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Most prosumer espresso machine’s with a heat exchanger boiler and E61 brew group require a significant cooling flush before an extraction to get to the proper brew temperature of approximately 201 to 203 degrees. But how long should a cooling flush be? And how long of a pause between the cooling flush and an extraction?
Note, all machines are a little different. For example, Expobar machines usually only need a very short flush of a couple seconds. But for many others, this technique will work.
To do it, draw water through the group with an empty single spout portafilter loosely engaged. As it pours listen closely for the hissing and watch for sputtering and unevenness in water flow. This indicates brew water that is flash boiling.
When the water calms down to a uniform stream it’s at about 206 to 207 degrees. From there, each second of additional flushing drops brew water by about 1 degree. So to get to a brew temp of 201 continue flushing for 5 to 6 seconds after observing a uniform water flow from the spout.
Following the cooling flush, allow 30 to 45 seconds for the new water in the heat exchanger to recover to brew temperature. Use this time to grind and tamp your coffee and then pull your espresso. The duration of the cooling flush will vary depending on how long your machine has been idle. So if you’ve pulled a shot within 10 minutes the flush will be much shorter.
There are minor variations from machine to machine. So, the proof is in the taste. If shots are consistently bitter it’s likely your brew water is to hot. And if they’re consistently sour it’s likely to cold.