Rule of Thumb for Fin-Tube Hot Water Baseboards Per Sq-Ft

 
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Post by lsayre » Sun. Dec. 27, 2015 10:03 am

For what its worth (I.E., nothing) my initial ballpark guess for residential 3/4" fin tube baseboard heating is that for every 18 square feet of indoor floor space that a room in the average home has it requires 1 linear foot of hot water fin tube baseboard. I realize that guesses based on averages for a given heating region do not fit all cases or regions. One home may be old and un-insulated and require more heat (more ft of baseboard per sq-ft), and anther may be newer and better insulated and require less. Plus location is a major determinant. So if you are designing a home around hot water fin tube baseboards, use this (if at all) only as a loose guide.

Assuming a nominal 450 to 550 BTU's delivered per foot of properly installed fin tube, my guess yields between 25 and 30 output BTU's per square foot of heated floor space.

For those of you with fin-tube baseboard heat, it would be interesting to hear how well this stab at a rule of thumb compares with your case.


 
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Post by Lightning » Sun. Dec. 27, 2015 10:26 am

I was hoping for such a ratio between fin tube and square footage. At some point when I install a boiler, my new living room needs a better way of heat distribution. Since it used to be an attached garage, the only place I can bring in the warm air supply is at the ceiling and then circulate it with a ceiling fan. Long story short, the room develops cool spots in the corners where the computer sits and under windows.

The room is about 450 square feet with 3 sides exposed to the weather.

Thanks for another cool thread Larry :)

 
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Post by lsayre » Sun. Dec. 27, 2015 10:31 am

Remember to place your baseboards along the outside walls (for rooms that have them). I noticed that in my home they are also often centered underneath the windows.

 
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Post by Lightning » Sun. Dec. 27, 2015 10:37 am

Right. So if I went over this ratio, say 1 foot of fin per 12 square feet, that wouldn't be a bad thing, right? If I had this room on its own zone?

 
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Post by lsayre » Sun. Dec. 27, 2015 10:47 am

Lightning wrote:Right. So if I went over this ratio, say 1 foot of fin per 12 square feet, that wouldn't be a bad thing, right? If I had this room on its own zone?
I would think that 1 ft per 15 sq-ft would be plenty for Olean, NY. And since it is new and I would assume well insulated, my 1:18 ratio may prove to be more than sufficient.

It seems that your area has roughly 20% more annual HDD's than mine, so on that basis alone 18/1.2 = 15

 
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Post by lsayre » Sun. Dec. 27, 2015 10:58 am

An improvement based on regional average annual HDD's:

~5,650 HDD's = 20:1
~6,000 HDD's = 19:1
~6,300 HDD's = 18:1
~6,800 HDD's = 17:1
~7,100 HDD's = 16:1
~7,600 HDD's = 15:1
~8,100 HDD's = 14:1
~8,700 HDD's = 13:1
~9,500 HDD's = 12:1
~10,300 HDD's = 11:1
~11,350 HDD's = 10:1
Last edited by lsayre on Sun. Dec. 27, 2015 11:15 am, edited 2 times in total.

 
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Post by Lightning » Sun. Dec. 27, 2015 11:00 am

Oh okay, perfect. 30 feet of fin, so 10 feet along each outside wall. That sounds about like what I was thinking. :)


 
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Post by lsayre » Sun. Dec. 27, 2015 11:23 am


 
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Post by Rob R. » Sun. Dec. 27, 2015 11:41 am

I have an old book that goes about this a different way. It has some general rules to calculate the heating load of each room, and you do that first. You look at the heat load of the room vs. the amount of wallspace available for radiators, and then use the charts to see what water temperature is required to do the job. The room that requires the highest water temperature sets the standard for the entire system, you then go back and resize the radiation for the other rooms based on the new water temperature. You need to use some common sense. e.g. Don't make the entire system run at 200 degrees because the kitchen didn't have enough room for baseboard radiation...instead use a toekick heater, or high-output baseboard.

 
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Post by lsayre » Sun. Dec. 27, 2015 11:49 am

Rob R. wrote:I have an old book that goes about this a different way. It has some general rules to calculate the heating load of each room, and you do that first. You look at the heat load of the room vs. the amount of wallspace available for radiators, and then use the charts to see what water temperature is required to do the job. The room that requires the highest water temperature sets the standard for the entire system, you then go back and resize the radiation for the other rooms based on the new water temperature. You need to use some common sense. e.g. Don't make the entire system run at 200 degrees because the kitchen didn't have enough room for baseboard radiation...instead use a toekick heater, or high-output baseboard.
This method would nicely even out the heat comfort of each room in the home.

 
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Post by lsayre » Sun. Dec. 27, 2015 11:52 am

Another consideration would be that since the water is losing temperature with every baseboard that it runs through, having it run through the colder side of the house first (if possible based on your situation) seems like that would even out the heat comfort.

 
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Post by Rob R. » Sun. Dec. 27, 2015 11:54 am

Yes, or just start with the room most challenged for radiator space.

 
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Post by lsayre » Sun. Dec. 27, 2015 12:26 pm

Another consideration is: How many maximum feet of baseboard can I (or should I) place on each single zone loop?

Using 3.5 GPM of flow as the target for each zone, and 3/4" Type M copper pipe, and standard 500 BTUH per linear foot output 3/4" HWB's, and factoring in the desire to achieve 20 degrees of drop across the loop, and *** assuming *** that 75% of the heat derived from your loop evolves from the baseboards, this can be guesstimated as follows:

3.5 GPM x 8.33 lbs./gal x 20 degrees of target desired drop x 60 minutes/hour ~= 35,000 BTUH per zone loop

35,000 x .075 = 26,250 BTUH of baseboards required for each zone loop

26,250 BTUH/500 BTUH/Ft. ~= 52 maximum feet of standard 3/4" fin-tube baseboard per zone loop

There are loads of variables here that can be manipulated to suit, so that allows for a lot of design leeway. You can increase or decrease the GPM's of flow. You can change the temperature of the water. You can get higher or lower nominal BTU output baseboards. You can change to some other desired temperature drop across each zone. Insulated or un-insulated pipe, etc... So this should be considered only as another guesstimated ballpark rule of thumb.

*** I wonder what the HVAC guys consider to be the typical percentage of heat evolution that is to be derived from the baseboards vs. the heat that evolves off of the rest of the loop (be it plain 3/4" copper piping, or whatever)? And should everything sans for the baseboards along the run be insulated or left uninsulated? ***

In my house proper (excluding here the garage loop) the average across my zone loops is about 40 feet of baseboard. Max 43 feet. Min 38 feet. So for my zones the average BTUH of delivery is probably only about 20,000 per each (assuming 20 degrees of drop).
Last edited by lsayre on Sun. Dec. 27, 2015 12:36 pm, edited 2 times in total.

 
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Post by Rob R. » Sun. Dec. 27, 2015 12:30 pm

75' is the generally accepted limit for 3/4" baseboard. Note that you can also split the supply and use a common 1" return.

 
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Post by lsayre » Sun. Dec. 27, 2015 1:07 pm

Rob R. wrote:75' is the generally accepted limit for 3/4" baseboard. Note that you can also split the supply and use a common 1" return
I would agree that if multiple of the variables that can be juggled are pushed more toward their upper range limits (instead of choosing mid-ranges) this would be the max.

And if the split return method is applied then wouldn't you effectively have created two zones, each with 37.5 feet of baseboard?


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