Ok so here is the problem, I have a free boiler given by friend it's a pennco 70kbtu my problem is I want to use it to run low temp radiant floor loops. I can't figure out what the best way to plumb this would be. I'VE looked at heat xchangers but can't meet the required gpm on radiant side. I've looked at a primary secondary loop but my load is low. I have 7 300' 1/2" pex loops. There is three thermostats which I've realized would be a mistake trying to achieve a min gpm. So far the only thing that seems would allow me to match the gpm on the boiler regardless of load would be a primary secondary with closely spaced tees or a injection pump with bypass. I have been reading for 2 weeks now trying to figure this out. What I have learned is low load low flow systems are tough to design.