We installed 750' of 1/2" pex piping into the floor for radiant floor heating and to help manage the heat flow into and out of the thermal mass. The 750' was split into 3 x 250' lengths which all run parallel.
As part of the pipe layout I did a number of calculations to determine the spacing of the tubes. Essentially I did the calculations according to the CSA standard to determine how much heat the flooring would need to provide (assuming it provides 70% of the heat for the house when it -20 C outside) and from there I calculated the total tube length spacing, as well as geeky things like the Reynolds number (to ensure the flow in the tubes is turbulent).
There is only one zone. This keeps the number of bends to a minimum and keeps the flow control simple. If you have multiple zones then you need to throttle down the flow in the shorter pipes to ensure you have even heating throughout the house. There is an art to throttling down the flow rate, requiring a good controller that costs a lot more than what I need for this simple system. Throttling down the flow rate is also inefficient due to head losses in the valves.
There is a lot of suggestion that radiant floor heating is a waste of money for a superinsulated house such as this. A big incentive to use radiant floor heating is to have nice warm floors underfoot, but for this house the floor temperature will be only slight higher than the room temperature and it will not feel warm.My argument is that I can use the water in the pipes to remove heat during the summer. To keep the house cool we'll pump some of the irrigation water through the floor before the water goes outside to water the garden. In the fall and spring it's more efficient to warm a lot of water a few degrees above room temperature, than run baseboard heaters. But to back up this claim I really need to do an analysis of the exergy (thermodynamics) of heating the house. Basically an exergy analysis tells us that it costs less to heat 2L of water by 15 degree than it costs to heat 1L of water by 30 degrees, even though the same amount of energy is being transferred to the water.
Additionally the cost of the pipe is minimal. Most of the cost of radiant floor heating is the boiler and controller. When the outside temperature drops below ~-0C we plan to use the wood stove, so we only need a boiler large enough to supply the heated water during the shoulder months and to help the wood stove during the winter. This reduces the cost of the boiler (by how much I have yet to determine).
We also poured the concrete at the start of July.
Radiant floor heating systems are installed under sub flooring and floor finishes such as hardwood or tile. It is completely hidden, consisting of tubes embedded in a concrete or gypsum slab. The tubes are filled with water or gas as necessary that is supplied by the heating source. The space above the floor is heated by radiation and also convection through the floor.
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