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Reverse Engineer Manual J

I did a rough manual J on each room of my house and got how many BTU/H I need to keep warm at min design temp and indoor temp of 68 degrees.

Then I gave my wife my phone, since hers broke. That's where the manual J app was. Now all that's gone.

I'd like to see how many BTUs I need to keep the house at 65 (due to an undersized radiator; I'd just like to get a sense of what that room will be like when others are at 68) and also get BTU/H at max design temp.

I figure I can reverse engineer this easily, but maybe not...


Comments

  • GroundUp
    GroundUp Member Posts: 1,865
    These are pretty linear; 3 degrees is likely within a couple dozen BTU of what it was before. Or you could simply do another heat loss calc
    mattmia2
  • mattmia2
    mattmia2 Member Posts: 9,410
    edited December 2023
    Do you now you design temp? Your delta t is probably changing from say 60 to 57 so the difference is about 5%.
  • Jamie Hall
    Jamie Hall Member Posts: 22,910
    Heat loss figures from Manual J or other methods are moderately close to linear over a reasonable range, so you should be OK if you remember your original interior and exterior temperature inputs, compute the BTUh required per degree delta T and then figure your new BTUh on the new delta T. The only time this really doesn't work well is if you had a high infiltration allowance.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 78
    @mattmia2 yeah, design temp is 14 degrees. @GroundUp , there's tenants in the 3rd floor and I froget dimensions, so it would just be spit-balling, really.

    Thanks.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 78
    @Jamie Hall that's helpful, I can do that. It is a 1895 house with some original windows, though (thankfully, not in the 2 rooms that seem to be short on radiator BTU/h) output, so pretty sure infiltration was pretty high.

    I did the manual J with the Shortfin app, which I've read elsewhere is as much as 25% over actual heat loss needs (as calculated vs actual fuel use). Which would explain why it seems I need 95 degree water to mantain a 68 degree interior temp when exterior temps are ... 68 degrees.
  • mattmia2
    mattmia2 Member Posts: 9,410
    Did the app have the type of emitters in it? The output of a standing CI radiator at 95 will still be significant. The output of fintube at 95 f will be close to 0.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 78
    edited December 2023
    oh, man, i don’t remember. this was last year that i did it. app is no longer on ios store; don’t have an android device to see (and it might not be there anymore, either) @mattmia2

    thinking about it, i’m pretty sure i just got the btu/h loss of each room. then i’ve got a spreadsheet that i made with all my CI rads in it; that calculates each rad’s BTU/h at different temps and compares it to the heat loss. at design temp (14 degrees), it that says i need to run at approx 172.

    If slantfin estimates 25% more btu/h loss than actual (as i’ve read elsewhere), that number could be as low as 146. which would be pretty awesome.

    i’ve been using a function i made on my phone to manually set the set point of my boiler based on the weeks low. I’ve been setting it at approx 160 for 30 degree nights and that seems to work pretty well (although this house is very drafty, and the t-stat is centrally located in a hallway, so other rooms get cold due to draft first)

    (other home improvements took priority over an ODR last year; this weal/winter was installing kitchen radiator that previous owner took out and setting up and ODR. i’ve run into some snags on that, but figured it out, and will be installing a ZVC next week. Maybe I’ll get some interior “storm windows” done this winter, too, but that’s looking like next winter).

    worst case scenario, i can just keep an eye on my boilers runtime and keep dropping the curve it until boiler is almost constantly running.
  • jesmed1
    jesmed1 Member Posts: 554
    edited December 2023


    thinking about it, i’m pretty sure i just got the btu/h loss of each room. then i’ve got a spreadsheet that i made with all my CI rads in it; that calculates each rad’s BTU/h at different temps and compares it to the heat loss. at design temp (14 degrees), it that says i need to run at approx 172.

    That's an unsually high water temperature for an 1895 house with cast iron radiators. Typically these old houses are very over-radiated (I live in one of them). With all the original radiators, you should be able to heat with supply temps 140 or less. In this old house, we could heat it with supply temps of 100-120 if I could make our boilers run that cold.

    Did someone remove half (or more) of your original radiators? What is the square footage of the house, and what is the radiator EDR sq ft? Do you have a rough idea what your total design heat loss is?

    And have you had a blower door test done? It sounds like you have a massive amount of air infiltration.

  • PC7060
    PC7060 Member Posts: 1,135
    edited December 2023
    @jesmed1 - over radiation of homes started after the 1918 pandemic in response to the fresh air movement which had people sleeping with windows open. This impacted heating system design such a as moving the radiators to the walls near the windows and adding more radiators so the house would livable even with all the windows cracked.

    For example, my neighbors 1910 home has 6 radiators total all located off central hallway. My 1928 house had 12 radiators all on outside walls. 
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 78
    edited December 2023
    @jesmed1 @PC7060

    hmm. that's all interesting. Total EDR is approx 660, 2700 sq feet.

    No blower test, and I'm hesitant to get one. A buddy got one done and afterwards the test administrator noticed asbestos and said they wouldn't recommend doing it again, as they don't know where all the asbestos might be.

    I know I've got a couple of planks hanging over my boiler and some asbestos shingles on my bays.

    Assuming the Slanfin manual J app is 25% over does get me to 146 degrees. I still have many original (not refurbished) wood windows, although it seems I have plenty of EDR in those rooms. Attic has been insulated. It's the hallway and the third floor front (w/ modern windows) that are slightly low on EDR (relative to the rest of the house).

    Hearing that temps of 110 would work for you makes me hopeful that a heat pump may work well in here. Hopefully this boiler lasts another decade and the tech continues to improve.
  • jesmed1
    jesmed1 Member Posts: 554
    edited December 2023

    @jesmed1 @PC7060

    hmm. that's all interesting. Total EDR is approx 660, 2700 sq feet.

    You're still over-radiated for a modernized old house. Some numbers for comparison. Our circa 1920 4-unit condo building in Boston was built originally as a 4-apartment house by an elderly man who wanted his adult children to live with him in their own separate apartments. We have EDR=960 and 4800 sq ft. That's an EDR/sq ft ratio of 0.2. Our total design heat loss is 96,000 BTU/hr at zero degrees outside design temp. That's 20 BTU/hr/sq ft.

    Your EDR/sq ft=660/2700=0.24. Which is even more EDR than we have, in this massively over-radiated house. So if your heat loss was the same as ours (20 BTU/sq ft), you could heat with even lower water temp than we do. Because we have large boilers, way too big for our design heat loss, they run about 45 minutes on, then 3-6 hours off depending on outdoor temperature. Since they're cold start, the water temp starts at 65 end ends up at 130-140 only at the very end of the cycle. So the average water temp during a heating cycle is like 100 degrees. For just 45 minutes out of 3-6 hours.

    That's how low your radiator water temperature could be with your EDR/sq ft=0.24. But only if your heat loss is similar to ours, ie 20 BTU/hr/sq ft. Even if your heat loss is double that, say 40 BTU/sq ft, you could still heat with water temps that low, but your boiler would just run twice as long, say 90 minutes every 3-6 hours.

    Here's where I think the disconnect is. When I calculated our total radiator EDR and then calculated our total design heat loss for the building, the radiator table I was using said that we'd need 140 degree water temps in our radiators, to give us about 90 BTU/hr/sq ft to heat the building. But in practice, even on a zero-degree design day, the boilers run only half the time, and the average water temp is maybe only 120.

    I haven't taken the time to figure out why the predicted water temp using the radiator tables was wrong. Maybe the radiator table I used is wrong, or maybe I miscalculated our EDR. I'm reasonably certain I used the right radiator dimensions and calculated the EDR correctly. But for whatever reason, using that radiator table vastly over-predicted our required water temperature.

    So I'm wondering if you have the same over-prediction problem, because by our EDR/sq ft comparison, you have more EDR/sq ft than we do, so you should be able to run similar water temps unless your heat loss/sq ft is massively more than ours.

    BTW, this is the radiator table I used which over-predicts our water temp:

    https://www.expressradiant.ca/pdfs/product_classic_sizing_how_to.pdf

    Your real bottom line is how many BTU's you burn over a heating season, or over one coldest month in the season. Keep track of your oil or gas usage, then calculate your actual design heating load using this method:

    https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

    That method is very accurate. Then once you know your actual heat loss number and your radiator EDR, you can estimate your required water temperature. But as I said, the radiator table above seems to over-predict our water temperature at 140, where in reality we're averaging well under 120.

    Also, I recommend this article by Ron Beck. Read the section headed "Cast iron radiation," where he says that, contrary to popular belief, cast iron radiators are low-temperature systems in modernized old houses:

    https://www.usboiler.net/outdoor-reset-doesnt-work.html



    PC7060