title_body: What is going wrong with this Espresso shot?
upvoted_answer: I don't think anyone can really give a definitive answer to this but I'd like to chime in to try to help. It's almost impossible to find your answer because of how many different variables are at play here. You should first focus on minimising the variance in your methods.
  
  
  You describe the shop shot as being a 'longer pull' and thereby expecting more bitter results. The problem with that assumption is that you can only truly compare shot times if you have extracted the same output weight in both cases. Since you have not, you have not only extracted much less in your shop shot, but, like you noted, it also took longer. I normally only ever measure my shots by weight because that's the number that tells me how much water got through and thereby how much real contact and subsequent extraction occurred.
  I use time as a secondary reference to diagnose other problems in my shot. If I suddenly start extracting a same weight shot in half the time that I normally do, I know something is definitely off. Since your shop shot extracted much less output weight, the sourness is clearly due to some level of underextraction from being stopped too early (relative to your home shot). In any case, I always try to aim for 30g in 30s, which means you probably need to grind slightly finer in the shop to be able to extract more and try to hit the same balance as your home shot.
  You mention your water sources being different. This is huge! You will never find the same shot if you are using filtered at home and unfiltered at the shop depending on how hard the water is when unfiltered.
  
  
  Compare shot times as a secondary to consistent output weight and see how grinding finer in the shop would fare.
  
  
  Higher boiler pressure does not necessarily point directly towards higher temperatures. Also, higher pressures could impact the puck suddenly when the valve is opened which could crack and cause channeling and therefore underextraction and sourness. Your main issues here are still in point 1.
  Weight difference is purely by choice. Crema production is mostly dependent on freshness of beans and less to do with specifics in extraction method. If preset weight quantities are being used as part of a single button-press on a machine, then your problem starts there. You need to weigh every shot and choose when to stop the shot yourself. Without this control, you're simply relinquishing responsibility of the shot and you are at the mercy of the machine, yet another variable to consider in being able to find the right shot.
  
  
  In conclusion, you MUST minimise variables. Make sure your grind sizes are the same at both locations, ensure your input and output weight are identical, analyse differences in taste and extraction time at same output weight, ensure your sources of water are identical, make sure your tamp pressures are the same. At this point the only real differences will be the machines and then you can start investigating how each machine is affecting the shots. Try to use bottomless portafilters in order to diagnose whether your distribution of your coffee bed was poor and whether channeling occurs during extraction.
  
  Combine all of the parameters above with the taste of your output shots and you'll know the next steps you need to take in finding the problem in your shot.