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Recently my boiler is leaking periodically through the pressure relief valve. I replaced with a new pressure relief valve. Still leaking so I hired a plumber he said my expansion tank is filled with water and it’s most likely the issue. I paid and had the expansion tank changed. Few hours later, it leaks again and now the condensate pipe has a small leak too. Every time it leaks, I noticed the pressure reading is very high up to 40 psi. I am completely lost now. What’s going on with my boiler? My plumber stated if the issue is not fixed, next will be changing the backflow preventer which will cost me about 500. I just want to know if this makes sense. I tried to research on the Internet finding relevant information, but couldn’t find the connection between backflow preventer and high pressure water leak.

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The line between the expansion tank and boiler should not be blocked by a normal backflow preventer (BFP). But some BFP are made for that location since they do have a tiny hole which is allowing a tiny back flow during heating up.

If that hole is blocked, the pressure can not be compensated by the expansion tank but must be released by the pressure relief valve.

And of course, the expansion tank needs the correct pressure setting on the gas resp. air side.

Here is some info for installers/engineers, allow some seconds for translation. https://www-bosy--online-de.translate.goog/ausdehnungsgefaesse.htm?_x_tr_sch=http&_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en-US&_x_tr_pto=wapp

BFP can be very small inlets, f.e. of the size of a coin that fits into pump/tube connections, i.e. it is not always visible from outside, if/where a BFP is installed:

https://www-bosy--online-de.translate.goog/Schwerkraftbremse.htm?_x_tr_sch=http&_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en-US&_x_tr_pto=wapp

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You may have a defective boiler feed water pressure regulator. (Looks like this.) This device takes water from your plumbing system and feeds it into the boiler until the pressure in the boiler reaches a certain level, typically 12 PSI. Then it stops adding water unless the pressure again drops below 12 PSI for any reason, such as a leak or the intentionally draining of some water. When the boiler heats up, expansion takes place and the water pressure rises a bit above 12 PSI. This excess pressure is absorbed by the expansion tank so the final pressure of the hot boiler may go up to 15-20 PSI typically, which is below the usual 30 PSI setpoint of the pressure relief valve.

Some regulators include a backflow preventer and some use a separate backflow preventer that is installed between the regulator and the boiler. The backflow preventer prevents boiler water from backing up into your household plumbing and potentially poisoning you.

If the regulator is faulty, it may be allowing water to continue to flow slowly into your boiler, raising the boiler pressure until it reaches 30-40 psi. At that point, the pressure relief valve would start dripping.

Your plumber may be planning to replace the feed water regulator and the backflow preventer at the same time, which should solve the problem. Or you could tackle this yourself if you're able.

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A problem I had (twice) with a boiler, was a pin-hole leak in the coil for the domestic HW supply. It let curb pressure cold water into the boiler which exceeded the relief valve. Eventually had to buy a better quality coil for the water type. A diagnostic would be be shut off that water feed and see if the problem abates.

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