Est Duty Cycle Of Units In Peak And Offpeak - HVAC - DIY Chatroom Home Improvement Forum

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08-27-2010, 05:21 PM   #1
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## Est duty cycle of units in peak and offpeak

I am doing a calculator to estimate costs relative to SEER rating and electric costs. I need to input the duty cycles and how many months each would apply. I am creating a peak and offpeak multiplier.

08-27-2010, 05:31 PM   #2
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For elec. heat, duty cycle is linearly related to heat output.

For AC you'd have to assume some COP curve. ASHRAE might actually have derived a formula using exponents for a COP curve that fits the curve reasonably well over a range of common temperatures.
This project might be pretty ambitious.

You gonna' use Excel or some programming language?

Last edited by Yoyizit; 08-27-2010 at 05:40 PM.

 08-27-2010, 06:03 PM #3 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,413 Rewards Points: 7,816 Peak and off peak? Whats is your definition of them. The formula is already in Manual J for operating cost analysis/estimates/comparisons.

 08-27-2010, 06:08 PM #4 Member   Join Date: Jul 2009 Posts: 244 Rewards Points: 242 Yeah, I just made a very dumbed down excel sheet to compare a few things. I wanted to look at energy costs as well as duty cycle compared to the final bill. I realize some stuff is already available but you have to look this up here, take this over here, etc. I just wanted it all on one sheet. Remember these are only estimates. I realize we can take this out quite a ways looking at deltaT and insulation classes to get a better duty cycle but all I want is something as simple as peak of summer 95-105F, duty is roughly XX. Right now I am figuring an average of 80% duty peak and 20% off peak.
 08-27-2010, 06:12 PM #5 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,413 Rewards Points: 7,816 The very simple way is in Manual J. And for what ever temp you want. Insulation value is not of a concern.
 08-27-2010, 06:38 PM #6 Member   Join Date: Jul 2009 Posts: 244 Rewards Points: 242 well, insulation determines actual load at certain deltaTs. I don't see how that would not be important. I will have to take a look at that. I was looking at it this way because I am building a circuit to monitor and log the actual duty time of my units month to month. Nice to actually compare that over time.
 08-27-2010, 06:45 PM #7 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,413 Rewards Points: 7,816 Insulation determines size. Or capacity used. Not actually duty cycle. A building with an R13 in the walls has the same heat gain weather you use a 2 ton or 4 ton unit. The duty cycle will change with the size of the unit. Look at Manual J. Every thing you need to know about the formula how to calculate the operating cost is there.
 08-27-2010, 06:52 PM #8 Member   Join Date: Jul 2009 Posts: 244 Rewards Points: 242 10-4. I will do that. Thanks
08-27-2010, 08:02 PM   #9
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Everything affects everything, and the strength of each effect is shown in the formulas.

If you can scrounge this book from somewhere
http://www.amazon.com/Mechanical-Eng...f=cm_lmf_tit_5
it has good stuff on HVAC. Not so technical as ASHRAE stuff and a click more complex than Manual J.
My library happens to have it.

With the formulas you can decide for yourself how accurate to make your estimation; +/- 10% or so like Manual J, or even better.

If the duct blower is a current source, the duct friction a resistor and the structure heat capacity a capacitor, this is an easy electrical circuit to analyze.
One problem is that the resistance value depends on the current through it, unlike resistors you buy at Radio Shack, so Excel is needed to zero in on the final value or you use a graphical solution.

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