edfromtampa, as far as I have learned, the tank is installed in an irrigation system to avoid the very two problems which you describe. Damage can occur to either the pump or the pressure relief valve if the system is allowed to operate as is.

hi..your not going to like me..... I read your post and started chuckleing to myself back in the early to mid eighties..the tandom water heater hook up was the "deal" ..and then in the early 90's or late 80's..we started switching back the heaters..your not getting more than 45 --55 gallions of hot water.. do you know how the piping is piped in your house we used to go in the attic..cut the pipes loose and blow thru them [open allvalves] figure what pipe goes where split the house up..between the 2 heaters..master bath ,laundry on one heater and a bath and k-sink on the other

Dave, Sounds like you may be confused between a PRV (pressure reducing valve or pressure regulating valve) installed at the water service entrance and the T&P (temp & pressure) relief valve installed on the WH. A PRV is installed at the service entrance to lower the street pressure to an acceptable PSI for the houses plumbing. Depending on water conditions they go bad and need to be replaced from time to time. They are usually set to around 60 psi. A T&P relief valve on the WH is a safety device and is installed so the water heater doesn't explode if pressure problems arise. When these T&P valves leak they MUST be replaced and with EXACTLY the same specification part or maybe... BOOM. The expansion tank should be check annually when you drain your water heater. You do drain your WH annually don't you?. To do that you put an air gauge on the air valve. If the bladder has failed then you'll get water out of the air valve. If you don't get water then you must drain all the water from the tank, which usually requires removal, and the air bladder needs to be pressurized to what the water pressure is when the tank is empty.

That's true, but what can we do about it unless we change the heating industry terms? The automatic coal stoker boilers often creep above boiling, but since they're under pressure, there's no actual boiling. Commercial boilers have a second aquastat with a maximum high limit of 240 degrees and manual reset, L4006E, so you want to avoid those near-boiling temps.