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What does Design Temperature mean in practice?

Let's assume an HVAC system has been installed in a house with Design Temperature of 94F and this this house experiences 15 days above 100F annually with 5 days above 105F with a peak temperature of 110F next year.

Let's also assume the HVAC system meets this definition of Design Temperature (DT) as the temperature that a heating or cooling unit must be able to maintain when outdoor conditions place the greatest demand on the system.

What would be the target behavior of such an HVAC system when the outside temperature of the house was greater than the DT?

Comments

  • PC7060
    PC7060 Member Posts: 1,445
    edited July 11

    HVAC System design should be based on the manual J report for the home. The manual J report lists the heat loss and loads at the cold/hot design day based on house location, orientation, window types/locations, insulation, wall thickness and material. .Assuming a secondary heat source for winter, a “perfectly match” cooling system would bring the house to target temperature (assume 75F) while running continuously with the outside temp at the design day temp. For every increase above the design day temp, you would expect the band to shift up linearly by same amount. Would for outside temp of 5 degree over design day, the HVAC would cool the house to 80f.

    No system is perfectly matched due to a variety of conditions so actual temperatures would vary accordingly.

    Just my 2 cents (and worth less)

  • hot_rod
    hot_rod Member Posts: 23,404

    Simply put if the heat load increases above what the system can manage, expect the indoor temperature to rise as the system is running non stop at full output capacity

    This could be couple degree , it could be 10 degrees for a short period

    Assuming the system is designed and installed to allow full capacity to be moved into the space

    Load calculation are a moving target it can and will change minute by minute

    Both internal and external loads or gains constantly change. That is why the load calcs use “assumption “

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • biosopher
    biosopher Member Posts: 21

    Thanks guys. The weather will be in the 100's over the next couple days. I'll track the inside/outside temps and post back then.

  • PC7060
    PC7060 Member Posts: 1,445

    @DCContrarian explains it well. Before we had air conditioning, my parent used to use fans at night to ”bank” the cooler temps inside the house. This was in the Midwest where the temp would drop to low 70’s at night. When sun came up, windows were closed and shade pulled to minimize heat from outside. Would typically keep it cool(ish) until mid/late afternoon. Once started to cool off in evening, the process would repeat.
    Of course, this did nothing to manage humidity 😑

    You can do same thing using AC by dropping the the thermostat to 70” in evening.

    GGrossdelcrossvpsb75
  • Jamie Hall
    Jamie Hall Member Posts: 24,876

    In some ways it would be kind of nice if we could all agree to rename "design temperature" to be "design delta T", wouldn't it? Because that's what it really is, and that's what we are really doing.

    But then we still talk about "pickup factor"…

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    PC7060
  • jumper
    jumper Member Posts: 2,385

    There is a technology called adiabatic cooling. When outside temperature is too high air for condenser is evaporatively cooled. A bonus is that the air is filtered so that condenser doesn't foul. I know that we aren't supposed to discuss $$$$ but now that labor is so expensive why don't customers pay for better equipment?

  • jumper
    jumper Member Posts: 2,385

    Design temperature can get tricky. In good old days residential heating was sized to maintain 70° inside when it was 0° outside. So most of the time heating system is over sized. Then to save energy old homes were tightened up and insulated. So now heating system is grossly over sized. So when boiler is replaced it is down sized. Radiators are now over sized. Is that good in theory? How about in real life?

  • pecmsg
    pecmsg Member Posts: 5,298
  • DCContrarian
    DCContrarian Member Posts: 683
  • DCContrarian
    DCContrarian Member Posts: 683

    This is a screen shot of a sensor in a room in my house on a day when the outside temperature got up to 6F above my design temperature. Thermostat was set for 77F.

    It's not a perfect line because it actually got down around 73F overnight. But ideally the line would look like a speed bump, flat while the cooling can keep up and then bumping up when it can't.

    PC7060
  • Jamie Hall
    Jamie Hall Member Posts: 24,876

    Why not? Adds to the fun. One can either make this whole business of designing heating or cooling equipment remarkably complicated — or just give up, calculate the still air load (Manual J or similar) for a given design temperature (look up the nearest city — maybe add a fudge factor for odd local environmental conditions), check the maximum and minimum heat sink/heat source conditions if you are planning on a heat pump or similar cycle equipment, find a catalogue, select a unit which fits, and that's it. Design the installation by the book, and follow the design, and you're done. We all do it. Most of the time it works just fine, and if it doesn't we can say — truthfully — that we followed the proper procedures.

    Something to be said for that! It would be a rare installation which required a more complete analysis.

    If it cheers you up any, highway engineers do the same thing for bridges. Most of them stay up… architects and structural engineers for whole buildings. And so on.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England