How to run radiant heat zone off boiler's tankless coil

wolfcry911

I have a small addition that I'd like to heat with under floor radiant heating - probably two loops approximately 250' each. My oil fired boiler has a tankless coil once used for domestic hot water, but that's being eliminated. Can I efficiently run the radiant off this tankless coil? And if so, how? TIA

HVACNUT

Why in the world would you want to do that? Is it a steam boiler?

wolfcry911

no, it's forced hot water. I just thought it would be easier to have the radiant (lower temp) separate from the other zone.

HVACNUT

Can you post pics of the boiler and piping? It shouldn't be too difficult to add a zone. The lower temperature would be from the mixing valve setting. The tankless coil temperature would be the same as the existing zone temperature. 
Others know better than me on radiant design, and you didn't say what size loops, but 500 feet through a tankless that gives you 5 GPM, I don't know. 

EBEBRATT-Ed

@wolfcry911

Just take the new radiant zone off the boiler. You will need a mixing valve and a circulator as well as some controls.

There is no benefit to using the tankless coil other than another part to fail and making things more complicated

EdTheHeaterMan

If you do the zone off of a tankless coil, you will need to treat that a a separate closed boiler system. It will need a relief valve, expansion tank, feed valve, air purger, circulator, and a mixing valve to regulate the temperature to the lower water temperature.

If you just add a zone to the existing closed system, all you need is a circulator with IFC, and mixing valve. Less stuff needed. The water feed, relief valve, air purger, and expansion tank are already there

And what @HVACNUT, @EBEBRATT-Ed, and @MikeAmann said

wolfcry911

Thanks for the advice guys. Looks like I'll abandon the tankless coil idea.