AH radiator cap numbers:
13 lb caps,
Duralast 7013, NAPA 703-1697, Murray 7013
I replaced my cap in 2020 with the Duralast. My AH has not been keeping any pressure (no leak though) so I just replaced it with a new Murray to see if that will solve the no pressure condition
New cap in place on the aquahot. AH up to full temp, still no pressure at all when removing cap to check.
Tank is lipping full to the top. New 1/4" hose to the overflow tank with new clamps. No loss of boiler fluid anywhere, tank and overflow tank maintaining same level.
So i used a vacuum service tool to fill aqua hot and all the lines. Got a lot of air out even though the tank was full
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The new Aqua Hot heaters have zero pressure tanks, On older heaters, 7 or 10 pound caps will work if 13 pound is not available.
The important thing is sealing the opening so hot expanding coolant goes to the overflow bottle and fluid returns to the coolant tank as it cools down each cycle.
I bought a new cap recently that did not seal the opening which was flat, not bent. I purchased a thermostat gasket, cut it to fit the top of the cap, put it in and no leaks.