Relay Q & A

Hi!  I’m Paul, and I just joined the team here at Kiln Frog.  In the coming months I’ll be posting some informative blog entries about things you always wanted to know about kilns, grinders, sandblasters, heat treating accessories, glass, and everything else you can find here at Kiln Frog.  I am a potter and a fused glass artist, and I’m always looking for something new to try or to learn.  Since joining the team here at Kiln Frog, I’ve also been exposed to a whole new world of knife making.  I’m absolutely fascinated with the Damascus process and can see a heat treating kiln in my future!  In my clay work, I specialize in cone 10 crystalline glazes and other unique finishes.  I like to work big, so I got into glass to learn how to make vessel sinks but along the way learned a host of other techniques that I like to combine into new and creative ways to manipulate glass.  If there is anything you would like for me to cover, just send me an email at and I’ll start working on a blog entry for that topic. 

Today, I’m going to talk about relays.  When ordering a new kiln, the question always comes up about what type of relays we would recommend.  A lot of people know that a relay is something in the kiln that has to do with electricity, but not much beyond that.  Hopefully, this informative blog will get you up to speed if you are scratching your head and wondering why you should care what a relay is.  They are really important to the operation of your kiln, and there are things you should know about the care and maintenance of your relays. 

As you will learn further down in the blog entry, mercury and solid-state relays are pretty cool!  Imagine if you never had to worry about changing the tires on your car.  Never a low tire pressure warning, never a blow out, never a day of your life spent at the tire store…  Mercury and solid-state relays are kind of like that for your kiln.  Spend a little more up front and take the worry of an under-fired, or even worse over-fired load of the best work you’ve ever done off your mind.  These types of relays will usually last the life of your kiln, and only require an occasional inspection. 

If you already have a kiln and it doesn’t have mercury or solid-state relays, don’t worry.  There is lots of great information here on how to care for your relays and even links to some tutorials on how to change out your relays when that time arrives.   So, let’s get started!

Relay Q&A 

Q: What is a relay.

A:  The relays in a kiln are the gatekeepers of electric current. When the relay is closed, the electricity has a completed path to the elements in the kiln, allowing them to heat up.  When the relay is open, the circuit is interrupted, thus no electricity to the coils and no heat.  Most larger kilns have multiple relays that control different sections of the kiln. 

 Q: How do relays work?

 A:  Relays in your kiln work in conjunction with the thermocouples and controller to allow the coils to heat up and maintain a desired temperature. When the temperature is below the desired set point, the relay closes and allows electricity to flow to the element and generate heat.  When the desired temperature is reached, the relay opens and breaks the current flow, allowing the coils to cool. That’s the simple explanation.  In real life, the relays go on and off many times during that ramp time so that the current isn’t constantly flowing.  This cycling is why you hear the clicking noises when your kiln is firing.  That’s the mechanical relays going on and off.  The controller is cycling the coils on and off via the relays in order to get a smooth transition from one temperature to another without anything overheating. 

Q: What are the different types of relays and how do they differ in function and longevity?

A:  There are three main types of relays found in kilns.

Mechanical relays - use electric current to move a contact to the open or closed position using an electromagnetic switch.  Because of the moving parts, mechanical relays have a limited life span and must be replaced periodically. They are usually good for around 200,000 cycles (clicks).

Mercury relays – use a tube of mercury with an internal weight that can drop down into the mercury, causing the mercury to raise in the tube and complete the electrical contact.  Because of fewer moving parts, mercury relays have a much longer functional life.  They are usually good for around 5 million cycles.

Solid-State relays – have no moving parts and operate using power semiconductor devices to open and close the flow of electricity.  Because they have no moving parts, they have a very long functional life, over 1000 years!

When you order a new kiln with either Mercury or Solid State relays, most manufacturers will use the more robust relays to control a regular mechanical relay.  The mechanical relay stays in the on (closed) position, but the mercury or solid state relay does all of the work of switching on or off to send current to the elements. 

All types of relays can burn out prematurely if you have loose wiring connections somewhere in the kiln that is causing an overheat situation in the electrical components.  This is why it is so important to inspect the wiring in your kiln periodically.  If you see evidence of overheating, you need to find the cause of the problem and get it corrected right away before it leads to more trouble.      

Q:  Why is it important to keep track of the health of the relays in my kiln?

A:  Let’s go back to the first question.  If a relay is the gatekeeper of electric current in your kiln and it fails, one of two things is going to happen.  If it fails in the open position, no current will flow and you won’t get any heat.  If it fails in the closed position, the current continues to flow unchecked and you will get too much heat!  No heat is a drag, because nothing will happen, but too much heat can be a disaster.  Your project could over-fire, you could end up with a mess in your kiln from unconfined glass flow, or even end up with a fire situation if left unchecked and unattended for too long.  Larger kilns with multiple relays are a little more forgiving, because one failed relay will be less likely to cause a catastrophe, but it will still not be a perfect firing and you will most likely know there is something amiss.

Q:  When should I replace my relays?

A:  There isn’t a hard and fast answer to this question.  If you have mercury or solid-state relays, they could very well last the life of your kiln.  A periodic visual inspection of the relays is usually all that is necessary. 

If you have mechanical relays, then the answer is much different.  As a general rule, mechanical relays will last around 18 to 24 months.  With that said, keep in mind that the reason mechanical relays fail is that they wear out.  If you are firing your kiln once a month, your mechanical relays will last much longer than if you are firing it two or three times a week.  It is best to replace your relays every 18 months to be on the safe side. 


If you own a Jen-Ken kiln, you should have current flow indicators on the controller for each relay.  Here is what Jen Ken says about this great innovation:

“Another thing that makes Jen-Ken Kilns different and unique is that they install a "pilot light" on every relay on every model. The relay is pretty important as it is what turns the element on and off. So, this added benefit helps the user identify if a relay is not firing, or is not turning off and is invaluable when troubleshooting problems. The light goes on every time the relay fires. If the light stays dark then you know you have a problem with the element, relay, or connection. This is especially helpful in kilns with multiple elements.” 


Evenheat Kilns introduced a great innovation a few years ago with their relay access door.  This door, which is really only applicable to the standard mechanical relays allows users much easier access to the actual relay and pulls the relay and appropriate wires out of the controller box. So, basically it’s a quick switch door that keeps your hands out of the box where all the other wires are. This eliminates the feeling that you are changing your relays in a bowl of spaghetti. It’s a nice little convenience that now will be standard on all of their kilns with mechanical relays.  


Here are some links to our manufacturer’s websites with lots of great info on changing out your relays. 


Evenheat Kilns


Paragon Kilns


Olympic Kilns


Jen-Ken Kilns

Small Relay

Large Relay


And here is a great tip from our resident expert, Arnold:

When replacing a relay, check for heat damage on the old relay. It will look like a melted spot in the plastic body. It is likely that a push-on terminal of a wire connected to the relay near that burned spot will be loose. This is because excessive heat reduces the spring tension of the terminal. If this is the case, replace the push-on terminal on the wire before you install the new relay. Otherwise, the loose terminal will destroy the new relay. This is because a loose connection produces tremendous heat.

Arnold Howard

And here are some pics from Arnold.

Loose wires on a relay can get hot enough to burn off as shown in the photo. Avoid using electrical tape in a kiln. The tape has a low melting point.


This kiln uses mechanical and MDR (mercury) relays. Vertical panel, the second row of components, left to right: 2 mechanical relays, and 2 MDRs. In this kiln, the mechanical relays are called driver relays, because they activate the MDRs. The mechanical relay on the horizontal panel activates a kiln vent (fan).


Make sure the wires are tight when you install a new relay. They should be difficult to push onto the relay terminals.

Different types of relaysEvenheat kilnsEvenheat kilns & ovensJen-ken kilnsKiln academyKiln relays healthKiln safetyKiln troubleshootingOlympic kilnsParagon kilnsRelay q&aRelays for kilnsRelays lifespan

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