- in HVAC by David Miller
What Size Dehumidifier Do I Need?
The question of what size dehumidifier you need is not one with a simple answer. When sizing a dehumidifier most people just look at the Pints per day specification. This is a misleading indicator of the effectiveness of the dehumidifier.
Knowing the best size dehumidifier to buy means taking a number of different factors into account and not just the amount of pints per day it can extract. It’s also not a trivial question because if you buy the wrong dehumidifier could end up spending a lot of money on electricity while not achieving the results you want.
Dehumidifiers work best when they are able to cycle on and off rather than running permanently. If you buy a dehumidifier that isn’t capable of bringing the humidity of your room down to where you want it then it’s never going to turn off. If it’s running all the time you can be sure that pretty soon it’s going to break. The information below will help you work out what the minimum size is that you need. Make sure that you choose a dehumidifier that has specifications that are 10% to 15% more than this.
While there is a lot of science and engineering that goes into controlling humidity levels in your home it ultimately comes down to room size and the humidity of the air. Once you determine these factors you will be able to work out the optimum size dehumidifier for your needs.
Dehumidifier Sizing - Room Size
Firstly it’s important to know how big the space is that you want to dehumidify? The size of the room determines how much air needs to be dehumidified. We will need to work out both the area and volume of the room. To calculate the area we simply multiply the length (in feet) by the width of the room. This gives us the room area in square feet. To calculate the volume of the room simply multiply the length by the width and then multiply that result by the height (length x width x height). This will give you the volume of the room in cubic feet. Now that we have the size of the room we need to understand how humid the air is.
Dehumidifier Sizing - Humidity Level
You can estimate the worst case humidity you expect or else measure it. To estimate it you can look at the weather page for your area to see what the highest humidity is that you experience there. If you’re sizing a crawl space dehumidifier or basement dehumidifier then you need to measure the humidity because these are affected by other factors besides the air outside.
To get an accurate measurement of the humidity of a room you need to use a hygrometer. You can get a cheap one for a few bucks like this one from Amazon. Once you’ve got a reading for the room humidity you can use the table below to work out how often the air needs to circulate to get it down to the optimum humidity level. This is measured in Air Changes per Hour, or ACH. Basically this tells you how often all the air in the room needs to pass through the dehumidifier.
Relative Humidity (RH) | Air Changes Per Hour (ACH) |
---|---|
60% - 70% | 3 |
70% - 80% | 4 |
80% - 90% | 5 |
90% - 100% | 6 |
Once you have the ACH figure you can now get a handle on how powerful the fans of the dehumidifier need to be. If a dehumidifier is great at extracting moisture from the air but has weak fans then it’s not going to be able to process all the air in your room. Air flow is measured in Cubic Feet per Minute, or CFM. To calculate the CFM required multiply the volume of the room (in cubic feet) by the ACH from the table above and then divide the result by 60. This will give you the required Cubic Feet per Minute (CFM) of air that the dehumidifier’s fan should be capable of circulating.
As an example: Let's say you measure the RH of the room to be 65%. From the table above we see that you need 3 Air changes per hour, or an ACH of 3. If our room is 30 feet by 30 feet and 10 feet high then the volume of the room is 9,000 cubic feet (30x30x10). Multiplying the volume of the room by the ACH gives us 27,000 (9000x3). We now divide that figure by 60 and get 450. So we need a dehumidifier that has fans capable of moving 450 cubic feet per minute.
Now that we’ve got an idea of the fan the dehumidifier needs we can finally get to the capacity, or the Pints per day figure. First we need the area of the room in square feet. We get this by multiplying the width by the length. Then we use the humidity measurement you got earlier and the area of the room to get the capacity required from the table below.
Dehumidifier Sizing Table
Area | Pints Per Day | |||
---|---|---|---|---|
Square Feet | 60%-70% RH | 70%-80% RH | 80%-90% RH | 90%-100% RH |
500 | 10 | 12 | 14 | 16 |
1000 | 14 | 17 | 20 | 23 |
1500 | 18 | 22 | 26 | 30 |
2000 | 22 | 27 | 32 | 37 |
2500 | 26 | 32 | 38 | 44 |
3000 | 30 | 37 | 44 | 51 |
So let's continue with the example we used above. The area of the room is 900 square feet (30x30). This means that we would be looking in the second row of the table because our room area is pretty close to 1,000 square feet. The RH we measured was 65%. That means that we would be looking in the second column (60%-70% RH) of the sizing table. From this we see that we will need a dehumidifier that is capable of extracting at least 14 Pints per day. It's always a good idea to allow for about 15% to 20% more so we would be looking at around 16 or 17 Pints per day.
People often ask "What is the best dehumidifier?" when they should really first ask “What size dehumidifier do I need?”. Hopefully from the above it’s clear that capacity in Pints per day is not the only factor to look at. Once you have a good idea of the size of the space you’re trying to dehumidify as well as the expected maximum humidity you can make a better decision. The first thing to confirm is the ability of the fan to circulate enough air. Once you have that you can work out the amount of moisture you can expect to remove per day. Once you have those two figures you can choose the best dehumidifier size that satisfies both of these requirements.
Product Images Sourced From Amazon.com