If the valve is leaking then I think it should be replaced.

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

The valve you are talking about is a pressure relief valve (PRV). It is a safety device to prevent WH explosions. Either the valve is bad and needs to be replaced with EXACTLY the same spec valve despite what a vest wearing know it all at a box store says... "this one fits everything". Or you have a closed plumbing system and you have a thermal expansion problem that needs to be corrected NOW. If you have a closed plumbing system, and that has been code since the 90's, there should be a small thermal expansion tank installed proximate to the WH. These tanks have a bladder that may have failed and the tank must be replaced. OR... you have a closed plumbing system and there is no thermal expansion device installed, which is not uncommon, and there needs to be one installed. In this case the PRV is acting as a thermal expansion device and it is NOT intended to. Here's some light reading on thermal expansion... http://www.watts.com/pages/learnabout/thermalExpansion.asp If this seems beyond your skill level call a plumber... and do it SOON.

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.