There are a couple of threads here about issues with the Semi's boiler fill. My problem is similar to the ones described but a little different.

Here are the symptoms:

- The boiler manual fill works properly.

- Shots pull normally.

- For about 10 minutes after a shot is pulled, the water level in the sightglass very gradually increases about 1/4 inch.

- When I do a cooling flush, the HX seems to be empty, and it takes a little while for water to begin flowing. The water is not overheated as would be expected.

It seems like there are two possibilities:

1. A slow leak in the boiler fill valve that allows water to backflow from the HX into the boiler via the valve; or

2. A (dreaded) pinhole leak in the HX.

Before I start taking things apart or buying parts, is there a way to test which of these two possibilities is more likely?

I'm thinking it's the valve. If there were a leak in the HX, I think the pressure/temperature in the boiler and the HX would equalize over time, and a cooling flush would produce overheated water. Does this sound right?

Any advice would be appreciated.

It's probably nothing but new machine anxiety.

The water level in the sight glass varies; it's not a cause for worry. I'm not sure why, but I guess bubbles develop from the dissolved oxygen in the water and affect the level. Either a leak in the HX or a leak in the valve would lead to the boiler filling while you pull shots, and there would be no flow through the puck, since the 1 bar resistance of the boiler is much less than the 9 bar resistance of the puck.

A completely steam filled HX is quite common if you haven't pulled a shot for a long time. Just flush till you get water, wait 30 to 45 seconds, and pull your shot (the machine runs at a steady temperature at a shot a minute, so the HX recovers in this time frame). If it happens soon after pulling a previous shot (like 1/2 hour), I'm not sure what the explanation is.

I should have mentioned that it's not a new machine. I've been using it daily for three years, and it's definitely not the normal behavior. The boiler level consistently increases 1/4 inch after each shot. Not during the shot, but slowly within a period of maybe 15 minutes after the shot. Cooling flushes after a long rest produce very little steam, as if the HX were empty and being filled with unheated water. A cooling flush after a short rest of a minute or two is more or less normal.

It sounds like there could be a pinhole leak in the HX. But I don't understand how the water would get from the HX into the the boiler after the pump shuts down; the pressure would equalize after a very small amount of flow, since water is incompressible.