When to use Heat Exchanger

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Hearth Supporter
Mar 14, 2008
West Coast
Our prior stove had one pump to the heat exchanger and one from the exchanger to the slab.
Using an open system, such as the Garn, is there a benefit to going direct?
The Garn sits 4' higher than the slab
I hope to hook up additional buildings if there is available heat in the future
Regards, Scotty
In this particular case, I would say there is reason to not use a hx.
The boiler is higher than the floor. You can pump tank water into the floor and back to the tank directly.
A Garn has corrosion inhibited water and the only reason I would use a heat exchanger here is if a back up boiler is in the loop (which would normally
be pressurized) or you wanted to antifreeze the system.
Separate buildings could warrant antifreeze unless you are in one of those weird West Coast places where it does not freeze often.
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HX is loss in the system, and extra pumps. If the chemicals are compatable with PEX and all else is a go then go direct!

If you are diligent in monitoring the water chemistry in the system, you can run it direct. Saves the cost of the HX.
I assume you'd still need a pump or zone valve off the main circulating loop to give on/off control to the floor while the boiler loop continues to run?
Don't forget a tempering valve, you don't want to be putting 180* water directly into your slab.
Don't forget a tempering valve, you don't want to be putting 180* water directly into your slab.
Water chemistry is critical in an open system, keep a handle on it.

Ideally a centrifugal circulator wants to have a bit of pressure, and that requirement changes, increases, as temperature rises. High operating temperatures and low, or no pressure can coax a circulator into cavitation conditions.

Grundfos suggest @ 4 psi at 190F temperatures, so you would need 9 feet of water above the pump to provide that. Keep the pump(s) as low as possible in the piping.

Hydronic pump manufacturers really don't like to see typical iron body pumps in open system, Taco offers a few pumps for open systems, either coated or non ferrous bodies. Grundfos is adding an "E" coating to some of their models for the open type GEO systems.
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