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Gaggia Classic preheater box

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All credits go to this person:





Since I was pulling apart my Classic today, I thought I should finally take some photos of the preheater mod I did a number of months ago.


I spent a while looking at possible electrical enclosures that might match. Specifically aluminium with IP68 rating. Of course this rating is for water getting *in* and not for holding pressure, but it was a place to start. Most importantly, the Bud Industries enclosures have a single continuous gasket with no join. Almost every other electrical enclosure I looked at had a gasket strip that was inserted and then cut to size. Not good for holding 9BAR of pressure!


I initially made the mistake of ordering a Bud Industries AN-1302 electrical enclosure from Mouser.com as it was the biggest that would fit inside the machine and had nice slots to place dividing inserts. Unfortunately it couldn't hold the pressure, bowed out the lid, and blew the gasket immediately.

Back to the drawing board I coughed up for another import - This time the AN-1301-A, which is equivalent to what the blogger used initially. It is smaller and the screws are closer together, so it holds the lid down and prevents it from bowing out with the pressure. I jammed a piece of aluminium sheet diagonally across the inside to create a partial divider for water flow.


The enclosure has a pretty small volume and I thought that having most of the preheated water vanish down the OPV was not useful, so I inserted another OPV right after the pump:


To complete the fit, the OPV needed an extra connector plus a 1/8 to 1/8 adapter, which I actually already had lying around.


@Norvin kindly donated a Gaggia coffee deluxe boiler inlet many (many!) months ago, so I completely removed my normal OPV. However, you can just adjust the normal OPV so that it doesn't open and all water flows through it.


Since the Gaggia Classic uses 6mm Teflon tube from the pump, connecting it all together is fairly simple.

I used these to get Teflon tube from the pump/OPV to the enclosure:


This involved tapping a 1/8BSP thread into each side of the enclosure. The push fit elbows provided a nice, solid, leak proof seal. The elbows swivel and allow the tube to rotate so moving the tubes around hasn't caused leaks.


The box mounts onto the flat of the boiler nicely. Pick your method of mounting it, but I used large cable ties scavenged from the street!

You could also file/sand and level the boiler surface and then use thermal paste, but I really just wanted to warm the incoming water and try to avoid having it boil when steaming. Letting the Gaggia warm up for 15 minutes is plenty of time to passively heat this box.


I have been using this daily for nearly 4 months without issues. Shots are more stable, with the PID showing no temperature drop when pulling a standard double basket shot. I would normally get a 2-3 degree drop registering before this mod. It is a lot simpler than ridiculously coiling copper pipe around the boiler, plus it is more effective since the preheated water is not wasted down the OPV.



Sorry, the internal machine pic is a bit confusing - I have a PID, integrated shot timer, plus a half completed volumetric counter designed to pause the shot timer when the preset volume is reached... Wires going everywhere!







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  • 4 weeks later...
I actually didn't do anything with the PID settings! I probably should run another auto tune but the machine has been heating without issue.

I've never been sure about when to actually tune it; do you do it when cold, or do it when fully warmed up?


I think (from reading things from others on here) you're supposed to let it get to Sv temp. for about 2 hours (so it's as stable as possible) and then start the auto-tune process. I doubt it's making much of a difference though - the preheater will probably do nothing but reduce the cooling mid-shot. Will definitely be doing this in the near future!


One question: how long did the enclosure take to arrive after you ordered it?

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  • 5 weeks later...
  • 2 weeks later...

As a quick update, I'm still just waiting for a 1/4-1/8 connector to turn up before I can crack on (as I'm careless and ordered a 1/4 OPV rather than 1/8...), hopefully tomorrow.


I'm going to use this as an opportunity to do a mini-overhaul on the machine, and add some insulation to the case, as well as replace some of the screws in the group head with brass.


Edit: Well, I'm not going to pretend it hasn't been a bit of a ball-ache; have gut the traditional cuts all over my hands and have been troubleshooting leaking issues. I think I've finally isolated it down to needing an o-ring for one of the connectors, so hopefully that'll be the final piece.

Edited by Quetzalcoatsy
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  • 2 years later...
  • 1 year later...

Sorry to resurrect an old thread, but @phario PM’d me about this mod to see how it was going and it’s been years since I’ve logged in!

It took me a good number of years to upgrade from the Gaggia as I found the shots to be consistent, repeatable and too my liking. The deciding factor was simply the single boiler faff. I still have the Gaggia in a cupboard though!
One thing I’ll note is that the outlet of the preheater box should really be on the top of the box. Having space at the top like mine allows air to accumulate and reduce it’s efficacy. Even worse, in my fiddling I actually have it upside down at the moment so it’s not working well with the outlet at the bottom and lots of air inside!

As it has aged, there is a little bit of calcification creep appearing along the boxes seal edge. Pretty typical of any aluminium water box as far as I’m concerned, and no worse than any Gaggia boiler.

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