TAKAGI BOILER CLOSED SYSTEM NO DOMESTIC HOT WATER

I can not recommend many online suppliers but some of the ones I can are:
www.pextubingstore.com  and www.bostonheatingsupply.com 
These are people I have worked with, they sell quality products, for the best prices and stand behind what they sell.
They will also give you 5% off on purchases if you use code Fred Seton for a reference.

Using a design, with loops up to 500' of 1/2" pex with a single manifold next to the heater in the mechanical area will heat very large homes. This design allows as many thermostat controlled zones as you need. It eliminates using a pump for each thermostat and manifolds scattered throughout the home. 

THE BEST TANKLESS HEATER IS THE TAKAGI, SEE   www.pextubingstore.com
I have used these in my radiant designs since 1995, I have three of them, one in my boat, one in my bus and one in my home to back up my wood burning boiler.

Not all instant water heaters are suited for this application.