Baseboard heat sizing report units?

asked 2020-07-29 08:46:01 -0600

TheEnergyTutor's avatar

updated 2020-07-29 20:07:05 -0600

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The E+ results of this baseboard heater autosizing indicate 1.43 GPM for 435 BTUH * F? What is the F? I am used to baseboard being sized at BTUH per linear foot like this other photo of a runtal.

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How do i get the actual total size of the heater needed? If i knew i needed 3.6 MBH i would specify 6' of UF-2, but i don't understand how to use the E+ report to match equipment.

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Does this post answer your question? Essentially, $F$ is the temperature difference between water and air across the baseboard in Fahrenheit. The baseboard length that is specified likely correlates to the temperature difference.

Aaron Boranian's avatar Aaron Boranian  ( 2020-07-29 10:00:40 -0600 )edit

You may have to be more specific as there are several temps. Entering water temperature, leaving wtr temp, and average wtr temp (this one is commonly used) and all 3 of these are known values to open studio as specified in the hot water system 170F sup, 140F ret, 155F ave. Regarding air temperature, I'm guessing this is what we call "Entering air temp", which is typically the space temperature at the floor, nominal design = 65F . So does this mean the heater size needs to be 435 * (155-65) = 39,150? =65 feet? I can't tell if the F is numerator or denominator. Where is all this documented? tx

TheEnergyTutor's avatar TheEnergyTutor  ( 2020-07-29 10:55:36 -0600 )edit

From the EnergyPlus Input-Output Reference, it's $(T_{water,avg} - T_{air,avg})$. The log mean temperature difference can also be used. For calculation details, check the EnergyPlus Engineering Reference.

Aaron Boranian's avatar Aaron Boranian  ( 2020-07-29 11:12:02 -0600 )edit

I uploaded some more selection tables. I'm not sure this is resolved but i appreciate the explanation. You can see the average air temperature is never used because it is not a known value - and also corresponds with the "AutoSize" setting in OS. While, i may agree that the formula noted provides a value, I don't know how they can use that calculation because the leaving air temperature of the convector is not known. Isn't the autosize supposed to report this result in some way?

TheEnergyTutor's avatar TheEnergyTutor  ( 2020-07-29 15:22:16 -0600 )edit

Also it doesn't define "Average Air Temperature." I assumed this was between the inlet and outlet of the baseboard but i am reading an article it says to use the average temperate in the space, so 70 degrees?

TheEnergyTutor's avatar TheEnergyTutor  ( 2020-07-29 15:32:52 -0600 )edit