No Expansion Tank for Water Heater

Hello,

I bought this house recently and I have Gas Water Heater and Weil McLein Water Boiler for heating. There is an expansion tank installed on the Hot Water side (where heated water comes out of boiler) of the boiler. And there is no expansion tank installed around the Water Heater. Is this configuration OK? I get this doubt becuase I have read in many posts that Expansion Tank must be installed on the cold water side of the Water Heater.

In my boiler I have Pressure/Temperature gauges in two places.
1. One where the heater water comes out of boiler
2. and another one is installed on the return water pipe taht enters boiler.

1st gauge reads 30 psi and 180 farenheit. 2nd gauge reads 25 psi and 170 farenheit. Why is my return water so hot? Shouldn't it be cold? Is this a problem?

Thanks for your help.

- Vasan

The addition of a boiler in your water system may make a difference, so hopefully HJ who is highly knowledgeable in boilers will respond to this, too. In a basic water system you need an expansion tank if the system is closed. This means there is a check valve located in the cold water supply. This can be a backflow preventer or a PRV. In a closed system, the exansion tank is placed in the cold water supply line between the device that checks the backflow and the intake to the hot water tank.

You might need two expansion tanks. A boiler system nearly always needs one in the closed loop heating portion (it is separated from the normal house supply, so can't perform the same function for your hot water heater). If you have a prv or a check valve in the primary house water supply, you may also need one to prevent the hot water tank from seeing excessive pressure. The same concept is at work here. Most boiler hot water circuits are closed - it needs a place to expand (boiler's relief valves are often set lower than a hot water tanks), as does your hot water heater.

The boiler expansion tank is isolated from the water system by the boiler's fill valve. It has no function other than to balance the heating system's pressure. The water heater needs its own expansion tank somewhere in the system, preferably between the tank and its shutoff valve, but it can really be anywhere in the cold water system as long as the heater's supply valve is not turned off.