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Heat Loss calc/ Boiler SIzing

rmelo99 Member Posts: 5
I was reading some other posts and wanted to see how "over" sized my

boiler is and what my real heat loss is. I am familiar with Manual J,

but have never fully completed one for my house. I kept seeing mention

of using historical Degree Day Data and Past Fuel Usage in other posts. I

gathered mine to identify where I stand.

Background: House is 100+ years old, 3story colonial, with cast iron

baseboard and radiators. Zoned into 8 heating zones. Heat is nat gas

with a grossly oversized conventional boiler 275k BTU. It has an

intellicon to help with my microzones and to prevent short cycling and

aide in efficiency.

I gathered my data and followed the guide here <a href="http://www.vonwentzel.net/HVAC/HVAC-Calculators/index.html#InDirectObservation">http://www.vonwentzel.net/HVAC/HVAC-...ectObservation

I used degree day data from the web and also from my gas company bill.

Both appear very close. Can someone help interpret the data and verify

my calc are correct? I did not adjust the BTU for the efficiency of my

boiler, since I don't know if it is 70%, 80% etc...

The way I read this is my heat loss is 20k BTU/Hr??? is that right? Sounds way too low, or am I crazy or wrong?

Pink area is summer non heating months where gas usage was for cooking and DHW only.</a>


  • furnacefigher15
    furnacefigher15 Member Posts: 514
    This method makes

    No sense to me.

    This will tell you how much fuel you have consumed.

    But, heating equipment should not be sized on averages. It needs to be sized large enough to handle the heat output needed on the coldest day in your area.

    What kind of boiler is it? steam or hot water?
  • rmelo99
    rmelo99 Member Posts: 5

    It is a hot water system, not steam. I understand what you are saying about those numbers showing me my fuel usage....but the way I read it is that when you do the formula with degree days, you should be able to show the heat loss over  a period of time.

    I found several sites that had similar heat loss calcs that used historical fuel consumption data and local Heating Degree Day data to show heat loss.

    I am not using this to size a new boiler, but more to have an idea on how oversized mine is and some planning for  a new boiler down the road. When it comes to that I will take my time to complete the manual J for my house.
  • meplumber
    meplumber Member Posts: 678
    The program is called CPAP

    CPAP takes fuel usage data over several heating seasons to provide a form of heat loss for the structure. The CPAP data should not be confused or used to replace a true heat loss calculation for the structure. Industry professionals often use the CPAP to confirm the heat loss when the calculation differs wildly from the output of the existing equipment.

    It is also used on varying occupancy structures, such as office buildings and institutional buildings where the heating plant is also used for minor process heating.

    Do not assume that this can give you a clear cut answer on the proper size of the equipment. It is merely one tool in the tool box that, when used properly, can assist the heating professional in making an informed decision.
  • rmelo99
    rmelo99 Member Posts: 5
    What does it show me

    Not being an expert at this, I'm just trying to validate if my numbers are right and if I'm interpretting the output correctly.

    It appears to me that the heat loss in in the 20k range. Is this possible, it seams very low.
  • meplumber
    meplumber Member Posts: 678
    I agree with FurnaceFighter

    The numbers in your chart don't make any sense.

    Something is amiss. Try this.

    Is your house warm enough? If so, then answer these questions.

    What is your location? What is the heat emitter in your home (baseboard or radiators)? How many square feet is your house? We can start there and help you come up with a ball park number.
  • rmelo99
    rmelo99 Member Posts: 5

    Located in CT house is about 3700 sq ft, 3 stories over full basement.

    The emitters are cast iron baseboards on 2 levels, and cast iron radiators on one level.

    I have no problem heating the house. I have done lots of research and am familiar with many concepts around hydronic heating.

    I re-plumbed the house from one zone into the 8 that are there now. It is a zoned with 8 taco circs, and taco zone controllers. It has been this way for 4years or so. I did this in anticipation of replacing the boiler, which wasn't and still isn't in the budget. I added the Intellicon a few seasons ago to help out.

    Some think I have gone overboard with my zoning but it allows for comfort on almost a room by room basis. We can have the baby's room and my mother's room up in the 74 range, while keeping our room a bit cooler and the heat very low in the unused guest bedrooms.

    I would like to make the system more fuel efficient with a more modern boiler that can maybe will be a one that modulates. Since I have nat gas I know I can get a 90%+ unit.

    Knowing my heat loss I can evaluate whether a simpler conventional unit that is smaller and more properly sized will make more sense, or a modcon type which will be more expensive. I want to do my fuel savings ROI and see what works.
  • meplumber
    meplumber Member Posts: 678
    edited January 2012
    Well it isn't 20k btuh

    Well 3700 sq ft at 100 yrs old, in CT, isn't 20k btuh. Probably something in the 150-160K as a raw guess.

    Since we know that the house is heating comfortably through the temperature swings, do a radiation survey. This means to go around the house and figure out the heat output at your current system temperature. You may need to pickup Dan's EDR book to figure out the capacity of the radiators. This number will give you an idea of what your radiation is capable of putting out.

    It was common in that era to over-radiate and over fire the boiler. Heat loss calculations were long drawn out, tedious calculations until the more recent advancements in computer programs.

    There is no real "rule of thumb", short of a heat loss calculation. My best advice would be for you to pick up a copy of HVAC Calc. It is a software package that is available to single use homeowners for somewhere around 50 bucks. It is a relatively simple program that can give you a much closer answer to your questions.

    Once you compare the heat loss of the structure to the heat output capacity of your radiation, then you will see how low you could run the boiler temp on a new mod/con. The lower the water temp, the more efficient the boiler.

    Here's the link to the software:

  • rmelo99
    rmelo99 Member Posts: 5
    I have HVAC Calc

    The trial version loaded and have about 60%of the information loaded into the program, but haven't had time to finish.

    I've tried running at lower water temp without great success...for the very reason you stated, trying to be more efficient.

    I am running at 180 deg water temp now. Is is really only 2 rooms that I believe would need more radiation added for me to run at a lower water temp.
  • Gordy
    Gordy Member Posts: 9,537

     The last column would be an average per hour for 24 hour period. Which is not a good indication of what the house is using each hour under varying conditions through out the day.  The numbers make sense to me. But remember HDD do not account for solar gain, or the lack of, and wind.

    I'm not saying this in lieu of a heat loss calculation. But if your thermostat records number of hours for a heat call each day. You could perform the calculations daily with HDD information IF you know that your boiler is truly firing according to the rating plate.

     I have done this in my own home, and it always works out to with in a couple thousand Btus of the actual calculated heat loss if I extrapolate those numbers for my design day in the heat loss calc. variations being solar, and wind.


     Take the run time for the heating system * the boiler output not input. / by the HDD for that day.  Then take that total and divide by the SF of the heated space. This will give you btus per sf per HDD. If you are below 10 btus a sf per hdd your house is pretty good on insulation compared to the national average.

    As stated this can be used as a comparision to a actual heat loss.

  • Gordy
    Gordy Member Posts: 9,537
    edited January 2012

    post deleted
This discussion has been closed.