulations and using common sense. Why do I need it - I like to be comfortable and healthy.
Cutting to the chase it became apparent that I needed to avoid mould growth, control humidity and remove odours. The answer I came up with was to use a humidistat to turn on an extract fan at a given level of humidity. Luckily I checked what humidity is and didn't rely on my remembered knowledge. This leads me to ask the question: If the humidity level is, as it is here today, 69% does that mean an extractor fan set to 60% would work constantly?

Positioning fans in the areas where moisture is most likely to be concentrated seemed sensible so I decided to talk to the experts. Thats when I discovered that the experts (technical department of extract fan manufacturer) are only concerned with selling whichever model of their fan meets regulations in a given situation.
My research suggests to me that mould starts to grow at around 55% humidity so I should look to extract damp air above that level but fans generally work on two settings 60% and 90% - is there a scientific reason for this and at what level of humidity should I set my fan to operate?
Experts talk about air flow in litres per second and how many air changes are required per hour. Taken together this suggests it is possible to calculate which is the best fan for a particular room but regulations get in the way and fan manufacturers say that the size of the room is irrelevant - maybe to regulations but surely not to achieving a healthy environment. Do I need a fan with a greater capacity for a larger room and if not, why not?

 

[bernardgreen]

It is not just the level of humidity, ( more accurately relative humidity ) but also the temperatures of the air, the walls / ceiling and dew point. Mould grows on damp surfaces, even with near 100% humidity surfaces can still be too dry for mould to grow on them.

 

[ericmark]

Humidity is in fact relative humidity it is relative to the temperature. If you want to know how much moisture there is in the air without the affect of temperature then you need to look at the dew point.

So my house today is 35% humidity at 23°C and outside is 95% humidity at 8.1°C so putting that into my Excel spread sheet I see dew point inside house is 10°C and outside at 7.2°C so changing air inside house with air outside the house will still make the house drier.

The problem with a fan is it takes time for replacement air to warm up, so set a 60% then it is possible the sensor is too cold to ever turn off. This can easy happen in the summer so tend to go for 90% to allow for air to be on the cool side.

 

[ericmark]

As an after thought have you considered a heat recovery unit? There are two basic types, one has two fans one in and one out it is a large box and installing needs some pipe work, the other has a single fan and needs a reasonably sealed room, it blows air out and the depression in the room sucks air back in through the heat exchanger, it is much cheaper and fits like a standard extractor fan the pipe is the heat exchanger. Not tried fitting either, in my house simply open a window and that is enough. Bathroom door always left open when not in use so dries out well, even with 40% and less relative humidity still get some mould growth, I blame my wife who never cleans the shower after she has finished. Quick wipe down with cleaner stops the mould not just keeping it dry.

 

[Edislaw]

I've come to a much simpler conclusion. Use the windows.

 

[endecotp]

If the humidity level is, as it is here today, 69% does that mean an extractor fan set to 60% would work constantly?

If 69% is the relative humidity of the OUTSIDE air, then the answer is NO, because the outisde air is warmed once inside your house, and so its relative humidity is lower than 69%.

Generally you can assume that when you create lots of moisture (shower, cooking, drying washing) you need to extract that damp air. Otherwise you don't. Do that, and your only danger of condensation / mould is if you have a particularly cold external wall somewhere e.g. in an unused room where the heating is off.

manufacturers say that the size of the room is irrelevant

Genenrally it's irrelevant to the number-of-changes-per-hour number, but not to the litres-per-second number.

 

[Jackrae]

I've come to a much simpler conclusion. Use the windows.

Or get a simple on/off fan with timer and avoid the eventual and inevitable failure of an automatic humidistat fan. If you live in the house then you control the fan - however if you are renting the property there may be a case for auto-systems that negate the possible negligence of leasers.

 

[Tipper]

Don't forget that 'open' gas burning appliances such as gas cookers and open fires have specific ventilation requirements to avoid CO build up. Also extract fans and tumble driers can pull combustion flue gases back through an appliance, e.g. a boiler unless room sealed, back into the house.
Can have deadly consequences!

 

[Jackrae]

That is true but only if the vent is actually in the same room as the appliance. If you have an open gas boiler in a bathroom it is time you spent some money and saved a life (maybe your own, or your childrens')