I have 10 gpm flow (measured at a peak time...8am on a hot day). Any recommendations on using 1/2" vs 3/4" (option #1 or #3 above)? I like 1/2" because my hose bib and sprinkler heads are 1/2" and it's simple. Yet I like 3/4" since it gives room for expansion (though now unforeseen) and small extra cost. I just wonder if there are any potential disadvantages to using 3/4" in this application?

Harvey - a complete repipe will be required to go to hot water heat for a couple of reasons - steam and hot water operate under different pressures, possible leaks from the old pipes/radiators, etc. Hot water systems can be either one pipe or two pipe. No matter what you decide to do, you'll need to hire a very experienced HVAC designer/contractor especially with your needs to separately bill your tenants. There are lots of ways to go with your system - multiple boilers, multiple zones, and on and on. Make sure whoever bids the work does a Manual J heatloss for your property. Hang around the site listed below to get up to speed on what Qs to ask the contractors and what options you have. Good Luck Here is a link that might be useful: Steam - Hot water radiant heat

Hot water chemistry is a bit different in that some things happen faster. Chlorine demand is increased so bromine, being a little more stable and still sanitizes some when combined, is frequently used. Continually changing the water will make it nearly impossible to maintain it properly. The pH, alk and sanitizer levels will change. The tub will slime up. Scott

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?