Radiant Hydronic Setup

gwi

I am designing a small in slab radiant system- 4 circuits, 3 zones controlled at the manifold.
As I understand one way to setup is to have a connection to the main supply to fill and makeup any drop in water (Figure 1.), or you can fill the closed loop at the manifold and maintain volume manually if needed (Figure 2.).

Any thoughts on the pros/cons of these two approaches, or a better way?

Any feedback on these system designs would be appreciated.

Thank you in advance.

GGross

Auto fill will add fresh water to your system, along with fresh water comes fresh minerals to be deposited on and begin destroying, your ferrous metal components in your system. One way that you can still have the convenience that is autofill, but also control the quality of fluid being added, is to use a dedicated system feeder. You also get the added benefit with this system of monitoring how much, if any, fluid you add. The downside here is that you would need to keep it filled, but realistically you need to just fix any leaks that arise, and the feeder will just keep your system up to pressure if a leak develops, monitoring the level of the feeder will allow you to see if you are using make up water and alert you to a leak. I use the Axiom branded ones, but you can make one yourself, or use any brand really.

https://axiomind.com/axiom_products_cat/system-feeders/

hot_rod

If you have low water cut off protection on the boiler you could have a system that is not connected to a fill valve or tank. I would over size the expansion tank, under charge it on the air side and "store" a few gallons of water there. Solar thermal systems are done that way all the time.

Now depending on the boiler you use, not many mod con can direct pipe like that drawing, especially zoned systems. Confirm you can make the minimum flow rate the boiler requirers.

gwi

hot_rod said:

If you have low water cut off protection on the boiler you could have a system that is not connected to a fill valve or tank. I would over size the expansion tank, under charge it on the air side and "store" a few gallons of water there. Solar thermal systems are done that way all the time.

Now depending on the boiler you use, not many mod con can direct pipe like that drawing, especially zoned systems. Confirm you can make the minimum flow rate the boiler requirers.

Thanks for the information. I was going to use a tankless condensing water heater (shouldn't have said boiler) so it only runs when it gets sufficient flow rate, in this case a LWCO is redundant right?

gwi

GGross said:

Auto fill will add fresh water to your system, along with fresh water comes fresh minerals to be deposited on and begin destroying, your ferrous metal components in your system. One way that you can still have the convenience that is autofill, but also control the quality of fluid being added, is to use a dedicated system feeder. You also get the added benefit with this system of monitoring how much, if any, fluid you add. The downside here is that you would need to keep it filled, but realistically you need to just fix any leaks that arise, and the feeder will just keep your system up to pressure if a leak develops, monitoring the level of the feeder will allow you to see if you are using make up water and alert you to a leak. I use the Axiom branded ones, but you can make one yourself, or use any brand really.

https://axiomind.com/axiom_products_cat/system-feeders/

These looks better to use and I can add glycol! Thank you.

hot_rod

Tankless water heaters are not a great matchup for radiant or hydronics. They are designed to heat blow gpm flows across a wide temperature range. Taking 50F water to 120F for example.
So it is opposite of how a boiler operates, or a heating system needs, higher flow rates, lower temperature rise.
You may end up needing a large circulator to push flow through the small tankless passageways. Some are unable to fire properly at hydronic design conditions, especially in multi zoned systems.

One thing they have going for them is a low price. Your drawing has a boiler icon, thats what you should use