Boilers - Steam and Hot Water Systems - system design/TRV questions

Now that I figured out what boiler to get I need to decide what kind of distribution arrangement I will do. we will have 3 floors to heat with cast iron rads in each room and radiant underfloor in the kitchen/eat in area. I am thinking of direct piping with TRVs to control the heat. Its my understanding that the circulator is always running and the valves open when heat is called for. But what tells the boiler to fire. does it work off the temp of the return water? will this setup cost more to run then using zones with thermostats?

First, the circulator pump, does not necessarily run all of the time. The boiler fires when a tstat calls for heat, then once boiler reaches water temp limit, the boiler shuts down, but the pump continues to run to achieve desired temp. As long as the system is calling for heat, if the boiler water gets too cool, dependent upon the low limit setting in the boiler, the boiler will fire again to maintain water temp. The zone valves, pretty much operate as you said. When a tstat calls for heat, it will open the corresponding valve. But the valves are also wired to the boiler, so when a valve is triggered by the tstat, it will also trigger the boiler to fire, or run the pump, depending upon the variables of water temp, etc. The boiler fires depending upon the water temp flowing through the boiler. As for cost, I could not tell you. NJ Trooper would have more knowledge upon this area.

The system circulator will just run. Turn it on when heating season starts. The MCBA control will tell the boiler when to fire based on outdoor temperature or other starting point.

Good brief summary (if you don't already have Siggy's Modern Hydronic Heating) is here:

http://hydronicpros.com/PDF/mhh2_sample.pdf

Home-run type system will be very efficient. Use an ECM smart circ like Wilo ECO or Grundfos Alpha. Low electrical consumption and will automagically adjust as the various valves open/close (no need for differential pressure bypass). But you will need to make sure that the boiler always sees at least the minimum flow rate. Some fiddling with the initial setup.

Thanks xiphias. Must have passed by that part in siggy's book. I like the idea of the home run system. seems a little easier to design as well. do you prefer trv's over thermostats in that setup?

I think it mostly depends on how you want to control the system. It's your comfort and lifestyle. If you're into the TRV approach, do that. If you're into the thermostat approach, do that.

With a nice modcon, home-run piping, and all that cast iron (and radiant-to-be), you already have a very efficient system.

The thing I'd pay most attention to is the heat loss and cast iron. See what kind of water temps you will need, then figure out if you can get the needed output from the radiant at the same water temperature without violating the max surface floor temp rule (about 85F). Single-temp system would really be ideal.

The system circulator will just run. Turn it on when heating season starts. The MCBA control will tell the boiler when to fire based on outdoor temperature or other starting point.

Good brief summary (if you don't already have Siggy's Modern Hydronic Heating) is here:

http://hydronicpros.com/PDF/mhh2_sample.pdf

Home-run type system will be very efficient. Use an ECM smart circ like Wilo ECO or Grundfos Alpha. Low electrical consumption and will automagically adjust as the various valves open/close (no need for differential pressure bypass). But you will need to make sure that the boiler always sees at least the minimum flow rate. Some fiddling with the initial setup.

Not sure what boiler we are talking about, but use the alpha or wilo (great use for these BTW) in a loop to the house and inject the boiler into that loop to hydraulically isolate the boiler flow, then you can use a globe valve in the boiler piping to set you flow rate / delta T