Nothing to add to the plate discussion, but as for pressure relief valves, all we need is a swing-type check valve. About $10-15 at any hardware store. Most are brass, but I've seen them in copper, too. Unlike the spring type check valves, these won't have a rubber o-ring seal; just machined brass or copper surfaces. They seal nicely(they're what we use with my well) and open under minimal pressure.; We could adjust the pressure required to open them by grinding away some weight to make them open sooner, or soldering-on some weight to make them open at higher pressures, but I suspect they're be right fine as-is. You can hold one to your mouth and see how much it takes to get them to open, and it's not much. Run a copper line from the outlet back toward the floor, and place the end of that line in a pail of water so that if it does release pressure, the vapors will condense immediately in the pail's water and not create an explosive cloud in your shed. Rescues your vapors for reuse, also, and would also show if the valve leaks(it would bubble slowly). It won't be able to siphon the water back to the column or boiler, since the valve is a one-way only device.
Gonna take my own advice and install one on my upcoming 3" column build; thanks for making me think of it, guys!
Like this:
Spooky, nice work! Regarding your leaks, there are usually three reasons for soldered joints that leak; not clean enough, not enough flux, or too much heat. A plumber friend who taught me to sweat joints told me to pull the torch away as soon as the solder went into the joint, and said most amateurs leave the torch on the joint too long, which makes the solder run right through it, leaving too little behind. I've never had a leak, using his method. I also clean my joints and the nearby surfaces as clean as can be, and use way more flux than seems necessary, which he also recommended I do. And don't use a dirty old flux brush; use a new brush for each new job, or as soon as you think the brush is dirty. He actually used his little finger more often than a brush. Anywho, hope this helps!