I've got a heat exchange Gaggia TE that has recently developed high pressure in the main line when the machine is "at rest." When the pump comes on to pull a shot, the pressure goes down to around 9.5, which is where it usually is. But then it goes pretty quickly up to 12-13, wheres the normal resting pressure is around 2-3. Seems like some kind of stuck valve or solenoid, but I'm not sure where to start looking. Any help much appreciated. I made a pot of French press today, and no one was happy.

It's normal for the brew pressure gauge to show higher pressure shortly after the shot. That is, pulling a shot introduces cooler water into a closed system, but then you suddenly close off the main exit at the end of the extraction. As the water in the heat exchanger warms up, it expands and the pressure rises. An expansion valve vents water to the driptray around 12 bar to avoid subjecting the hydraulic system to excessive pressure.

Dear Dan,

Thanks. I didn't know about the expansion valve. But here's the problem. The pressure goes to 12-13 bars on start up and after pulling a shot it stays there. Indefinitely. But I take it the expansion valve is the place to start looking for trouble? Or might a good descale help out?

Best, Chris

Sorry if I'm being slow here... as I understand your observations, the brew pressure is correct and the expansion valve is clearly doing its job. If so, why does the idle pressure concern you?

Dear Dan,

My apologies for not being clear. The idle pressure had been 2-3 bars. Now it's always at 12-13. At 12-13 I get a bit of a leak from somewhere in the main line, but not at 2-3, or 9.5. It's an 8 year old machine that hadn't been cared for by its previous owner. I'm just a bit worried at the change from idling at 2-3 to idling at 12-13 constantly. Thanks again.

Best, Chris

You can always adjust the expansion valve to open at a lower pressure like 10 bar, but I'd fix what's leaking rather than working around it.

Without knowing more about your machine's hydraulic system, I can only speculate at the cause. For example, you can relieve the system brew pressure on E61s by raising the lever enough to open the evacuation valve, but not enough to start the pump (i.e., the pressure will drop to zero, then raise to ~4 bar as the water warms up again). Yours probably has a three-way solenoid and the symptoms you observe could (?) be due to scale/dirt buildup slowing the valve's movement, resulting in less water being evacuated at the end of the shot and subsequent increased pressure.