Designing an efficient and cost-effective radiant floor heating system takes a lot of planning and consideration. Such systems are not pre-manufactured nor are they pre-designed. Instead, each installation is custom and consists of different parameters and variables. The factors that must be measured in the design process include: area size, heat output surface area, and heat loss. Based on this data, calculations are made and a design is compiled that indicates the required size and spacing of PEX pipe, length of circuits, water flow rate, and temperature of the system. A use of special design programs is recommended to ensure accuracy and save time on designing the plan.
While tubing can potentially be installed under the whole floor surface, placing it efficiently can bring in significant benefits. For example, there is no need to install PEX tubing under the kitchen cabinets. Taking this factor into consideration, will not only save on tubing cost during installation, but most importantly, will result in constant savings by reducing the loop size and releasing the heat output in the areas where its actually effective. For best results, an area that will not be heated should not be counted in the plan; it should be deducted from the total area, resulting in Adjusted Floor Area.
The next step is to figure out the rate of Floor Output Needed. This can be accomplished by dividing the Total Heat Loss of the area by the Adjusted Floor Area. Once the rate of the Floor Output Needed is established, it can be used to find the Required Floor Surface Temperature.
After selecting an appropriate PEX tubing size, the Floor Output Needed rate is used to determine spacing and maximum length of circuit (the higher the btuh/sq.ft. rate, the shorter is the spacing between tubes).
At this point, it is possible to figure out the total amount of tubing that is required for the installation. That is done by multiplying Available Floor Area by Tube Spacing Factor (that is correlated to the Tube Spacing).
The length per circuit and the amount of circuits needed can also be calculated by first dividing the total tubing needed by the max tubing length (once rounded, the result will represent how many circuits are needed) and then divide total tubing needed by the number (rounded) of circuits needed (this will establish actual length per circuit). It should be noted that while recommended, circuits are not required to have the same length.
When calculating an ideal system supply temperature, the materials that are used for flooring are a very important factor. Materials that are highly heat conductive will require lower temperatures, while materials with low conductivity will need higher supply water temperatures. Generally, the higher the R-value (thermal resistance) of the surface, the higher supply water temperature is required).
The last part of the system design is to figure out the System Flow Rate, Circuit Head Loss, and last but not least – pump size.
The System floor rate is measured by Gallons Per Minute (GPM) and is generally calculated by dividing BTU by 10,000. GPM is further divided by the number of circuits to establish the flow rate per circuit.
Flow rate and tube size are used to calculate a correct circuit head loss. When it comes to the pump, an efficient circulator must be able to function under the required head loss and flow rates.
Manufacturer’s guidelines should be used as a reference when making calculations and system design.
