Aprilaire is the humidifier brand of Research Products Corporation of Madison, Wisconsin, and its units are among the most common furnace humidifiers in Ontario basements. The 500, 600 and 700 are evaporative: a solenoid valve opens, water trickles down a coated aluminium Water Panel, warm furnace air passes through it and the surplus water runs to a drain. There is no reservoir of standing water, which is the reason the design has lasted.
Halton and Hamilton have hard water, and that sets the maintenance schedule. A Water Panel here is crusted with scale after one heating season, and the same minerals clog the tiny orifice that meters water onto it. Most Aprilaire units we are told are dead turn out to have a blocked orifice, a seized valve or a panel that was last changed several years ago.
Timing is simple. A humidifier that will not run is a comfort problem, and it can wait for a booked visit. A humidifier that is dripping should be shut off at its water valve today, because the water runs down the plenum into the furnace cabinet, where the control board and the heat exchanger sit. Our answer on a leaking furnace humidifier covers the first steps.
In 1970s to 1990s subdivisions we often find an Aprilaire that outlived the furnace it came with: the furnace was replaced and the old humidifier was hung back on the new plenum with whatever wiring was handy. Those are the units that run water with the furnace off or never run at all. ZK Mechanical is independent and services Aprilaire along with every other make of furnace humidifier, so the recommendation is based on your house, not on a brand target.
Aprilaire lines we install and service
The smaller bypass humidifier, sold as the 500 with automatic control and the 500M with a manual humidistat. It takes the Water Panel 10. A bypass unit has no fan of its own: a round duct carries warm supply air through the panel and back to the return, so the damper in that duct must be open in winter.
The large bypass unit, rated by Aprilaire at 17 gallons a day, with the bypass damper built into the cabinet. It takes the Water Panel 35, and the automatic version ships with a digital control and an outdoor temperature sensor. This is the model we install most when a house has room for the bypass duct, at $500–$900 installed.
The 700 mounts on the supply plenum and has its own fan to pull air through the Water Panel 35, so it needs no bypass duct and a nearby electrical outlet. It suits furnaces in tight closets and larger homes that need more output. Fan-powered humidifiers run $700–$1,200 installed.
An electrode steam humidifier: it boils water in a replaceable canister and injects steam into the duct, rated by Aprilaire at up to 34.6 gallons a day depending on voltage. It works with or without a heat call and runs on ordinary tap water. Aprilaire calls for a new canister every year, and a steam unit runs $1,500–$3,000 installed on its own electrical circuit; see steam humidifier installation.
What goes wrong on Aprilaire equipment
No water reaching the Water Panel
The furnace is heating, the humidistat is calling and the panel is bone dry. Three parts cause it: a solenoid valve that no longer clicks open when it gets 24 volts, an orifice or in-line strainer packed with scale, and a saddle valve on the water pipe whose needle hole has closed up with minerals. We test for voltage at the valve first, then for water at each point downstream. A plugged saddle valve is replaced with a proper shut-off, because a saddle valve that clogged once will clog again.
Scaled Water Panel and a blocked drain: water on the furnace
A scaled panel no longer wicks water evenly, so output drops and water sheets off the crust instead of soaking in. The same scale washes into the drain fitting and hose until the drain blocks and the pan overflows onto the furnace. The fix is a new panel, a cleaned distribution tray and drain, and a hose that slopes down the whole way to the floor drain or condensate pump. Replace the panel every fall here; it is a $20–$40 part that prevents most of these calls.
Model 700: the fan motor has failed
On a 700, water can be running over the panel perfectly and the house still stays dry, because the built-in fan is what moves air through it. A seized or burnt-out fan motor shows as a silent unit with a wet panel. We check that the outlet is live and that the control is switching the fan before replacing the motor. On an older 700, compare the price of the motor with the $700–$1,200 for a new fan-powered unit installed before you decide.
Control faults: outdoor sensor and wiring
The automatic humidistat lowers the humidity target as it gets colder outside so the windows do not sweat, and it needs a working outdoor sensor to do that. A sensor that has failed, was mounted in the sun or was never connected leaves the control guessing. The other common fault is wiring: a humidifier powered so that it runs whenever humidity is low, even with the furnace off, sends cold water over the panel and straight down the drain. It should open only with a heat call or with the blower running, and we rewire it that way.
Aprilaire warranty: what to know
- Aprilaire's owner's manuals for these humidifiers give a five-year limited warranty counted from the date of installation. Confirm the term in the manual for your model, and use Aprilaire's online warranty registration.
- Water Panels and steam canisters are maintenance items that you replace on a schedule. They are not warranty parts.
- Water damage from a blocked drain or a panel left in for years is a maintenance failure, not a manufacturing defect. Keep the receipts for your annual panel changes.
- Keep the installation invoice showing the date and the installer. It is the starting point for any claim on a valve, a fan motor or a control.
Parts availability in the GTA
Aprilaire parts are easy to get. Water Panels 10 and 35, solenoid valves, orifices, feed tubes, drain fittings and the Model 80 steam canister are stocked by GTA HVAC wholesalers, and the panels are also sold in hardware stores. Fan motors for the 700 and digital controls are usually a short order. Buy the genuine panel in the right number for your model: an off-brand pad that is slightly thin lets water run past its edges, and a 10 will not fit where a 35 belongs.
Common questions
Why does my Aprilaire send water down the drain all the time?
Some drain flow is normal. The 500, 600 and 700 are flow-through humidifiers, so more water runs over the panel than evaporates and the surplus carries minerals away. That flow should stop when the furnace stops. If water runs with the furnace off, the solenoid valve is stuck open or the unit is wired to run without a heat or blower signal.
Should the bypass damper on my Aprilaire be open or closed?
Open in the heating season, closed in summer. On the 600 the damper is on the cabinet and marked for each season; on a 500 it is in the round bypass duct. Left open in summer, it lets cooled air short-circuit from supply to return, which costs you air conditioning capacity. A fan-powered 700 has no bypass damper to set.
Can I change the Aprilaire Water Panel myself?
Yes, and you should, every fall. Shut off the water, open the cover, lift out the frame, swap the panel with the coloured mark at the top and rinse the scale out of the distribution tray and the drain. The 500 takes a Water Panel 10; the 600 and 700 take a 35. If the unit still will not wet the new panel, see why a furnace humidifier is not working.
Is an old Aprilaire worth repairing?
Usually. A valve, an orifice, a panel and a drain cleaning bring most of them back, and the cabinet itself rarely fails. Replace it when the housing is cracked or warped, when the repair approaches the $500–$900 installed price of a new bypass unit, or when you are changing the furnace anyway. Our furnace humidifier repair page explains the visit.
When does the Aprilaire 800 steam model make sense?
When an evaporative unit cannot keep up. Bypass and fan-powered humidifiers depend on warm furnace air, so their output falls in a house heated by a heat pump with cooler supply air, and in large or very dry homes they run out of capacity. Steam does not depend on furnace runtime. The trade-off is a $1,500–$3,000 install, a yearly canister and higher electricity use.