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Q & A: heat from dehumidifier

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Most recent answer: 10/19/2015
Q:
I'm wondering if you can tell me if running a dehumidifier and then dumping the water down the drain will result in a net heat gain for the room the dehumidifier is in?I live in Vermont and have to manage humidity in my house in the winter. I have fans in my bathroom and over my kitchen stove to remove steam produced by showering and cooking. This of course sucks warm air out of my house as well. I could not use the fans and then run a dehumidifier to keep the moisture under control.The dehumidifier uses electricity, but if using it produces a net heat gain then this is less energy I am using to heat the room.Considering it is winter time and I'm heating the room, which approach do you think is more energy efficient: 1.) Blow steam (and hot air) out of my house with fans. This reduces the electricity needed to run the dehumidifier, but increases how much energy I use to heat or 2.) run a dehumidifier?Thanks a million for taking the time to answer my question.
- Jeremiah (age 28)
Cabot, Vermont, USA
A:

Running the humidifier definitely dumps heat into the room. Say it has drawn 1kWh of energy. Where did that energy go? There's some energy change involved in getting water vapor to condense, but that change is negative, leaving even more than 1kWh of energy to go into heating the room. So that's more than 100% efficiency as a heater. I don't think that it's close to the efficiency of a good electric heat pump, which can be around 500%, but it's better than a simple electric heater, which is close to 100% efficient. (I'll update when I get better numbers for the dehumidifiers.*)

Simply sucking air out is not very efficient, unless the temperature outside is close to the inside temperature. You could get a heat exchanger, in which the outgoing warm air transfers heat to the incoming cold air. Those are fairly expensive but they do allow you to get fresh air with little heat loss, or little heat gain in the summer.

*Update: A typical home dehumidifier removes roughly 2L of water per kWh of energy used. That soaks up a bit  over 1 kWH of latent heat. So the room warming is about 2kWh per kWh input energy. The efficiency is over 200%, not bad if you want to dry your house air also.

What surprises me is that you have too much moisture in the winter. Usually houses are too dry in the winter, so that you'd want to spread the moisture from the shower around the house rather than blow it outside.

Mike W.

 


(published on 10/19/2015)

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