The “Boiler Room” forum on Hearth.com had some folks willing to try to answer my concerns about the system pump overcoming the vent pipe and pushing boiler water into the overflow tank. Unfortunately they had some concerns of their own, mostly about my competence. That was a bit wounding but I tried to just get the info I needed and move on. This is the forum discussion on the topic.
Someone mentioned that with an open system the boiler pressure cannot be higher than the opposite pressure from the elevated tank and it’s “head” or water would flow into the overflow tank.
But for the open tank there were several cautions and requirements especially about the corrosive influence of the water in the overflow tank bringing oxygen into the system that seemed to make it a poorer choice than a closed system. The relationship between the vent and the feed pipes remained murky–wasn’t obvious if both are needed, and I can’t really find a full explanation anywhere. Since I have a pressurized fill system instead of the gravity feed of the open tank too, it seemed obvious that I had to move away from the open tank.
Once I understood that the open tank overflow system was specified in the installation instructions to keep the pressure in the boiler under the maximum system operating pressure of 2 bar, then it made sense to forgo the open tank and its issues of design and operation and just use a low pressure relief valve.
Atmospheric pressure at about 30 inches of mercury (the standard measure) equals 14.7 psi or about 1 bar. So operating the system with a 15 psi pressure relief valve will ensure that the system stays well under the maximum operating pressure. These conversions are made easy because of the online measurement converters for air pressure. I used this one–from a free source code site–enter any measure and all others are converted to match!
I ordered a 15 psi pressure relief valve and a 4.7 gallon pressurized boiler expansion tank last night and will install them instead.
The question still remained whether the pump would have provided too much pressure in the system, tripping the 15 psi relief valve, but that is not going to be the case. (Coincidentally, the head from the open tank would have had the same effect on the water in the boiler as the 15 psi pressure relief valve–it would not have moved up the pipe due to the opposite pressure of the height of the water in the vent pipe (head) and atmospheric pressure.)
The explanation from a forum expert was, “Although the pump is intended to build pressure, it is also pulling suction at the same time. Since your system will always maintain a positive head pressure, the entire system will remain flooded at all times. This means that instead of the pump “building pressure” it will just serve to move water.”
The Taco 006 is a high head lower flow pump because there is so much water pumping into the boiler and pulling from the storage tank and pipes. It would have to overcome the total head in the boiler system to build enough pressure to blow the pressure relief valve.
This is the new piping diagram.