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Sizing dehumidifer to protect house and belongings

Luke Wood | Posted in General Questions on

I was wondering if anyone has any practical experience in sizing a dehumidifier in an equatorial climate. Our open-plan house is on three floors and. An open staircase connects the following room sizes:

Basement:          270 square feet

Middle Floor Living Area: 750 square feet

Top Floor Living Area: 1210 square feet

The daytime RH is around 70-80% in the basement and can rise to over 90% at night. The RH in the connecting middle and top floors are between 5-10% and 10-15% less, respectively. The outdoor temperature at night is usually between 24°C (75°F) and 29°C (84°F)

I plan to install solar and upsize the rating to allow me more energy to play with. I will run one 1.5 HP split AC throughout the day. This will reduce the RH in the basement to 60-65%, which is almost acceptable.

I understand that the most cost-effective way of reducing the RH at night is to place a dehumidifier in the basement that will run through the night. The dehumidifier could also support the AC in the daytime. There is no ducting, so I need to use a portable model.

I am struggling to size this. I have checked a lot of websites, but the recommendations vary greatly. The two largest models are:

A 35 litre 350 watt

A 60 litre 780 watt

I prefer the 35-litre if possible as the 60-litre would cost a lot to run. Would it be enough to keep the RH at around 60%?

Any help would be greatly appreciated.

Kind regards,

Luke

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Luke,

    Both daytime and nighttime humidity levels are high. Do you have a bulk water issue in the basement? Does your house have gutters and grades that move water away from the foundation?

    If your house is tight, you might need a ventilating dehumidifier. If you have a ventilation strategy, you should verify that it's not introducing too much outdoor air.

    In the meantime, I would buy an Energy Star Rated dehumidifier and put it in the basement. If this is a new house, you may be dealing with some residual moisture as well.

  2. Luke Wood | | #2

    Hi Steve,

    Thanks so much for your response and your advice. The house is about eight years old. We do have an issue with a small damp patch on the wall. There is a playground next to the basement, so about a third of the wall is underground. We are currently trying to fix this with the council. However, the nighttime humidity outside often exceeds 90%.

    The house isn't tight at all. We have seven sets of sliding doors. They aren't the best quality so sealing them is challenging.

    I had a look at ventilating dehumidifiers. They look fantastic, but I think I would have to import one as they are not available in Malaysia.

  3. James Boris | | #3

    I'd recommend sealing the house up as best you can before shelling out on a fancy dehumidifier. If the house is even average leakiness, you're gonna be dehumidifying the rainforest around you. Not saying a dehumidifier is a bad idea, just that if you have $X to spend now, throw it at some nice caulk and I bet you'll see a bigger improvement on your dollar.

  4. Steve Knapp CZ 3A Georgia | | #4

    Hi Luke,

    I didn't realize the house is in a tropical country. If it were me, I would try to mimic what the locals do to stay comfortable. (And be sure to check out Martin's article on living with high humidity: https://www.greenbuildingadvisor.com/article/living-with-high-humidity.)

  5. Luke Wood | | #5

    Hi James and Steve,

    The article is interesting; this is basically what the locals do. We just use AC to sleep and in rooms on an ad hoc basis as needed.

    I just worry about the extreme humidity. As the temperature drops, the RH can rise to the 90s. I can't have smoke alarms for instance as they are only rated up to 80% so they go off in the middle of the night and wake us all up. Any electronics in the storeroom stop working after a few months, which is suspect is related.

    I might try sealing up all the windows and seeing if a 35-litre will make the house more comfortable. I have noticed that having one 1.5 HP AC running reduces the humidity pretty quickly.

    Thanks so much for your help.

  6. DCContrarian | | #6

    The amount of dehumidification you have to do depends on the amount of outside air getting in. There really is no such thing as "standard" construction, houses can vary in the leakiness by a factor of 20 or more depending on how they're constructed. So you can't really size equipment based on the size of the house.

    Generally houses in warm places are nearly as well sealed as they are in cold places. If you really want to keep your house below the outside humidity level, you need to seal it well. You probably don't need much insulation, but you need to fill all the cracks. Depending on how the house was constructed this could be a simple task, or it could be nearly impossible.

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