Electric Furnace Parts List And Functions Explained

Furnace Breakers —Protect Wires Feeding Furnace From Main Breaker Panel.
24 Volt Transformer —Reduces Voltage From 240V to 24V for controls.
Elements —Produce Heat When Power Is Applied.
Sequencer —Turns Elements & Fan On In Sequence Upon A Call For Heat.
Thermal Disc Limit —Disconnects Element From Power When Air Flow Is Reduced.
Blower Motor & Wheel —Moves Air Through The Furnace.
Fan Relay —Turns On Fan Motor From Thermostats Fan On/Auto Switch.

What Would Cause A Furnace To Trip A Breaker?

Their has been some argument about the breakers in a furnace being just a switch, but if that switch has an amp rating on the toggle then it’s a breaker and it can trip.

Breakers are an automatic switch that detect heat, excessive current or shorts circuits to a ground.

I have a perfect example out in my shop where a customer let his maintenance get behind. The filter wasn’t installed correctly and lots of stuff collected in the blower and on the indoor air conditioning coil. This was one of those times in the high desert when you want some cooling in the day and some heat in the night.

So they ran their AC during the day and ice formed on the coil which dramatically cut down the flow of air across the elements. Then when they turned on the heat that night the elements overheated, drooped down onto the elements frame to ground and the element wire burned in two.

electric furnace element burned out

Fortunately the breakers tripped and saved the wires from overheating and possibly the house from a fire.

Breakers in electric furnaces are generally rated at 30 or 60 amps. You could have a few combinations of those breakers. In our area I’ve seen a few 90 amp breakers fed from one circuit. That takes some large wire and expense.

One 60 amp would feed two elements at 10 KW each and a fan. Or, a 30 amp and 60 amp would feed three elements adding up to 15 KW furnace and a fan motor. The 60 amp breaker usually feeds power to 2 elements which draw about 20 amps each.

Wire size is also important to each breaker. This list is not complete but will give you an idea of the principle behind wire size and breakers.

  • 15 amp breaker not less than #14 wire
  • 20 amp breaker not less than #12 wire
  • 30 amp breaker not less than #10 wire
  • 60 amp breaker not less than #6 wire

When you look into your main breaker panel you should see breaker ratings that match the breaker ratings in your furnace. If your furnace has two 60 amp breakers then your panel should have two 60 amp breakers.

Those breakers, according to code, should be plainly labeled “furnace” so that anybody could identify them. Your main service panel will have two types of breakers. The breakers that feed your lights and plugins throughout your house will be 115 volt or single breakers.

Those are the skinnier or thinner breakers in the panel and rated from 15 or 20 amps on the toggle of the breaker. Some breaker manufacturers label the breaker on the face instead of the switch toggle.

A little safety tip to use on your breakers is to hold the back of your finger on the outside surface of the breaker. The back of my fingers are more sensitive to temperature than my finger tips. If it feels warm or hot it’s a sign of a loose connection or an overload. You should contact your electrician or HVAC expert and get it examined.

The 24 Volt Transformer Reduces The Voltage From 240V to 24V For Furnace Controls

Electric furnaces are fed from the main panel to the furnace with 240 volts. Most controls like thermostats and relays that control the furnace are rated at 24 volts. A transformer is necessary to step that voltage down. This lower voltage is a safer voltage to handle and more convenient to work with. Also the wire is less expensive to run wires to items like thermostats and other controls like dampers and zoning systems.

24 Volt Transformer

This transformer has a glass fuse to protect itself and other low voltage components like your thermostat. I prefer transformers with the push button reset. At least then you’re not running to the auto parts or hardware store trying to find the replacement.

Transformers have load limits just like a breaker. If those limits are exceeded then the transformer will run warm, hot or even burn up. They can cause quite a stink when they burn up.

Three to five amp fuses are sometimes in the transformer circuit to prevent the burn out. Some transformers also come with a reset button that will allow it to try again. If the reset continues to trip it’s time to find the problem before more damage is done.

If you are adding equipment like an air conditioner or a heat pump to your existing electric furnace, it would be a good time to check the load limit on your 24 volt transformer. Transformers are rated in VA or volt amps. To add equipment such as air conditioners or heat pumps you should have at least a 40 VA transformer.

Their are transformer with multi taps for connecting to multiple voltages. If you are looking to replace a transformer in an electric furnace then you need only be concerned if it has voltage ratings for 240 volts and 24 volts. It’s okay to use multi rated transformer, but they generally cost a bit more.

I see transformers on Amazon for less than $15.00 but I would recommend spending at least $25.00 and get one with a built in reset or breaker.

Elements Produce Heat When Power Is Applied

Just like your hair dryer or toaster. When you’re cooking your morning toast and see those red coils heating up, that’s what you could see deep inside your furnace when your thermostat calls for heat.

Elements are nothing more than a coil of wire that when electricity is applied they will produce heat. They have to be engineered to produce just the right amount of heat that can be cooled enough by the fan or airflow across them so that they don’t overheat and burn out.

A Sample of Electric Furnace Parts

Not all electric furnaces have a control board as pictured. Control boards such as this one convert ac to dc voltage and operate relays like these sequencers. Why dc voltage you ask? That’s another post.

Electric furnaces are unique in that their aren’t too many parts that can’t easily be replaced and that make sense to replace as opposed to replacing the whole furnace. A combination of repair parts would have to be a consideration to replace the furnace. I have seen some elements that are horrifically expensive probably due to lack of availability and some manufacturers like to push you into replacing your equipment.

Sequencer Turns Elements & Fan On In Sequence Upon A Call For Heat

Sequencers or stack switches as these are sometimes referred to control the elements and fan. The bottom stab on connectors that are sort of gold colored are the low voltage or 24 volt connections. The jumper wire between them can be cut to allow the elements to be staged on and off.

A two stage thermostat would be required to two stage a furnace. Sometimes it is more prudent to stage the elements and use a little less heat. Be careful when staging electric furnaces. Sometimes a longer run time can mean more energy costs. I like getting the job done and getting the furnace off.

The top or silver colored stab connectors are switches for the elements and the fan. The fan should come on with the first element to be powered and off with the last element to prevent them from getting too hot as we discussed earlier in this post.

A failed sequencer is a common problem in electric furnaces. The symptom is usually very little heat or just a fan blowing cold air.

Sometimes a contactor is used in place of a sequencer. They are a little noisy. If you hear a sharp snap or click when the furnace starts then you probably have a contactor controlling the elements. Both are good and it’s hard to say which is longer lasting.

A Thermal Disc Limit Disconnects Element From Power When Air Flow Is Reduced

To help protect the furnace system and your home, elements are protected by temperature limits that open the circuit to the element in case they overheat. Remember what overheating is caused by? Lack of air flow. Most of these limits reset automatically. They get hot and open and then they cool down and close allowing the element to come back on and heat.

You know of those plastic soda pop lids on the cups at the fast food place. The server will punch the little bubble on the lid to ID the kind of pop you ordered if their is more than one order, to keep them straight. A thermal disc limit works just like that only it pops when heated and then pushes on a little plunger that opens a switch.

Excessive opening and closing will most times cause the limit to fail in the open position. Another reason for not enough heat coming out of your registers.

Blower Motor and Wheel Moves Air Through The Furnace

Number one force for a happy furnace and duct work system is air flow. Not too much not too little. To big of furnace or blower causes noise and inefficient heating and cooling. To small a blower is the leading problem in iced up air conditioner evaporator coil and burned out furnace elements.

Blower motors for residential furnaces are what they call fractional horse power motors. Mostly from quarter horse power to 3/4 horses. Their main problems come from dust and other particles such as dog and cat hair getting in around the winding’s and over heating the motor. Most are sealed bearings now days and require no oiling or maintenance other than keeping them clean.

The blower motor is probably the most expensive part in an electric furnace to replace and should get the most attention when maintenance time rolls around.

A lot of noise issues come from lack of duct work or location of duct work in relationship to the furnace blower. Filters also have a great deal of influence on the proper amount of air flow across heating elements. Funny how these items seem to be the most neglected.

Blower wheels have a propensity for collecting dust. These wheels are constantly sucking air from every cavity and crack in your duct system and even through the doors in the furnace itself. Take a look at the front of your furnace a you may be able to see fine dust lines all around the doors and fitting of the furnace cabinet.

We were taught early on that even a small amount of dust collected on the surface of a blower wheel will greatly diminish the efficiency of a blower. It’s a good test to see how sharp your service company is if they present the issue of cleaning your blower wheel as a maintenance item.

Fan relay Turns On Fan Motor From Thermostats Fan On/Auto Switch

When you want to run your furnaces fan on constant to filter the air, it’s the fan relay that gets switched by the thermostat and powered by the 24 volt transformer to make the blower turn over and run.

Newer digital thermostats also have a setting that’s called circulate. It turns the blower on and off about every ten minutes to also filter the air and save a little money in electricity by not running the fan motor constantly.

Their is not right or wrong in where the fan switch should be set. Either in the on position or auto or circulate. It’s strictly a matter of choice. The fan will operate or not as needed by the thermostat and sequencer.

These relays have different configurations as far as switching goes. Their is single pole single throw (SPST), single pole double throw (SPDT) , and double pole double throw (DPDT). It’s a preference of the designer of the equipment.

If you are replacing a fan relay be sure to get an exact replacement. Rewiring could be a monster head ache if not.

Chad Peterson

Chad Peterson is a veteran of the HVAC industry since 1977. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

Recent Posts