Adding a radiant manifold to existing single zone loop

What kind of boiler do you have? Atmospheric? Condensing?

If your house is under 4,000 square feet, a Taco 007 or Grundfos 15-42 should work.

Not a condensing boiler.

Look at the vent pipe. A condensing boiler would use a plastic vent pipe because all the flue gas condensation would rot the galvanized vent connector away in a year. According to the manual this is a better way to do the near boiler piping if you want less problems with air in the system.

Assuming that you are connecting to standard Baseboard heating units with 3/4" copper tube. The circulator on the return indicates the installer didn't read the manual and followed the directions his father who taught him to do it that way.

Now adding a radiant floor zone to your existing system will require draining water from the boiler (so you may want to upgrade your near boiler piping at this time) and connecting the zone this way. This is the bare minimum that will work.

The controls will need to be added so when thermostat that operated C2 calls for heat , C1 will not operate but the burner will operate. depending on what is currently controlling the boiler, that can be done with a ZC or ZR terminal and a Taco SR501 or equal. Or a Taco SR 503 or equal.

I can draw you the wiring diagram if you want me to.

Any Questions?

Since you have baseboard hot water radiators, there is not much water volume in your system. For that reason you never actually needed boiler temperature protection. The return water heated up quickly. By adding a low temperature radiant heat zone, then you will be returning much colder water if only that zone is operating. So this is a redesign that I might use. In your "proposed" drawing the radiant heat zone circulator will not get any hot water from the boiler unless the boiler C3 is operating to move heated water past the closely spaced tees. Whenever C3 is operating there will be a tendency for the baseboard loop to get some ghost flow or thermocycling or even forced flow even though C1 is not operating. that is because the baseboard loop is not connected to the primary boiler loop

I didn't see your proposed diagram when I first looked at your post. That is why I offered the "Minimum Diagram". Now that I can see that you are using three circulator pumps, I will offer you this diagram with four major corrections.

1. I have pasted the near boiler piping from the I/O manual from Pensotti.
2. I have corrected the closely spaced tees. The idea is to have next to zero pressure drop between the supply to and the return from the secondary loop. By putting one of the Tee fittings on the corner of a run in the primary loop, you are creating a pressure difference between the supply to and the return from a secondary loop.
3. I have placed the hotter secondary loop first on the primary loop. The second lower temperature secondary loop will be able to operate with lower temperature water from the first secondary loop’s return. Since both primary and secondary can operate at the same time and also only one at a time, the water temperature feeding the low temperature secondary loop will be different if the high temperature loop is operating or is not operating. Since this happens without notice, the manual mixing valve idea will not work so that brings me to fix #4
4. Install an automatic mixing valve.  

I hope you find this helpful

Mr. ED

if the radiant us around 10% of the boiler btu output , I would not be concerned about return protection