We have seen a number of installers call in with the pressure relief valve on their storage tank venting. They assume it is either a defective pressure release valve or that the temperature in the top of the tank is a lot hotter than the temperature in the bottom. They might see a temperature of 135 degrees on the bottom sensor and still have the release valve releasing. Is the temperature at the top of the tank 80 degrees hotter than the bottom? No.
What these installers are most likely experiencing is the pressure relief valve releasing due to too much pressure. In many areas of the country it is written into the code that a backflow prevention device must be installed in the line. By not having any place for the heated (and therefore expanded) water to go, the system pressure may spike and cause a release through the P&T valve. But, there is a way to prevent this.
An expansion tank should be used on the potable side of any system where there is backflow prevention (or a check valve) installed in the house line. You can assume that any home built within the last 5 years would have one of these. By installing a properly sized expansion tank in the line you will eliminate the call back for the pressure relief valve blowing.
The exact size of the expansion tank required depends on incoming water temperature, tank high limit temperature, and incoming water pressure. As a rule of thumb; you should use an ST-12 Therm-x-trol (4.4 gallon) on the potable side for all of your 80 gallon solar storage applications. We have these in stock.
If you would like to size the expansion tank yourself use the following link;
http://www.amtrol.com/pdf/MC4090%2006_07%20TXT%20Brochure.pdf Standard city water pressure runs around 40 p.s.i…
Note: Only use expansion tanks with polypropylene liners for potable water applications.