Brewing Heating Options Part 2 – Direct Fire Sytems


We are going to look at direct fire for brewing today, as the second in our series of brewery heating solutions. The first article was on electric heating and can be read here.

Direct fire as the name applies uses an open flame to heat up your brew kettle. There are few different options open to brewers and we’ll look at them all in this article.

The technology has evolved over the years and there exists good option for brewers in most situations. Historically direct fire brewing used coal as the main fuel source however, in modern brewing gas or oil is used to run the burner.

What Does Direct Fire Brewing Look Like?

The burners are generally housed in a cast iron combustion chamber underneath the kettle distributing heat across the kettle bottom.

It can be one main flame or more commonly several flames allowing for better heat distribution across the kettle. A newer design I’ve recently seen “pushes heat through a coil” inside the kettle for more efficient heat transfer, as the coil is immersed in the liquid.

Options using direct fire for mash tuns exist but they’re rare, as the mash can easily stick or scorch due to localized hot spots. The other main issue with most direct fire systems is their inefficiency.

The open flame heats up the air around the kettle as well as the wort. You can lose up to 60% of the “heating” depending on the system in place. In modern craft brewing 10 US Bbl. (1,170 litres) is the usual top end for a direct fire brewery and 15 US Bbl. (1,760 litres) for indirect fire, although bigger systems do exist.

The Difference Between Indirect Fire and Direct Fire for Brewing

Direct fire brewing means your heating source is directly under the brew kettle. The heating source can be a firebox or an open flame gas burner running on either gas or oil.

If you look at the picture above you can see the gas burner is actually outside the kettle. The heat generated by the burner goes into the firebox underneath the kettle heating up the wort.

As we said before, this type of direct fire system is good to about 10 US Bbl. (1,170 liters) with most suppliers and experienced brewers recommending steam for a larger brewhouses.

Indirect Fire for Brewing

The system is similar to direct fire but the burner “blows” the heat into a jacket rather than a firebox. They work similar to steam jackets in brewing, the advantages being more even heat distribution and greater efficiency over simple direct fire.

Internal Coil Direct Fire Solution

The last and maybe most interesting option when it comes to using a direct fire for brewing is an internal helical coil. A burner is still used, but the heat is pushed through an internal coil inside the kettle which is immersed in the wort.

With the coil immersed in the wort there’s greater contact time and surface area to heat exchange with the liquid, leading to more efficient heating. It’s likely the most expensive option out of the three but it’s a tradeoff due to increased efficiency.

Direct Fire for Brewing Safety Issues

Anytime you’re working with an open flame there are going to be some safety considerations. The are some standard safety measures you can put in place to help keep the systems safe, such as the following:

  • When turning the brew kettle heating on, it triggers a solenoid valve allowing the electric ignition of the pilot to light.
  • If the systems detects an issue the whole system automatically shuts down.
  • If there’s no issue detected and the pilot light is lit, it triggers another solenoid allowing for full flow of gas to the main sensor.
  • There’s also an air blower feeding the burner to help it burn too.

Direct Fire for Brewing and Mash Tuns

I’ve not seen too many direct fire mash tuns incorporated into brewing systems. Although I recently saw one being proposed for a smaller system in South America. So, they do exist.

They’re usually a (forced air) burner under the mash tun to heat it.

Some of the advantages of using a direct fire mash tun are:
  • They heat up the mash quickly
  • The mash doesn’t need to pumped elsewhere (unlike an electric system)
  • It’s possible to do a decoction boiling or gelatinization rest directly in the mash tun, with the liquid then pumped to the lauter tun.

The Disadvantages of a Direct Fire Mash Tun

The heat from a direct fire system can be strong, hard to control leading to localized hot spots and/or scorching of the mash plus denaturing of enzymes.

If you scorch the grains in the mash it can lead to undesirable off-flavors in the final beer. Additionally, it’s going to hard to clean as some of the mash will be “caked” on to the mash tun. Lastly the general wear and tear on the metal from the open flame is also worth noting.

Mash Tuns and Indirect Fire for Brewing

The advantages are the same as direct fire, but with the additional benefit of more even heat distribution with the use of jackets. The heating up of the mash would likely be even quicker to boot.

The downside to the jackets is they’re hotter than steam so denaturing could be an issue. Also, the whole jacket needs to covered. So, decoction might not be possible unless you’ve more than one jackets in the mash tun and they can be controlled independently.

The Advantages of Direct Fire for Brewing

Costings and Space Requirements

Steam is the most expensive heating solution to install when brewing. Direct fire is cheaper to install and calibrate. Plus, direct fire takes up less space as there is no need for a boiler. Direct fire is also cheaper to run than electricity in most places.

Easy to Use

As we said it’s cheaper than steam to set up, but it’s also easier to install too. Direct fire doesn’t require a boiler, boiler room, piping and traps unlike when using steam. It means you’ve less ongoing operational headaches or parts which can go wrong.

Option Fuel Source

If you are smart with your direct/indirect fire set up, you can have the option to switch between propane and natural gas to run the burner. If supply may be issue, having the option to switch is an advantage.

Disadvantages of Direct Fire for Brewing

Local Regulation and/or Building Codes

As you’re working with an open flame with direct fire, there’ll be local regulations you need to understand and follow. Operating a forced air direct fire system in your brewhouse might trigger certain requirements to pass building code and local regs. It could be as simple as installing a fire suppression system but needs to be addressed.

Heat Dissipation

The inefficiency of direct fire systems often means the exhaust off the flue can get very hot. Having proper venting or insulation to deal with the heat is key to avoid any health and safety issues.

Maillard Reaction / Caramelization

The chances of increased Maillard compounds being produced with direct fire are real. Caramelization is the reaction taking place when sugars are exposed to temperatures in excess of 176°C (350°F).

In wort boiling temperatures doesn’t really go over 100°C (212°F) however, Maillard compounds are produced, ranging in flavors from caramel to toast.  The increased chance of localized hotspots and scorching with direct fire systems means more Maillard compounds are likely formed.

In some beer styles like a Scottish Wee Heavy these compounds are desir, in others like lager not so much. It’s something you need to be aware of as a brewer.

Also, one last issue to note is even after flameout your firebox will have some residual heat transferring to your wort. It’s key when installing a direct fire system, your burner supplier comes to test and dial in the system.

The Clean Up

As we mentioned above direct fire systems can be messy. The heat of a firebox on the bottom of a kettle can lead to hard to remove deposits inside the kettle. The fire over time will “char” the outside of your tank, which will look unsightly and be hard to clean up after.

Direct Fire for Brewing Conclusions

There’s a sweet spot for brewers where the choice of direct fire for brewing represents the right option. The initial start-up is easier plus less expensive than steam, takes up less space and easier to maintain too. Furthermore, as a heating solution its cheaper than electric in most cases.

Taking into consideration your local regulations and building codes, for a smaller brewing operation direct fire maybe the idea solution. Always, remember there’s an increased likelihood of Maillard compounds being produced with direct fire which can affect lighter beers adversely.


As we said localized hotspots can make clean-up harder than with other heating solutions. Plus, there’s increased wear and tear on the tanks too. I’d also suggest paying extra to have the system set-up correctly, because I’ve seen brewing vessels warped or burnt due to poor installation.

If the option to have a “turbo” system, exists the extra costs are worthwhile. The use of air blower will help the burn. A naturally aspirated system (without a forced air blower) takes much longer to heat up wort and is less efficient.

Overall, the best option takes into account many factors, if you’re unsure then a good equipment manufacture can help guide you, or hiring a consultant to help with the commissioning could be a sound investment.



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