Can you have too many fans in a damp house?

pdej4

We've just moved into a 1930's 3 bed Semi detached house that has a fair bit of condensation.

The bathroom had no extraction fan and at the recommendation of several damp surveys and the original surveyor we've had a Vent Axia Centrif Duo unit put into the bathroom (installed yesterday). Prior to this going in, if the two of us had a shower each in the morning, the whole bathroom would be sopping wet, with the walls covered in water (waterproof paint, so the walls cannot breathe). With the addition of the fan, which runs on a trickle setting all the time and uses its boost when the light switch is turned on, it manages the steam in the room brilliantly, and whereas before the room would be wet (note, not damp!), the walls are dry and the window condensation disappears within 20 minutes of the light being turned off (trickle setting clears most of this up, as the boost is only on for 2 minutes after the light is turned off).

Similar scenario in the kitchen. There was an extraction hood, but it was not hooked up to extract the air outside, so it just made a lot of noise and circulated the air within the room. Now that is working too, so less condensation and moist air in the kitchen when cooking.

The issue I have is that I didn't realise how good these fans would be, but we've also not had a really cold morning this morning like we did last week, when I agreed to put in a Drimaster 2000 unit too which would help the other rooms. The master and third bedroom (box room) have air brick vents in them which currently let loads of cold air in, particularly noticeable overnight. The damp guy who came and recommended that rather than replace them with valved one way vents we should consider a drimaster 2000 unit which would create the positive pressure we need to expel the stale air. I'm happy with the idea of the unit etc and have read the pro's and con's, but with the more powerful Vent Axia humidity controlled extractor fan in the bathroom will the two be overkill?

I know the bathroom is now ok, but I don't know about the rest of the house. I know there are higher damp meter readings in the hallway/staircase, and have been warned about mould due to condensation in those places so thought the Drimaster would still be a good addition for this purpose.

Finally, would we be good to turn off the trickle on the Vent-Axia fan in the bathroom otherwise presumably the Drimaster air will just seek the bathroom?


It may also be worth noting, we have a mix of new and old double glazing, as well as some single pane glazing. The house has no cavity, and is solid brick construction.

anotherbaldrick

You can go overboard with too much extraction . You need enough to stop condensation but beyond this you are throwing expensive heat out. Have you checked that you are not producing more moisture than you need to ? in the kitchen,personal and clothes washing ? and are your external walls as warm as you can get them with cavity insulation ? I think the philosopy has to be keep the house warm, make as little moist air as possible and ventilate enough to keep Relative Humidity below 65% . you can buy RH meters cheaply on the high street or net.

Gloomendoom

If you are blowing moist air out of your house it has to be replaced by air coming in from outside. If you stop air entering from outside by fitting one way vents to the air bricks, all you will do is reduce the amount of moist air blown out so your condensation problem will return.

Insulating the walls or fitting a proper heat recovery ventilation system, or both, is probably your best bet.

Jaynne

A one way vent helps a bit with draft but really ANY vent causes a draw of more outside air into your house from somewhere otherwise you'd pump all the air out and then suffocate

Anotherbaldrick has it spot on - you want just enough ventilation to stop the build up of moisture on walls but no more otherwise it causes your heating bills to rise.

In a normal house you really should only need serious ventilation in the bathroom and kitchen and possibly anywhere you dry clothes inside. The other places that sometimes need it are conservatories where they can get lots of moisture laid air either from the house or outside then undergo rapid air temperature changes resulting in condensation on the nice cool walls/roof.

ListysDad

Personally I'd put the second system on hold for a while and see how things pan out with just the bathroom fan.

What is actually happening is you are creating a negative pressure area in the bathroom. This draws air in from everywhere else to replace it so chances are that it will seriously reduce moisture levels elsewhere too. What you do have to do is consider where you open windows though. Ideally you have the kitchen one cracked and teh replacement air will flow through that to the bathroom assuming no others are open thereby taking the kitch moisture with it.

Air movement is an odd thing. You have to be continually aware of how it moves and understand it will always take the path of least resistance. So if you have two windows open, one close to the bathroom and one far away, the impact of the one far way will be almost negligible - you have to manage teh air flows to get the best from your system

HTH

Owain_Moneysaver

If the bathroom vents are sucking air out, the replacement air will be coming in through the air bricks and the rooms with the air bricks will be colder. Hence you will need to add additional heating to those rooms. Cold rooms can be insulated internally with something like Marmox board. This is DIYable and can be done room by room to coincide with room redecoration.

You might be better running the bathroom extractor through a heat exchanger so the replacement air is pre-warmed. Otherwise you will just be sucking out all your expensive warm air and might as well leave the window open.

Whole-house heat recovery is also excellent if you have the tumble dryer vented through it (with a fluff filter) as about three-quarters of the tumble dryer heat gets returned to heat your hosue.

Individual extractor fans with heat recovery are here

pdej4

Thanks for all the help people, and the links owain. I've cancelled the drimaster as I think the bathroom fan is doing a great job, to the point where I think in a couple of weeks I will try it without the trickle setting. It doesnt even give the steam a chance to settle on the walls whilst we're having a shower because the boost is so good.

The vents in the bedrooms make those rooms quite cold and I've experimented by covering the one in our bedroom whilst leaving the box room one uncovered. It does seem to make quite a bit of a difference. What options are there for covering, blocking or replacing them?

llandyrnog

You mentioned you have solid walls as your house construction. A Govt initiative called The Green Deal is just about to start up and Insulating solid walled houses will be an item that qualifies for Green Deal finance. It is a loan on the proerty , not the occupier and it works on the principle that the savings made on your fuel bills will be more than the repayments.

Gloomendoom

pdej4 wrote: »

Thanks for all the help people, and the links owain. I've cancelled the drimaster as I think the bathroom fan is doing a great job, to the point where I think in a couple of weeks I will try it without the trickle setting. It doesnt even give the steam a chance to settle on the walls whilst we're having a shower because the boost is so good.

The vents in the bedrooms make those rooms quite cold and I've experimented by covering the one in our bedroom whilst leaving the box room one uncovered. It does seem to make quite a bit of a difference. What options are there for covering, blocking or replacing them?

Good call on cancelling the Drimaster. I had a look at it and it should create a positive pressure in your house but it appears to do so by pumping huge quantities of air air in from the attic. I have a remote monitor that shows outside, inside and attic temperature in my house and, unless the sun is shining directly on the roof, the attic space stays about 1 degree warmer than the outside temperature. I certainly wouldn't want that blown into my house in the middle of winter.

As somebody else pointed out, the air coming in to replace the air blown out by your bathroom extractor will take the path of least resistance so, if you want to reduce the cold air sucked through your bedrooms, you need to make it easier for air to get into the bathroom, and keep the door shut while the fan is running.

anotherbaldrick

Another point to check for condensation is in the roof space. Have a look to see if there is a waterproof membrane under the tiles . If the roof space has been well insulated on top of the ceiling joists you can get water dripping from the u/s of the membrane . This has come from moist laden air which has made it's way up from below , through the insulation and hitting the stone cold membrane. To get some ventlation into this cold void is the answer