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Old 02-18-2012, 10:53 PM   #1
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Calculating time to heat/cool a place from/top specific temp


Is there a catch all formula for calculating how long it will take to go from one temp to the other, if I manually time certain values?

Ex: If I know that it takes 1 hour to get the house from 15 to 20 when it's -20 outside, am I also able to calculate any other combination such as how long it takes to bring it from 10 to 15 when it's -40, etc? What if I know two values, is it linear where if I have two points I can figure out the rest?

I'm working on a hvac control app and my original plan was to make it "learn" over time all the different situations and it would make a best judgement based on existing values but if there's a catch all formula, then it may save me from keeping track of all that data and just need to manually input a few numbers and it will know the rest.

Also would the same apply to cooling, when the temp outside is hotter?

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Old 02-19-2012, 06:41 AM   #2
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Calculating time to heat/cool a place from/top specific temp


No, no catch all formula.

While a balance point graph is done linear. Its done based on night time conditions.

A structure that is losing 30,000 BTUs when at 2:00AM when its 30 degrees outside. Isn't losing 30,000 BTUs when its 2:00PM and sunny outside.

Next, air infiltration changes by how much warmer or cooler the structure is then the outside temp. At -40 outside and 70 inside, more fresh air is coming in then when its 10 degrees outside and 70 inside. And of course the more fresh air coming in, the more heat that is needed to maintain indoor temp.

So the question becomes how accurate do you nee or want your program to be.

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Old 02-19-2012, 02:10 PM   #3
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Calculating time to heat/cool a place from/top specific temp


Not looking for too much accuracy, just ballpark. Basically given the outside temp, the inside temp, and how hot I want it to be at NN time, I want to find out how early should the furnace start to reach that temp by NN time. There will be a sensor outside.

But if there's not really a formula then I'll stick to my original plan, I'll just have it learn over time, and take best guesses based on existing data. Also did not take into account the sun and wind. That's also going to change things. I guess if I really wanted to get fancy I could make it look at the weather conditions (sun/cloud/wind) as well as the time of day. Think I'll just ballpark it with the learning system. As long as I'm not hours off, it will be fine.
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Old 02-19-2012, 03:29 PM   #4
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Calculating time to heat/cool a place from/top specific temp


Honeywell uses P+I to determine recovery time.
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Old 02-19-2012, 04:55 PM   #5
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Calculating time to heat/cool a place from/top specific temp


ASHRAE has a formula somewhere but it'll cost you.

If
instantaneous room temp = To
register air is constant at Tf
thermal time constant of the air in your house is Tc
initial room air temp Ti
delta Td = Tf-Ti
and t is elapsed time
then

To = Tf(1-{e^[-t/Tc]}) where e is 2.718. . .
gives you a not too realistic math model

1.1 >enter Tc in hours
85 >enter Tf in degrees F
65 enter Ti in degrees F
20 = Td
0.8 >enter t in hours
0.727272727 =t/Tc = A
0.483225081 =e^A
0.516774919 =1-(e^A)=B
10.33549838 =BTd
75.33549838 =To = BTd+Ti

To add the time constant of your building walls
is more complex formula-wise but can be done
with a spreadsheet that clunks through each
degree F at a time.

Also, register temp tracks room temp.
A spreadsheet can do this.

Last edited by Yoyizit; 02-19-2012 at 05:09 PM.
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Old 02-20-2012, 04:55 PM   #6
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Calculating time to heat/cool a place from/top specific temp


Thanks, I'll play around with that and see what I can do. For now think I'll just make it start at the time I set it at, once I see my app is working then I'll add the predictive start feature in it.

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