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
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

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
100	95	90	85	80	75	70	65	60	55	50	45	40	35	30	25	20	15	10
110	110	108	106	104	102	100	98	95	93	90	87	84	80	76	72	65	60	51	41
105	105	103	101	99	97	95	93	91	88	85	83	80	76	72	67	62	55	47	37
100	100	99	97	95	93	91	89	86	84	81	78	75	71	67	63	58	52	44	32
95	95	93	92	90	88	86	84	81	79	76	73	70	67	63	59	54	48	40	32
90	90	88	87	85	83	81	79	76	74	71	68	65	62	59	54	49	43	36	32
85	85	83	81	80	78	76	74	72	69	67	64	61	58	54	50	45	38	32
80	80	78	77	75	73	71	69	67	65	62	59	56	53	50	45	40	35	32
75	75	73	72	70	68	66	64	62	60	58	55	52	49	45	41	36	32
70	70	68	67	65	63	61	59	57	55	53	50	47	44	40	37	32
65	65	63	62	60	59	57	55	53	50	48	45	42	40	36	32
60	60	58	57	55	53	52	50	48	45	43	41	38	35	32
55	55	53	52	50	49	47	45	43	40	38	36	33	32
50	50	48	46	45	44	42	40	38	36	34	32
45	45	43	42	40	39	37	35	33	32
40	40	39	37	35	34	32
35	35	34	32
32	32

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