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Choosing a dehumidifier

Moisture in house air can be a problem when there is either too little or too much. Air that is “too dry” can cause discomfort, dried and itchy skin and nasal passages, cracked or rickety furniture, and sparks when you reach for a doorknob or another person.

Air that is “too damp” can cause itchy skin and nasal passages, ongoing condensation on windows, water damage to materials, mould growth and even rot of wood materials in your house.

What Is Relative Humidity?

Relative humidity is a percentage. It tells you how much moisture is in the air relative to the maximum amount the air can hold at that temperature. For instance, when air at a given temperature contains all the water vapour it can hold at that temperature, the relative humidity is 100 per cent. If the humidity is higher than 100 per cent, moisture will begin to condense from the air. If the air contains only half the water it can hold at that temperature, the relative humidity is 50 per cent.

Warm air can hold more moisture than cool air. The relative humidity of a sample of air will change as the temperature changes, even though the actual amount of moisture in the sample air does not. For example, as a sample of air cools, the relative humidity rises.

Note that both excessively dry and overly damp conditions can both lead to the same problem of dry and itchy skin and nasal passages. In the first case this is because the air is dry and in the second case because it contains mould debris and spores that are toxic.

What is the “right” moisture level?

Generally, the “right” moisture level— the relative humidity—in your house is less than 50-55 per cent. At less than 50 per cent relative humidity it is unlikely that mould will grow indoors.

There are cases when 50 per cent relative humidity is too high. For instance, if there is condensation on your windows in cold weather, it's a good idea to lower your relative humidity to as low as 30 per cent.

Another instance: if you, or someone in your family, is asthmatic, you should consider keeping the humidity level in the bedroom at 40 per cent or less.

Dust mites prefer relative humidity of 50 per cent and higher. Dust mites leave debris in bedding, and the debris aggravates asthma. Keeping the relative humidity at 40 per cent or less controls the dust mites and reduces their effect on asthmatics.

Sometimes, reducing relative humidity won't solve moisture problems. Defects in insulation or the air barrier in walls and ceilings can cause cold spots in your house. They show up as areas where there is always condensation, even if relative humidity is 50 per cent or less. A dehumidifier won't solve the problem. You will need help from a qualified builder, renovator or insulation specialist.

Where does moisture in air come from?

Moisture can come into your home from many places. Outside sources include the soil around your house, surface water drainage and damp outdoor air. Breathing and perspiration by you, your family and your pets is a major source of indoor moisture. So are showering, bathing, drying clothes indoors, venting clothes dryers indoors, washing dishes and floors and humidifiers.

Most houses have more than one source of moisture. Moisture can cause problems once in a while, or all the time. A little prevention can keep excess moisture out of your home's air and prevent both occasional and continual problems.

Catastrophes—such as plumbing leaks or floods—can cause serious problems very quickly. You will need emergency repairs to deal with them.

Checking the moisture level in your house

A "hygrometer" measures relative humidity. A hygrometer is an inexpensive, easy-to-use instrument, sometimes called a humidity sensor or relative humidity indicator. There are mechanical and electronic hygrometers. Electronic hygrometers cost from around £10 to £60.

Stopping moisture

Preventing moisture from entering your house is the best way to solve moisture problems.

Easy preventive measures include shutting down humidifiers, drying clothes outdoors and venting the clothes dryer outside the house. One of the best ways to reduce moisture is to use a good quality, quiet bathroom fan. It vents moisture from showers and baths.

Air out your house when there's a dry spell and no chance of moisture problems. However, you can reduce relative humidity in dry, cold weather by increasing ventilation. A whole-house ventilation system, such as a heat recovery ventilator (HRV) or an exhaust fan coupled with fresh air intakes, will increase ventilation and dry out house air.

In the summer you can use an air conditioner that removes water from incoming air instead of just cooling it. Look for an air conditioner with a high “latent heat” rating rather than a good “sensible heat” rating.

In regions where there are months of cool, damp weather or hot, muggy weather, ventilation just adds more moisture to indoor air. A dehumidifier is an effective way of preventing moisture problems.

Dehumidifiers and how they work

Heat pump dehumidifiers

How a dehumidifier works

Dehumidifiers use a heat pump (similar to an air conditioner's heat pump) or chemical adsorbents to remove moisture from the air without cooling the air.

A heat pump dehumidifier uses a fan to draw indoor air over a heat exchange coil. The coil can be almost be at  freezing point but this depends on the air temperature that enters the machine. The water in the air condenses on the coil and is drained. A second heat exchange coil reheats the air, which the dehumidifier exhausts into the room.

A heat pump dehumidifier dumps heat lost from the compressor and fan motors into the air. It returns to the indoor air the heat generated by the dehumidifier after turning water vapour to liquid.


If you need some product advice? Call 01225 436236