How effective is a desiccant dehumidifier for room heating?

Thermometer with humidity readingElectric oil filled radiators, fan heaters, halogen heaters, bar heaters, convertor, panel heaters and so on are all 100% efficient at converting electric power into heat.  However, as for how efficient they are at heating the room varies from one type to another.

Another form of electric heating involves a heat pump, where heat is taken from the outdoor environment and pumped into the house like a refrigerator operating in reverse.  Air to air heat pumps do this by extracting heat from the outdoor air and humidity and releasing it indoors.  Technically, a dehumidifier releases heat from indoor humidity as it condenses it to a liquid, so just how effective is one at warming a room?

It takes 2265kJ of latent heat to evaporate 1kg of water without any change in temperature.  1kg of water is roughly 1 litre and 2265kJ converts to 0.6291kWh of energy or 629.1 watts for one hour.  An evaporative air cooler exploits this by evaporating water to absorb heat from the air, so an evaporative air cooler is effectively a humidifier marketed as an air cooler.

A dehumidifier works by condensing moisture from the air, in turn releasing the latent heat.  For every litre of water a dehumidifier collects, it releases around 0.6291kWh of heat in addition to the waste heat from its power consumption.  So technically a dehumidifier operates in reverse to an air cooler by releasing additional heat as a side effect of condensing moisture from the air.

An electric heater can heat up a room in one or more ways depending on the type of heater.  Convector heaters tend to be cheapest, but these can be the least effective.  Oscillating fan heaters tend to be the most expensive, but are better at spreading the heat.  Let’s first briefly take a look at how efficient each type of heater heats a room.

Convector heaters

Convector heaterThis type of heater relies on the natural convection of heat.  This heated air raises straight to the ceiling, in turn pushing down the cooler towards the ground.  By the time the heat reaches the occupants, much of the heat will have escaped the ceiling and upper walls.  To overcome this issue, convector heaters typically come in high wattage models such 3kW, so while they can warm the room reasonably quick, they are quite inefficient at doing so. On the other hand, as they have no moving parts, they are silent and reliable.

Oil filled/free radiators and panel heaters

Oil filled radiatorIn addition to providing natural convection heat, these radiate some heat directly around the heater making the room feel warmer to anyone in view of the radiator.  In the case of a panel heater, these radiate most of heat from the side they heat up.  As the radiator surface temperature can be controlled, the temperature of the air raising directly to the ceiling is much lower than that of a convector heater, in turn wasting less heat out the ceiling and upper walls.  In addition to the radiated heat, these types of heaters are cheaper to run than convector heaters for the equivalent comfort.

Halogen heaters and electric bar heaters

Halogen heaterThese heaters radiate most of their heat directly out from the heater like an open fire place, making the room feel warm to anyone in close or reasonable range of the heater.  Most of this radiated heat in turn warms up anything else in view of the heater, including the walls, floor and ceiling, which in turn is lost out of the room.  Thus, these heaters are inefficient at heating a room over a long period as they must remain switched on to make the room feel warm.  On the other hand, they are effective at providing temporary heat such as in a garage where it does not make sense to warm the whole place just to do an hour of work.

Fan heaters, including ceramic and oscillating type

Fan heaterA fan heater works by blowing out hot air, warming the surrounding air in the process.  These are the most effective at warming the air in the room, particularly in comparison to a convector heater where most of it heat heads straight for the ceiling.  The ceramic type has a ceramic heating element that inherently cannot exceed a certain temperature and in turn are safer than resistive wire heaters, such as in the event of the fan failing.  An oscillating heater continuously changes the airflow direction, improving the heating effectiveness further by evenly warming the air around the room.

Desiccant dehumidifier as a heater

Dehumidifier runningFor this article, we will see how effective a desiccant dehumidifier is at heating a room.  Although desiccant dehumidifiers are not as energy efficient as compressor dehumidifiers in heated rooms, we are more interested in seeing how effective they are as a room heater as they emit a lot more heat than most compressor based dehumidifiers.  This would best simulate the scenario where one uses it in place of an existing electric heater, rather than to supplement it.

For the comparison, we will look at how effective an oil filled radiator and a basic fan heater are at heating a room.  For the fan heater test, I ran it on the low (1kW) setting until the energy monitor accumulated 1kWh.  For the oil filled radiator, I ran it on its medium (1.2kW) setting for 30 minutes and lowered its temperature so it cycled for part of the final 30 minutes so that it accumulated 1kWh on the energy monitor once the hour had elapsed.

For the desiccant dehumidifier, I ran it on continuous mode for the 1 hour duration, after which it accumulated 0.63kWh on the energy monitor.  For the second desiccant dehumidifier test, I ran it on mute (low power) for the full 2-hour duration, after which it accumulated 0.72kWh on the energy monitor.

Each test was carried out on a separate day around midday with the central heating off and no to little sunshine.  The room door was kept shut throughout the test period and I only entered the room each 10 minute interval to take the temperature and humidity readings from the digital hygrometer.  The graphs show how much the temperature elevated in °C since the start of the test:

Heating comparison between fan heater, oil filled radiator and desiccant dehumidifier

As the fan heater provides instant heat, it is the quickest to warm the room.  The oil filled radiator takes at least 10 minutes to warm up and only caught up with the dehumidifier’s low setting test-run after 20 minutes.  Impressively, the dehumidifier on high power managed to keep the lead over the oil filled radiator for the full test hour despite consuming a third less power than the oil-filled radiator.  By the end of the hour, both increased the room temperature by just under 3C.  The fan heater managed to heat it by nearly 4C, while the dehumidifier on low power warmed the room by about 1.5C.

When it came to switching off the appliances apart from the dehumidifier on the low power test, the room heated by the fan heater experienced a sharp drop off in temperature, falling to that of the oil-filled radiator after 10 minutes.  As the oil-filled radiator keeps giving out heat until it fully cools off, the room temperature climbed slightly over the 10 minute period.  Although the dehumidifier does not retain heat like the oil-filled radiator, the room temperature fell off at a lower rate to the point that the room temperatures were similar between the fan heater and dehumidifier high setting test after about 30 minutes.

30 minutes after switching off the appliances, the room heated by dehumidifier on low power caught up with the temperature of the room that was heated by both the fan heater and dehumidifier high power test.  At this stage, the oil filled radiator had cooled down and its room temperature fell-off at a quicker rate to where it was less than a half a degree warmer than the fan heater test at the end of the two-hour mark.

For the dehumidifier on low power test, I left it running until the energy meter accumulated 1kWh.  I then switched it switched it off and continued taking readings until the 3 hour mark:

Room heating graph with dehumidifier on low

Although the room was slow to warm up, the low constant heat helped maintain the temperature well after the two heaters were switched off.  Had the dehumidifier been run on high for the first hour and switched down to low for the second hour, it would likely have had the lead by the end of the second hour while using the same 1kWh of power that each heater consumed.

The recovered heat

As mentioned earlier in the article, condensing water vapour releases 0.6291kWh of heat for every litre of water condensed.  For the two dehumidifier tests, it collected the following amount of water:

  • 1 hour on high – 247ml = 0.155kWh
  • 2 hours 40 minutes on low – 350ml = 0.220kWh

In high power mode, the dehumidifier was providing an additional 155 watts of heat in addition to the heat released from its power consumption.  On low power mode, the 0.22kWh was spread over 2 hours 40 minutes, so this works out at about 83 watts of heat.

Electric input vs heat output for the above test runs:

  • High setting – 630W input, 785W heat output – 1.25 COP
  • Low setting – 375W input, 458W heat output – 1.22 COP

Although the COP figures may seem tiny compared to any heat pump, this still puts out around 25% more heat than the energy it consumes, which is unusual in a portable electric-only appliance.  The collection rate will also vary depending on the room’s humidity level as the lower the humidity level, the less effective it will be due to the lower extraction rate.

Potential side effects

Low humidity levelWhile this method may work well in home with a relatively high humidity level as background heating, excessive run times can potentially lead to the side effect of low humidity levels, particularly if the humidity level falls below about 35%.  Very low humidity levels can also make the air feel cooler than it is, negating the recovered heat benefit.  Other potential side effects include a dry throat, dry eyes or issues with static electricity.

Attempting to add humidity to the air such as placing clothes on an aerator or running a humidifier will also negate any recovered heat from the dehumidifier.  All humidifiers absorb heat from the air or a built-in heater to evaporate water.

Drying clothes on an aerator will draw heat from the air to evaporate moisture from the clothes.  On the other hand, this is still more efficient than vented tumble dryers that vent their waste heat to the outdoors.

Finally, although a desiccant dehumidifier provides significantly warmer air than compressor dehumidifiers, they are generally less energy efficient at collecting water and in turn have a lower recovered heat to energy consumed ratio than compressor dehumidifiers, particularly at elevated humidity levels and when the room temperature exceeds about 15C.

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