If you’re tired of sticky rooms, musty odors, or an AC that runs but never quite feels dry enough, adding a whole house dehumidifier to your HVAC can be a game-changer. This guide walks you through how to install a whole house dehumidifier the right way—where it goes, how to duct it, how to handle drainage and controls, plus common mistakes to avoid. We’ll keep the language simple and the steps practical, while naturally covering whole house dehumidifier installation terms people search for.
Heads-up: You’ll be cutting into ductwork, wiring a control, and routing a condensate drain. If you’re not 100% comfortable with dehumidifier installation, hire a licensed HVAC pro. Many homeowners do—partly because a pro will size the unit, pull permits if required, and commission it correctly.
The “right-way” overview (how a furnace/central dehumidifier ties in)
- Dedicated return from a central area of the home (hallway, open living space).
- Supply discharge to the HVAC supply plenum (downstream of the air handler/coil), with a backdraft damper.
- Keep at least 24 inches of straight duct before the first branch so dry air mixes thoroughly.
Manufacturers publish the same basic geometry: pull air from a dedicated return and inject to the supply; many also allow fresh-air intake as an option on “ventilating” models. Avoid using kitchens/bathrooms as returns and place the wall control in a central living area.
Why not “return-to-return” only? It can work, but you must interlock the HVAC blower so the main fan runs when the dehumidifier runs; otherwise you risk poor distribution and odd pressure effects. If you inject return-to-supply, the dehumidifier can run without the HVAC blower, but mind the mixing distance called out in manuals.
Where should a whole-house dehumidifier be installed?
Near the existing HVAC system—in a basement, utility room, or attic—to keep duct runs short and service access easy. That’s why most installs happen right by the air handler. If ducts travel through unconditioned spaces (attic/crawl), insulate and seal them to code (often R-6 to R-8 minimum).
You can browse typical capacities and features (including models with fresh-air options) on TheDryAir – Whole House Dehumidifiers to match coverage (e.g., ~2,300–4,500 sq ft) before you plan duct sizes.
Pre-install checklist (read this once)
- Sizing: Choose capacity for your square footage and moisture load.
- Location & support: Solid platform or strapping kit; allow service clearance.
- Duct plan: Dedicated return from a central area; supply to supply plenum with backdraft damper and 24″ straight before the first branch. No bathroom/kitchen returns.
- Electric: Most residential units plug into 120V and draw ~6–7A; verify nameplate and circuit capacity. Always disconnect power before work.
- Drainage: Gravity drain with continuous slope ≥ 1/8″ per foot (1%); use a trap if the manual calls for it. Add a secondary pan with float switch if above finished space. Use a condensate pump only when gravity isn’t possible.
- Control: Mount the humidistat/remote control in a central living area (not the attic/basement unless specified).
Tools & materials
Aviation snips, hole saw/duct collar kits, drill/driver, foil tape + mastic, sheet-metal screws, insulated flex/round duct (per manual: often 8–10″), backdraft damper, ¾″ PVC for condensate with fittings & trap (if specified), pipe hangers, level for slope, condensate pump (if needed), secondary drain pan + float switch (if above finished areas), 120V receptacle as required, low-voltage cable for control.
Option A (most common): Dedicated return → supply plenum
- Power down the air handler and dehumidifier branch circuit. Verify with a meter.
- Set the unit on its platform or hang per the manual. Leave filter access clear. If above finished areas, set a secondary drain pan with float switch under the unit.
- Cut in the dedicated return.
- Choose a central hallway/living return grille. Run insulated duct to the dehumidifier inlet.
- Don’t pull from bathrooms or kitchens; avoid rooms with doors typically closed.
- Cut in the supply collar on the HVAC supply plenum (downstream of the coil).
- Install a backdraft damper in the dehumidifier discharge to prevent system back-flow.
- Maintain 24″ minimum straight duct before the first branch to mix dry air. Seal all joints with mastic/tape.
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Drainage:
- Run ¾″ PVC with a continuous 1/8″ per foot slope to a code-approved drain. If gravity isn’t possible, use a condensate pump.
- Install a trap if the manual requires it; some units have internal traps. Support the line; avoid sags.
- Electrical & control:
- Provide a 120V, 15A grounded receptacle where required; follow the nameplate amps.
- Mount the humidistat/remote in a central living area; pull low-voltage cable to the unit. Set the control to ~45–50% RH as a starting point.
- Commissioning:
- Restore power. Check for air leaks, confirm damper orientation, verify drain flow.
- Run the unit alone and with the HVAC blower. You don’t have to run the HVAC blower for return-to-supply setups, but confirm good supply mixing.
Option B: Fully ducted, stand-alone
For homes with tricky duct access (or to treat a specific zone), you can duct inlet and outlet to dedicated grilles—still using a central return and discharging to rooms that need drying. This is common in basements or additions. Follow the same drain, power, and control rules as above, and mind return placement rules (no kitchens/baths). Manufacturers list this configuration as acceptable, alongside return-to-supply.
Alternative configs (when you must deviate)
- Return-to-return: Acceptable in some manuals, but wire the unit to run the HVAC blower whenever the dehumidifier runs to move dry air through the supply ducts. E.
- Return-to-supply (short distance): If space is tight, keep the 24″ minimum before the first branch for proper mixing.
- Ventilating dehumidifiers: Some models support outdoor air intake with a motorized damper. This adds fresh air while controlling RH—plan for balancing and code compliance.
Drainage rules that prevent callbacks
Gravity wins. Run the drain line with a minimum 1/8″ per foot slope (1%) toward the termination—this language is mirrored in the International Mechanical Code. Support the line and keep the slope uniform. Where the unit is above finished areas, add a secondary drain pan with float switch.
Ducts in attics/crawls? Insulate & seal
Any supply/return duct sections running through unconditioned spaces should be insulated (typical code minimums: R-6 to R-8) and sealed so the dry air doesn’t pick up heat or leak. This is now standard energy-code language across many states.
Combustion safety (important)
If your home has atmospherically vented gas appliances (draft hood water heater, older furnace), avoid installation choices that create strong negative pressure zones. Backdrafting is a real risk in tight homes with exhaust devices running. If in doubt, have a pro conduct a CAZ (combustion appliance zone) depressurization test when you modify ductwork or add fans.
Common mistakes to avoid
- Return-only tie-in without interlocking the HVAC blower. (Poor distribution.)
- No backdraft damper at the dehumidifier discharge when injecting to supply. (Backflow risk.)
- No mixing distance—injecting dry air right before a supply branch. (Uneven results.)
- Flat condensate lines or no secondary pan over finished areas. (Leaks.)
- Uninsulated attic ducts carrying dry air. (Re-humidifies and wastes energy.)
Is a whole-house dehumidifier worth it?
If humidity routinely sits above ~50–60% RH, yes—comfort jumps, musty smells fade, and you can often run the AC fan less for the same dryness. The EPA suggests keeping indoor RH below 60% (ideally 30–50%) for health and mold prevention; a furnace dehumidifier makes that automatic.
How much does whole home dehumidifier installation cost?
National cost guides put installed totals commonly around $1,300–$2,800+, depending on capacity, ducting complexity, access, drainage, and electrical. Expect higher for attic installs or new duct runs.
Frequently Asked Questions
Can I install my own whole house dehumidifier?
If you’re skilled with ductwork, low-voltage wiring, and condensate plumbing, it’s possible. Many homeowners still hire pros due to code, combustion-safety checks, and commissioning.
Where should a whole house dehumidifier be installed?
Near the air handler—basement, utility room, or attic—with short, insulated duct runs and easy service access.
How much does it cost to install a whole house dehumidifier?
Typical projects fall around $1,300–$2,800+ installed, depending on size and complexity.
Is it worth getting a whole house dehumidifier?
Yes if your indoor RH lives above ~50–60%. Keeping humidity 30–50% improves comfort and helps prevent mold.
Do I need to run the HVAC blower when the dehumidifier runs?
Not if you inject from return to supply (the dehumidifier can run alone), but you must keep 24″ mixing distance before the first branch. If you go return-to-return, interlock the HVAC blower.
Simple commissioning checklist (save this)
- Set humidistat to ~45–50% RH; later fine-tune for season and comfort.
- Verify backdraft damper orientation and that supply has 24″ straight before first branch.
- Confirm drain slope (≥1/8″ per foot) and test the pump (if used).
- Seal and insulate any attic/crawl duct runs.
- Place control in a central living area; avoid kitchens/baths.
When you’re ready to compare sizes and features, explore TheDryAir’s Whole House Dehumidifiers collection to match coverage, duct options, and controls to your layout.
