Set the first water heater slightly above your required output temperature and the second water heater at the required output temp. Example; Set the first water heater at 135 and the second one at 125. In this configuration the water entering the second water heater is already above the demand temperature therefore the second water heater doesn't fire but just acts as a storage vessel. This will give you the full 40gal in the second water heater + the first hour rating of the first water heater which would typically be about 60gal/hr and in the event that the demand is still greater than what the first water heater can produce, as the water entering the second water heater drops below the demand temp, the second water heater will fire thus you have both units firing at the same time and they will produce a nearly limitless supply. Not quite limitless however. Under an actual test at my cousins house in mid winter with incoming cold water at 42degF we set 3 showers to run at 105degF and timed the water-not quite limitless, we only got 74 minutes continuous run before the shower temps began to fall off so I went back to my drawing board and came up with a slight modification that grossly improves on that.

I had a conversation with a local pro, and he believes adding a mixing valve and turning the aquastat up from 100* to something like 140* will do wonders. He said it's normal in any water heater to have much higher temps at the top and lower at the bottom, with the average around the aquastat setting, meaning we have ours turned way down. I'll give that a try. On the DHW priority wiring, I'm having a little difficulty finding straightforward info. Most diagrams/discussions include so many references to controllers and various models of components that I can't tell what the basic schematic should be. Here is my current setup:We have a single circ pump controlled by the boiler control. I just learned that the "DHW End Switch" terminal in my boiler goes to the HydroStat controller and only bypasses the thermal targeting in order to get full power from the boiler during DHW demand. My plan now is to simply add a DPDT relay (in red below) with the coil powered by the aquastat, that disconnects the signal to the two heating zone valves from their thermostats. This will will close those valves while the DHW is heating and create the priority setup. Anyone see any problems with this setup?

"Klem1, what's your point?" If I were to condense it to a single point it would be "common sense will always serve one better than asking strangers on the net". Thee single thing that helps prevent being ripped off by bottom feeders posing as service providers is spend time talking over the fence and on Social media telling neighbors about blood suckers as well as reliable pros instead of chatting with strangers 2k miles away. Although the majority of USA citizens believe they need not make an effort because Big Brother is looking out for them,that's not true. If you want it,go and get it because the yahoos that convinced majority of voters they will handle it are not going to come through when you need them. Have you noticed all the surveys from people you favor by spending money with them? Don't waste your time,give them publicity by word of mouth and other means. One of those "other means" that is underutilized is local news media. Make the effort to get the goods on scammers,outright inept jazz-axes that claim to know what they are doing. If you get 3 or more individuals willing to stand up,you can get a local story. Once news breaks,the contractor will straighten up or get out of business. Often when they go out of business,they start another business and ride it long as possible. Stay on top and twist their tail if they go back to old tricks. Getting back to the problem at hand. Common sense would have a homeowner check and record incoming water temp,check and record temp of supply,run water til burner come's on,turn facuet off until burner go's off, attach a garden hose to a hot faucet,stick hose in 55 gallon barrel with a secound barrelstanding by ,record time (alternatly you fill your tub if you calculate tub capacity (1 cu ft= 7.5 gallons). A helper can man one post or you can run back and forth to see how long til burner come's on and test and record temp on about 5 minute intervals. If temp hold's up after burner come's on (it should) keep testing temp at facuet at 1 minute intervals until it get;s less than would be comfortable for showering. Turn facuet off, record time and record time that burner go's off. Subtract starting cold temp from starting hot temp. Time to burner off and temp differential of hot and cold alow's calculating recovery which is the true test of heater function. A builder grade 50 gallon should heat 40 gallons of 30f incoming to 120f per hour (re-heat cold 50 gal tank 90f above incoming) Those #s will tell a pro where to begin. Nancy that fall's under common sense. I didn't see anyone mention doing or suggesting anything like that. If non of that ring's a bell,perhaps the fact suffing the net for 3 years hasn't solved Daniel's problem so common sense should tell us there must be a better approach. (:

Nothing wrong with flex pipe on a water heater. 99% of the problems come from people not knowing how to install it properly. If you look at a new water heaters supply and outlet nipples. You will notice a red and blue plastic insert. Those inserts are mainly for sealing the connections. Now when you use a nipple to connect two lines you do not have the plastic inserts to protect the rubber gasket in the hose connecter flange. Thus making them very easily to damage and causing leaks. Once the rubber gasket is damaged you can never get it to stop leaking no matter how tight you tighten it. Been using them for years and have had very little problems.