# Thread: Heating Degrees Per Hour

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## Heating Degrees Per Hour

On a new electric system, forced air, about how many degrees per hour per cubic s.f. should it take to heat?

2. This will vary according to how much internal load there is at the time, how much solar gain there is, how much loss from wind, door openings, etc, and what the temperature difference there is between inside and outside. The other variable is how close the equipment was sized to the design load, i.e. load 50k equipment 100k or load 50k equipment 52k.

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With all variables considered is there a basic range or rule of thumb? I know its hard to answer when your a tech due to what has to be considered first but can it be simplified down to a range / like 5 degrees per hour is good 10 degrees is better and 20 degrees is perfect for say 8000 cubic feet?

4. The only constant I know of is internal building materials, furnishings, etc can only absorb heat at a rate of 3 degrees an hour. So with an oversized system the space will reach temperature, shut down, then come back on shortly as the cold soaks up the heat. If it is probably sized it will just run a long time and the walls/etc will be absorbing the heat at a similar rate but when it cycles off it will stay off a while.

5. At design conditions 0°F temperature rise!

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on one of the units at my job we figured 3 degrees per hour from 67F to 70F with the outside at 30F for 9500 cubic feet and so we wanted to be able to say either "that is good" or compared to others "that is bad".

7. Originally Posted by AlexCollins
on one of the units at my job we figured 3 degrees per hour from 67F to 70F with the outside at 30F for 9500 cubic feet and so we wanted to be able to say either "that is good" or compared to others "that is bad".
When it is -10 outside does that unit keep up or does the indoor temperature drop? If it holds but doesn't cycle it is probably about right.

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and I also figured the heat load of the space is 30K and the coil is producing 28,400K

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You are basically requesting
1. _ WHAT is the composite (Total) Heat Capacity of the building Interior ?
2. + What is the EXCESS Heating Capacity available at Any given OutDoor Temperature?
3. + What time of day is it?

1. How much concrete, steel, drywall, water, furniture, etc., is in the building?
__
2. Indoor Temperautre: 70'F
___ Design Temperature: 0'F

___ ODT: -10'F or 65'F

___ Manual J or N Calc
___ Manual S calc

... looks like 1,000,000 + permutations

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Originally Posted by AlexCollins
and I also figured the
heat load of the space is 30K and
the coil is producing 28,400K
Heat Loss = 30,000 BTU/HR at 0'F
… … ___ ___ 428.57 BTU/ HR- 'F [ 70'F Interior Temp. ]

… … Heat Loss at 20'F = 21,429 BTU/HR
_____________ 40'F = 12,857 BTU/HR

Equipment Capacity = 8,323.6 Watts =28,400 BTU/HR

Excess Heat Capacity at 20'F = 6,971 BTU/HR
___________________ 40'F = 15,542 BTU/HR

Next ..
___ divide the Excess Heat Capacity
___ ___ by the TOTAL Heat Capacity { BTU / 'F } of ALL Interior materials.

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Originally Posted by dan sw fl
Heat Loss = 30,000 BTU/HR at 0'F
… … ___ ___ 428.57 BTU/ HR- 'F [ 70'F Interior Temp. ]

… … Heat Loss at 20'F = 21,429 BTU/HR
_____________ 40'F = 12,857 BTU/HR

Equipment Capacity = 8,323.6 Watts =28,400 BTU/HR

Excess Heat Capacity at 20'F = 6,971 BTU/HR
___________________ 40'F = 15,542 BTU/HR

Next ..
___ divide the Excess Heat Capacity
___ ___ by the TOTAL Heat Capacity { BTU / 'F } of ALL Interior materials.
Thank you that makes so much sense now,I am sure I will understand this in the future, feel free to break it down in laymans terms.

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Post # 10 Follow-up
___ ____ _____

Heat Loss = 30,000 BTU/HR at 0'F
… … ___ ___ 428.57 BTU/ HR- 'F [ 70'F Interior Temp. ]

… … Heat Loss at 20'F = 21,429 BTU/HR
_____________ 40'F = 12,857 BTU/HR

Equipment Capacity = 8,323.6 Watts =28,400 BTU/HR

_________ Excess Heat Capacity at 20'F = 6,971 BTU/HR
________________ _____________ use 9,000 BTU / 'F for an Example
for TOTAL INTERIOR MATERIAL HEAT CAPACITY
…...……………...……...…………...………...……… … 0.77 'F Increase / Hour when it is 20'F outside

_______________ ___ ___ ___ at 40'F = 15,542 BTU/HR
___________________ ___ ___ … … … … 1.73'F Increase / Hour when it is 40'F outside

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OK , So when its 30 outside your increase per hour would be 1.27 F and I figured 3 F per hour actual increase so that means I am exceeding and must have really good air flow. Right ?

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