Sizing a Dehumidifier

Sizing a dehumidifier is not always as simple as it may appear. There are guidelines that should be followed, about how many "pints per day" a unit should be able to remove from the air to control humidity in a room of a given size.

There are also factors which cannot be reduced to a simple formula where individual priorities and judgements play a part. In this article we will try to cover the most important things you may wish to consider when sizing a dehumidifier for an area of your home.

First, the simple part. Rooms in our homes have ceilings that vary in their height from the floor but in the vast majority of homes the variation is so minor as not to matter. Simply measure the floor area and you have the size of the room.

If you happen to have exceptionally high ceilings then some allowance may need to be made as it is the volume of the air space that is critical, not its area. If we say that a typical ceiling height is 7.5 feet but your ceilings are ten feet from the floor, the volume of air in a room of a given size is one third greater in your home than in most others. If you measure the floor area and add a third you will have a revised "floor area" for sizing a dehumidifier. So, for example, if your room is 600 sq ft you need a model that is rated for at least 800 sq ft.

It is quite confusing when you go to buy your appliance as the figures for area coverage quoted by manufacturers for models of the same capacity vary wildly. The sizing guide we would recommend is the one produced by AHAM (Association of Home Appliance Manufacturers) and reproduced on the Energy Star website. You may read or download the guide by going to Energy Star (opens in new window).

The information is presented in the form of a table which gives the minimum capacity, in pints per day, a unit must have to control a range of areas, from 500 sq ft to 2500 sq ft, at four levels of "wetness". It is, to the best of our knowledge, the most reliable and objective guide available for sizing a dehumidifier.

Having studied the guide you will easily be able to see the minimum size needed for your home. What else might you want to consider when sizing a dehumidifier?

First you need a "margin for error". If you work out that a 30 pint unit is just big enough for your needs we recommend that you buy at least a 40 pint unit. The 30 pint model you buy may actually have a capacity of 28 pints, due to errors in testing or simply rounding the capacity up to a "neat" figure. Its performance may deteriorate a little over time. You may have classified your room as one level drier than it is, or you may even have made a mistake when taking the measurements. When sizing a dehumidifier it is better to play it safe and choose one a little bigger than you apparently need.

If your intention is to maintain a level of relative humidity much below 50%, say 40 or even 35%, you will need a slightly larger capacity appliance to achieve this. If you select a unit that is "just big enough" it is likely that it will run continuously rather than cycling on and off as it should.

A larger model will run for fewer hours per day to maintain your chosen level of relative humidity than a smaller one. These appliances are noisy and larger models are noisier than smaller units. The noisier one may, however, be less of a nuisance if it can be switched off when the noise is most intrusive, when you have company or while watching TV for example.

Larger models consume more energy than smaller ones but they remove more water per energy dollar than smaller units so are more efficient. Since energy will almost certainly cost more over the lifetime of your appliance than the unit cost to buy this may make the larger model cheaper over all.

As you can see there are a number of arguments for buying a unit larger than a the best available guidance suggests. This is an entirely individual decision but you may wish to take our comments into account when sizing a dehumidifier for you home.

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