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Estimating heat load based on gas usage

hvacuser | Posted in Energy Efficiency and Durability on

I’m following the advice here

My numbers are:
Gas usage in Jan: 11.4 GJ
Gas usage in Sep 2020: 2 GJ – I use this to account for usage of stove and water heater that both consume gas
Gas usage by the furnace in Jan: 11.4 – 2 = 9.4 GJ
GJ converted to BTU: 9.4 * 947,817= 8,909,479
Account for 96.7% efficient furnace: 8,615,466

So total BTUs delivered to the ducts is 8,615,466

Using the 60F balance point, in Jan I get 647.6 HDDs. 
8,615,466 / 647.6 / 24 = 554 degree-hours
My design load/outside temp is 28F, so 65-28 = 37
37 * 554 = 20,498
Account for 1.4x oversize = 28,700 BTU/h

The above number seems good in the sense that it’s close to Manual J calculations. Through a rough Manual J calculation I get a load of 28379.

I have two questions:
– Is it expected that the number matches Manual J only after accounting for 1.4x oversize?
– I don’t quite understand the math. If 8,615,466 were delivered total in January, then given 30 days in Jan and 24 hours in a day 8,615,466/30/24 = 11,965 BTU/h. This means on a daily average I need 11,965 BTU/h in January. What’s wrong with this way of calculating the load? Why is this number wrong compared to the heating degree days calculation?

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  1. paul_wiedefeld | | #1

    1. The fuel usage generally comes in lower, arguably because it’s based on reality. It’s so hard to perfectly complete a manual J and if you have the actual data, why bother? That it’s 40% oversized this time is probably just chance.
    2. Also just a coincidence that you had 647 HDD in January and that’s kind of close to 744 (31x 24), so the division works out within 15%. Heat loss is very linear and depends on outdoor temp (HDD_60 is just 60 minus average temp, if average temp < 60, otherwise 0). Hourly average is irrelevant, you’re sizing for the day that happens 1% of the year, not the average day.

    1. Jon_R | | #2

      > you’re sizing for the day that happens 1% of the year

      More accurately, you are using 1% data, but you are sizing to keep comfortable even when it goes below this (which happens frequently and sometimes for extended periods). Manual J (and fuel use with 1.4x factor) does NOT predict design day load. But it does provide a value useful for equipment sizing.

      1. hvacuser | | #4

        Thanks, then if I understand it those are predicting slightly worse conditions than design load.

    2. hvacuser | | #3

      1. I didn't realize this is the most accurate method. Then I'll confidently use a 2 ton pump.

      2. Ok, it makes sense why the overall average is not a useful number. I still don't understand one thing though. I calculated 554 degree-hours, and then we calculate 65F - 28F=37F. I'm usually setting the thermostat to 72F, why should I use 65F instead of 72F to get the difference? Looks like this calculation is being done without accounting for indoor temps.

      I know there is a section in the guide "What about thermostat settings?", but I didn't quite follow that part since the example wasn't clear enough. Would you mind helping me pick the right number?

      1. Expert Member
        Akos | | #5

        Because of internal gains (sun, people, appliances) most houses have a balance point between 60F to 65F.

        At balance point, when it is 65F outside, the furnace does not need to deliver any heat to keep the building at 70F. This is why it is used for this calculation instead of your thermostat setpoint.

        1. hvacuser | | #6

          Thanks for explaining.

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