Reversing Airflow on a HotTop Roaster

The HotTop drum roaster is by far the most sophisticated and functional home roasting machine, with features that match, or exceed what can be found on some of the finest commercial roasters. But it does have limitations. It’s designed for the home, so typically you are expected to roast one batch of beans at a time, and then allow the machine to cool down before roasting another batch. One reason for the cooling-down period is because the way a HotTop is designed, as hot air is exhausted, it is passed from the drum to the back of the machine, heating the air around the drive motor and speed reduction gears. This causes the nylon speed reduction gears to become too hot, leading to premature failure.

A way around this is to reverse the airflow in the HotTop, so that cool air flows from the motor section to the drum, which is then exhausted by drawing the hot air through a duct by means of a fan that conducts the air to a chaff collector and smoke filter. There are various designs that others have implemented, and my own design was inspired by what others have done before me. My design is somewhat unique in that I chose to use a free-standing chaff collector that sits next to the roaster, rather than attaching it to the unit. This has several advantages, with the one disadvantage is that the entire assembly cannot be moved without disconnecting the parts.

The process of reversing the airflow is fairly simple. You need to add an exhaust pipe that attaches to the roast chamber, and a fan to pull the air from the roasting chamber through the exhaust pipe. Because the heat generated by the roasting chamber can reach 500°F, the components that attach to the roaster need to withstand high temperatures, and since smoke is generated with dark roasts, some air filtration is needed to reduce smoke.I chose to use 1.5″ stainless steel waste pipes that are found in the plumbing section of your local hardware store for my exhaust pipe. I removed the top vent from the roaster and made a 1.5″ hole to insert the pipe, then cut 4 slots and bent the metal between them to form tabs to secure the pipe inside the vent.

To move the fan, I used 8″ steel pipe that is typically used to exhaust the smoke from wood stoves. A 12″ lengh of pipe forms the body, and 2 caps form the top and bottom. Two holes were cut in the top, one for the entry port for the exhaust pipe, and one for the air flow generated by the roaster’s 4″ exhaust fan.

Initially, I was concerned about the power of the fan, and whether or not it would draw enough air to remove smoke from the roasting chamber. On my first test, it did not. This was primarily because I had allowed some small leaks in the exhaust air passage and it wasn’t creating enough of a vacuum. I reinforced my connections by adding rubber seals, tightening the joint between my two sections of waste pipe, and better securing the aluminum tape on the roast chamber vent. My second pass was much more successful, so I took the extra step of adding a filter to reduce smoke.

There are different options for the smoke filter. It needed to go anywhere in the exhaust path, but the logical place was inside the exhaust canister. After considering several options, I decided to make a filter that could be attached directly to the fan base, as this would be easiest to change out. It is likely that I will be able to find a suitable filter that will fit the size of the fan, but I only had a 4″ x 9″ filter at hand, so this was cut down to fit the fan and secured with rubber o-rings. If the o-rings fail in the future, I will replace these with steel springs.

Now that I have produced several roasts with the reverse air-flow system attached to my HotTop, I am finding that using the fan now has a much more dramatic impact on the environmental temperature inside the drum. Running at just 10% fan speed can cause the drum temperature to drop more rapidly than before. In order to better control the air flow, I may be adding a butterfly damper to the exhaust pipe. This will allow even finer control of air than the HotTop currently allows. The only downside is that I will not be able to record the damper position automatically with Artisan, so my hope is to find a single restriction amount that will work for all of my roasts. Alternatively, I will recalibrate my roasting technique to compensate for the greater airflow. These things take time to sort out.

5 comments

  1. Update on this modification… I didn’t need the air damper after all. Once I got used to the roaster running these mods, I found that by charging the drum at 420°F and dropping the heat to 80% at end of dry, I could run the fan at a minimum of 20% through the remainder of the roast to eliminate smoke, and control the roast with a small reduction in heat before going into first crack and ramping up air through the rest of the roast to maintain even environment heat at around 410°F.

    After running a few roasts, I opened the canister to see if there was any chaff and I found that the plastic of my modified Bissel air filter had severely warped. I removed it and instead am now using 6 layers of the static filter only, and that is working beautifully! The only smoke I see is coming from the drum when I drop the beans at the end of the roast. This has turned out to be a highly successful mod.

    1. Chip Chrisman

      Hello, question about your HT mod–did you pull the rear fan to use in your chaff canister? Can you be more specific about that please. Also, have you been able to build more heat earlier in the roast (before FC) where the ET curve could actually begin to decline before FC and have enough heat going thru FC…? Thanks,

      1. Tim Piazza

        Chip, yes. I used the rear fan and moved it to the chaff collector. I made an extension cable that plugs into the main board where the fan would normally plug in, and then plugged the fan into the other end. Just an FYI, there isn’t a tremendous amount of directed airflow through the Hottop the way a conventional gas drum roaster has. This is partly due to the perforated drum and partly due to the surrounding chassis not being a sealed system. There are plenty of gaps for undirected air to pass through and as a result, must of the chaff ends up in the collection tray in the bottom of the roaster rather than in the external canister. I suppose with a more powerful fan, that could change.I have considered stacking multiple fans to increase airflow, but at this point, I seldom use the Hottop as it has been repurposed as as sample roaster for small batches.

        In regards to your second question, you have to consider the entire system. The hottop doesn’t have the thermal mass of a traditional drum roaster, so you can’t directly apply the same principles. Instead of thermal mass, you have the electric heating element. The element is positioned to be close to the perforated drum but as far as possible from the bean mass. My method of roasting is to drop at a high temp and keep the heat on full, then slowly drop the heat and ramp up the airflow to control the heat progression. This is very different from how I roast on my 2 kilo, where I soak the bean mass with thermal energy built up in the drum until the approach of the turning point, then hit the heat hard to get a fast rate of rise that I slow before FC by reducing gas, then slow the drop in rate of rise by increasing airflow until the approach of FC end, at which point I am working to control the exothermic momentum of the bean mass to keep the rate of rise from climbing. I don’t have this issue with the Hottop. On the small roaster, it’s more a matter of maintaining the right amount of heat and less a matter of fighting the heat energy of the bean mass.

        I really hope that answers your question. If it doesn’t please reply and I’ll do my best to answer.

  2. What kind of pipe did you use for the 12 in body, ABS? Are the static filters just plain vanilla kind?

  3. Tim Piazza

    The pipe is a section of steel vent pipe, used for venting a wood stove. I don’t think ABS would stand up to the heat without warping.

    The static filters are plain, but I have since switched to charcoal filters since I found a good source for them and I think they work a little better.

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