How do DEhumidifiers help save on energy bills?

_Sam_

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!

Are you saying that for people who have gas central heating, dehumidifier will not be a good option? 

I confess I haven't done the math. We have gas central heating and intuitively I assumed that using dehumidifier will help us spend less energy on heating. 

But I'd be very interested to read a more scientific comparison, dehumidifier vs replacing inside air with outside air and then heating it by gas or electric. (I suppose a room contains a specific number of cubic meters of air, plus specific humidity, then heating efficiency, should be possible to calculate for those good at math!)

Effician

Qyburn said:

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.

As long as you replace moisture heavy warm indoor air with relatively dry incoming outdoor air then the home will contain less water in the air.

Today we opened a lounge window wide open then opened a loft window wide open , you could feel the warm exiting the loft window & the cold air entering the lounge window, 20 mins later we shut both windows , due to the fabric of the house being  between 15-18c before starting & with 20 mins of 4c air rushing through,  the air in the lounge had dropped to 13c , the fabric of the room temp now comes into play & on closing the windows the temp soon rises to 17c. Yes we lose a little room temp but negligible cost to reheat should we need to, often a little sun through the windows is enough.

_Sam_

Re: sticky heatwave, I wouldn't dream of using dehumidifier in those conditions. So I've been researching air conditioners hoping to buy one now while no one needs it (in MSE style!)

The snag is they so much more expensive than dehumidifiers. Also the portable units with just one hose are not very efficient it would seem (although we might just bite the bullet figuring it would normally be needed when it's hottest the sun should be out and solar panels would take care of whatever electricity it needs...)

Novice_investor101

Not science but personal experience. I always used to feel chilly in the downstairs of my house even though the temp was up to 21c & the heating blasting away. Upstairs was always fine though. 

One day I moved a hygrometer/thermometer I had in the bedroom to the lounge to check the thermometer in there was reading correctly. The humidity reading shot up! 

A week after buying & using a Meaco ABC model dehumidifier I really noticed the difference, I felt warmer & more comfortable! & two small damp wall patches dried up. The first week there was a damp/earthy smell as the moisture was evaporated from the walls. It saved me money cos I stopped turning the thermostat up & up trying to feel warm even though it was already 21c in the lounge & kitchen. 

masonic

_Sam_ 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!

Are you saying that for people who have gas central heating dehumidifier will not be a good option? 

I confess I haven't done the math. We have gas central heating and intuitively I assumed that using dehumidifier will help us spend less energy on heating. 

But I'd be very interested to read a more scientific comparison, dehumidifier vs replacing inside air with outside air and then heating it by gas or electric. (I suppose a room contains a specific number of cubic meters of air, plus specific humidity, then heating efficiency, should be possible to calculate for those good at math!)

If you are heating your home using electricity (either directly, or indirectly through a dehumidifier), then you'll generally pay a lot more for each kWh heat given today's pricing, so it will certainly shift the economics between dehumidifying vs reheating cold air from outside. It would depend on the efficiency of your central heating system and your tariff as to how much it shifts it. As above, it is unlikely to have significant cost implications, but the sums would need to be done.

diystarter7

Scot_39 said:

Think you mean Dehumidifier.

A humidifier adds moisture to air - a dehumidifier removes it.

Take Laundry airing

Basically these are cheaper to run for a few hours at a given rate than say a conventional tumble dryer -  as they use less energy / kWh.

And take the dampness out of the air without having to open windows and let cold air circulate instead to dry out the moisture it puts into air.
And saves the often even more significant added cost of reheating spaces afterwards if do open windows etc to air to remove.  To get a through flow/draft - I would be cooling 2 rooms, halls and stairway.  Over half of house.

And reduce the time need to air house for other sources of humidity - cooking, shower, even with vent fans, my vent fan struggles after baths especially,  even breathing etc.

I can get 1/2 pint to a 1pint out of air after a few bath towels or heavy cotton sheets wash etc.  And I have a 1600rpm spin washer dryer. Lower spin speeds leave more moisture in clothes as a general rule.

If you get moderate to heavy condensation on Windows or worse in some respects walls - chances are a dehumidifier will help not only reduce it, but also the risk of dampness.  As most say need 14-15C min temp to avoid mould etc.

Others say they use theirs in bedrooms to cuts down on humidity so can sleep more comfortably.  Others say just sleep with a window open - not so easy if asthmatic etc in depth of winter.

Dry air said to reduces level of dust mites, so a potential health benefit.. 

And then for many, damp air just feels colder, so need a higher heat setting.  And every deg C counts.

But then need to factor in costs to buy - my cheaper basic 10l model was £150.  And although not light, has handle so portable to move around. But would maybe need a bigger model to do larger rooms or house etc as quickly - these can run to easily over twice that.

Costs to run - uses 0.6-0.7kWh over 2-3 hours.

I won't be spending an extra £200 on a washer dryer next time.

Hi

What an incredibly informative and helpful post and one of the best posts I've recently read out the hundreds I read daily.
Thank you very much indeed.

Astria

diystarter7 said:

Scot_39 said:

Think you mean Dehumidifier.

A humidifier adds moisture to air - a dehumidifier removes it.

Take Laundry airing

Basically these are cheaper to run for a few hours at a given rate than say a conventional tumble dryer -  as they use less energy / kWh.

And take the dampness out of the air without having to open windows and let cold air circulate instead to dry out the moisture it puts into air.
And saves the often even more significant added cost of reheating spaces afterwards if do open windows etc to air to remove.  To get a through flow/draft - I would be cooling 2 rooms, halls and stairway.  Over half of house.

And reduce the time need to air house for other sources of humidity - cooking, shower, even with vent fans, my vent fan struggles after baths especially,  even breathing etc.

I can get 1/2 pint to a 1pint out of air after a few bath towels or heavy cotton sheets wash etc.  And I have a 1600rpm spin washer dryer. Lower spin speeds leave more moisture in clothes as a general rule.

If you get moderate to heavy condensation on Windows or worse in some respects walls - chances are a dehumidifier will help not only reduce it, but also the risk of dampness.  As most say need 14-15C min temp to avoid mould etc.

Others say they use theirs in bedrooms to cuts down on humidity so can sleep more comfortably.  Others say just sleep with a window open - not so easy if asthmatic etc in depth of winter.

Dry air said to reduces level of dust mites, so a potential health benefit.. 

And then for many, damp air just feels colder, so need a higher heat setting.  And every deg C counts.

But then need to factor in costs to buy - my cheaper basic 10l model was £150.  And although not light, has handle so portable to move around. But would maybe need a bigger model to do larger rooms or house etc as quickly - these can run to easily over twice that.

Costs to run - uses 0.6-0.7kWh over 2-3 hours.

I won't be spending an extra £200 on a washer dryer next time.

Hi

What an incredibly informative and helpful post and one of the best posts I've recently read out the hundreds I read daily.
Thank you very much indeed.

I typically use a dehumidifier in the bathroom when having a shower instead of opening the window at this time of year, I run it for at least an hour even though I'm only in there for about 10 mins - no longer have to wipe down the window or mirrors and costs about 5p - less than it would cost to put the equivalent amount of heat back in the room once it's all disappeared out of the window. It'll also automatically turn off once the room is at an appropriate amount of humidity. It cost £99 from Amazon for a 12l/hour unit - so I've got 2 to save carrying them up and down stairs all the time. The price has now increased to £149 - seemingly because of the amount of reviews and possibly purchases - I bought it when it had no reviews.

StocktonFlyer

We had a cold damp house and were advised to run a dehumidifier.

It did reduce our (gas) heating bill by some margin, and the house was a lot more comfortable.

However, obviously it increased our electric bill which possibly cancelled out the savings on gas.

But the house is better, so I take that as a win.

Petriix

It's utter nonsense to suggest that heating up a whole house of cold air uses a 'negligible' amount of energy. In the current times where some people are genuinely struggling to pay their spiralling energy bills, understanding the difference in costs is really important.

The most significant factor is that the energy used by the dehumidifier is all retained in the house as heat while the ventilation method simply pours the energy out into the atmosphere.

Standard rate electricity is approximately 3 x the cost of gas heating (factoring in losses) so that's an important consideration. Running a dehumidifier overnight on Economy 7 (or another time of use tariff) is almost certainly cheaper than the equivalent gas heating. 

If you do ventilate then do it in the warmest part of the day to minimise lost heat. Don't leave your bathroom window open for hours because, once it's cold, the air becomes far less effective at evaporating the moisture from the surfaces. It's far better to extract the warm, moist air. 

Effician

Petriix said:

It's utter nonsense to suggest that heating up a whole house of cold air uses a 'negligible' amount of energy. In the current times where some people are genuinely struggling to pay their spiralling energy bills, understanding the difference in costs is really important.

The most significant factor is that the energy used by the dehumidifier is all retained in the house as heat while the ventilation method simply pours the energy out into the atmosphere.

Standard rate electricity is approximately 3 x the cost of gas heating (factoring in losses) so that's an important consideration. Running a dehumidifier overnight on Economy 7 (or another time of use tariff) is almost certainly cheaper than the equivalent gas heating. 

If you do ventilate then do it in the warmest part of the day to minimise lost heat. Don't leave your bathroom window open for hours because, once it's cold, the air becomes far less effective at evaporating the moisture from the surfaces. It's far better to extract the warm, moist air. 

We heat our 1870's terraced  house ( solid walls /18yr old DG & negligible draughts ) with one direct air fed log burner & some solar gain so you would think that a dehumidifier would make sense to retain the little heating we use but it doesn't,  our average winter electric usage of under 100kWh / month is comfortable but adding a dehumidifier just adds extra cost & too dry air without addressing the main reason for condensation forming.

As far as i'm concerned the aim is to get the most out of the energy our house uses & having run a dehumidifier all last winter ( even on E7)  found  it was not worth it.