slipchuck wrote:From what I read the thermocoils heat the water "on the fly" so it should in theory stay the same temperature no matter what the temperature of the source is
The temperature of the thermoblock/thermocoil itself stays the same. But not necessarily the water which has passed through it. Guessing why? Thermocoils have a huge problem which, probably, is the reason why they are not used widely even though their idea is looking super-cool (cheap in production, have very short warm-up time and close to zero recovery time, and you don't keep same water hot in the boiler for weeks). They look ideal, but they are installed literally in a few models among hundreds on market. Have you already guessed what the issue is? OK, here is the answer: brew temperature is flowrate dependent. A slow flowing shot will be hotter, a faster flowing shot will be cooler. Copypase from the Internet:
small flowrate differences will have a rather dramatic effect in temperature. For instance, a shot that flows 10% faster will have a brew temperature that is about 7°c lower. Same goes for faster flowing shots
When you have dialed in, you will have drinks of a same temperature since the water will move through the thermocoil with the same speed, so the heat will being applied during the same time. But, while you are dialing in, and before you have found the correct grind size, water will enter the group having different temperatures. Don't forget, that temperature is one of the factors which affect extraction (and, hence, the grind size)! So, water flow speed affects temperature in the group > temperature in the group affects grind size > grind size affects water flow speed - it's like a snake which bites its own tail (or recursive call, as we, programmers, say). You get unpredictability on top of all the variables espresso hobbyists struggle with. But even after successful dialing in, you don't have full control on the value of temperature. Let's say, you programmed the PID for 93 C. But with which flowrate was it tested by the engineers? An average one? And what about different brew ratios? When it's 1:3, the water is flowing fast and, maybe, doesn't have enough time to get properly hot, while in 1:1.5 brew ratio water is flowing slowly - will it be hot like hell while you see the same number on the display?! In the regular classical boiler, the water will come to the group head with the same temperature regardless the flowrate.
I didn't know all that when I was buying the Ascaso (I found that info later) and bought it because it had volumetric dosing. Which... didn't work! Weight of each shot was too different, looking more random than pre-programmed. Soon, I paid attention, that the dosing was programmed by time (even though it's officially called "volumetric"). Unfortunately, that kind of programming doesn't work for me since my habit is to use the time as the indicator of grind size correctness while the brew ratio (i.e. the shot weight) is kept constant. For example, for the same brew ratio, the difference in the taste of a 20 sec and a 35 sec shots will definitely be felt, but it will not be super-drastic (any of these shots will be drinkable even if not ideal), so I have enough room for a mistake in grind size. But if the time pre-programmed, and I get 30 g once, and then 40 gr (absolutely different brew ratios!), the difference in taste will be huge. So, I returned the Ascaso uno w/PID to the store and bought another machine (third, after two attempts - Lelit Victoria and the Ascaso).