A recent thread brought back a question that's been nagging me for some time. Why do HX machines require long flushes? That's one of the most common complaints about HX machines. You commonly hear of people flushing as much as 8oz/250ml. This doesn't make sense to me.

Assumptions:
1) Most home HX machines have a heat exchanger volume of about 100ml (3.5oz). There are exceptions, (e.g., Cimbali), but I believe 100ml is fairly common.
2) The primary purpose of the HX flush in an idling machine is to eliminate overheated water in the heat exchanger.

If these assumptions are correct, then why would a large flush be of any benefit? If the HX volume is 3oz, and you flush 3oz and replace it with fresh water, what does a larger flush accomplish? At some point you'll start to lower the boiler temp, but that's certainly not the purpose of the flush.

Perhaps an E61 machine with an overactive thermosyphon needs a larger flush to lower grouphead temp. But I thought the grouphead idled cooler than thermosyphon temp, not hotter, due to radiative heat loss.

FWIW, I do not find large flushes to be of benefit on my equipment. On my Vetrano, with the boiler pressure set to 1.3 bar (raised slightly to compensate for 4000' elevation - tip of the hat to Eric Svendson for pointing this out), a 3oz flush appears to be perfectly adequate.

I thought I understood heat exchangers pretty well, but apparently I'm still missing something. Maybe my guesstimate on HX volume is off. Any thoughts?

RapidCoffee wrote:Perhaps an E61 machine with an overactive thermosyphon needs a larger flush to lower grouphead temp. But I thought the grouphead idled cooler than thermosyphon temp, not hotter, due to radiative heat loss.

John, what does your grouphead idle at? I thought you had a thermometer adapter now? If 3 ounce flushes are working for you, I would suspect that your grouphead idles very close to desired temp.

RapidCoffee wrote:
I thought I understood heat exchangers pretty well, but apparently I'm still missing something. Maybe my guesstimate on HX volume is off. Any thoughts?

My HX volume is 250 ml each, or nearly eight and a half ounces. I pretty much fill my cup and then lock in, pull the shot after drying the cup, and end up at 196 - 200. If I want to go cooler, I flush a little longer and I can get a cup ready to go right after the flush, and I can get it somewhere in the range of 191 - 196, hotter is easier, just take three seconds and I'm in the 200 - 208 range. Yeah, I'm guessing that you're comparing oranges to apples a bit.

RapidCoffee wrote:Assumptions:
1) Most home HX machines have a heat exchanger volume of about 100ml (3.5oz). There are exceptions, (e.g., Cimbali), but I believe 100ml is fairly common.
2) The primary purpose of the HX flush in an idling machine is to eliminate overheated water in the heat exchanger.

If these assumptions are correct, then why would a large flush be of any benefit? If the HX volume is 3oz, and you flush 3oz and replace it with fresh water, what does a larger flush accomplish? At some point you'll start to lower the boiler temp, but that's certainly not the purpose of the flush.

Perhaps an E61 machine with an overactive thermosyphon needs a larger flush to lower grouphead temp. But I thought the grouphead idled cooler than thermosyphon temp, not hotter, due to radiative heat loss.

I have an Anita with Eric's thermometer adapter installed, and I definitely flush more than 3oz. Depending on how long the machine has been on, the GH temp at idle is usually around 212, but will sometimes creep as high as 216 if it's been sitting for a few hours.

I think you're right in assuming that the HX is only about 100ml of overheated water, but that's at the same temp as the rest of the boiler, 250-ish right? So I flush that out which is also adding heat to the brewpath and GH. Then I keep flushing to cool off the overheated GH so I don't end up with a shot around 212. Works for me, anyway. My normal technique is using flush and rebound, so obviously I'm giving the water in the HX some time to heat back up. I guess if I didn't do that the hotter GH temp wouldn't heat the water too much.

EDITED: So it seems you're right about the GH idling cooler than the thermosyphon, but the TS loop is close to the temp in the boiler, right?

Weird, it's never done that before...

I've found that on my Anita I like a 5-ounce flush give or take a half-ounce (boiler @ 1.3 bar for steaming performance). I also like to flush lock and pull. The espresso is darn hot with good chocolate, caramel and raisin notes (this weeks home-roasted Liquid Amber from Sweet Maria's).

I wonder what the volume differences are between flush lock and pull, and the rebound methods are? Sorry if that has been discussed before.

My Levetta needs more than 8oz of cooling flush. The group temp is around 216 before 30 minutes of warm up (according to the Eric's digital adapter) and the boiler pressure set to .95.

3 oz of cooling flush, would be great.

Ale

I am grateful that it is 11:45 PM EST because this is one of my favorite espresso subjects. Certainly there are several ways in which an hx espresso machine can be operated and even slight variations within those methods. On one end of the scale, it is entirely possible to pull a very good shot with absolutely no flushing whatsoever. This feat was first brought up by Ian Eales (CafeIke) using his PID'ed Vibiemme with his temperature controller set such that boiler pressure was around 0.70 bar. I duplicated this with Anita and provided a thermofilter shot with my pstat turned down to 0.70 bar (max reading). Steaming performance was merely "OK". Machine recovery after a shot is, well, not great. Machine recovery with the Vibiemme would be better than Anita.



The above graph has been previously posted but it serves to vividly demonstrate what different settings can do with respect to recovery time.

Now, here's the other end of the scale. Personally, I am a proponent of large flushes (~ 8 oz) to cool the grouphead down and then pull the shot as the grouphead is climbing back up to temp. I find that this provides for greater consistency in the espresso shot and I am able to easily adjust the temperatures if I happen to be using a bean with a known affinity for something either higher or lower than my norm (~200 F). I also start grinding and weighing and spooning in my WDT "clumpless " grind at about the same time that I start flushing. It takes me an inordinate amount of time to "prepare the puck" - especially at 7:00 AM but the timing always seems to work out pretty well.

This above chart is also available in the adaptor tidbits document on my "poor man's" website:

http://users.rcn.com/erics/

jesawdy wrote:John, what does your grouphead idle at? I thought you had a thermometer adapter now? If 3 ounce flushes are working for you, I would suspect that your grouphead idles very close to desired temp.

Tonight my grouphead is idling at 209.5F. Boiling point of water is ~205F at my elevation (depends on barometric pressure). HX flush spikes at ~214F and typically settles down (quits flashing) at ~206F on the thermometer adaptor.

Psyd wrote:My HX volume is 250 ml each, or nearly eight and a half ounces.

This is your 2-group Astoria, right? I would expect a 250ml heat exchanger to require a 250ml flush on an idling machine.

jgriff wrote:EDITED: So it seems you're right about the GH idling cooler than the thermosyphon, but the TS loop is close to the temp in the boiler, right?

That's my understanding. When idling, the grouphead is continually radiating heat and cooling the thermosyphon. Thermosyphon temp should be hottest as it exits the boiler and coolest as it reenters the boiler.

erics wrote:Certainly there are several ways in which an hx espresso machine can be operated and even slight variations within those methods. On one end of the scale, it is entirely possible to pull a very good shot with absolutely no flushing whatsoever.
...
Now, here's the other end of the scale. Personally, I am a proponent of large flushes (~ 8 oz) to cool the grouphead down and then pull the shot as the grouphead is climbing back up to temp.

Hi Eric, and thanks for chiming in so late at night. I agree, there are several different ways to operate an HX machine effectively. But that doesn't answer my question: why are long cooling flushes seemingly required on some machines? (Expobar owners, in particular, complain that their machines run hot and require long cooling flushes.)

Let's ignore temp stability issues for the moment, and concentrate on a flush & go scenario. After flushing overheated water from an idling HX, shouldn't you be able to pull a shot immediately, without burning the grounds? On machines that seem to require a longer flush, does the initial flush overheat the grouphead, and the extended flush cool it down?

Still searching for enlightenment...

you answered your own question...

RapidCoffee wrote:Perhaps an E61 machine with an overactive thermosyphon

RapidCoffee wrote:(Expobar owners, in particular, complain that their machines run hot and require long cooling flushes.)

i probably am unique because i have actually owned both the pulser and the vetrano. in fact i have had both machines running together at the same time. (vibe vs rotary shootout anyone? ill pull the pulser from the basement and set it up before i sell it.)

erics TC has been in both group heads using the same probe and the same reader.

i have used the pulser with and without the thermosyphon restrictor. with it installed and with the pstats running 1.1-1.3 bar. i can flush ~3 ounces from idle and get within brewing temp. without the restrictor it would idle much higher than without the restrictor. i had it working really well was a "flush and go" type of system. i flushed very little water to get to the brewing temp of ~200° .

then i got the vetrano and everything changed. its seems to work well for the "flush and wait". i still only flush very little water. about 3 ounces even from idle. i am at about 4500 feet above sea level in SLC, UT. the vetrano runs between .9- 1.2 bar it idles at about 210.

i have noticed that with either machine that more than a few ounces will be too much especially if you flush until it stalls. i still dont get the stall. i try to avoid it.

it is late here so i don't remember the details of everything but i have posted on the subject on HB and EOG.

thanks for the time.
jon