I had alot of challenges getting my DHW working right. I installed/plumbed everything. During the 1st winter when the t-stat called for heat and you were in the shower, you got cold/luke warm water on you. It sucked! Then somebody on these boards gave me a great idea. I plumbed the hot water exit pipe from the DHW in the 520 into the NG 50gallon hot water heater (into the cold side). THEN I installed a T onto the cold water pipe going into the cold water of the 520. I soldered a hose bib on the one side of the T and connected a hose from there into the water drainage of the NG hot water heater. Make sense? So what happens here is that by gravity, I am banking all the unused hot water from the boiler into the NG hot water heater. The hose drainage is acting as an overflow that pushes the cold water back into the boiler.
So now when I'm in the shower and the t-stat calls for heat, my hot water supply is coming from the NG hot water heater and I get a nice even hot water shower!
I can tell you that in Oct/Nov and December I would have the shower valve almost on cold to get a nice shower because the water was soo hot. The in Jan and Feb the valve was almost on full hot to get the same temperture. This is because the boiler was always sending the hot water into the radiators and there was hardly any window to let the gravity method replenish the hot water in the NG tank.
I'm very happy with the setup. In the summer, I turn off the coal and use NG hot water. I don't have to touch anything.