Types
Aprilaire 500 vs 600
Two bypass humidifiers from the same maker, separated by one number that Aprilaire publishes and almost nobody compares: 12 gallons a day against 17.
By Scooter M. · Published · Updated
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| # | Product | Best for | Gal/day | Price |
|---|---|---|---|---|
| 1 |  Aprilaire Model 500Top pickTwelve gallons a day against a stated 3,600 sq ft, on the same published basis as the 700. It is the bypass design, which means it borrows the furnace blower instead of bringing its own fan. | A ducted house, without the fan-powered price | 12.0published · 3,552 sq ft | |
| 2 |  Aprilaire Model 600Seventeen gallons a day to a stated 5,000 sq ft - the 700's capacity class in the 500's bypass body. It still borrows the furnace blower, so it humidifies only while that blower runs. | A large ducted house that still wants the bypass price | 17.0published · 5,032 sq ft | |
| 3 |  Aprilaire Model 700Eighteen gallons a day against a stated 5,300 sq ft, cited to AHRI Guideline F. It is ducted, it needs installing, and it is the only sizing claim here with a standard behind it. | An actual whole house, sized against a standard | 18.0published · 5,328 sq ft |
Most whole-house comparisons on the internet are 500 against 700, because that is bypass against fan-powered and it is a genuine difference in kind. The 600 sits between them and gets skipped, which is a shame, because for a particular size of house it is the correct answer and neither of the other two is.
The useful thing about comparing these two specifically is that almost everything cancels. Same design, same installation, same annual maintenance, same 24 VAC transformer, same humidistat in the box. What is left is one published number, and you can work out which side of it your house falls on with a calculator.
We earn a commission if you buy through the links on this page, at no extra cost to you. It does not change what we recommend or the order the picks appear in. Full disclosure. Prices checked October 13, 2026.
| # | Product | Best for | Gal/day | Price |
|---|---|---|---|---|
| 1 | Aprilaire Model 500Top pickTwelve gallons a day against a stated 3,600 sq ft, on the same published basis as the 700. It is the bypass design, which means it borrows the furnace blower instead of bringing its own fan. | A ducted house, without the fan-powered price | 12.0published · 3,552 sq ft | |
| 2 | Aprilaire Model 600Seventeen gallons a day to a stated 5,000 sq ft - the 700's capacity class in the 500's bypass body. It still borrows the furnace blower, so it humidifies only while that blower runs. | A large ducted house that still wants the bypass price | 17.0published · 5,032 sq ft | |
| 3 | Aprilaire Model 700Eighteen gallons a day against a stated 5,300 sq ft, cited to AHRI Guideline F. It is ducted, it needs installing, and it is the only sizing claim here with a standard behind it. | An actual whole house, sized against a standard | 18.0published · 5,328 sq ft |
What Aprilaire publishes for each
Aprilaire is the only maker in our entire set that states a daily output in gallons and a coverage in square feet on the same page, which is why this site's arithmetic is built on its range. Both of these units do it.
| Model 500 | Model 600 | |
|---|---|---|
| Output | 12 gal/day | 17 gal/day |
| Coverage claimed | Up to 3,600 sq ft | Up to 5,000 sq ft |
| Implied rate | 300 sq ft per gal/day | 294 sq ft per gal/day |
| Design | Bypass evaporative | Bypass evaporative |
| Water panel | Model 10 | Model 35 |
| Panel interval | Annual, per Aprilaire's maintenance page | Annual, per Aprilaire's maintenance page |
| Control | 24 VAC transformer, Model 60 digital humidistat | 24 VAC transformer, Model 60 digital humidistat |
| Reservoir | None - plumbed, drains to waste | None - plumbed, drains to waste |
Note the third row, because it is the reason we trust these figures at all. Aprilaire's two claims imply 300 and 294 square feet per gallon per day, and the 296 figure this whole site sizes against is the mean of three of its models. A maker whose numbers agree with each other across a range is telling you something different from a maker who prints a square footage on a box.
The only question that separates them
How many gallons a day does your house need? Under about 12, take the 500. Between 12 and 17, take the 600. Above 17, you are looking at the 700 and the conversation has changed.
Do this with your own number rather than ours. The whole-house sizing chart has the square footage worked out in both directions, and gallons per day explains why this is the only unit of capacity worth comparing - tank size, which is what most of the category advertises, does not exist on either of these machines.
Why they are the same machine in every other respect
Both are bypass units, and that word is doing more work than the model numbers are. A bypass humidifier has no fan of its own. It is mounted on the supply or return plenum with a duct running between the two, and it borrows the furnace blower to push air across a wet panel.
- Same installation, same requirements. Ducted forced-air heating, a water supply, a drain, and a route for the bypass duct between supply and return. If one of these fits your mechanical room, so does the other.
- Same limitation. Neither humidifies while the blower is off. On a mild day when the furnace barely cycles, a bypass unit barely runs - which is the single functional argument for the fan-powered 700 and it applies equally to both of these.
- Same maintenance. Aprilaire's maintenance page puts Models 300, 500, 600, 700 and 720 on an annual water panel replacement with an annual distribution tray inspection. One job a year, either way.
- Same absence of a reservoir. Both are plumbed and drain to waste, so nothing stands still inside them. That removes the pink film and the mold that dominate portable-humidifier ownership entirely, which is the real argument for ducted over portable.
If the bypass limitation is the part that worries you, that comparison is its own page: bypass vs fan-powered works through when borrowing the blower stops being good enough. It is a different decision from this one, and choosing between two bypass units does not settle it.
The one difference that is not capacity
They take different consumables. The 500 uses a Model 10 Water Panel; the 600 uses the Model 35, which is the same panel the fan-powered 700 takes.
On an annual schedule that is one purchase a year either way, so this is a smaller difference than it would be on a portable unit eating a wick every 30 to 60 days. But it is not nothing, and it is worth checking the price of the panel your model actually takes before you buy the machine rather than after. Furnace humidifier pad replacement covers which panel fits which model, and wick vs water panel explains why a ducted panel lasts an order of magnitude longer than a portable wick doing the same job.
What about the Model 400?
Fair question, because the 400 publishes the same pair as the 600 - 17 gallons a day to 5,000 square feet - and it is a bypass unit too. The difference is that the 400 is drainless: it recirculates rather than sending water to a drain.
That saves water and it costs you the property that makes ducted humidifiers low-maintenance, because a drainless unit is holding water. Aprilaire's own maintenance page reflects it - the 400's panel goes twice per heating season where the 500, 600 and 700 go annually. If the reason you are buying ducted is that you are tired of cleaning a tank, the drain is the feature.
Which we would buy
The Model 500 for most houses, and it is not close. Twelve gallons a day serves about 3,550 square feet of circulated space, which covers the large majority of single-family homes once you have subtracted the parts the furnace does not serve. It is the cheaper unit, it takes the cheaper panel, and buying capacity you cannot use is the most common way to overspend in this category.
The Model 600 when your own arithmetic lands between 12 and 17 gallons a day - a genuinely large house, or one where the ducted air reaches nearly all of it. This is the band where the 500 will run continuously and still fall short and the 700 is more machine than the job needs, and it is exactly the case the internet's 500-versus-700 comparisons leave unserved.
The Model 700 when you need more than 17 gallons a day, or when you need humidity at times the furnace blower is not running. Its own fan is the thing you are paying for, not the extra gallon a day over the 600.
One honest caveat that applies to all three: Aprilaire publishes no warranty term on the product pages we read, and none of these is a purchase you complete yourself. Budget for a contractor unless you are comfortable cutting into a plenum, and read whole-house installation cost before you commit to the category at all.
The picks, in full
Top pick · A ducted house, without the fan-powered price
Aprilaire Model 500
Twelve gallons a day against a stated 3,600 sq ft, on the same published basis as the 700. It is the bypass design, which means it borrows the furnace blower instead of bringing its own fan.
This is the cheaper half of the whole-house decision and, for a lot of houses, the correct half. Aprilaire calls it a small bypass evaporative humidifier, and the published pair - 12 gallons a day to 3,600 square feet - works out to 300 square feet per gallon per day, which is within two percent of the rate we derive from its whole range.
The bypass design is the reason it costs less. Instead of carrying its own fan it takes a duct between the supply and return plenums and lets the furnace blower push air across the water panel. Fewer parts, less to install, less to fail - and one real consequence, which is that it moves air only when the furnace blower is moving air.
At 12 gallons a day it covers most single-family houses. The arithmetic for what your house actually needs is on our sizing chart; if it lands above about 12 gallons a day, or your ductwork has nowhere sensible for a bypass duct to go, that is the point at which the fan-powered 700 earns its extra cost.
What works
- Publishes capacity and coverage together, which almost nothing else in the category does
- No reservoir, so none of the standing-water maintenance that dominates portable humidifiers
- Water Panel on an annual schedule rather than a wick every 30 to 60 days
- Materially cheaper than the fan-powered model, installed or not
What does not
- Humidifies only while the furnace blower runs, because it has no fan of its own
- Needs somewhere to route a bypass duct between supply and return, which not every install has
- Requires ducted forced-air heating, a water supply, a drain and realistically an installer
- Wastes water: the drain that keeps it clean also sends water down it
Skip this ifyour house needs more than about 12 gallons a day, or your ductwork has no practical route for a bypass duct. Both point at the fan-powered 700 instead.
The arithmetic
- Published output (continuous)
- 12.0 gal/day
- Covers, at 296 sq ft per gal/day
- 3,552 sq ft
- Maker’s coverage claim
- 3,600 sq ft
- Output that claim would need
- 12.2 gal/day
Output as the manufacturer publishes it. Coverage rate derived from Aprilaire’s own published capacity-and-coverage pairs.
Published specification
- Tank
- not publishedThere is no tank. It is plumbed to the water supply and drains to waste.
- Output/day
- 12 gal/daySource
- Coverage
- Up to 3,600 sq ftAprilaire states this for tightly constructed homes. It is one of the three published capacity-and-coverage pairs our 296 sq ft per gal/day rate is derived from.Source
- Power
- 24 VAC via the supplied transformerAprilaire ships a Model 60 digital automatic humidistat, an outdoor temperature sensor and a saddle valve in the box.Source
- Filter
- Model 10 Water PanelAprilaire's maintenance page puts Models 300, 500, 600, 700 and 720 on an annual schedule, plus an annual distribution-tray inspection.Source
- Cleaning
- Drains to waste, so no water stands inside the unitThe same design property as the 700: nothing sits still long enough to grow a film.Source
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Pick 2 · A large ducted house that still wants the bypass price
Aprilaire Model 600
Seventeen gallons a day to a stated 5,000 sq ft - the 700's capacity class in the 500's bypass body. It still borrows the furnace blower, so it humidifies only while that blower runs.
The 600 is the model people miss. It sits between the two whole-house units most buyers compare - it has the 700's published capacity, 17 gallons a day to 5,000 square feet, and the 500's bypass design, which is the cheaper of the two ways to move air across a water panel.
What you give up against the 700 is the fan. A bypass humidifier has none of its own: it takes a duct between the supply and return plenums and lets the furnace blower do the work. That is why it costs less to buy and less to install, and it is also the one functional limit - on a mild day when the furnace barely cycles, the humidifier barely runs.
What you gain against the 500 is five gallons a day, which on our derived rate is about 1,400 square feet of extra capacity. If your house arithmetic lands between 12 and 17 gallons a day, this is the unit the category tends to skip past.
What works
- The 700's published capacity class at bypass prices
- Publishes capacity and coverage together, which almost nothing else in the category does
- No reservoir, so none of the standing-water maintenance that dominates portables
- Model 35 Water Panel on an annual schedule rather than a wick every 30 to 60 days
What does not
- Humidifies only while the furnace blower runs, because it has no fan of its own
- Needs somewhere to route a bypass duct between supply and return, which not every install has
- Requires ducted forced-air heating, a water supply, a drain and realistically an installer
- Wastes water: the drain that keeps it clean also sends water down it
- Aprilaire publishes no warranty term on the product page we fetched
Skip this ifyour ductwork has no practical route for a bypass duct, or your heating is short-cycling by design and you need humidity when the blower is idle. Both point at the fan-powered 700.
The arithmetic
- Published output (continuous)
- 17.0 gal/day
- Covers, at 296 sq ft per gal/day
- 5,032 sq ft
- Maker’s coverage claim
- 5,000 sq ft
- Output that claim would need
- 16.9 gal/day
Output as the manufacturer publishes it. Coverage rate derived from Aprilaire’s own published capacity-and-coverage pairs.
Published specification
- Tank
- not publishedThere is no tank. It is plumbed to the water supply and drains to waste.
- Output/day
- 17 gal/daySource
- Coverage
- Up to 5,000 sq ftAprilaire states this for tightly constructed homes. Against its 17 gal/day that is 294 sq ft per gal/day, within one percent of the 296 rate we derive from the 500, 400 and 700.Source
- Power
- 24 VAC via the supplied transformerAprilaire lists a Model 60 digital automatic humidistat with the unit.Source
- Filter
- Model 35 Water PanelThe larger panel, the same part the 700 uses - not the Model 10 the 500 takes. Aprilaire's maintenance page puts Models 300, 500, 600, 700 and 720 on an annual schedule, plus an annual distribution-tray inspection.Source
- Cleaning
- Drains to waste, so no water stands inside the unitThe same design property as the 500 and 700: nothing sits still long enough to grow a film.Source
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Pick 3 · An actual whole house, sized against a standard
Aprilaire Model 700
Eighteen gallons a day against a stated 5,300 sq ft, cited to AHRI Guideline F. It is ducted, it needs installing, and it is the only sizing claim here with a standard behind it.
This is the reference point for the whole site. Aprilaire is the only maker in our set that publishes a capacity in gallons per day and a coverage in square feet together, and cites the residential sizing standard as the basis for pairing them. Three of its models do it consistently: 12 gal/day to 3,600 sq ft, 17 to 5,000, and 18 to 5,300. Those three pairs are where our 296 square feet per gallon per day comes from.
Everything else follows from the fact that it drains. There is no reservoir, so there is no standing water, so the pink film and the mold that dominate portable-humidifier ownership simply do not arise. The Water Panel goes once or twice a heating season instead of a wick every 30 to 60 days, which changes the recurring cost by an order of magnitude.
The honest counterweight is that this is not a purchase, it is a project. It mounts in ductwork, it needs a water line and a drain, it needs a humidistat and an outdoor sensor, and unless you are comfortable cutting into a plenum it needs a contractor. Our whole-house cost page works through the break-even against running several portables.
What works
- The only sizing claim in this set with a published standard behind it
- No reservoir, so no standing water and none of the maintenance that follows from it
- Water Panel changed once or twice a season instead of a wick every month or two
What does not
- Requires ducted forced-air heating, a water supply, a drain and, realistically, an installer
- Installed cost is a different order of magnitude from any portable here
- Wastes water: the drain design that keeps it clean also sends water to waste
Skip this ifyou rent, or you heat with anything other than a ducted forced-air system. Neither is a fixable objection.
The arithmetic
- Published output (continuous)
- 18.0 gal/day
- Covers, at 296 sq ft per gal/day
- 5,328 sq ft
- Maker’s coverage claim
- 5,300 sq ft
- Output that claim would need
- 17.9 gal/day
Output as the manufacturer publishes it. Coverage rate derived from Aprilaire’s own published capacity-and-coverage pairs.
Published specification
- Tank
- not publishedThere is no tank. It is plumbed to the water supply and drains to waste.
- Output/day
- 18 gal/daySource
- Coverage
- Up to 5,300 sq ftAprilaire states this for tightly constructed homes and cites AHRI Guideline FSource
- Power
- 24 VAC via the supplied transformerSource
- Filter
- Model 35 Water PanelAprilaire's maintenance page puts this on an annual schedule, plus an annual distribution-tray inspection. Its product page says once or twice per heating season; the two documents differ and we take the more specific one. Either way it is by far the longest interval of any unit here.Source
- Cleaning
- Built-in drain flushes mineral build-upWater does not stand inside the unit, which removes the standing-water problem entirelySource
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Questions people actually ask
What is the difference between the Aprilaire 500 and 600?
Capacity, and almost nothing else. The 500 publishes 12 gallons a day to 3,600 sq ft; the 600 publishes 17 gallons a day to 5,000 sq ft. Both are bypass evaporative units with no fan of their own, both are plumbed and drain to waste, both take an annual water panel. The 600 uses the larger Model 35 panel where the 500 uses the Model 10.
Is the Aprilaire 600 worth the extra money over the 500?
Only if your house needs more than about 12 gallons a day. At our derived rate of 296 sq ft per gal/day, the 500 serves about 3,550 square feet of circulated space, which covers most single-family homes. If your own arithmetic lands between 12 and 17 gallons a day the 600 is the right unit; below that you are buying capacity you cannot use.
Is the Aprilaire 600 a bypass or fan-powered humidifier?
Bypass. It has no fan of its own and relies on the furnace blower to push air across the water panel, exactly like the 500. That is what distinguishes both of them from the fan-powered 700, and it means neither adds humidity while the blower is off.
Do the Aprilaire 500 and 600 use the same water panel?
No. The 500 takes the Model 10 Water Panel and the 600 takes the Model 35, which is the same panel the 700 uses. Aprilaire's maintenance page puts both models on an annual replacement plus an annual distribution tray inspection, so it is one purchase a year either way - but check the price of the panel your model takes before buying the machine.
What size house does the Aprilaire 600 cover?
Aprilaire states up to 5,000 square feet for tightly constructed homes, and its 17 gallons a day works out to about 5,030 square feet at the 296 sq ft per gal/day rate we derive from its own range - so the claim holds up against its own data. Size against the space the furnace air circulates through, not the floor area on the listing.
Aprilaire 600 or 700?
The 700 for its fan, not for its capacity - the two are only one published gallon a day apart, 17 against 18. If you need humidity when the furnace blower is idle, or you have nowhere to route a bypass duct, the 700 earns the difference. Otherwise the 600 does the same job for less, and our bypass vs fan-powered page works through the trade properly.
Sources
Every figure on this page came from one of the documents below, read on the date shown. We do not cite other review sites, forum posts or video reviews as sources of fact.
- Aprilaire Model 500 - manufacturer spec page (retrieved September 1, 2026)
- Aprilaire Model 600 - manufacturer spec page (retrieved September 1, 2026)
- Aprilaire Model 700 - manufacturer spec page (retrieved September 1, 2026)
- Aprilaire Model 400 - manufacturer spec page (retrieved September 1, 2026)
- Aprilaire - Humidifier Maintenance (per-model water panel schedule: annually for Models 300, 500, 600, 700 and 720, twice per heating season for the Model 400; annual distribution tray inspection; steam canister annually and electrode wires every three years)
- AHRI Guideline F - Selection, Installation and Servicing of Residential Humidifiers (the residential sizing standard; ahrinet.org returned HTTP 403 to this build, so we cite it as the governing document without paraphrasing text we did not read)
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Found a number that is wrong? Tell us and we will fix it and say so. How we pick shows every formula used on this page.