How do DEhumidifiers help save on energy bills?

MattMattMattUK

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

Personally, what drives my decision to use a dehumidifier vs an 'air change' (opening windows on opposite sides of the house to get a brief through-flow), is pollution levels outside.

Whilst I agree with the information on humidity, it also takes a lot less energy to dehumidify the air in a home and keep it at 15c than increase that same volume of air from 0c to 15c 1-2 times a day.

_Sam_

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

That is true, but to get this dehumidifying effect you'll need to heat that air from 0 to 15C. How much energy would it take? And how would this compare with the energy dehumidifier takes to achieve the same level of humidity without the need to replace the inside air with outside air?

I suspect the difference is the reason the article referred to by OP suggests dehumidifiers help conserve energy.

Effician

MattMattMattUK said:

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

Personally, what drives my decision to use a dehumidifier vs an 'air change' (opening windows on opposite sides of the house to get a brief through-flow), is pollution levels outside.

Whilst I agree with the information on humidity, it also takes a lot less energy to dehumidify the air in a home and keep it at 15c than increase that same volume of air from 0c to 15c 1-2 times a day.

Not sure where that came from but seems extreme to drop the air temp to 0c once or twice a day in the home , are you implying  to just dehumidify & not ventilate.

masonic

MattMattMattUK said:

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

Personally, what drives my decision to use a dehumidifier vs an 'air change' (opening windows on opposite sides of the house to get a brief through-flow), is pollution levels outside.

Whilst I agree with the information on humidity, it also takes a lot less energy to dehumidify the air in a home and keep it at 15c than increase that same volume of air from 0c to 15c 1-2 times a day.

Are you sure about that? I need to run my zeolite dehumidifier for about an hour at 330 W (0.33 kWh) to bring my 40 m3 room down from 60% to 40% RH. A 5 minute through-flow, followed by a quick 5 minute blast from a 2 kW fan heater (0.17 kWh) seems to get me to the same place for slightly less energy if anything, and much more quickly. Clearly the energy used by the dehumidifier is also lost to the room as heat, but these are not significant numbers.

Looking at the specific heat capacity of air of ~0.0002 kWh/kg/K and density of ~1.2 kg/m3, it suggests that raising the temperature of 40m3 air by 15 C would cost a measly 0.14 kWh.

k_man

The lowest energy option would depend how much moisture is being added to the air in the house.

The air replace method (assuming a large proportion of the air is replaced) uses almost the same energy to reheat, no matter how humid the air was in the house.

The dehumidifier uses more energy if the air is more humid.

If there is not much moisture being added, dehumidifier wins.

But if lots is being added, then air replace is preferable.

Which is why running an extractor fan or opening a window after a shower, or while cooking, is worth doing.

_Sam_

masonic said:

MattMattMattUK said:

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

Personally, what drives my decision to use a dehumidifier vs an 'air change' (opening windows on opposite sides of the house to get a brief through-flow), is pollution levels outside.

Whilst I agree with the information on humidity, it also takes a lot less energy to dehumidify the air in a home and keep it at 15c than increase that same volume of air from 0c to 15c 1-2 times a day.

Are you sure about that? I need to run my zeolite dehumidifier for about an hour at 330 W (0.33 kWh) to bring my 40 m3 room down from 60% to 40% RH. A 5 minute through-flow, followed by a quick 5 minute blast from a 2 kW fan heater (0.17 kWh) seems to get me to the same place for slightly less energy if anything, and much more quickly. Clearly the energy used by the dehumidifier is also lost to the room as heat, but these are not significant numbers.

Just had a look in the manual of our dehumidifier (Meaco Zambezi) it says there:

"The heat used during the drying process is mixed with the dry air to ensure that the air that comes out of the dehumidifier is not just drier but 10-12°C warmer than the air that came in."

To me this side-effect of dehumidifying seems to be a definite plus in favour of the method.

masonic

_Sam_ said:

masonic said:

MattMattMattUK said:

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

Personally, what drives my decision to use a dehumidifier vs an 'air change' (opening windows on opposite sides of the house to get a brief through-flow), is pollution levels outside.

Whilst I agree with the information on humidity, it also takes a lot less energy to dehumidify the air in a home and keep it at 15c than increase that same volume of air from 0c to 15c 1-2 times a day.

Are you sure about that? I need to run my zeolite dehumidifier for about an hour at 330 W (0.33 kWh) to bring my 40 m3 room down from 60% to 40% RH. A 5 minute through-flow, followed by a quick 5 minute blast from a 2 kW fan heater (0.17 kWh) seems to get me to the same place for slightly less energy if anything, and much more quickly. Clearly the energy used by the dehumidifier is also lost to the room as heat, but these are not significant numbers.

Just had a look in the manual of our dehumidifier (Meaco Zambezi) it says there:

"The heat used during the drying process is mixed with the dry air to ensure that the air that comes out of the dehumidifier is not just drier but 10-12°C warmer than the air that came in."

To me this side-effect of dehumidifying seems to be a definite plus in favour of the method.

Yes, they are very good in winter assuming you'd be heating your home with electricity anyway (those with gas central heating might be better off using their boiler), but not so good during a sticky heatwave!

MattMattMattUK

masonic said:

_Sam_ said:

masonic said:

MattMattMattUK said:

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

Personally, what drives my decision to use a dehumidifier vs an 'air change' (opening windows on opposite sides of the house to get a brief through-flow), is pollution levels outside.

Whilst I agree with the information on humidity, it also takes a lot less energy to dehumidify the air in a home and keep it at 15c than increase that same volume of air from 0c to 15c 1-2 times a day.

Are you sure about that? I need to run my zeolite dehumidifier for about an hour at 330 W (0.33 kWh) to bring my 40 m3 room down from 60% to 40% RH. A 5 minute through-flow, followed by a quick 5 minute blast from a 2 kW fan heater (0.17 kWh) seems to get me to the same place for slightly less energy if anything, and much more quickly. Clearly the energy used by the dehumidifier is also lost to the room as heat, but these are not significant numbers.

Just had a look in the manual of our dehumidifier (Meaco Zambezi) it says there:

"The heat used during the drying process is mixed with the dry air to ensure that the air that comes out of the dehumidifier is not just drier but 10-12°C warmer than the air that came in."

To me this side-effect of dehumidifying seems to be a definite plus in favour of the method.

Yes, they are very good in winter assuming you'd be heating your home with electricity anyway (those with gas central heating might be better off using their boiler), but not so good during a sticky heatwave!

During a "sticky heateave" people are not going to be trying to heat air up so there would be no heating cost, but then most people would open the window. The reason that they would not do that in winder is because it is zero degrees outside and heating the cold air up costs more than the dehumidifier. 

Whilst it might also not work for everyone,  a room a few degrees warmer than ambient but much lower humidity feels more comfortable than a high humidity one a few degrees cooler so a dehumidifier can still be of benefit though it is very situation specific.

Qyburn

Effician said:

Not sure where that came from but seems extreme to drop the air temp to 0c once or twice a day in the home , are you implying  to just dehumidify & not ventilate.

I think what Matt is meaning is that drawing in outside air and warming it up doesn't get rid of water, it just holds it as vapour. To get rid of it you need to dump that warm air back outside, replacing it with more cold outside air.

masonic

MattMattMattUK said:

masonic said:

_Sam_ said:

masonic said:

MattMattMattUK said:

masonic said:

_Sam_ said:

Effician said:

_Sam_ said:

Effician said:

No need for a dehumidifier to dry the indoor air at this time of year , plenty of relatively dry air on tap from outside if you know how to use it.

Hmmm 88-89% humidity outside, wouldn't call it a dry air on tap! https://www.metoffice.gov.uk/weather/forecast/gcpk9yegm#?date=2023-01-28

You don't seem to have grasped the meaning of relative humidity.

I have by now! We had a good discussion on it in the other thread.

Moisture which is present in the air doesn't disappear when you raise the temperature of the air. When you let the outside air in, you let in the exact amount of moisture that was outside, measured in grams per cubic meter of air. 

Raising the temperature of that moist air only means that the same cubic meter can now accept more moisture, hence your relative humidity reading. But the actual water you let in via the "outside tap" stays the same.

At 0C, 88% humidity air contains 4.3 g/m3 water. Bring in that air from outside and heat it even to 15C and the humidity is 33%. This is very dry compared with the typical indoor humidity of 60%+. The ideal moisture content of indoor air will be around 6-8 g/m3, assuming your home is heated to a reasonable temperature. Replacing air with 8-15 g/m3 moisture with that of 4-6 g/m3 will have a dehumidifying effect.

Personally, what drives my decision to use a dehumidifier vs an 'air change' (opening windows on opposite sides of the house to get a brief through-flow), is pollution levels outside.

Whilst I agree with the information on humidity, it also takes a lot less energy to dehumidify the air in a home and keep it at 15c than increase that same volume of air from 0c to 15c 1-2 times a day.

Are you sure about that? I need to run my zeolite dehumidifier for about an hour at 330 W (0.33 kWh) to bring my 40 m3 room down from 60% to 40% RH. A 5 minute through-flow, followed by a quick 5 minute blast from a 2 kW fan heater (0.17 kWh) seems to get me to the same place for slightly less energy if anything, and much more quickly. Clearly the energy used by the dehumidifier is also lost to the room as heat, but these are not significant numbers.

Just had a look in the manual of our dehumidifier (Meaco Zambezi) it says there:

"The heat used during the drying process is mixed with the dry air to ensure that the air that comes out of the dehumidifier is not just drier but 10-12°C warmer than the air that came in."

To me this side-effect of dehumidifying seems to be a definite plus in favour of the method.

Yes, they are very good in winter assuming you'd be heating your home with electricity anyway (those with gas central heating might be better off using their boiler), but not so good during a sticky heatwave!

During a "sticky heateave" people are not going to be trying to heat air up so there would be no heating cost, but then most people would open the window. The reason that they would not do that in winder is because it is zero degrees outside and heating the cold air up costs more than the dehumidifier. 

Whilst it might also not work for everyone,  a room a few degrees warmer than ambient but much lower humidity feels more comfortable than a high humidity one a few degrees cooler so a dehumidifier can still be of benefit though it is very situation specific.

The difference in energy is negligible, as my calculations above show, so I'm more swayed by other factors, such as the time it takes and the quality of air I'd be letting in. Some day's I'll even keep the extractor fan off when I take a shower and run the dehumidifier afterwards, rather than let in smoggy air. That does equate to a good couple of hours extra running time in an area I wouldn't otherwise be heating though, so I consider that more costly.

Regarding summer usage, I've always regretted putting the dehumidifier on during a sticky night, but my tolerance to high temperatures is perhaps more limited than most.