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How To Figure Heat Loss Anomalies

D107 Member Posts: 1,798
edited November 2017 in Gas Heating
1800 sq ft well insulated house, a little north of NYC, 40kbtu heat loss, three zones: basement baseboard, main and 2nd floor large cast iron rads.
1. How would one factor in that fact that main and second floors are each piped in series. 6 and 4 rads respectively? My guess is that a)this is not worth re-piping into direct return, monoflo etc, and that b) any delay of heat or imbalance has been unnoticeable in such a small house.
2. Our side porch is very poorly insulated, with three external walls, two of which are brick and has two inadequate Governale radiators. It is always 15 degrees colder than the rest of the house. We leave it that way. In a heat loss some years ago this room factored in at 10K of the 40K heat loss. So when I do a new real-time heat loss based on btus used and degree days, I'll have to add in btus to know what the usage would be if I heated that room--maybe with some insulation put in--to the same temperature as the rest of the house, which in our case is 65degrees. But I will probably factor in a new inside set point of 75º as a worst case scenario.
3. We just had our attic rafters closed-cell spray foamed--now unvented. R36. With my original heat loss, I factored in a cold ceiling of .110 given that the attic floor had some fiberglass insulation. So now do I no longer call that a cold ceiling given the foam above it? Or still factor in a less cold ceiling since some house heat is still going to heat the attic's conditioned space to about 3 degrees colder than the rest of the house. (Most of the very old fiberglass is still in the attic floor but with plenty of upward leakage.)
4. On the internet I ran into Robert C. Obrien's 'real time' heat loss method http://mechanical-hub.com/sites/hydronics/heat-loss-calculation-on-every-residential-boiler-replacement/ which bases the loss on actual usage per heating degree day over a certain time period. I think others have promoted this as well. I love the idea since it cuts through so much guesswork. So I multiplied my 777 therms used for heating last year X 100kbtus per therm = 77,700,000btus X .75AFUE old boiler = 58,275,000btus/4995 degree days = 11,667btus per DD/24 hours = 486.111 btus per Heating Degree Hour/1800 sq ft X 65 (differential between set point of 65º and 0º design temp) = 31,600 btu/hr. And this is conservative since design temp here is really more like around 13ºF. On the other hand I'd probably want to raise the set point to 75º and account for that cold porch. So my question is: Does this method really tell you if you'll have enough heat on a design day? In looking for a boiler replacement in two years, contractors are already ballparking me in emails for a 155Kbtu boiler which seems nuts. Our boiler is under-fired at 170kbtu and with all our new insulation I'm worried we'll be short-cycling and condensing too much. Our heating savings are around 33% since the foam insulation. No one can believe a house actually has a 30K heat loss. Which leads me to my last question:
5. With any new boiler we would get an indirect hwh. Our potable water btu requirements are about equal to the heat loss, though I'd probably want to get a boiler that netted at least 40-50kbtu to cover the series piping, higher indoor temps, cold porch, etc. (Our current 10 yr. old stand alone 40gallon gas hwh nominally fires at 40kbtu, but I measured the gas meter dials and found it closer to 30K.) Our hot water is more than enough for my wife and I. So seems to me net 50kbtu boilerworks, yes? Which would be about 74Kbtu firing rate.


  • D107
    D107 Member Posts: 1,798
    Oops--my calc is based on pre-foam insulation usage. So if the 33% useage decrease holds or the whole winter then my loss will be even less = smaller boiler.
  • mikeg2015
    mikeg2015 Member Posts: 1,187
    1) The upstairs radiators could be oversized to factor in the lower water temp or, downstairs are undersized a little.... or both. Really, not a bad way to save some piping and even increase delta T. Depending on flows, Downstairs could have 180F in, 165 out, upstairs 165 in, 155 out.

    Do you mean each vertical riser is a series, or all downstairs are one series zone and upstairs a 2nd series zone? In that case, the only solution is to "overpump", over/under size radiators according to the series temperature. Can work either way.

    That enclosed porch is likely an add-on. Very common. They are always messed up. Best you can do depending n the piping, is add some baseboard radiators in series or a wall convector or floor convector. There are some nice units out there. They work well with lower water temps so would work in series with the radiators. Other option is radiant floor heat. Staple under the floor if you have access or add a subfloor above, and insulate well accordingly.

    Given the size of your house and insulation being good, 31k wit ha 13F design temp is probably pretty close. I was heating a 2200sqft newer construction with 3/4 finished basement with 25k BTU at 5F and windy. Was still cycling off low stage. I installed a 40k 96% 2 stage furnace and the contractor thought I was nutz replacing the original 100k 80% furnace.

    First make sure you are factoring in the temperature you set it at vs. the 65F used for heating degree days. That assume a 68F setpoint, and 3F margin for internal heat gain (cooking, light, showers). I'd add another 10% if you have a lot of natural sunlight. It will skew things a bit on a daily average. Smaller homes also loose heat faster.

    But if you use the IBR rating that includes jacket loss and pickup factor, I think you have plenty of margin. 155k is crazy.

    However, all that being said, you still should go back and look at the EDR of your radiators and see what they add up to. Then take a EDR calculation table and figure out what water temp you'd need at design. You might want a modcon possibly if design says <150F is adequate, you'll be condensing all winter.. Then you can oversize very slightly with less issue. Could even consider a combi.

    Smallest cast iron boiler they make is about a 50-60k input. DOE around 45, IBR around 38.

    Other data points for myself. I spray foamed and new Low-E storm windows (42 of them!) in a 1925 3200sqft stucco home, no wall insulation. High ceilings, full basement, partially heated. 60k furnnace downstairs (never modulated over 45k after recovery, upstairs I estimate used only 20-25k (cycled the 60k furnace on low stage). This is at 0F! I replaced 170k BTU of furnace capacity.

    Current home is 3800sqft 1905 victorian, higher deilings, full basement and attic, drafty, but I still calculate only a 95k heat loss at 0F based on my heat pump run time. Downstars heat pump output is 21k BTU and it' hold temp down to 40F! My downstairs is larger than your whole house and about 50k will hold it at 0F. Sorry, not bragging, it will still burn $400+ in gas this Jan. and I need new storm windows and some spray foam to seal some leaks on the perimeter of attic and basement.

    You are not crazy. Sorry for long post. I love building science stuff.
    EBEBRATT-Ed Member Posts: 11,985
    to me your heat loss sounds low but trust your #s if they are good. I have not used the "real time" heat loss

    The attic with spray foam depends on where the attic is vented. If vented below the spray foam it doesn't count as insulation but will keep the roof cold and prevent ice dams.

    As far as the piping on a series loop if done correctly the radiation installed increases slightly in size along the loop as the water temperature decreases.

    I would stay with a design temperature indoors of 70 deg. Your going to sell the house someday. Don't increase to 75 to compensate for the porch. Design the porch heat for the porch and it would work best as a separate zone.

    Boiler sizing and the indirect sounds ok if the HL is correct. Try a couple of different methods and come up with a consensus You certainly don't need anywhere near 150,000 btu boiler. You can use the Slant Fin app as well,some believe it is on the conservative size.
  • SeymourCates
    SeymourCates Member Posts: 162

    You have two errors in paragraph four. The first error is the requirement to subtract the total hot water usage from the fuel consumption prior to figuring the heatloss of the building. To do that, you need the total hot water usage for the period in gallons. Multiply by 667 to get the total BTU usage for the period.

    The second error is when you achieve 486 BTU/hr/degree. To get BTU/hr, you must multiply by degrees. Figuring a base temperature of 65F and a design day of 6F, the multiplication factor is 59 and the result is 28674 BTU/hr on the design day.

  • D107
    D107 Member Posts: 1,798
    edited November 2017
    @mikeg2015 Thanks for the great details. I had measured first floor loss at 20kbtu, radiation 43K; piping goes from rad to rad on that floor, that's zone 1. Zone two loss was 12Kbut, radiation 20Kbtu; supply piping rad to rad again; basement zone loss about 7500K, baseboard 17Kbtu. Note I have boiler high limit set at 160deg. Yes, radiation is ideal for low temps. To be clear on the series piping, supply for zone 1 goes out from boiler up to first rad in main floor bathroom, then out rad and across to living room rad riser, through rad, down riser across to next living room riser etc. this is old converted gravity system; old steel risers are maybe 1.25"; new horizontal copper piping is 3/4". If the foam savings hold, I'll be looking at about 500 heating therms a year instead of 780, then maybe a 21Kbtu loss. Hard to imagine. The foam was after blown in high density wall insulation, chimney top damper, some door air-sealing. Still have the boiler draft hood sucking out warm house air.
  • D107
    D107 Member Posts: 1,798
    edited November 2017
    @EBEBRATT-Ed Thanks. There is no venting in the attic. foamed under sheathing in bays-even over the joists to prevent thermal bridging, and also gable walls foamed 4inches +. Also I made inside set point 75 not because of the cold porch but I've seen how my 93 year old Mom now keeps her thermostat at that temp. And yes, definitely would like to put the porch on its own circ and t-stat--first I'll see how i can insulate it without losing serious square footage in the narrow room.
  • D107
    D107 Member Posts: 1,798
    edited November 2017
    @SeymourCates Thanks. I could only estimate hot water usage since it's lumped in with the drier and stove, but all in all in summer months our gas usage is 12 therms/mo, so I subtracted 144 for the year for non-heating therms. (total usage 921 therms.) As for the degree day hour, I did multiply by degrees, but being conservative I had 0º as a design day not 6. Your 6 is very likely more realistic.
  • D107
    D107 Member Posts: 1,798
    edited November 2017
    @EBEBRATT-Ed Re: Series piping, I do see that the rad sizes increase along the run--up until the very last one in each zone--but that's due more to the room sizes rather than system planning. The insulation has made short cycling more likely; for the year or two I'll still have this boiler I'm concerned with flue gas condensation more than boiler on-off-on wear and tear. I understand that generally return piping at boiler is supposed to reach 140deg. within ten minutes of the call for heat. (This is cold start.) I doubt it does that on morning recovery, and during the day cycles probably don't exceed five-ten minutes, even before the foam insulation was done. Perhaps I should raise the pump speed on the Grundfos 3-speeds to medium for 2nd Floor. (First floor is Taco 007, stuck with that.) or raise high limit from 160 to 170 or 180--though with short cycles I'm sure wouldn't reach that anyway.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,449
    You can also install a bypass loop with a small circulator. Then you won't have condensation issues. If you have a zone controller just use the end switch to control the bypass circulator. Maybe I miss read, but why not install a mod/con? Then those low return temps are your friend!
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Gordy
    Gordy Member Posts: 9,514
    edited December 2017
    Your therms used for DHW usage needs subtracted, and any other same fuel appliance subtracted from your therm usage.

    The btu per hdd is a great way to track btu loss when done correctly. Some points are hdd s do not factor in solar gain, internal gains, and wind.

    It is quite possible your boiler could be less than 75% efficient.

    Remember mod/cons are better clawing their way at the load verses being oversized even with the high tdrs now,

    As far as DHW loads if you are getting by fine with a standard 40 gal HWH you’ll do fine with a small mod/con unless you have an insatiable desire for speedy hot water recovery that stems from large Roman tubs, car wash showers, and or long shower periods.

    A 40gal gas water heater only has a 35k burner.
  • D107
    D107 Member Posts: 1,798
    @Gordy my 777 therm usage was heating only. I had subtracted 144 non-heating (hw, drier, stove) to get the heating. (12 therms a month use in the summer months x 12.) hwh is 50 gallon, nominally 40kbtu but really firing at 30k. no roman tubs for us.