choosing-dehumidifier

Choosing a dehumidifier

We would like to offer you some dehumidifier advice on using your dehumidifier during extreme cold weather where you may find that your dehumidifier may not be extracting very much water yet you may still see signs of condensation. Don’t be disappointed as there may not be anything wrong with your dehumidifier.

The reason could be that you have not set your humidistat to a low enough level. If you have a dehumidifier with a normal minimum setting of 50% then our advice would be to put the machine into an over-ride condition to extract more moisture. But don’t expect too much extraction as in very cold conditions the cold air can not hold very much water vapour.General dehumidifier advice would be to have an ideal relative humidity setting anywhere between 40 & 60%. 100% is saturated air and at 0% you have no moisture. For some reason many customers are using a dehumidifier when there is no real need to do so.

Our advice would be to purchase a thermo-hygrometer so you can measure the conditions within your home. You may be running a dehumidifier needlessly and therefore just wasting electricity.If you still have condensation and have a fully automatic dehumidifier and you have an over-ride or boost function then use that function so that the machine will run constantly as you may need to try and achieve around 40% rH if you are experiencing condensation.

During cold weather such as we have experienced this year, internal surface temperatures on windows, window frame work, walls, roofs etc may be very low and close to the dew point temperature. The only way to deal with this is to lower humidity levels to a much lower level by using a dehumidifier. An example would be a room heated to 21 deg C with a relative humidity of 50%. The dew-point or the point that moisture will form will be 10.19 deg C so if you have a window surface at that temperature you will experience condensation. Raising your temperature by 1 deg C with a reduction of relative humidity to 47% will only alter the dewpoint to 10.18 deg C.

If we raise the air temperature to 23 deg and maintained the same humidity we would actually have condensation form on surfaces at 12.03 deg C so a you can see we need a dehumidifier to take moisture out of the air to where the dewpoint is lower that any surface temperature. Using a dehumidifier to drop the relative humidity down to say 40% would see a dewpoint of 6.9 deg C +/- 1.06 deg so unless any surface temperature is around that level we would not see condensation. In extreme low external temperatures you may need to lower your relative humidity even further and that is not easy with a compressor type dehumidifier so all you can do is consider raising the internal temperature so that dewpoint temperatures will be higher therefore allowing the dehumidifier to operate more efficiently.

Dew Point – the point at which moisture formsTo determine the Dew Point from the charts below, find the temperature of the air in question on the left side of the table.  Next, locate the relative humidity of the air in question across the top of the table.  The intersection of these two numbers in the matrix identifies the temperature at which Dew Point is reached
When air comes in contact with a surface that is at or below its Dew Point temperature, condensation will form on that surface.
Example:If the temperature in a facility is 75º F (24º C) and the relative humidity is 35%, the intersection of the two shows that the Dew Point is reached at a  temperature of 45º F (7º C), or below.  This means that moisture vapor in the 75º F / 35% RH air will condense on any surface that is at or below the Dew Point temperature of 45º F.This example could represent the inside of a building that is 75º F and 35% RH during the day.  At night the outside temperature drops.  It is not likely that the inside air of the building will cool from 75º F to 45º F, but it is quite possible that the framing and any exposed exterior surfaces will reach the Dew Point temperatures, creating condensation.
If the temperature in a facility is 75º F (24º C) and the relative humidity is   35%, the intersection of the two shows that the Dew Point is reached at a  temperature of 45º F (7º C), or below.  This means that moisture vapor in the 75º F / 35% RH air will condense on any surface that is at or below the Dew Point temperature of 45º F.This example could represent the inside of a building that is 75º F and 35% RH during the day.  At night the outside temperature drops.  It is not likely that the inside air of the building will cool from 75º F to 45º F, but it is quite possible that the framing and any exposed exterior surfaces will reach the Dew Point temperatures, creating condensation.

Air Temperature in Degrees Celsius

AirTemp °C% Relative Humidity
100959085807570656055504540353025201510
434342414039383735343231292724221816115
41413938373635343332292827242219171383
38383736353433323029272624221917141170
3535343332313029272624232119171512940
323231312928272624232220181715129620
29292827272624232221191816141210730
2727262524232221191817151312107420
2424232221201918171614131197520  
21212019181716151413121087430
18181717161514131210976420
16161414131211109765320
1313121110987643210
10109877643210
7766443210
4443210
2210
00

Example: Read the air temperature in the left hand column and the humidity at the top of the chart.  If the temperature of the storage unit is 75ºF (24º C) and the relative humidity is 35%, the intersection of the two shows the dew point of the area to be 45ºF (7ºC).  If the metal coming in is below 45ºF (7ºC), water will condense on the metal.  
 

Air Temperature in Degrees Fahrenheit

AirTemp °F% Relative Humidity
100959085807570656055504540353025201510
11011010810610410210098959390878480767265605141
10510510310199979593918885838076726762554737
100100999795939189868481787571676358524432
9595939290888684817976737067635954484032
9090888785838179767471686562595449433632
85858381807876747269676461585450453832
80807877757371696765625956535045403532
757573727068666462605855524945413632
7070686765636159575553504744403732
65656362605957555350484542403632
606058575553525048454341383532
5555535250494745434038363332
505048464544424038363432
45454342403937353332
40403937353432
35353432
3232

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