Hello Dr. Brew! My name's Austin and I run a coffee shop/brewery in Silverton, CO. One of the customer service reps gave me your email in hopes you could help me with some questions about my old (but new to me) Bunn Dual.
I must preference we're above 9000 ft and we had to tinker quite a bit with our single head fetco to brew good coffee at this elevation. The lesson we learned from our old brewer is that we needed longer brew times and more coffee to compensate for the fact that water boils at 187 at our altitude and we're unable to reach a proper extraction temperature. We've been using 4oz coffee for .5gal, 7oz for 1 gallon, and so far haven't been brewing 1.5gal batches.
Our Bunn machine so far doesn't seem to be giving us the full flavor coffee we're used to. The Bunn Dual in question is an older model, so as far as I know, we are only able to change the thermostat temperature and the 6 knobs for time of brewing .5gal/1gal/1.5gal for each head. We don't have the set/lock switch to adjust the 2 second duration of the brew. It seems that the longer we set the brew, the more water we have in the final pot, i.e. the .5gal setting at 3 mins brews .5gal, the .5gal setting at 5 mins brews .75 gal. The very kind rep is sending me some of the 6 hole nozzles with smaller diameter holes to help limit the flow of water. I've also shut off the water bypass, since that didn't seem like it would help.
Any help at all would be appreciated. Are we on the right track? Do you have any other questions for me? Can you think of anything else we could be doing to achieve a fuller flavor coffee at our elevation?
Thanks in advance!
Brewing at high altitude is a challenge. The temperature of 186°F to 188°F would be the top range for 9,000 to 10,000 feet. From your description the Dual you have is an older model without any capability of setting a pulse sequence to lengthen the brew water delivery time. The spray heads with fewer and smaller holes will do that, although the pattern and uniformity of saturation may suffer. Appears the weight of coffee has been increased to compensate for loss of temperature. You may also want to grind as fine as you can to expose additional surface area (but not so fine you create an overflow issue).