Useful Information & Advice

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 as the cold air can not hold very much water vapour.

Dehumidifiers and Internal Temperatures

With refrigerated dehumidifiers achieving a low level of relative humidity in a cooler or cold area is difficult so our advice would be to consider the use of dehumidifiers such as desiccant dehumidifiers as these operate far better at lower temperatures.

Extraction Rates

Compressor dehumidifiers work on the principle of moisture condensing on a cold refrigerant pipe inside the unit. As a result, compressor dehumidifiers work well in very warm conditions, but as the temperature falls so their effectiveness drops off dramatically. All compressor driven units are rated in test conditions of 80% humidity and, critically, 30 degrees centigrade temperature.

A three degree drop in temperature to 27 degrees results in up to a 50% loss of performance with Compressor units. At 21 degrees centigrade (which is room temperature) a compressor unit will tend to extract around 30% of its theoretical maximum. Below 15 degrees centigrade most compressor units extract about a tenth of their theoretical maximum and in cooler temperatures below eight degrees basic compressor units stop working as the evaporator plates ice up.

Therefore a compressor unit rated at 20 litres per day extraction will extract up to 5 litres in a normal domestic environment; a unit rated at 18 litres will extract up to 4.5 litres and a 10 litre will extract up to 2.5 litres.

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, the dew point, the point at which moisture will form could be extremely low and therefore you need to either raise your internal temperature to have the condensation evaporate off the windows or lower your relative humidity to a much lower level.

Though raising an internal temperature will decrease relative humidity it will also raise the dew-point temperature and therefore condensation on surfaces will occur at a higher surface temperature if you were to continue raising relative humidity by creating excess moisture from internal laundry drying, cooking, bathing, showering etc. and therefore the only way to deal with this excess moisture is to use a dehumidifier.

Dew Point - the point at which moisture forms

To 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

Air Temp °C	% Relative Humidity
	100	95	90	85	80	75	70	65	60	55	50	45	40	35	30	25	20	15	10
43	43	42	41	40	39	38	37	35	34	32	31	29	27	24	22	18	16	11	5
41	41	39	38	37	36	35	34	33	32	29	28	27	24	22	19	17	13	8	3
38	38	37	36	35	34	33	32	30	29	27	26	24	22	19	17	14	11	7	0
35	35	34	33	32	31	30	29	27	26	24	23	21	19	17	15	12	9	4	0
32	32	31	31	29	28	27	26	24	23	22	20	18	17	15	12	9	6	2	0
29	29	28	27	27	26	24	23	22	21	19	18	16	14	12	10	7	3	0
27	27	26	25	24	23	22	21	19	18	17	15	13	12	10	7	4	2	0
24	24	23	22	21	20	19	18	17	16	14	13	11	9	7	5	2	0
21	21	20	19	18	17	16	15	14	13	12	10	8	7	4	3	0
18	18	17	17	16	15	14	13	12	10	9	7	6	4	2	0
16	16	14	14	13	12	11	10	9	7	6	5	3	2	0
13	13	12	11	10	9	8	7	6	4	3	2	1	0
10	10	9	8	7	7	6	4	3	2	1	0
7	7	6	6	4	4	3	2	1	0
4	4	4	3	2	1	0
2	2	1	0
0	0