Water Heaters - Hot Water Tank "leaking"

second floor with and additional bath. All the homes were built between 1999 and the last ones were completed in 2002. All the homes have 40 gallon Bradford White gas hot water heaters installed with no thermal expansion tanks and no pressure regulators. The city water pressure is 80 PSI and has been fairly constant since 2006, prior to 2006 there were some fluctuations, and the city remedied that by closing a loop in our area. Several people have noticed the water heater “leaking,” until last July, mine was working properly. I noticed a small amount of water under my heater, and checked the TP valve, when the heater was running, and it was releasing water. I replaced the valve and it started to release the other day. I checked the “street pressure” in and outside, to verify the pressure is still 80 PSI (it is). I checked the pressure on the discharge of the dank and waited for the heater to start, the pressure went up to 170 PSI, as soon as the heater went off the pressure went down to 80 PSI,. I assume this is the problem I.E thermal expansion. I checked my neighbor’s system and the pressure went form 80 to 100 PSI when the heater was on. His system is identical to mine with no expansion tank or water softener. Can you explain why one system works different from the other? We have over 100 units in our development and some people are nervous and are replacing their water heaters before they have any problem, because some basements are finished and others are “Snow Birds” and do not want the units to fail when they are not available to see any damage. I’m sure some shut the units off when they leave for the winter.

Thanks,

with the pressure being 80 PSI plus, you need to install a PRV so you can get the pressure down to around 50 PSI where it should be. The T&P valve on the WH is just going to be the first of many problems if you don't.