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methods of heat loss calculation

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Member Posts: 54
edited April 2023
I'm getting some different numbers from two different heat loss calculators. Manual J says 79k and Slant Fin's says 120k. I realize Manual J is a very common standard and so I'm more likely to go with 79k, but I'm also questioning its methods for determining room by room heat loss.

With Manual J, it apparently calculates the heat loss for one whole story of a building, and then uses square footage to determine the heat loss for each room. One thing that seemed to make more sense about Slant Fin's calculator is that it takes exterior wall length and window square footage into account when calculating the heat loss of an individual room.

We have two bedrooms of equal square footage, but one has double the exterior wall length (on a corner) and 3 times the window square footage. According to Manual J, these would have the same heat loss. We are using low mass radiators with TRVs in each room, so I want to make sure I'm sizing these properly. It makes sense to me that the corner bedroom would have a larger radiator.

• Member Posts: 2,827
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Well yes , The square footage of exterior of walls ,floors and ceiling exposed to non condition space will increase heat loss in the room .. Also tank into account the wind , The North-West corner will be cooler ... I would used your J for sizing the boiler .. Its better to oversize then undersize if your using TRVs

What are you installing Baseboard (low mass )with TRV's ? Baseboard you need to take in account some of it may be blocked by the furniture . I would try to steer you to panel radiators under the windows if you wanted my advice ..

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• Member Posts: 3,547
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You can try this
http://mechanical-hub.com/sites/hydronics/heat-loss-calculation-on-every-residential-boiler-replacement/

Those two numbers are very far apart as well as both pretty high, at least with a design temp of 0F or thereabouts and reasonably sized house. I'll use the BTU/sqft as a sanity check. If you had 2000 sq/ft for example you would be at 40-60 BTU/sqft and neither of those numbers make sense unless it's a real old house. You'd want to recheck your inputs.
• Member Posts: 15,833
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I respectfully suggest something is wrong with your calculations.

They should be a lot closer than that. I would re check. The two bedrooms mentioned must have different heat loss, no way they are the same
• Member Posts: 23,635
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You can try this
http://mechanical-hub.com/sites/hydronics/heat-loss-calculation-on-every-residential-boiler-replacement/

Those two numbers are very far apart as well as both pretty high, at least with a design temp of 0F or thereabouts and reasonably sized house. I'll use the BTU/sqft as a sanity check. If you had 2000 sq/ft for example you would be at 40-60 BTU/sqft and neither of those numbers make sense unless it's a real old house. You'd want to recheck your inputs.

Just to add a little perspective: the main place I care for is about 7,000 square feet, varying good to non-existent insulation, built between 1780 and 1893. The heat loss on the design day (-10 F) is, from actual fuel usage measurement, 25 BTUh/square foot.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,160
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I'm using slant fin right now to figure out heat loss as I'm planning a new 1-pipe steam installation in my home, so I need room by room loss to size the rads. I plan on using cast iron rads that I just slavaged out of a home that's same sq ft to my own.

With slant fin I noticed that a slight changes in any of the inputs will change btu loss per room, so just make sure you are plugging in all the correct data.

Slant fin gave me rating of just under 109kbtu at 1 degree design day, compared to my current forced air furnace output rating of 112kbtu... That's pretty close!

Mine is a 106 year old brick house, 2,400 sq ft, newer windows, no insulation other than under roof on 3rd level finished attric, a shot-gun style house, and I have 3 walls exposed to the outside in 2 of 3 rooms on each floor as well as 2-3 36x48 windows in each room. 1st and 2nd floors are same footprint, but being over unheated basement, 1st floor BTU load is about 25% higher than 2nd floor.

Slant fin, to me, makes more sense.
• Member Posts: 54
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This is for a 4000 square foot, new 2x6 construction, reasonably tight house. I am using panel radiators under the windows. It is a 4 level house including the basement, and is more or less a cube which is good for retaining heat.

These calculations are for -10 design day. The areas microclimate occasionally will see -25 in the depth of winter.

I'll likely go with the manual J total heat loss calcualtion to size the boiler. And then look at the room to room ratios from Slant Fin and size them down so that the ratios stay the same but they add up to the Manual J total. Sound like a good idea?

• Member Posts: 1,160
edited June 2017
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As was mentioned above, 2 numbers should be relatively close to one another. What you are planning sounds like the best of both worlds.
• Member Posts: 1,160
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Fyi - free online manual j tool
https://www.coolcalc.com/products/manual-j
• Member Posts: 3,547
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This is for a 4000 square foot, new 2x6 construction, reasonably tight house. I am using panel radiators under the windows. It is a 4 level house including the basement, and is more or less a cube which is good for retaining heat.

These calculations are for -10 design day. The areas microclimate occasionally will see -25 in the depth of winter.

I'll likely go with the manual J total heat loss calcualtion to size the boiler. And then look at the room to room ratios from Slant Fin and size them down so that the ratios stay the same but they add up to the Manual J total. Sound like a good idea?

What type of boiler? Over radiating will allow lower boiler temp.
• Member Posts: 22,472
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Every calculation is based on some guesstimating, like wind speed, infiltration rates. Other than cost, no harm in oversizing the panel rads by a couple inches.

I might design them for covering the -10 load with 120 supply temperature. This give you a lot of flexibility with heat sources, wiggle room for below design conditions, and the most efficient operating condition for mod cons or heat pumps.

Size tends to be about 40% larger on the panel rads. Cost increases also.

Good choice on using panel rads, more jobs should utilize them. TRVs are a nice addition, since I’m spending your money😁
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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EDSTech said:

It's been 6 years...

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Looks like ChatGPT !
• Member Posts: 6,505
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Looks like ChatGPT !

You're probably right.

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• Member, Moderator, Administrator Posts: 2,345
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It was spam. Thanks!

President
HeatingHelp.com

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