Five Troubleshooting Tips For Coffee Roasters, From Coffee Technicians

by Arno Holschuh

Most coffee technicians know how vital it is to get a broken espresso machine back up and running. Without the machine, the café cannot create the drinks that drive revenue, tempers flare as the whole cafe goes into cardiac arrest, and a tech walks in and starts pulling the machine apart. 

But as stressful as that can be, having a roaster go down is an entirely different level of problem. Now you’re not worried about disappointing a regular location; you face an existential threat to the entire business. Wholesale customers will walk, eCommerce orders will turn into customer service time bombs, and a whole labor force will be idle while this most crucial piece of equipment is serviced. 

True expertise in roaster repair is rare. But many simple malfunctions have relatively simple repair pathways open to the well-rounded tech. I’ve collected five common and pretty straightforward examples here. It's not meant to be an exhaustive list but a helpful guide to some of the most common and linear issues.

First, some elementary precautions:

● Always turn the roaster off. 

● Always turn the natural gas off. 

● Always unplug the power supply. 

● Wait for the roaster to cool before working on it.

● Wear a mask and gloves — the dust inside a roaster is intense. 

● Never work on natural gas plumbing unless you know exactly what you are doing. 

This is a non-exhaustive list; please use your common sense, and above all, be careful.

Roaster, roaster burning bright

Roasters are, in essence, giant convection ovens. They use hot air (and exhaust gases) to heat coffee beans, aided by radiative and conductive heat coming from the metal of the roast chamber. 

One of the most common issues, especially on older roasters, is a less-than-ideal combustion of natural gas right at the burners. If there is inadequate oxygen or a mixing of oxygen and natural gas, then incomplete combustion can occur. In most cases, this is due to some clogging or reduced flow at the natural gas apertures in the burners. When some openings get clogged, more gas than intended has to flow through the other openings. 

You can usually diagnose the issue by visually inspecting the burners during use. If any of the flames are bright yellow, you are watching incomplete combustion. This will reduce the amount of heat produced by the burners; if some burners are more clogged than others, it can also cause less even heating. In addition, incomplete combustion will produce soot, which itself can clog burners or otherwise gunk up your beautiful system.

To address the issue, pull the burners off and replace them or carefully clean out the pores in the burners. This is a labor-intensive process.

Air supply is not just a cool band name

As convection ovens, roasters are highly sensitive to the flow of air through their roasting chambers. In general, the more airflow you have through a drum, the brighter and cleaner a coffee will taste (although there are certainly those who will also say high airflow makes for baked flavors). Conventional wisdom states that reducing fan speed or throttling airflow will make the coffee ‘rounder’ and more full-bodied.

Whatever airflow strategy a coffee company has decided best meets their flavor goals, they will want to get that airflow every time they turn the roaster on. But there are a lot of ways in which that airflow can degrade if not tended to. 

So, if you get reports of the coffee from a particular roaster tasting “muted” or “lacking clarity”, airflow is a great first thing to check. The biggest culprit here is a build-up of soot and other hydrocarbons in the exhaust tubing. Take the exhaust apart (they’re all disassemblable) and take a gander into the tubing. If there is any thickness of build-up in there, you have to scrape it out. 

Consider this: If you have a 4” exhaust pipe and you get just 1/4” of build-up all the way around, you have lost 25% of your airflow, maybe even more. And it is not at all hard to get 1/4” of build-up — that can quickly happen in a week’s roasting. 

No one wants a bomb in their roastery

As giant natural gas furnaces, roasters can present a fire and burn hazard. One danger in particular has been the focus of a lot of design effort: a fuel-air bomb going off in your roaster.

A ‘fuel-air bomb’ refers to a type of explosive in which fuel is allowed to spread and mingle with atmospheric oxygen and then ignited. While simple-sounding, they are among the most powerful conventional explosives around. Also, it is an excellent description of what would happen if you let lots of natural gas collect in your roaster and then hit it with a spark: a massive boom. 

To avoid this, roaster manufacturers deploy very robust measures to ensure that no unignited natural gas can accumulate. These technologies will keep the flow of gas shut off, usually with a solenoid, until they “know” the gas will be burnt off as it is dispensed.

These systems take two primary forms: temperature sensors and optical sensors. Temperature sensors can be as simple as a thermistor over a pilot light, much like you might find on a water heater in your home. If your roaster won’t light, it’s an excellent bet to inspect that temp sensor and ensure it is in good working order and, crucially, positioned correctly. Optical sensors ‘see’ the flame, so in addition to placement and inspection, cleaning the lens can help. 

Important note: these safety interlocks are also typically present on afterburners, so if your afterburner doesn’t light, these are still the things to check. 

Tuning the motors

All roasters will have motors driving the drum agitator, fan, and cooling tray. Classic models may have a single motor and chain, or even leather belts, to drive these. Modern roasters may have three separate motors, each controlled by a variable frequency drive. 

In all cases, a malfunction of these motors will seriously impact coffee quality (or the ability to roast at all). If your drum motor is not working correctly, your coffee will not be accurately mixed, and you won’t even be able to get it out of the drum. Faulty cooling tray motors will result in unevenly cooled coffee. And finally, a malfunctioning fan motor will impact airflow, as noted above. 

The best thing to do is get a sense of what these motors are like when performing correctly. What is the RPM of your drum? What does a healthy motor sound like? This will give you a better sense of when things are going bad. Inspect your chains and belts, replace and lubricate as necessary. 

Getting your bearings 

Finally, closely related to belts and motors are bearings. Keeping up with bearing maintenance and understanding the basics of bearing adjustments can save you a lot of time and headache. 

There is no upper limit to how many bearings your roaster might have, but they pretty much all have at least two: the rear and front main bearings for the drum (or drum agitator). As the name implies, these are at the front and back of the roaster and are used to allow the drum or agitator to rotate while supporting its weight. 

An essential step is to keep them lubricated. Almost all bearings will have a grease fitting or ‘zerk’ where you can attach the business end of a grease gun. Make sure you learn what grease type is recommended, as greases are not compatible with each other — mix the wrong two, and they’ll turn into a liquid mess that oozes out and leaves your bearings dry.

On older designs, the drum itself can be adjusted forward and backward. This is done to allow a technician to precisely tune the distance between the drum and the faceplate. Too close, and they run against each other when the metal gets hot and expands. Too distant, and beans can fall out of the space between the two. This adjustment is usually made with a custom spanner turning an adjustment nut right behind the front bearing.

Final thoughts

Again, these tips are not meant to be a comprehensive repair guide; they are more accurately a beginner’s guide to the simplest, highest-impact repairs you can make. Most roasters are not complicated machines, but their expense and critical nature to their owners make them intimidating. Our role as techs is to take the intimidating and make it clear, logical, and predictable so that our teams can do their work. These steps can start you down that path.