Heating Degree Days.. Detail info!!

gogreen123gogreen123 Registered Users Posts: 2
The base temperature for calculating heating degree-days is lower for the same indoor air temperature as house becomes more insulated. Why is it so?


  • gogreen123gogreen123 Registered Users Posts: 2
    Re: Heating Degree Days.. Detail info!!

    The heating degree day temperature base is the average daily outdoor air temperature at or above which no supplemental heat will be needed. This is called a temperature“lift”.
    Internal heat generation (sensible heat from cooking, lights, humans, appliances, large dogs, etc.)adds enough heat to the airspace of the house that they can keep the house a few degrees abovethe outdoor air temperature with no supplemental heat. Solar gain through windows permits afew more degrees of temperature lift.
    Poorly insulated houses lose heat with sufficient ease that only a few degrees of lift can becarried – 10 F in the United States was shown during initial development of the heating degreeday concept.
    If a house is more completely insulated, heat loss is lessened for the sametemperature difference between indoors and outdoors and internal heat generation plus solar gaincan hold the house at its desired temperature when the lift is more than 10 F.One could hypothesize a house so well insulated that it might never need to be heated – theinternal heat and solar gains would more than suffice even during the coldest weather. This pointis not reached in practical construction, but a highly insulated, or super insulated, house limitsthe heat loss rate so the lift become very high.
    This can be seen in the equation for steady stateheat loss, Q = UA∆T. Notice that the current heating degree day data for the United States is based on an indoor air temperature of 75 F.
    If the home is better insulated, this set point can be lowered and provide thesame degree of comfort, which also lowers the base temperature and number of heating degreedays.The ability to hold a greater temperature lift occurs on cold days, where the heating degree day value for the day is reduced by the increased lift.
    When days are warmer, a larger temperature liftwill mean some days that would have provided some heating degree days to the yearly total will be totally eliminated from that summation.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Heating Degree Days.. Detail info!!

    i thought it was based on 65 degrees f.
  • Lee DodgeLee Dodge Solar Expert Posts: 112 ✭✭
    Re: Heating Degree Days.. Detail info!!
    niel wrote: »
    i thought it was based on 65 degrees f.

    Maybe these two values are sort of consistent. gogreen is stating that the assumed indoor temperature is 75 F, but unless the outdoor temperature drops 10 F below that value, or 65 F, then the house does not require the main heating system to come on to provide heat since there is some heat supplied by appliances, passive solar gain, people, etc. So the tables that refer to a 65 F reference temperature mean that the degree day calculations are based on the difference between 65 F minus the outdoor temperature, and do not assume an indoor temperature of 65 F. This approach assumes that person doing the heat load calculations is not already including the appliance loads and passive solar in their calculations (which I do include in my calculations).

    The 10 F differential, and resulting 75 F from the "standard" base 65 F tables, both sound higher than what I have always assumed, which is more like a 5 F or 7 F differential, corresponding to a winter indoor temperature of 70 F or 72 F for the 65 F reference tables. I think these values are closer to the European assumptions about the "lift" between the indoor temperature and outdoor temperature where heating is required.
  • BB.BB. Super Moderators, Administrators Posts: 32,740 admin
    Re: Heating Degree Days.. Detail info!!

    Also depends on "how efficient" your home is... An old fridge, lots of filament lamps, TV/entertainment system going, cooking, etc... And you have more "waste heat" to heat the home.

    Do lots of conservation, and you may need a bit more "heat" in cold weather (and less A/C in the summer).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Lee DodgeLee Dodge Solar Expert Posts: 112 ✭✭
    Re: Heating Degree Days.. Detail info!!

    I prefer a different approach. Like many folks on this forum, I know exactly how much electricity that I use per month, as well as natural gas for heating. Since I do not have or need air conditioning, most of the electricity that I use ends up as added heat in the house. An exception is the electric clothes dryer. I also have a pretty good idea of how much passive solar that I collect. Therefore, I can compute how much energy that I am adding as heat into the house, both from the furnace and these other unintentional sources. Then I can mostly avoid this 5 or 10 degree F fudge factor in doing heat balance analysis for the house.

    I also know the R-value of all the components in the house, and that allows a computation of U x A, where U is the thermal conductivity, and A is the area. That value multiplied by the heating degree days gives an estimate of the conductive heat loss for a year. Using results from a blower door test, the natural ventilation rate can be estimated, allowing a computation of the heat loss due to ex-filtration. Then the unintentional heat losses inside the house can be added. This sum can be compared with the measured natural gas usage. For my house, the agreement is pretty good.
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