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Heat Loss Calculations

Hi again everyone,
So I used this heat loss calculator, https://www.builditsolar.com/References/Calculators/HeatLossOld/HeatLoss.htm

and this R value calculator for the walls
https://ekotrope.com/r-value-calculator/


I have balloon framed house with an EIFS exterior (which includes a layer of styrofoam) and mineral wool on the inside. To be conservative, I gave the assembly an R-value of 7.

To give an example, I have a bedroom with two exterior walls, 1 window, 1 sliding door, and the dimensions are about 14 x 16. It is on the second floor of a three-story house. I come up with 209 sqft of wall, 11 sqft of window, 32 sqft of door, and the calculator says the heat loss is 4460 BTU/hr. This seems to be plenty, as the room currently has a 4 foot section of fin tube, which I estimate to put out 4000 BTU, and the room heats just fine. So, that just confirms my R-value is on the conservative side.

1. Does that sound right to you? I read somewhere that a rule of thumb is the square footage x 40, which gives me 8960. Is it possible I am off by a factor of two, or is it just the difference between rule of thumb and actually calculating it out?

2. If I go through the house and replace some radiators with smaller ones based on the heat loss calculation, will that mess up the balancing for the old radiators that stay? Will I need to replace the air vents with different sizes to keep everything balanced? (I have one-pipe steam, BTW) My gut says no, but I wanted to check.

Thanks in advance, you guys are awesome.

~Jackie

Comments

  • nicholas bonham-carternicholas bonham-carter Posts: 7,763Member
    You can compare the heatloss from the SlantFin app, and see how close they are.
    If you change the radiators for smaller ones, that will make your present boiler oversized, and less efficient. There could be some practical problems in replacing radiators, such as needing new inlet valves.—NBC
  • hot_rodhot_rod Posts: 11,124Member

    Hi again everyone,
    So I used this heat loss calculator, https://www.builditsolar.com/References/Calculators/HeatLossOld/HeatLoss.htm

    and this R value calculator for the walls
    https://ekotrope.com/r-value-calculator/


    I have balloon framed house with an EIFS exterior (which includes a layer of styrofoam) and mineral wool on the inside. To be conservative, I gave the assembly an R-value of 7.

    To give an example, I have a bedroom with two exterior walls, 1 window, 1 sliding door, and the dimensions are about 14 x 16. It is on the second floor of a three-story house. I come up with 209 sqft of wall, 11 sqft of window, 32 sqft of door, and the calculator says the heat loss is 4460 BTU/hr. This seems to be plenty, as the room currently has a 4 foot section of fin tube, which I estimate to put out 4000 BTU, and the room heats just fine. So, that just confirms my R-value is on the conservative side.

    1. Does that sound right to you? I read somewhere that a rule of thumb is the square footage x 40, which gives me 8960. Is it possible I am off by a factor of two, or is it just the difference between rule of thumb and actually calculating it out?

    2. If I go through the house and replace some radiators with smaller ones based on the heat loss calculation, will that mess up the balancing for the old radiators that stay? Will I need to replace the air vents with different sizes to keep everything balanced? (I have one-pipe steam, BTW) My gut says no, but I wanted to check.

    Thanks in advance, you guys are awesome.

    ~Jackie

    The load calc is an educated guess really, based on some assumptions as far as building construction and most importantly infiltration, how much leakage around the windows and structure. Assumptions on how the home or rooms are used, doors opening and closing, internal gains form lighting and appliances, etc. Most believe there is some fudge factor built into the programs also, maybe 10%?

    The best indication of the actual load may be the current heat emitter. If these maintain all the rooms at the temperature you desire, stick with them or emitters of the same output.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • delta Tdelta T Posts: 724Member
    "I read somewhere that a rule of thumb is the square footage x 40"

    This is an old school rule of thumb, and you know what.....you will NEVER under size an emitter or boiler if you use it ;)

    Your 4k sounds pretty reasonable to me.
  • foresthillsjdforesthillsjd Posts: 23Member
    @nicholas bonham-carter
    Slantfin gave me 5120 BTU/hr, which is about 10% over my calculation. Good to know I am in the ballpark. Why would one need new inlet valves? Is it because they are probably so old that if I touch it, it will probably need to be replaced?

    Also, if my boiler ends up being oversized and less efficient, does that mean it will cost me more to heat, or just that the thing will by cycling on and off more than ideal?
  • nicholas bonham-carternicholas bonham-carter Posts: 7,763Member
    The connection between the inlet valve, and the radiator consists of a matched set of spud with union faces, and valve. The pieces are particular to each other, and will not seal in a mix"match situation. Removing the spud from the radiator is very difficult, so it is best to keep the radiators in place.
    One property of steam is to only condense what is needed in the radiator, even though on a mild day, it may only fill halfway.
    any overheating rooms can be adjusted using the venting-to some degree.--NBC
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,219Member
    Op stated copper fin tube.

    The more fin tube you install the lower water temperature you can run =more fuel saved. Less fin tube=higher water temp=more fuel burner.......within reason.

    If you replace the existing I would keep it the same size if your heating ok. I would not install less. keep the same amount in each room .... so you don't upset the balance. Or do a complete new heat loss and install fin tube accordingly
  • kenjohnsonkenjohnson Posts: 23Member
    If you really wanted to get exact with the heat loss calculation, you could do it in Excel and factor in a different R-value for the windows, walls, ceiling (if it has an attic above it) and floor (if cold basement below it).

    At least that is what I did to get exact estimates because every off-the-shelf program gave me answers that seemed 1.5 to 2x too high (they were too high, it turns out).

    R-7 for your wall sounds a little low...but a conservative value will ensure that you have enough heat.

    But, the answer you are getting may be fine for what you are trying to do. If you are trying to perfect-size new radiators for each room to solve some issue with localized room overheating, or something like that, I would try to get more precise on the calculations.
  • kenjohnsonkenjohnson Posts: 23Member
    Just saw your other post...it's easy to setup in Excel if you are proficient in Excel.

    Heat Loss = U * Area * Delta T

    U = 1/R-value (the inverse of R-value). Units are BTUs per Hour per (Square Foot * degree F)
    Area = in units of square feet
    Delta T = Temperature differential inside vs. outside (in degrees Fahrenheit).

    When you multiply them all together, Area units and Temperature units cancel and you are left with BTUs/hour.

    Be sure you treat the walls, windows, floors, and ceiling separately. If you don't know exactly how good the insulation is inside the wall cavity, be a little conservative. Also, there are studs and headers in the wall that are not as insulative as the mineral fiber, so reduce by 20% to your perfect wall R-value calculation to account for framing wood thermal bridging.
  • JUGHNEJUGHNE Posts: 5,439Member
    Is this 1 pipe steam or hot water?
  • VoyagerVoyager Posts: 196Member

    Just saw your other post...it's easy to setup in Excel if you are proficient in Excel.

    Heat Loss = U * Area * Delta T

    U = 1/R-value (the inverse of R-value). Units are BTUs per Hour per (Square Foot * degree F)
    Area = in units of square feet
    Delta T = Temperature differential inside vs. outside (in degrees Fahrenheit).

    When you multiply them all together, Area units and Temperature units cancel and you are left with BTUs/hour.

    Be sure you treat the walls, windows, floors, and ceiling separately. If you don't know exactly how good the insulation is inside the wall cavity, be a little conservative. Also, there are studs and headers in the wall that are not as insulative as the mineral fiber, so reduce by 20% to your perfect wall R-value calculation to account for framing wood thermal bridging.

    I used Excel also after getting rather different results in a few of the online tools such as the Slant/Fin. The conductive heat loss was the relatively easy part. The harder part was making a good estimate of the loss due to infiltration and that has a significant affect on the total heat loss.
  • kenjohnsonkenjohnson Posts: 23Member
    Voyager said:

    The harder part was making a good estimate of the loss due to infiltration and that has a significant affect on the total heat loss.

    True enough in an older home. My renovated house has a 2.8 ACH50 before drywall, so I can basically ignore infiltration losses. If the home is not tight, adding 20% to the heat loss calculations would be a reasonable assumption.
  • foresthillsjdforesthillsjd Posts: 23Member
    @JUGHNE, I should have clarified that it's 1-pipe steam. I have been thinking about this system for so long that I forget other types exist!

    @kenjohnson I play Excel like the piano (queen of the index-match and pivottable), so doing my own heat loss will be way easier than using these online calculators. Thanks for the tip!
  • foresthillsjdforesthillsjd Posts: 23Member
    @kenjohnson what is the formula for infiltration losses?
  • SuperJSuperJ Posts: 464Member
    A word of caution to the excel people, the math is not hard, but the secret sauce in is in some of the assumptions (about leakage, construction quality, thermal bridging, etc) in the various models. That's why you see a 10-50% variation even when you specify the same size and construction details in the different calculators.

    loadcalc.net is a good one, it's not pretty but seems to be more accurate in the real world than the slantfin one, IMHO.
  • VoyagerVoyager Posts: 196Member
    SuperJ said:

    A word of caution to the excel people, the math is not hard, but the secret sauce in is in some of the assumptions (about leakage, construction quality, thermal bridging, etc) in the various models. That's why you see a 10-50% variation even when you specify the same size and construction details in the different calculators.

    loadcalc.net is a good one, it's not pretty but seems to be more accurate in the real world than the slantfin one, IMHO.

    I agree. The reason I ended up with Excel is because I wanted to see exactly what affect varying the assumptions would make. With the online tools, none told me exactly what they were doing or what assumptions they were making behind the scenes. With Excel, I put in the equations so I knew exactly what the variables were and I could do sensitivity analysis. The hardest was the edge loss through my slab as it is neither insulated nor open to ambient. It has a 1/2” asphalt expansion joint and then abuts an 8” hollow CMU. So, I guessed at a low R value and went from there. Infiltration loss was the second hardest assumption to make. I didn’t want to bother with a full blown measurement as I have only a workshop, not a McMansion, so I took a reasonable guess.

    So far, things look pretty good. I designed for a 60 degree delta T and I am seeing almost exactly that. We had a few days where the temps were in single digits during the day and -15 at night with 30 MPH winds. The shop got down to a low of 52 with the boiler running flat-out. I am pretty happy with that as it will maintain my desired indoor temp of 60 in all but the coldest and windiest conditions and the gas usage is so low I almost can’t tell it is there compared to what my home has used historically. The added gas usage of the workshop is literally almost not noticeable. So, I am a happy camper and I just hope that the little TT boiler lives a long life since I decided to spend the extra $1400 or so above what a water heater would have cost.
  • hot_rodhot_rod Posts: 11,124Member
    WAG as far as the infiltration number. You could have a blower door test run, but again it would be just for that one point in time.

    Good, better best is usually enough to get you in the ballpark for infiltration number. Try it several ways to see how much the load changes.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Jamie HallJamie Hall Posts: 9,997Member
    There is no usable formula for infiltration. As @hot_rod said, WAG is the way to go. To get any closer you'd need a terrific amount of computing power to model the aerodynamics of the building, and much more information about its construction than is ever available.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • SuperJSuperJ Posts: 464Member
    True, one bad defect in a building envelope can skew things mightily. And they aren't always obvious.
  • BrewbeerBrewbeer Posts: 565Member
    If you have good records, you can use gas consumption compared to HDD to get a check on the program or spreadsheet methods.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • JohnNYJohnNY Posts: 2,185Member
    edited February 22
    Your screen name being ForestHillsjd, I'm going to assume you're in Queens, NY. That "square footage x 40" RoT is for a part of the world colder than ours. I'm in Manhattan. Here in NYC we tend to use sq. ft x 30, or cubic feet x 3.5 (or "x 4" if we're feeling like there's excessive infiltration or low R-value to deal with). But be careful. All the rules-of-thumb tend to be geared toward standalone buildings. Not row houses. I've done actual heat loss calculations for attached Brooklyn and Manhattan townhouses and brownstones that have a heat loss of not more than 17-19 Btus per square foot. That means big (by our standards) 4-story $6,000,000 homes being heated very well with 150,000 BTU boilers.
    For installations, troubleshooting, and private consulting services, find John "JohnNY" Cataneo here at :
    "72°F Mechanical, LLC"
    Or email John at [email protected]
    John is a professional Master Plumber, licensed by The Department of Buildings of The City of New York, and works extensively in NYC while consulting for clients in and out of state.
  • SuperJSuperJ Posts: 464Member
    Brewbeer said:

    If you have good records, you can use gas consumption compared to HDD to get a check on the program or spreadsheet methods.

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    This works pretty well. Mine was pretty accurate once I accounted for the baseload consumption of NG (from my dryer, stove, and DHW).
  • JUGHNEJUGHNE Posts: 5,439Member
    So you want to keep the existing boiler but downsize some radiation?
    Just to clarify, is there fin tube in the future picture?
  • kenjohnsonkenjohnson Posts: 23Member
    If you know your ACH50, you could get a rough estimate of natural ventilation, see https://www.energystar.gov/ia/home_improvement/home_sealing/ES_HS_Spec_v1_0b.pdf

    I made my house renovation super-tight, and the blower door test came in at 2.8 ACH50. I basically assumed no infiltration loss, but I also assumed no gain from background heat sources (refrigerators, lighting, people, etc.). If you read the link, my calculated natural ventilation would be about 0.15 air-change per hour - pretty low (and based on how my heating system is operating, it might be less than that). But this will change based on how windy it is outside.

    If your house was really leaky (maybe 15 ACH50), then you might have 1 air change per hour. If your house was that leaky, you'd probably feel a lot of uncomfortable drafts. More likely, you are probably about 0.3 or 0.4. If you thought of it as 0.2, that would probably account for ignoring the ambient heat generation. That's why I said just assume 20%.

    When you do your Excel calculations, make sure to separately calculate rim joist areas, treat the trim around windows as perhaps the same R-value as the windows (if there are leaky, uninsulated pockets), account for wall framing, etc. It was easy for me - I had open walls and could see what I had. Then, I modeled the walls with the exact wall framing factor. It was a lot of calcs, but I'm an engineer and I wanted to model it right. If you know Excel and are patient, it's not hard to get this right. And as another poster pointed out, you can change your assumptions and see the result, whereas the canned programs are just black boxes.
  • foresthillsjdforesthillsjd Posts: 23Member
    @JohnNY, yup, you're right, I'm in Forest Hills Queens (but not the fancy part). It's a freestanding house. The previous owners put EIFS on the exterior, so it was pretty tight already, and now that all the walls are open, we've also stuffed mineral wool between all the studs (which are, unfortunately, only 2 x 4) My conservative calculations are averaging out to about 20 BTU/sf when I give the exterior wall an R-value of 10.

    @JUGHNE, yes, I am keeping my boiler, but I'm replacing almost all the radiators. Assuming I get my heat loss calculations correctly, will there be any downside to reducing the size of my radiators? Will I notice a difference in comfort? Once I downsize the radiators, I figure that when the boiler dies (it's about 10 years old already), I can replace with a smaller boiler.

    @kenjohnson Yeah, the EIFS (which, albeit ugly and not my first choice for cladding a house) makes the house pretty tight. I don't feel any drafts in the winter, so I agree that 20% is the right number for infiltration.
  • SuperJSuperJ Posts: 464Member
    You may be able to shuffle around many of your existing radiators to rooms that are a closer match to their size.
  • ethicalpaulethicalpaul Posts: 555Member
    I love that you're willing to replace/move your radiators in order to match your actual heat loss.
    1 pipe Utica 112 in Cedar Grove, NJ, 1913 coal > oil > NG
  • Steve MinnichSteve Minnich Posts: 2,248Member
    @hot_rod I always use poor, below average, and not so bad. B)
    Steve Minnich
    "The wages of carelessness is failure."
  • foresthillsjdforesthillsjd Posts: 23Member
    @ethicalpaul I like that you put a sight glass on your riser! I'm glad there is place like this for folks to congregate. My journey started about four years ago when I moved into an old house with one-pipe steam and the realtor said I had to put a bucket under this tap and drain a little water out every week. I grew up on forced hot air, so I thought, "Uh oh, I better learn about this steam stuff," so I bought and read all of Dan's books, and started tweaking the things in my house. The only problem is that now when I walk into a steam building with poor balance or pitch or malfunctioning vents, my eye starts twitching. ;)
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