Everything You Need to Know About Controlling Your Kiln

Learn all about kiln relays and how to prevent kiln relay failure.
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Scott Shannon

Understanding and Preventing Kiln Relay Failure

Three words no artist wants to hear: kiln relay failure. At best, kiln relay failure can be an annoyance that disrupts your project mid-schedule. At worst, kiln relay failure can pose a threat to the safety of your household and your property. However, the good news is that with foresight, proper maintenance, and the right equipment, kiln relay failure can mostly be avoided.

Over the course of this article, we’ll be discussing:

  • How kiln relays work.
  • Why (and how) relays fail.
  • How to choose a kiln relay that minimizes the chance of failure.
  • How to easily track preventative maintenance to know when to replace relays.
  • Kiln safety equipment to invest in to mitigate damage in case of relay failure.

How Kiln Relays Work

To understand why kiln relays fail, first, it’s important to understand how they work. In simple terms, kiln relays are a switch. Based on input from the kiln controller, relays cycle on and off – closing and opening the circuit to the kiln’s element.

When a relay closes, electric current flows to the kiln’s elements, increasing kiln temperature. When a relay opens, no current flows to the element, and the kiln cools down. Relays open and close at variable rates, allowing the kiln to reach precise firing temperatures.

Types of Kiln Relays

Different types of kilns relays have different types of mechanisms that allow them to cycle on and off. In modern kilns the most common types of relays are mechanical relays and solid-state relays. However, some older kilns still use mercury relays – so we’ll be explaining all three mechanisms:

  • Mechanical Relays: Mechanical relays have the most moving parts. Mechanical relays use an electromagnetic switch that physically moves each time the relay cycles on and off. That’s why you can hear them click each time they cycle when the kiln is operational!
  • Mercury Relays: Mercury relays work through gravity and displacement. Mercury relays use a metal weight in a tube of mercury. Gravity causes the metal weight to fall, displacing the mercury until it closes the circuit. A coil at the top of the tube electromagnetically lifts the weight when it receives current, causing the mercury to recess and open the circuit.
  • Solid-State Relays (SSRs): Solid-state relays use semiconductors and electronic parts to turn the circuit on and off based on signals from the controller. They do not rely on any moving parts to open or close the circuit.

Why Relays Fails

Kiln relay failure largely comes down to failure of their switch mechanism. As you might expect, mechanical relays have the most moving parts and are subject to normal wear and tear. They will fail eventually, it’s just a matter of when.

Mercury relays only have a single moving part and aren’t subject to the same contact erosion in high current applications. They’re extremely reliable, lasting for millions of cycles.

Solid-state relays have no moving parts. As such, they are supremely reliable and aren’t subject to any wear and tear. Their only Achilles heel is heat. Solid-state relays must be appropriately cooled via heat sink to maintain proper performance and prevent melting.

Kiln relay failure can occur in one of two positions. A relay can get stuck open, in which case your kiln will fail to heat up. While this is inconvenient and can put your project on hold, it’s far preferable to when a relay gets stuck closed! When kiln relays get stuck in the closed position, the kiln heats up indefinitely, posing significant risk to your household and property.

However, there are safety precautions you can take to circumvent any potential damage, which we’ll be discussing later in the article.

Choosing the Right Type of Kiln Relay

As you can see from the descriptions above, mechanical relays are far more likely to result in kiln relay failure. Mercury relays, while reliable, use the highly toxic liquid metal, mercury, to perform switching. As such, they’re largely obsolete and are even outlawed in states such as California.

That means that, realistically, your best option to prevent kiln relay failure is to purchase a solid-state relay. Not only are SSRs superior in lifespan and environmental concerns, but they cycle on and off significantly faster than other types of relays, resulting in superior performance and precision.

Most new kilns have the option for solid-state relays. While TAP Kiln Controllers are compatible with all three types of relays, we strongly encourage our customers to invest in a solid-state relay. The upside far outweighs the cost. If you’ve got an older kiln with mechanical or mercury relays, consider swapping them out for solid-state the next time they need replacement!

Additionally, whether you’re shopping for mechanical relays or solid-state relays we suggest purchasing from reliable, reputable manufacturers.

Preventative Maintenance

In addition to selecting the right type of relay, preventative maintenance is key to preventing kiln relay failure – especially if you’re using a mechanical relay.

As far as preventative maintenance for solid-state relays, really the biggest consideration is making sure that they are rated properly for the application and that you install the relay correctly with a heat sink to prevent the relay from overheating. Beyond that, we encourage you to store your kiln inside to avoid exposing electrical components to the elements and pay attention to your controller’s error notifications (assuming you’re using an advanced controller such as TAP that provides real-time diagnostics, as well as detailed error and firing logs).

Mechanical relays require much more proactive preventative maintenance. Mechanical relays have a finite lifespan that’s impacted by how often you use your kiln and the types of kiln firing schedules you use. However, you can find an estimated life expectancy for the number of cycles your mechanical relay is rated for from your kiln manufacturer or the relay manufacturer’s product sheet.

With advanced controllers from temperature controller manufacturers like SDS Industries, you can input life expectancy for your relay(s), elements, and thermocouples. The controller will then track the usage of these components and send you preventative maintenance alerts. This helps you replace your kiln relay before it fails.

The TAP Kiln Controller tracks kiln relay usage to provide preventative maintenance alerts to help users replace relays before kiln relay failure occurs.
The TAP Kiln Controller tracks kiln relay usage to provide preventative maintenance alerts to help users replace relays before kiln relay failure occurs.


Kiln Safety Equipment

Investing in the right type of kiln relay and being diligent about preventative maintenance significantly reduces your chances of facing kiln relay failure. But there will always be a non-zero chance that your relay fails, and an ounce of prevention is worth a pound of cure!

We strongly encourage our customers to invest in kiln safety equipment that keeps you safe in case of kiln relay failure. This equipment includes:

  • A reliable fire extinguisher to keep nearby your kiln to mitigate any damage in the case that your relay gets stuck closed.
  • An advanced kiln controller that provides precise inputs to your kiln relay and monitors your kiln for discrepancies to provide you with real-time error alerts.
  • A kiln control mobile app that provides real-time temperature monitoring and push notification alerts and alarms to your smartphone, so that you can spot any warning signs in real-time and respond appropriately.
  • A safety limit controller (such as the soon-to-be released TAP Monitor!) that can be wired to a safety relay to provide max temp safety shutoff even if your primary kiln relay fails.


Explore Kiln Control Solutions by SDS Industries

The TAP Ecosystem includes a host of solutions to enhance kiln safety and help you prevent (or safely respond to) kiln relay failure. In addition to being the most advanced, precise, and easy-to-use programmable digital kiln controllers on the market today, the TAP and TAP II Controllers by SDS Industries provide advanced diagnostics, preventative maintenance alerts, and remote real-time kiln monitoring and error alerts via TAP Kiln Control Mobile app. These features help you prevent and proactively address any potential failures in your kiln or oven build.

Additionally, our soon-to-be released TAP Monitor Digital Pyrometer & Limit Controller can be added to any kiln or oven build, regardless of your current kiln control method, to provide safety redundancy and real-time remote kiln monitoring.

We invite you to explore our selection of programmable kiln controllers, pyrometers, standalones, and conversion kits on our online store. You can also purchase TAP Digital Controllers or TAP Controlled Kilns and Heat Treat Ovens through one of the following distributors:

Shop programmable temperature controllers.


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