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How to Install a Whole-House Dehumidifier | Ask This Old House

Ask This Old House plumbing and heating expert Richard Trethewey shows how to control humidity throughout the house all year round. (See below for a shopping list, tools, and steps.)

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Richard visits New Orleans to help a family deal with high indoor humidity.

Shopping List for How to Install a Whole-House Dehumidifier:
– Whole-house dehumidifier

Steps for How to Install a Whole-House Dehumidifier:
1. In the attic, attach four steel hanging bars and the metal pan from the collar ties. Position the pan very close to the air-conditioning unit.
2. Set the whole-house dehumidifier in the metal pan.
3. Cut a square hole in the hallway ceiling and install a new return-air grille.
4. Attach a flexible return-air duct to the port on the end of the dehumidifier.
5. Secure the duct to the roof framing with plastic straps.
6. Cut the duct to length, then attach it to the return-air grille installed in Step 3.
7. Cut a new supply-air duct fitting into the air-conditioning unit.
8. Connect a flexible supply-air duct to the fitting. Attach the opposite end of the duct to the existing air-conditioning duct system.
9. Run a new PVC condensate line from the dehumidifier to the outdoors.
10. Install the dehumidifier’s filter; be sure to replace the filter every two to three months.
11. Mount a humidistat to the wall beside the existing thermostat.
12. Start by setting the humidistat for 35 percent humidity, then adjust as necessary.

Richard installed the Ultra-Aire XT105H whole house dehumidifier, manufactured by Ultra-Aire [https://www.santa-fe-products.com/].

Expert assistance with this project was provided by Surgi’s Heating & Air Conditioning [https://www.surgisac.com/].

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How to Install a Whole-House Dehumidifier | Ask This Old House


  1. My previous house had a bad humidity problem. It was running over 70% and causing mold/mildew throughout. I got a larger dehumidifier and put it in the basement, hardlined it to a drain and began running it 24/7. It took months but eventually the whole, three story house was healthy. I was able to keep it between 40 and 50% year round, top to bottom. Mold and mildew ceased and desisted. I was pleasantly surprised how well it worked.

  2. Imagine having a small basketball gym in your attic.

  3. Why can't one just put the intake on the fresh air return lines and the out on the supply line?

  4. 35% seems awfully low. I have a portable unit that I keep set at 50%. It maintains right at 48%. Makes it much more comfortable indoors in the summer in NC. Since I got the dehumidifier I’m able to set the ac a couple degrees higher.

  5. My original intention for this humidifier was to use it for plants that require high moisture levels. After using it in my room, https://www.youtube.com/post/UgkxtD9aJ2m6GU-X1IChQxjn9l31K1A7Kpwj however, I think that's where it is going to stay in the fall and winter season at least. The increased humidity in my room made the room warmer and more comfortable. I had first ran the humidifier for 4 hours with the timer setting for it. After the first hour, I checked my room and can feel the difference. When the full 4 hours was up, I checked again and noticed that the humidifier was shut off as it is suppose to. The moisture level never seem to get passed 56/57%. I then let it run until all the water is used up, which it wasn't all used up. There are still some left in the base that you are not suppose to add water to. I filled up the water tank all the way again, being careful when removing it from the base. Some left over water may spill out. After putting it back on and setting the unit to auto, I let it operate. It didn't stop running, rather I don't know if it did at any point since I didn't check it regularly, for 24 hours after which the water in the tank ran out. The moisture reading never passed 56/57%, which may be a good thing for me. Being situated right next to my bed, the noise was not very noticeable with the occasional water gurgle from time to time as the unit ran.

  6. She said basement I New Orleans 😂

  7. That's the biggest AC in a house that i've seen

  8. That HVAC tech must hate his life, or doesn’t like people; the dude never spoke when spoken to.

    Side note: do not use any other kind of tape with duct work other than foil tape. Don’t use gorilla tape, and especially don’t use duck tape.

  9. Missed the backdraft damper at connection between dehumidifier and HVAC supply

  10. Why do the pipes in my wall vibrate when I turn the shower water on??

  11. nice stripped head on the bolt on the dehumidifier lol

  12. I wanted to do my attic with insulation like there's as my insurance company said hell no.

  13. What will happen when you turn on the AC/ Heat is it not going to blow back the air in to the Dehumidifier ??

  14. how can the small fan in that unit overcome the whole house static pressure unless it triggers the blower on HVAC unit ??

  15. So you have to run a whole other drain line? To can't tie into the ah one?

  16. Romulan is such a good technician.

  17. A variable speed air handler with a thermidistat works better…

  18. Personal preference, for sure. I'm happy getting our rooms down to 50%, am comfortable at that level. Our hvac is in the basement, not sure if this apparatus would fit. Do these draw enough power to need a dedicated circuit?

  19. 4:04 What a great and perfectly timed question. Couldn't have timed that better if they had planned it. :/

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