Using the procedure outlined here, you can accurately calculate the length of finned-tube baseboard required in each room served by a series piping circuit. First, you have to determine the design heating load for each room (for a detailed discussion on heat load calculations, see “Trouble-Free Forced-Air Heat”; the load calculation is same for all heating types). The formulas presented here are based on using water as the system fluid.

Graph A shows how the heat output of baseboard varies with the water temperature in the element. The four curves represent finned-tube elements with different rated outputs. Each curve has a b value, which is the baseboard’s heat output (in Btu/hr/ft. of finned element) at a standard water temperature of 200°F and a flow rate of 1 gpm. You can get this value from the ratings table in the manufacturer’s literature. For the most accurate results, divide the table value by 1.15 to remove the heating effect factor before finding the nearest b value on the graph.

Heat output also depends on the flow rate through the element. The higher the flow rate, the higher the heat output. Graph B gives the flow rate in a series piping circuit powered by a typical 1/25 horsepower wet-rotor circulator.

Step 1: Select a boiler outlet temperature; 180°F is typical. The inlet temperature to the first baseboard is assumed equal to the boiler outlet temperature.

Graph B. This graph gives the estimated water flow rate in piping circuits of different lengths. An allowance has been made for a typical assortment of fittings and valves.
Graph B. This graph gives the estimated water flow rate in piping circuits of different lengths. An allowance has been made for a typical assortment of fittings and valves.

Step 2: Estimate the flow rate through the baseboard using Graph B (above).

Step 3: Calculate the average water temperature in the first baseboard as follows:

Graph A. This graph shows the heat output (in Btu/hr/ft of finned element) of four different models of finned-tube baseboard. The graph assumes the air entering the baseboard at floor level is at 65°F. The b value, which can be found in the manufacturer’s literature, is the baseboard’s heat output with 200°F water and a flow rate of 1 gpm.
Graph A. This graph shows the heat output (in Btu/hr/ft of finned element) of four different models of finned-tube baseboard. The graph assumes the air entering the baseboard at floor level is at 65°F. The b value, which can be found in the manufacturer’s literature, is the baseboard’s heat output with 200°F water and a flow rate of 1 gpm.

Step 4: Use Graph A (above) to estimate the heat output of one foot of baseboard at Tavg. This value is designated as q.

Step 5: Calculate the required baseboard length as follows:

Round off to the next higher whole foot length, referred to as Lrounded.

Step 6: Calculate the outlet temperature of the baseboard using:

Use the outlet temperature of this baseboard as the inlet temperature to the next baseboard, and repeat the procedure starting at Step 3. Repeat for each baseboard in the series circuit, each time treating the outlet temperature from one baseboard as the inlet temperature for the next.

This would be the assumed inlet temperature to the next baseboard. Computer Assistance This procedure cries out for spreadsheet programming. Once programmed, you can compare the effects of varying design factors like flow rate, boiler outlet temperature, and so forth, in seconds. The Hydronics Design Toolkit software (written to accompany my book, Modern Hydronic Heating for Residential and Light Commercial Buildings) has both a heat load estimating program and a complete baseboard sizing program that does all these calculations, as well as allowing you design options such as speccing antifreeze instead of water or plugging in different air temperatures for each baseboard. The program costs $70 and is available from Iris Communications (541/767-0355) or the Radiant Panel Association (800/660-7187).

Example

Size the first baseboard in a series piping circuit for a room heating load of 4,000 Btu/hr. Assume the overall piping circuit is 150 feet long, and built with 3/4-inch tubing. Also assume the boiler outlet temperature is 180°F, and the baseboard used has a rated output of 600 Btu/hr/ft. with 200°F water.

Step 1: Tinlet = 180°F

Step 2: From Graph B, the circuit flow rate is estimated to be 5.5 gpm.

Step 3: Calculate the average water temperature in the first baseboard:

Step 4: From Graph A (blue line), the output of the baseboard at 179.3°F is about 480 Btu/hr/ft.

Step 5: The required length of the first baseboard element is:

Step 6: The outlet temperature from this baseboard is:

This would be the assumed inlet temperature to the next baseboard.

Computer Assistance

This procedure cries out for spreadsheet programming. Once programmed, you can compare the effects of varying design factors like flow rate, boiler outlet temperature, and so forth, in seconds. The Hydronics Design Studio software (written initially to accompany my book, Modern Hydronic Heating for Residential and Light Commercial Buildings) has both a heat load estimating program and a complete baseboard sizing program that does all these calculations, as well as allowing you design options such as speccing antifreeze instead of water or plugging in different air temperatures for each baseboard.

This article originally appeared in JLC as a sidebar to the main article: Hydronic Baseboard Basics.