The explanation is at your fingertips. The same question has been asked far too many times before.

Search this forum or Google the net for thermal expansion, PRV (pressure reducing valve), and T&P valve.

Did the plumber check the water pressure? If not, find a different plumber.

If you have a closed plumbing system the PRV may have failed. The T&P valve may have failed or it may be doing its job and saving you an explosion. If you have an expansion tank it may have failed.

Here's a little light reading for you... click here

If your pressure relief valve is actually leaking then you must make sure you get the correct replacement valve as they are set to open at particular pressures. Use teflon tape on the threads and do not use a wrench or spanner on the body of the valve, there should be a hexagon provided.

Here is a link that might be useful: How to install a french drain

Justalurker is right...

You need to rule out that this was caused by a pressure event rather than just replacing the pressure relief valve. The way to do that is to install a pressure gauge that records the maximum pressure and leave it on for a day or longer.

The one shown in the link includes a hose bib connection -- which would make it easy to connect. I can't vouch for this particular one, but it gives you the idea...

Here is a link that might be useful: pressure guage - max pressure & hose connector

I use this gauge

They sell it at HD for about $11

Thanks for all the advice. I will definitely pick up the gauge. Where should I install it? The drain valve on the water heater doesn't release any water, so I'm not sure it will do any good to put it there.

Just on any available garden hose spout - make sure you measure the pressure with no taps running - or you can hook up to the washing machine connection. Just turn on the tap with the gauge connected and it'll tell you what the pressure is there and then. It's best you get the type with the second needle that records the highest pressure, then leave it that way overnight. Your pressure should be no higher than 70psi, really.

If the drain valve on the bottom of the hw heater doesn't run, it must be majorly clogged. You should drain/flush your hw heater periodically - it's been explained on this forum before - but if it's that clogged, you may need to replace the tank.

That's not the correct place to put a gauge anyway, you need to connect it to an outside garden hose bib or the washing machine cold hookup.

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

wat?
that works just fine you can run in series or in manifold but if they are in series you have to set the first htr to a lower temp like 90 degrees and second htr too like 140 degrees you have plenty hot wtr
good luck