When things are wetter inside the house than outside, it's time to take action. But is a dehumidifier the answer? Tom McKinlay reports.
So much water was running down the window panes during the winter months that Meridian Energy popped by to assess the flow for hydro-electric potential. Mould spores found the children's bedroom so convivial you thought of redecorating in black to complete the effect.
It was time to take action, head down to the appliance shop and pick up a dehumidifier. It's a well-trodden path. About a quarter of New Zealand homes run a dehumidifier. Figures from 2005 show there were then a quarter of a million in the country and 45,000 sold annually. So they must be good.
Well, yes and no.
Although dehumidifiers will start to address some problems straight away, they are not likely to be a long-term solution.
And in many Dunedin homes, they will perform poorly.
Emeritus Prof Gerald Carrington of the University of Otago's department of physics says dehumidifiers are not designed to work at low temperatures.
"Unless the room is a reasonable temperature, the efficiency of the dehumidifier collapses," he says.
They are designed to operate at above 20degC.
"By the time the room gets down to 15 they are getting pretty manky and by the time they get down to 10 degrees they are awful.""One of the major reasons is as the temperature drops, the dehumidifier has to turn the water into ice in order to remove it from the air."
A paper published by Prof Carrington, Dr Zhifa Sun, PhD student Sam Lowrey and others at the department, identified that dehumidifier capacity is tested by manufacturers only down to a temperature of 18.3degC. Studies in New Zealand have found that average bedroom temperatures are about 10degC.
"I think many people think 'we'll just dry the room out, we will not heat it' but, because they are just drying it out, the dehumidifier efficiency is awful and so it is still costing them more than if they did the right thing, which is to improve the insulation and then try to keep the temperature up," Prof Carrington said.
There's something of an urban myth that reducing the humidity of a room means less energy is required to heat it, but Prof Carrington says this is not the case, citing the second law of thermodynamics.
"If you have high humidity in your house, often it is because you are not heating it or you don't heat very much," he says.
There is some gain in terms of temperature when running a dehumidifier but it is small.
"The energy that is extracted from the vapour to turn it into water goes into the room, so the room gets that energy," Prof Carrington says.
"But you are dealing with quite small power. You may only be getting a few hundred watts from a dehumidifier and you are putting in 150, 250 watts."
A dehumidifier uses about the same amount of power as a refrigerator.
Fellow energy expert Associate Prof Bob Lloyd, also of the university's physics department, says many people heat cyclically.
"They don't heat 24 hours a day, they heat for short periods. When you heat for short periods and then it cools down, all the moisture from breathing and so on drops out as condensation. It actually affects your building fabric quite badly. And it is really a matter of cost."
Certain kinds of heating can even make matters worse.
"There's a strong correlation between people who use unflued gas heaters and those who use or need dehumidifiers, because the gas always produces a lot of humidity," Prof Lloyd says.
"People who are in difficult circumstances will often go to a [hardware store] and buy a portable gas heater and these have no flue," Prof Carrington says. "So the products of combustion - which are carbon dioxide and water on the whole, plus a few noxious things - go into the room."
It makes for terrible humidity problems.
"A month after having bought a gas heater they see the solution, which is to buy a dehumidifier. After nailing one foot to the floor they nail the other one to fix it."
Both physicists agree that dehumidifiers can be effective in terms of removing moisture from a room.
The biggest benefits can include reducing mould and condensation on walls. By reducing moisture in the environment, it is possible to reduce house mite numbers, mould and fungi - the sorts of things than contribute to asthma, Prof Lloyd says.
To survive, the house dust mite needs warm temperatures and high humidity (70%-80%). One study showed that when humidity drops below 60% the population stops growing and dies out. Dust mites do not drink, as such, but absorb water through their bodies, and at lower humidity levels they die of dehydration.
The ideal humidity for room comfort is between 30% and 50%.
Tips:
• The best way to reduce moisture in a room is to achieve the correct balance between heating and ventilation. If you don't get that balance right, any condensation that is produced will settle on cold surfaces, such as external walls and windows.
• The easiest way to ventilate a house is to open the doors and windows at either end of the building for 10 minutes - that will do the job of a ventilation system or a dehumidifier.
• Don't put a dehumidifier outside the bathroom, as it just draws the steam out of the bathroom into the house. Better to ventilate the bathroom by opening a window or using an extractor fan (the same advice applies to steam in the kitchen).
• Drying clothes inside will increase moisture. Placing them under a heatpump will just spread the moisture throughout the building.
