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185 posts

Master Geek

#62686 12-Jun-2010 14:23
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Our old house has a bit of a mould problem in one of the bedrooms. All of the windows are covered in condensation in the mornings and the back side of the house gets very little sun. My other half thinks a dehumidifier will help to solve the problem. I have never used one and am concerned about power use, and how long/when it would need to be on. I hate noise when I sleep so would not want it on at night, would it be effective during the day? We'd only be able to afford an older 2nd hand one. Would somebody be able to tell me more about them please? Thank you.

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249 posts

Master Geek
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  #340961 12-Jun-2010 14:58
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A dehumidifier will certainly reduce / eliminate your bedroom problem.  If you get hold of a temp / humidity gauge combo from Dick Smiths etc you'll see that the relative humidity in your room is high (70% or more) when the temperature is low.  This will cause mould in the room in places where air doesn't circulate and, because you don't get any sun on the room, it will be hard to reduce the moisture in the room.  Opening a window on a dry bright day for a couple of hours  will certainly help and you'll see that the relative humidity will go down (on a bright sunny Chch day our dampest room will go to about 40% RH), but because the moisture is also in the walls, floor etc as soon as it gets cold outside your condensation will be back and your RH will be back up.

A dehumidifier pulls the moisture out of the air, the walls, floors etc and dumps it in a container for you to pour away.  They do chomp thro' the electricity (a small unit will use about 220 watts and remove approx 10 litres of water a day in ideal conditions but they don't need to run for very long - you can determine how long by trial and error - to make a difference.

Ideal conditions for a dehumidifier are damp air and a high temperature; as temp falls so does the effectiveness of the unit so warming the room slightly will allow the unit be more efficient and help reduce the damp in the first place.

You will of course only be treating the symptoms so at the same time try and find a cure for your dampness.  The best bet here it to insulate walls, floors, ceilings etc, get some heat into the room so it doesn't suffer from large temperature swings and air it as much as you can on dry days.  Some form of ventilation is important.  We found that retro-fitting double glazing on our south facing rooms made a huge difference to condensation as so much warmth is lost thro' a single glazed window.

They make the same sort of noise as a fridge when running and, as above, it is better to run them when the room is at its warmest.


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Uber Geek


  #340973 12-Jun-2010 16:27
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May be a false economy to buy an old one.
The newer technology may mean you save more in power in the long run.
But as said above look for what is causing the symptoms not just try to fix them.

How wet the it under your house? e.g. is the soil very wet if so a moisture barrier may be of some benefit.

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4783 posts

Uber Geek


  #341146 13-Jun-2010 11:10
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If you're intending to heat the room then you may find that the dehumidifier pays for its own electricity usage as it's more efficient to heat drier air. If you're not intending to heat the room then make sure you get one with a defrost function as it otherwise won't work in temperatures below about 15 degrees. You may also want to buy one with a programmable timer so that you can run it when you're out of the house if you find the noise bothersome; the compressor in mine is quite noisy.

Overall I have found that mine is very effective, and seems to do a good job of sucking moisture out of bedding and furnishings.


185 posts

Master Geek

  #341936 15-Jun-2010 14:55
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thanks. we have decided to see how we go for this winter as far as a dehumidifier goes, and if we feel we need one for next winter we'll get one on special in a few months time, rather than getting one that is 10 years old and probably not too efficient! i've seen mini heat pumps for sale (portable ones), i wonder if they would do a similar job?

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Uber Geek

  #341947 15-Jun-2010 15:14
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i've seen mini heat pumps for sale (portable ones), i wonder if they would do a similar job?

Sounds a bit strange, a "heat pump" by definition moves heat from an inside unit to an outside unit,  ( ie you have to have a hole in the wall for the piping to transfer the heat)

Anything portable would have to have to still be connected to an external condenser some how, and the only way I can think of for a portable unit to do this is out a window, - which would seriously negate the efficiency of the unit.

7116 posts

Uber Geek


  #341975 15-Jun-2010 16:13
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You need to think of the whole picture. You have condensation because you have moist warm air touching a cold window. There are serveral ways to address this:

Remove the cold window, ie double glaze.
Remove the moisture, ie dehumidify but this could just as easily mean ventilate the room with less moist air from elsewhere, usually outside.
Remove the warm air too I guess, ha but who is going to cool the room?!

What is causing the moist air?
Do you have a range hood over the stove and an extraction fan in the bathroom?
You could remove the animals/humans breathing out warm moist air.....

Yes a dehumidifier will work. It just passes the moist air over some very cold coils. To really make a dehumidifier work you need to warm the air so you get more of the rooms water/moisture actually out into the air. To be effective either run it during a warm part of the day or pay for another heater to heat the room. Best best is to open the windows first, then shut them and run the dehumidifier to extract remaining water from the floor/furnishings themselves.

Expect this to cost a bit. Passive ventilation (opening a window when you can) is cheaper but not as expensive. Forced ventilation will work better, it HRV etc, but this will cost more to run a fan and it brings in more outside air temp air, so cools your house during winter and you'll have to run a heater to offset the low air temp you're bringing in.

1923 posts

Uber Geek

  #343013 18-Jun-2010 13:46
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Are you using a gas heater to keep warm???
These pump moisture and warmth into a room... trouble is the damper air requires more energy to heat.

Portable heatpumps can be got used for $300-$400, or new for about double that. Mitre10 had them on their shelves. You need to vent (duct) the exhaust air outside the building (as it's ice-cold air) and drain the collected moisture out as well (although you can let this collect in the machine and empty the tank every few hours..).

They are easy to use, provided you vent them properly, but do make a lot of noise.
They'll heat your house more efficiently because they're removing moisture (drying) the air at the same time. And with drier air in the house at night if you need to run a quieter heating solution it will work more effectively too.

BUT. If you own the house first put insulation in the ceiling otherwise you may as well set fire to your money on a plate... the heat will last about the same amount of time.



185 posts

Master Geek

  #343027 18-Jun-2010 14:30
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its just our bedrooms that suffer from the problem. Our house is very old (the part with the bedrooms in particular) but is well insulated. we are on a hill and one of the bedrooms gets very little (if any) sun, the other 2 bedrooms do get sun. they are well away from the kitchen and bathroom, but yes we do have extractor fans in both kitchen and bathroom. we've started religiously opening the bedroom windows for a few hours each day, hopefully this is helping things as we really cannot afford any new appliances at the moment. we are not using a gas heater to keep warm, we do use a heat pump or multi burner in our lounge/kitchen area, and have 2 oil heaters we use at night in our bedrooms. we have ideas of getting a dvs home venhilation system or a 2nd heat pump in the future but not sure what would be better for our house. whatever we do end up doing it has to be something that adds value to the house!

1923 posts

Uber Geek

  #343070 18-Jun-2010 17:05
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You can get a kit from Bunnings or Mitre 10 that you fit yourself that will shift air from room to room. Given you already have a heat pump, simply moving the dry air into the damp zone would work wonders I'd think.

They cost a few hundred and there's also full on DVS/HRV clone options you can install yourself (that will take air from the roof space). You'll save thousands doing it this way, but the key to getting the best out of these systems is in getting the inlet and outlet ports in the right places.

Simply moving the air around will help.

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