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Guide to dehumidifiers

Why is humidity a problem?

Excess humidity can cause problems with both health and property.

Dust mites enjoy humidity so populations increase exacerbating allergies.
Respiratory problems like flu, colds and chest infections can all be products of excess humidity.
Condensation forms on all surfaces. If it forms on walls it may cause paint to blister and wallpaper to peel. Damp spots on the walls are an indicator of high humidity.
Metals rust and wood will form mildew and rot. In the worst cases wood will warp causing doors and draws to stick.
In severe cases mould can cause structural defects in a home.

· Condensation in electric items is never a positive thing!

· Mould, mildew, fungus and musty smells will appear. Even disregarding the serious health implications, living in a damp environment is not pleasant.

How can a dehumidifier help?

Simply put: dehumidifiers dry the air. Dehumidifiers draw in the moist air, remove the excess humidity and release drier air into the atmosphere. They are used in a variety of locations from all types of domestic application, through to gymnasium, office, warehouse and museum environments. At a domestic level, as well as acting as a remedy for the problems listed in the section above, they can be used in other applications. Examples of this are to keep boats dry in the winter, dry washing at way lower running costs than a tumble dryer, dry a room after painting or plastering, keep classic cars in optimum condition, keep damp from a boat in the winter and keep tools from rusting in a garage or shed.

Dehumidifiers can also help with heating bills. Dry air is substantially easier to heat than moist air and thus requires less energy to get to the desired temperature. This will result in lower bills through lower consumption. Excessive relative humidity makes us perceive temperature as greater than it actually is, so keeping your home drier will lead to less use of heating. Although running a dehumidifier may seem like it’s costing money, in the wider scheme you will be saving money.

Larger scale dehumidifiers are used in commercial applications such as swimming pool and spa areas, as well as drying out buildings suffering from flood or water damage.

Post by Intelligent Climate.

How do dehumidifiers work?

There are two types of dehumidifier available; compressor models and those that use a desiccant to dry the air.

Compressor Dehumidifiers

These work on the same principle as domestic refrigerators. A compressor raises the pressure of a refrigerant when it is in gas form. The gas is then led to a heat exchanger (condenser) which cools the gas, changing its state to liquid. The liquid is then forced through a small valve into an evaporator unit where there is low pressure. The liquid evaporates back into gas, it is this evaporation process which cools the gas and hence the evaporator unit.

A fan draws the surrounding air over the evaporator unit, the temperature is lowered and the water held in the air reaches its dew point and condenses onto the evaporator surface. This water then runs off to be collected in a reservoir or through a pipe. The cold dry air is then directed over the condenser coil where it assists in the condensing process. This warms the air, which then exits the dehumidifier.

Desiccant Dehumidifiers

These use a desiccant material, typically Zeolite, which absorbs water vapour in much the same way as silica gel (a packet of which you would often find in a pair of new shoes). A fan draws air into the dehumidifier and passes it through a section of a slowly rotating wheel which holds the desiccant. The material draws the moisture out and dries the air which then exits the unit. The moisture is then extracted from the desiccant; this is done by heating a different portion of the wheel not being used to dry the air. The water then evaporates from the desiccant, is passed over a cold plate where it condenses, runs off and is collected in a reservoir or a permanent feed.

Desiccant vs Compressor

Compressor dehumidifiers work best at higher temperatures and humidities. They are rated at a temperature of 30°C and a relative humidity of 80%.

Desiccant dehumidifiers work better at the lower temperatures, this for example would suit the conditions found in UK boatyards during the winter. In fact, if the temperature drops too low then parts of a compressor dehumidifier will ice up, a problem desiccant humidifiers do not have. Different compressor models have different ways of dealing with this: they either switch off, or they can have a reverse hot gas cycle which quickly heats up the frozen evaporator plates and melts the ice.

Compressor dehumidifiers do warm up the air slightly but desiccant models greatly warm it. This is obviously advantageous in a situation such as in a boat in winter helping reduce the chill and the likelihood of parts freezing up.

Features

Most dehumidifiers also have a humidistat, allowing you to set your desired level of relative humidity. This has two parts; a sensing element and a relay. The sensing element has metal conductors measuring the change in humidity based on the conductivity between the two parts. The relay measures this resistance in turn and decides if the dehumidifier should be running or not.

Dehumidifiers usually have a built-in reservoir to collect the condensed water. On anything other than highly portable devices there is a point to connect a hose as a permanent feed. This saves having to keep draining the reservoir and is useful for situations where the dehumidifier is left unattended for long periods of time.