Fan compartment safety switch: this article describes the door interlock safety switch found on air handlers used in both air conditioning and heating furnaces.

We explain how the blower door safety switch works, how it is tested, installed, and wired.

A/C or Heat Pump Air Handler Blower Door Safety Interlock Switch

In forced warm air furnaces as well as air conditioning and heat pump system air handlers the indoor air handler's blower compartment houses a fan, usually a round squirrel cage fan and electric motor (to operate the fan) which moves air from the return air plenum on towards the cooling or heating coil.

An access door to the blower compartment should have a blower door interlock switch - also referred to as a blower door safety switch - a safety device which will prevent the blower from operating if the door is opened (see below).

Shownabove, an Armstrong interlock furnace door safety switch, distributed by American HVACparts.com. The steel bracket is used to secure the blower compartment door switch such that when the door is properly closed and secured the button is depressed, permitting electrical power to flow to the blower motor.

[Click to enlarge any image]

What & Where is the Blower Compartment Door Switch

An air handler blower compartment access door switch is present on newer units, and can be seen as a button or switch which is depressed when the blower compartment door or cover is properly in place.

This switch shuts off the blower fan as a safety control if the door is opened.

You'll find one or more of these switches along the edges, usually at one side near the top, of the blower compartment frame of your furnace or air conditioner indoor air handler or blower unit.

Look along the edge of the door frame (not the door itself but rather the opening into which the door is inserted) for a button that will be depressed when the door is in place.

If your air conditioner won't run and someone has been fooling with it, be double sure that the blower compartment door is properly closed and that the interlock switch sensor button or lever is properly depressed to convey that fact to the system.

Quoting Trane's typical equipment instructions:

The blower door safety switch will prevent or terminate furnace operation when the blower door is removed 

 

Above is an illustration of a Carrier® blower door safety switch used on many Carrier air handlers.

This blower door safety switch # HR54ZA006 fits Carrier air handlers and is widely distributed including by SupplyHouse.com Tel: 1-888-757-4774 in the U.S.

Many air handlers of more than one manufacturer may accept the same air handler door switch. Often the same OEM interlock door switch will fit Bryant, Carrier, and Payne equipment using 115/120V 2-wire installations.

At below-left is a Trane furnace door safety switch for Trane equipment # DRS652132B (about $35. U.S) that replaces blower door switch part numbers SWT01254, CARSWT1254 used in American Standard Trane HVAC equipment.

A virtually identical switch (below) is sold for BDP Carrier, # HR54ZA101, AR54ZA100, HR54ZA100, CARHR54ZA101 and fits many Bryant, Carrier and Payne air handlers.

Air Handler Door Safety Switch Voltage & Wiring Details

Usually a furnace or air handler blower door safety switch carries 120 volts.

Testing the air handler door interlock switch

Watch out: for dangerous shock hazards. Be sure all power to equipment is off before touching any switches, wiring, etc.

When power must be turned on to perform an electrical test, for example for the presence of voltage, do not attempt such testing yourself unless you are properly trained in safe procedures and know how to make safe use of your test equipment.

See DMMs VOMs SAFE USE OF

A typical 120V door interlock safety switch should conduct power (and show 115-120V across the switch) when electrical power to the air handler or furnace is ON and the door is latched (or the switch button is depressed).

• When the fan compartent door is opened and the switch button extends outwards the switch should open and interrupt the circuit - no voltage should appear across the switch though it will remain on the switch input side.
• When the blower compartment door is properly shut and secured the interlock switch button is depressed and closes to permit current to flow to the equipment.

Wiring the furnace or blower door safety switch

Typically a replacement door interlock switch will come with a simple wiring diagram, and you may also be able to identify the original safety switch on the wiring diagram for your air handler.

[Click to enlarge any image]

Shown at left is an excerpt from a Trane® gas fired furnace. Our red rectangle shows the location of the blower door switch on the wiring diagram. Be sure to refer to the proper wiring diagram for your own HVAC equipment. - "TUX-D-2 Up flow/Horizontal Left Downflow/Horizontal Right Condensing Direct Vent Gas Fired Furnace XR90 TUX040-series, single-stage fan assisted combustion system" from Trane, retrieved 11/5/2014, original source: http://www.trane.com/commercial/uploads/pdf/1112/tux_c.pdf

Hook-up is so simple that you might simply use the existing door switch wiring, connecting it to the replacement switch.

The switch is connected in series between a relay in the air handler that operates the blower fan and a terminal on a control circuit board in the unit. The switch is not powered independently. It's power comes through the circuit board.

You'll be able to see this by following wires from the blower door safety switch back into the air handler.

But if your air handler or furnace is an older one that did not include a door interlock safety switch you'll want to wire the switch so that it interrupts power to the blower fan.

Replacement blower door switches are easily obtained for your air handler by brand and model via your local HVAC supplier, from the manufacturer, or often online. Some safety switches will work properly on several air handler brands and models.

I add that the blower door switch is an important safety feature that helps avoid chock or chopped-off fingers or worse.

Be sure that your replacement switch is properly mounted so as to be reliably operated by the blower compartment door(s) - the switch opens- cuts off power - when the door is opened, and is depressed in to close and permit power to the blower when the door is properly closed.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: Where is the blower door located?

Where is the blower door located at ? - 2016/08/14 Linda said:

Reply: A photo-guide to finding the blower door on an air handler

Linda:

At AIR HANDLER BLOWER DOORS I've added some photos and created this article to help you see what you're looking for when you need to find the blower compartment access door or cover on an air handler.

How to bypass a blower door safety interlock switch

Please help fast

How can I bypass a furnace door switch that has three wires - On 2018-11-02 by Franklin

Reply by (mod) -

Franklin

Be careful, no one in their right mind is going to bet your life and safety on telling you what three wires are when they really don't know. You need to look at the wires and look at the wiring diagram for your furnace.

Usually a blower door switch is designed to prevent the blower from running if the door isn't closed. That's an important safety feature because they don't want somebody to reach into the blower compartment and get their fingers chopped off by the blower motor, fan, or belt.

Blower door switches typically are interrupting a hot wire to the blower motor. That would just take two wires to the switch if there's a third wire I don't know what it's doing. It could come from a control or it could be a ground wire or it could be something else.

In Sum even if you can bypass a blower door switch you're doing something dangerous and you should be aware of that.

What's the part number for a Janitrol Blower Door Switch?

I need to purchase and install a new interlock or safety switch for a Jantrol Model GUX120-5,Serial #871200223, furnace Blower door but i am not sure what the part # or what replacement part i can use. Can someone please help me? - On 2016-07-06 by mark

Reply by (mod) -

Mark, these switches are some and pretty standard. Just take the old one to your HVAC SUPPLIER for an equivalent or comparable match, or order one from Jantrol.

Some example part numbers are given above eon this page, but you should also see a manufacturer and part number on the blower door switch that you've got in hand, too.

Blower Furnace Won't Stop Running - A/C door interlocking switch is beeping

The blower furnace switch can be the problem? Thanks

Hi, I have my blower running constantly it doesn't stop I checked a couple things and still doing it... my question is the furn switch

My A/C door interlocking switch is beeping. The A/C is working properly, but a short beep is sounding every 45 seconds or so. Why is this and what do I do? On 2016-05-25 by Rhonda

Reply by (mod) -

Jose

Blower door switches do not, themselves, beep nor contain an alarm. A blower door switch opens when the blower compartment door is removed or opened - that will turn OFF the system. That's not the problem you're describing. Instead you want to

see

• BLOWER FAN WONT STOP - LIMIT SWITCH
• BLOWER FAN WONT STOP - THERMOSTAT SWITCH
  

Door interlock safety switch not working properly - taped "closed"

Furnace Heat cycle works fine ... when I have interlock taped to "close", and the door off.

But, when I put door back on, with the interlock still taped "closed", the blower slows down to a stop.

I discovered this by accident. I replaced the Primary Limit, (2) Roll Outs, the blower housing Secondary Limits. It is a Lennox Elite w 2 phase Heat, with an ECM 2.3 variable speed fan, 1 HP, 1075 RPM. - On 2020-11-06 by Andrew Pgher

Reply by mod: repair tips

When you tape the door interlock button down (as if the door were pressed in place or closed) that allows the blower to run as it should (on a call fodr heat) but of course it's unsafe as someone could reach into the blower compartment and be badly hurt by the spinning blower fan.

When the furnace will not run once you put the door back in place I suspect that the frame of the blower compartment is being bent slightly askew enough that the switch opens.

Check that the switch is mounted securely and that the blower compartment door is not bent.

If all of the wiring connections are snug and the blower door frame is dead square and the door dead flat and the door latches snugly in place but the switch is still giving you trouble, replace the blower door switche itself - I suspect that the switch button may be loose or damaged.

 

Furnace fan runs only if I fool around with the blower compartment door

I have a thermostat that is set to kick the fan on and off for heat. (heat comes in from an outdoor wood burner)

When the thermostat calls for heat, it clicks to turn on but the fan does not kick on.

I can go down to the furnace and take the panel off, and put it back on and most of the time the fan will kick on. any suggestions. - On 2012-10-11 by Doug Kline

Reply by (mod) -

Doug

If the door frame and door are flat and square and the door is pushing the switch button in all the way but the fan still won't work, then

check for a bad blower compartmen door safety switch. or just replace the switch.

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Continue reading at CONTROLS & SWITCHES, A/C - HEAT PUMP or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR CONDITIONER BLOWS WARM AIR
• AIR CONDITIONER WON'T START
• AIR HANDLER BLOWER DOORS how to find & open the blower compartment door on an air handler or furnace
• AIR HANDLER / BLOWER UNITS - home
• BLOWER FAN OPERATION & TESTING
• FURNACE CONTROLS & SWITCHES
• HEAT WON'T TURN ON
• MANUALS & PARTS GUIDES - HVAC - home

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BLOWER DOOR SAFETY SWITCH at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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