While I do not know the specific reasons for that recommendation, it is well known that reactions between metals and chemical elements occur more rapidly at higher temperatures. Thus if some interaction with the water might render the T&P device inoperative, a second one on the cold line might remain functional longer and protection from excessive pressure (caused by excessive temperatures) would be available. Comply with the recommendation.

The reason is that calcium and magnesium (the elements that cause water to be hard) are less soluble in hot water than they are in cold. This is the exactly the opposite from most things - most things get more soluble (you can dissolve more in water) as temperature increases. When you have hard water, calcium and magnesium compounds drop out of solution and form scale when they are heated, which is what happens in your water heater, so a pressure relief valve on the hot side is likely to see build-up that could cause it to fail.

It should also be mentioned that the valve on the cold water line IS NOT a T&P valve, it is a "pressure relief valve.

These type of Pressure Releif valves are also used on water heaters when the water heater is installed in a basement for vault where they cannot establish a gravity flow drain for the TPRV.

Electric water heaters have built in high temperature limit switches but when a gas or electric water heater is installed below grade where there is no drain the TPRV is removed and we install a "Watts 210" T&P type gas shutoff valve and the pressure relief valve is then installed at a higher location in the structure where the drain line from the relief valve can be discharged outdoors or into an approved indirect waste receptor.

Thanks for the info. Since i am on well water the plumber stated i did not need an expansion tank which i did had in another home with city water. Does the pressure tank act as an expansion tank and/or should i still install one??

Your plumber is correct, the well pump tank serves as an expansion tank.

thanks Lazy. What would the pressure relief valve look like??

The Watts "pressure relief" valve looks amost the same as a Temperature & Pressure Relief Valve except the PRV has no thermal sensing probe extending out the inlet fitting.

To install the PRV you install a Tee with a threaded side port or a Tee & Female thread adapter on the cold water line with the side port on the top of the pipe, then apply 3 layers of yellow teflon tape or thread dope on the theads of the PRV and screw it into the TEE.

You then have to install a full bore drain line from the side outlet of the PRV to the outdoors or to an approved indirect waste receptor.

see photo below

It didn't post the photo