Hi everyone,

Hoping I could get clarification on a few expansion tank questions I had in addition to some feedback on moving the location of the expansion tank. Little background on my project. Its a closed loop radiant heat system running off of a A.O. Smith Vertex (100BTU).

My plan calls for a "potable water" expansion tank near the cold out side of the manifold. The first diagram is from the phoenix manual. It is very similar to my heating diagram. Here the manual also states " A thermal expansion tank suitable for potable water must be sized and installed within this piping system between the backflow preventer and the cold  water inlet".



Q: If the system is "closed" why is there a need for a potable water expansion tank? An atmospheric backflow preventer for example should stop any sort of backflow. The rest of the system to the right of the heat exchanger has no contact with the domestic water supply.

My manifold (rifeng) has air eliminator's built into the manifold, however I decided to get an Spirotherm Air Eliminator that has an expansion tank connection at the bottom. This is where my next expansion tank question comes into play. Is a "potable water" expansion tank still what I should be installing here? From my research, expansion tanks made for potable water purposes are not made to take constant hot water because of there construction.

I have attached a second drawing which shows the expansion tank below the air eliminator. The supply make up water coming off of the cold return line. A watts 9D atmospheric back flow preventer and then the fill valve aka pressure reducing valve (taco).



Thanks in advance!

For all practical purposes, hydronic fluid is incompressible, will significantly expand when heated, and may generate high pressure that may destroy an unprotected hydronic radiant floor heating system. Typically, both an expansion tank and a backup pressure relief valve are used to protect a hydronic radiant floor heating system from this potentially destructive hydronic fluid expansion.

Diaphragm-type expansion tanks should always be mounted vertically with their inlet connection at the top and should always be installed in the system very close to the inlet side of the circulator pump(s). Failing to do this may result in air being trapped on the fluid-side of the diaphragm (which may result in premature tank corrosion/failure), may reduce the system's ability to expel air (or even result in air being drawn into the system via the air separators), and may encourage destructive circulator pump(s) cavitation and premature failure.

Borst Expansion Tank Design Software

This calculator enables you to properly design/size and setup a diaphragm-type expansion tank for use in a hydronic radiant floor heating system. A properly designed/sized diaphragm-type expansion tank allows the system pressure to reach within about 5 PSI lower than the pressure relief valve opening pressure setting when the system reaches its maximum operating temperature.

The pressure on the air-side of the diaphragm must be adjusted to the required design pre-pressurization value BEFORE the system is filled with hydronic fluid. An under-pressurized tank will perform like an under-sized tank and may cause the relief valve to open each time the system is heated. A properly designed/sized and pre-pressurized diaphragm-type expansion tank will not contain any hydronic fluid until the system first begins to get heated and the fluid begins expanding.

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Q: If the system is "closed" why is there a need for a potable water expansion tank? An atmospheric backflow preventer for example should stop any sort of backflow.

Unless something goes wrong and the backflow valve fails. Or maybe potable tanks use chlorine resistant bladders.