Why the page asks for two rating points instead of one
Fin-tube element is sold with an output table, not an output number. The table has a row for each average water temperature, and the output per foot climbs steeply with temperature because the fins are shedding heat to room air by convection and the driving force is the difference between the water and the room. Give the calculator a single output figure and it has no way to know what happens when you run the water cooler, which is precisely the question worth asking. Two rows let it draw the line the table implies and read off any point on it.
Over the range published tables cover, the relationship is close enough to straight that a line through two nearby rows lands within a few percent of the intermediate rows. It stops being straight if you extend it a long way past the ends, which is why the page flags an extrapolation rather than pretending it did not happen.
Average water temperature, not supply temperature
This is the mistake that survives longest. Element ratings are stated at average water temperature: the mean of what goes in and what comes out. If the boiler supplies 180 and the circuit drops 20 degrees, the element is rated at 170, not 180. Read the wrong row and every length on the drawing is short.
The drop also is not fixed. It is set by the flow rate and the load, through the same relationship the hydronic flow calculator uses, so a circuit designed around a 20 degree drop at design load runs a smaller drop at part load. That is a second-order effect for a length calculation and a first-order one for how the room behaves in April.
The cliff, and what it does to a boiler swap
Take the defaults on this page: 580 BTU/h per foot at 180 degrees average water, 430 at 150. That is 5 BTU/h per foot for every degree. A room losing 8,000 BTU/h at 170 degrees average water needs 16.7 feet of element. Run the same room on 140 degree supply — 130 average — and the output per foot falls to 330, and the room now needs 24.2 feet. Nearly half as much again, on the same wall, in a house where the wall was full already.
That arithmetic is the whole story of a condensing boiler dropped into an old baseboard house. Condensing boilers earn their efficiency by returning water cool enough to condense the flue gas, and the emitters that were sized for 180 degree water cannot deliver at those return temperatures. The boiler is not faulty and the installation is not wrong; the emitters were sized for a different system. Either the emitters grow, the load shrinks, or the boiler spends the winter running hot and never condensing, which buys the efficiency of the old one at the price of the new one.
| Supply water | Average water | Output per ft | Feet for 8,000 BTU/h |
|---|---|---|---|
| 190°F | 180°F | 580 | 13.8 |
| 170°F | 160°F | 480 | 16.7 |
| 150°F | 140°F | 380 | 21.1 |
| 140°F | 130°F | 330 | 24.2 |
Those output figures come from the default rating points on this page, which are a plausible fin-tube product and not a universal one. Put your own table in and the shape stays the same while the numbers move.
What the length does not tell you
Feet of element is a heat balance, not a layout. A run under a window and a run on an interior wall deliver the same BTU and do not feel the same. Furniture over an enclosure blocks the convection the rating assumed. Two rooms on one series loop share water, so the second room sees the temperature the first one returned, and its output is lower than a table read at supply temperature suggests. None of that is in this arithmetic.
For the load itself, the heat loss calculator sums conduction and infiltration from your own areas and R-values. For the flow rate the circuit needs, the hydronic flow calculator; for what a lower water temperature does to a whole system, the outdoor reset curve calculator. If cutting the load is on the table, the insulation calculator and the air sealing payback calculator price the two sides of it.
Questions people ask
How many BTU does a foot of baseboard put out?
There is no single figure, and the range across products and temperatures is wide enough that quoting one does real damage. The same element can be rated near 600 BTU/h per foot at 180 degree average water and near 330 at 130, and a different product at the same temperature can be a third higher or lower. Take the number from the rating table for the product you are actually buying, at the row matching the average water temperature and the flow rate your circuit will run. That is why this page asks for two rows instead of offering a default.
Do I use supply temperature or average water temperature?
Average, which is supply minus half the temperature drop across the circuit. Ratings are published that way. If the boiler supplies 180 and the circuit drops 20 degrees, the row to read is 170. Using the supply figure overstates output by roughly half the drop times the slope of the table, which on a typical element is 50 BTU/h per foot for a 20 degree drop, and that quietly makes every run about ten percent shorter than it should be.
Why did my new condensing boiler leave the house cold?
Almost always because the emitters were sized for the old water temperature. A condensing boiler only condenses when the water coming back is cool, and the control strategy that gets it there lowers the supply temperature. Baseboard output falls sharply as water temperature drops, so the same feet of element that heated the house at 180 degrees do not at 140. Nothing is broken. Run the defaults on this page at both temperatures and the required length grows by about 45 percent, which is the missing capacity in one number. The fixes are more or better emitters, a lower load, or accepting a higher supply temperature and less condensing.
Does the flow rate change the output?
Yes, though far less than temperature does. Higher flow raises the film coefficient inside the tube slightly and, more importantly, reduces the temperature drop across the circuit, which raises the average water temperature and therefore the output. Most published tables have separate columns for different flow rates so you can see the size of it. Read the two points from the column matching your design flow rather than mixing rows from different columns, because the difference between columns is smaller than the difference between temperature rows and mixing them buries a small error inside a large one.
Can I just add more baseboard to fix an undersized system?
Sometimes, and the constraint is usually wall rather than arithmetic. The calculation is straightforward: work out the length the room needs at the water temperature you can supply, subtract what is installed, and that is the shortfall. What stops it is that rooms which are short on emitter are usually rooms which are short on wall, because both are consequences of windows and doors. When the length does not fit, the options are a higher-output element in the same space, a different emitter type, or reducing the load. Adding element also adds water volume and flow resistance to the circuit, and past a point that becomes the circulator sizing question rather than the emitter one.