I'm designing a DIY radiant heat install for my home and have a question concerning boiler loops.

If I have figured out how to run one temperature of water (radiant floors first floor, radiant walls basement) can I run the radiant manifold as the boiler loop or do I still need a mixing valve and two pumps / two loops?

If I used one loop the way the controls would work is like this??? :
1. Thermostat calls for heat after drop below set point
2. Taco zone controller opens actuator for zone
3. Taco zone controller also jumps boiler T+T terminals
4. Grundfos pump senses pressure drop and starts to pump water through system
5. Boiler will fire only if water passing through it drops below it's built in aqua-stat (outdoor reset determined)
5.5. Grundfos pump can continue to run with or without the boiler pump and the boiler turning on?


6. Call for heat ends. Zone valve closes. TT terminals are open again. Grundfos pump stops due to dead-heading.

Products I'm using are:
Taco 5004 3/4" mixing valve
Grundfos Alpha 15-55 F cast iron pump
Taco 6 Zone valve control module
Navien NCB-180E natural gas combi-boiler
Uponor EP manifold & 4-wire actuators
Tekmar 519 Thermostats w/ slab sensors


Single loop:



Dual loops:

The point of Outdoor reset is to alter the boiler temp to deliver more or less heat to the system to offset greater or lesser heat losses from the structure due to the outside temps. With that in mind, wouldn't a mixing valve, I am assuming set at a fixed temperature, buried in the middle of the flow override the purpose of the outdoor reset boiler temp changes...

Does the boiler you are looking at have return temperature limitations(shock cooling/cast iron)? That is usually the reason to run the primary/secondary loop configuration. If your boiler must run this way I would say maintain a steady boiler temp and use a powered outdoor reset mixing valve controlled by an outdoor sensor to manipulate the water temperature delivered to your radiators. IF your boiler doesn't need this I would definitely opt for the simpler 1 loop system...

I'd do roughly the first one, except read the installation manual, paying particular attention to the use of closely spaced Ts for the boiler. Partially bypass some zones if the smallest zone can't absorb 14K btu (ie, avoid short cycling).