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

 
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Rob R.
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Post by Rob R. » Thu. Dec. 31, 2015 1:01 pm

What are you guys using for measuring supply and return by zone?
I have never checked it in my own home. I have adjusted the dampers on a few baseboards and converted the upstairs to a "home run system" so I could balance it better, but otherwise I just let it ride.

 
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lsayre
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Post by lsayre » Thu. Dec. 31, 2015 1:07 pm

Rob R. wrote:I have never checked it in my own home. I have adjusted the dampers on a few baseboards and converted the upstairs to a "home run system" so I could balance it better, but otherwise I just let it ride.
I'm curious. What is a "home run system"?

 
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Post by Rob R. » Thu. Dec. 31, 2015 1:11 pm

Separate piping circuit for each radiator or baseboard, piped off a manifold.

 
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Post by Rob R. » Thu. Dec. 31, 2015 1:12 pm



 
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Post by CoalisCoolxWarm » Thu. Dec. 31, 2015 2:38 pm

Nice. It's amazing that we have all this modern stuff available, yet nothing beats a sharpie! I have both black and silver for most things 8-)

Larry- I don't think adding 2 ft for every 20 ft of HWB is a very difficult or adds too much complexity- especially for a rule of thumb. You know people will start using it instead of calculating, so may as well get it as close as reasonable now.

 
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Post by lsayre » Thu. Dec. 31, 2015 4:23 pm

We are doing the same thing, only going about it differently.

I just played with some actual BTU figures and now I have decided that for 3 rooms on a zone loop, rather than adding 2 sq-ft to the calculations for the first room and taking away 2 sq-ft from the calculations for the thrid room, I now believe that better results can be had by adding and subtracting only 1.5 sq-ft.

For a nominal 1 ft. of HWB's for every 15 sq-ft of floor space
-------------------------------------------------------------------------------------------------
Room 1 would get 1 ft. of HWB's for every 16.5 sq-ft of floor space.
Room 2 would get 1 ft. of HWB's for every 15 sq-ft of floor space.
Room 3 would get 1 ft. of HWB's for every 13.5 sq-ft of floor space.

For water starting out at 180 degrees and terminating at 160 degrees, the "nominal" temperature drop across each of the 3 rooms is 6.667 degrees.
Room 1's HWB's experience a nominal midrange of 176.66 degrees
Room 2's HWB's experience a nominal midrange of 170.00 degrees
Room 3's HWB's experience a nominal midrange of 163.33 degrees

If we extrapolate from the 4 GPM line on the HWB chart, we get:
176.66 degrees ~= 550 BTUH per foot
170.00 degrees ~= 510 BTUH per foot
163.33 degrees ~= 463 BTUH per foot

Room 1: 550/16.5 = 33.33 BTUH per sq-ft
Room 2: 510/15.0 = 34.00 BTUH per sq-ft
Room 3: 463/13.5 = 34.30 BTUH per sq-ft
All are better than ballpark close. If room 1 was to be the room on the warmest side of the house things would be even closer.

 
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Post by lsayre » Fri. Jan. 01, 2016 7:47 am

I'm attaching here a crude spreadsheet that I just whipped up for calculating a homes initial HWB design requirements. It does not factor in the matter of the water cooling as it progresses along through each zone loop. It also does not suggest which zone loop to assign to each room. All of that you will need to do yourself. But its free, and you get what you pay for. You must supply data in the blue regions. The peach regions are the calculated output of the spreadsheet. As with all rule of thumb based calculators, use at your own risk. It took me all of about a half an hour of playing around, so its level of sophistication is nill. It does scale to HDD's, so along with getting your room dimensions correct, you must also supply a decent value for your local HDD's.

Attachments

Hot_Water_Baseboards.xls
.XLS | 18.4KB | Hot_Water_Baseboards.xls

 
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lsayre
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Post by lsayre » Fri. Jan. 01, 2016 8:14 am

The spreadsheet also makes a crude (ballpark guess) attempt at sizing a boiler to your homes sq-ft and your regional HDD's. it does not however factor in DHW demand. I believe I read somewhere that the average homes DHW energy requirement is about 17% of the total, so if also using for DHW, multiply the spreadsheets calculated output BTUH requirement by 1.17 to ballpark account for this.

Also, be realistic in determining input BTUH's based on the spreadsheets output BTUH calculation. The efficiency factor is what I'm alluding to here. Since boilers are rated for input, you must divide output by some realistic efficiency factor to match the spreadsheets BTUH's to manufacturer rated BTUH's. Be wary of manufacturer supplied efficiency ratings.


 
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lsayre
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Post by lsayre » Fri. Jan. 01, 2016 8:33 am

Warning: Do not use my spreadsheet to include or calculate a garages (or any other outbuildings) needs. Do not use it even if the garage is attached. It is only intended for the main living areas of a home, and it will not work for garages, since it has no provision to do so. Put a garage on a zone of its own, the needs for which (baseboard and boiler wise) would be in addition to anything the spreadsheet has calculated. Did I mention that it is free?

 
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lsayre
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Post by lsayre » Sat. Jan. 02, 2016 6:38 am

When I put my own homes rooms, room dimensions, and regional historical annual HDD's into my spreadsheet it delivers recommended baseboard lengths for each room that are amazingly nigh on identical with the ones I actually measure in our home (as it should, since I used our home as my guide in setting up the spreadsheet, and programs can only output what the programmer expects them to output).

The bottom line is that the baseboards found in my house were calculated without any compensation for the water temperature dropping as it travels through each zone. This shows that the spreadsheet as is works just fine, and mimics the logic used by the builders of our house back in 1964 when it was built, even though with some thoughtful tweaking (compensation for temperature drop) it could clearly be made better from the comfort perspective.

For the case of our home the boiler BTUH output figure as calculated by the spreadsheet is about 46.5% more than I have found is actually required to heat our home, so if one assumes that coal boilers require an over-sizing "pick-up" factor of on average about 30% (forum member Pacowy being the best source to elaborate on the logic behind this requirement), and that DHW typically requires an additional 17% of over-sizing for a family of 4-5, the spreadsheet seems to have all of this pretty closely covered with regard to the coal boiler BTUH output.

 
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Post by lsayre » Sun. Jan. 03, 2016 7:47 pm

I'm slowly working toward a means by which to incorporate into my spreadsheet the fact that the water is cooling and losing BTU's as it flows through a loops fin-tube baseboards. I have added a few things to my spreadsheet to assist here, The new stuff is the stuff with the magenta background.

First the spreadsheet computes the individual BTUH requirements for each room.
Then for each zone you must manually (and off spreadsheet) outline which room (which set(s) of baseboards) will receive the flow of boiler water first, second, third, etc... for said zone,
Then you match the sum total of BTU's by foot on the scale on the extreme right of the spreadsheet to the spreadsheet computed BTU requirements for room 1, by adding up the BTU's by feet of baseboard until it reaches reasonably close agreement with the BTUH calculation for the room. If you highlight and drag a range of BTUH's by feet and look at the bottom bar of your spreadsheet you will find that it has summed up the highlighted range for you, making this quite easy to do. You will find that it is less than the "initially suggested" feet of baseboards for room 1 on zone 1.
Then you start room 2 on zone 1 where room one left off on the right hand scale, and when you have added up enough feet of BTU's to meet its calculated BTU requirement, that is the number of feet of baseboard room 2 requires. It will be closer to the ideal calculated feet than room 1 was.
Then you move on to room three, etc... if/as required to complete the zones feet of baseboard calculations.

Do this for each zone to complete the task.
Hot_Water_Baseboards.xls
.XLS | 22.5KB | Hot_Water_Baseboards.xls
The revised spreadsheet is attached here. It also has slightly downwardly revised the boiler BTUH output requirements. This also has been done (and scaled) to account for the BTU's that are declining as the water flows through the baseboards.

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