I have a a few questions related to the radiant floor system I am installing. Context: The previous owner of our homes (two located on one property 60 feet from one another other) had ½ PEX oxygenated lines run throughout the entirety each structure. Both homes are Insulated concrete form construction (12 inch exterior walls), slab foundation, and PEX embedded 4 inches in a polished concrete floor. The two homes are both circles. 1 is 50 feet in diameter the other is 30 with a small 150 foot addition. So home 1 is 2000 square feet and home 2 is 900 square feet . Sounds odd, but those are our new tanks of homes. Unfortunately the radiant floor pipes were never hooked up. The family relied on parlor stoves to heat the two homes. Supposedly they worked so well that they never completed the radiant install. There is a pair of 1 inch PEX leading from the utility room in the big house to the utility space in the small house. The plumbers notes (the homes were constructed 8 years ago) say that each loop is no shorter than 200 and no loner than 300 linear feet though most are closer to 300. There are 11 ½” pairs in the big house and 6 ½” pairs in the small house, with one designated for snowmelt that is currently just a stub out. I pressure tested everything and it all checks out. The previous owners left a brand new 199,999 BTU Takagi/T-H3-DV-NG to be used to heat both systems. I intend to use it and another tankless water heater for the domestic water supply in each home. Question: I am in the process of figuring out the system layout and am unsure of the appropriate pump size and installation locations. If I am to maintain .6 gpm and have a total of 16 300’ loops what pumps are recommended for this sort of configuration? Where would you recommend locating the pump or possibly two separate pumps for this system. Generally, I am concerned that the head from the 60’ run between the homes will force me to use a pump that may be too strong for the rest of the system. Much appreciated,

16 1/2" PEX circuits that are each 300 feet long with each having a flow rate of 0.6 GPM will require a pump that can deliver 9.6 GPM at a 5.9 feet head. Just pick a pump that puts that maximum operational point near the center of the pump performance curve. You can use the hydronic radiant calculator on our website to play with and refine the design.

Thank for confirming the math. I am planning on a primary/secondary system. Because of the open layout of each home I will use a single Grundfos ALPHA2 15-55 pump to power each home's manifold of loops.

It seems that the ICF home construction may create too much of a heat envelope for the Takagi T-H3-DV-NG as it's lowest BTU output is 18,000. For that reason I'm wondering if that may be better used for the domestic water supply (two baths, washer and kitchen) and I purchase a T-k4-IN-NG for the radiant floors. The T-k4-IN-NG has a range from 11,000 - 190,000 BTUs, that should give me a bit more control over the heat especially in the 900 sq foot home. Thoughts?


Much appreciated.

You really need to accomplish a room-by-room heat loss analysis to properly design a hydronic radiant heating system and size the heat source. However, you already largely have a design and installation... So at this point, you can only alter what loops are associated with each heating zone and size the heat source. You could accomplish an ACCA Manual J (or ASHRAE) room-by-room heat loss analysis (we have a free DIY calculator on our website to do this) to properly define the heat zones and size the heat source. You could also just accomplish an existing building energy usage analysis (we have a free DIY calculator on our website to do this) to just size the heat source.

I would speculate that 190 kBtus/h is way too much for 900 sf ICF building in any location, however, the lower modulation levels might work fine...but I don’t really like to speculate and one should really run the numbers... You might be able to make $1,200 Nextgen electric boiler accomplish the floor heating mission. The Nextgen boiler would provide everything required to complete your hydronic radiant floor system (pump, expansion tank, differential pressure valve, safety relief valve, air separator, controller, etc.) except the manifold...so DIY friendly plug and play...

I should also add that I suspect 0.6 GPM for each circuit may be excessive...and one should really run the numbers based on the room-by-room heat loss analysis...