I want to share this with you all. Recently, on a vacation home, I was installing flow checks on the return piping of a system because of convection. At the same time, I was to install a boiler feed tank, because there was no make-up connection. The system was installed about the year 2000. For the past two years, there was excessive air in the system, which I thought was due to there being no fluid feed. I was partially right, however read on.
Every zone can be isolated and purged without having to shut down the entire system. The system had a Weil-Mclain Gold Plus 40, Indirect Fired Water Heater. I drained each zone individually, using air pressure to blow out any excess fluid, to make my job a little easier. As I got to the Water Heater zone and applied air pressure, I could hear gurgling in the tank. Knowing it was a tank within a tank, the sound just didn't seem right to me. So I did some further testing. Able to totally isolate the DWH in every aspect, I closed all ball valves, on the domestic and boiler side. I attached a pressure gauge to the boiler return side of the loop and pressured the the zone with 25 psi of air. The pressure gauge rose to 25 psi, but as suspected, dropped at a steady rate. I then moved the pressure gauge over to the domestic cold water supply, and administered the same test, through the boiler supply pipe. Just as I thought, the pressure in the domestic water supply side of the DWH started to rise. And the last test I administered was basically the same, I just change locations as to where the pressure was being added and measured. I moved the pressure gauge to the boiler return piping and supplied air pressure to the cold water fill on the domestic side, the result was the same thing. I was confident that the Indirect Fired Water Heater had a bad heat exchanger.
Keep in mind that this is a vacation home, the house is heated @ 55 when vacant and to 68 when occupied. The water heater and domestic water is turned off when the house is vacant. Knowing this and all the above, the puzzling thing is this: Why didn't the domestic water pressure (70 psi) override the boiler pressure (15 psi) when the home was occupied? My testing with air clearly proved there was a leak, somewhere between the two tanks. However, the pressure in the hydronic system never rose above 20 psi, that I know of. Being air is lighter than water, I can only assume that the heavier domestic water, was in fact, leaking into the boiler side of the water heater, but at a very slow rate, not enough to raise the pressure. I also think that is was possible that when they were using DHW, it may have siphoned fluid from the heating system, thus causing fluid loss to the heating system, and with no make-up water, creating air pockets.
I did cut part the outer tank and pull the inner tank out. The inner tank is corrugated, however, 1/3 of the the tank was swelled at the bottom, making it very difficult to pull apart. I didn't see any any cracks or pinholes on the inner tank, but I couldn't see the very top part where the nipples are welded to the tank. However, on the inside of the outer tank, I could see rust in one particular spot, that ran from the top of the tank to the bottom.