# Do setbacks make sense for an oversized system?

Discussion in 'General Discussions' started by jk96, Jan 6, 2017.

1. ### docjenserWell-Known MemberIndustry ProfessionalForum Leader

COP gets complex. Antifreeze correction factor, power factor, etc. High precision is tough with the COP. At the end of the day it is energy out divide by energy in. Let us know what you come up with and we can discuss.

Minus 3% in the CFM can easily be a -20% in the energy use of the blower. You are doing it right, the sweet spot is the key matrix.

2. ### jk96Member

I have been focusing on my south unit only. I have something strange going on with my north unit (more on that later). My numbers are suspect without a more precise pressure gauge and flow chart. I do know that my runtimes remain the same and I am seeing as low as 2505 watts depending on how many zones open and how hard the fan has to work to maintain cfms. As soon as I get a better gauge I'll take a stab at figuring COP.

3. ### jk96Member

One additional question. I have an energy monitor measuring watts on each unit. The power factor adjustment is set to 1. Am I correct in what I have read that my power factor should be set to slightly less than 1 on my monitor? I've seen the number of .85 floating around and used for COP calculations but didn't know if that was fairly uniform from different units?

4. ### pfer10Member

Does you energy monitor have leads to measure the voltage or just current? If it has voltage most of the time those compensate for power factor. If it doesn't then it probably just uses a set voltage and the current to calculate power.

5. ### docjenserWell-Known MemberIndustry ProfessionalForum Leader

Forgot to mention that the COP in the ratings is at 41F EWT at first stage, with some pumping and blower penalties. If you are getting close to 4 in first stage, you are doing really well. 0.85 is good for power factor. You will see, there are a fair amount of assumption you have make for COP calculations. You are not having a \$2 million lab to do all the exact measurements like a major manufacturer.
Mots importantly: Do not fall for the illusion of precision.

6. ### jk96Member

Thanks Doc. As soon as I get my low pressure gauge I'll take a shot at heat of extraction and COP.

Now on to my north unit. This is the one that I labeled as my "trouble unit" way back when I was first started asking for help a year or two back. This unit has never seemed to run as efficient as my south unit. Power consumption on this unit has always constantly fluctuated up and down by 100-150 watts on my energy monitor. I chalked it up to a problem with my monitor. A few days ago when checking EWT and LWT I noticed the LWT water temp constantly fluctuated up and down. It does not hold constant. Temps will quickly draw down then float back up then draw down again. Any thoughts? I have a call into our installer to come out and take a look.

20170112_163814_80750109900138 (1) on Flickr

Last edited: Jan 14, 2017
7. ### docjenserWell-Known MemberIndustry ProfessionalForum Leader

It sounds like that the expansion valve is hunting the pressures....nothing to unusual.

8. ### jk96Member

So should it be doing this throughout each cycle? I'm not getting this on the south unit, only the north and seems to never settle in.

9. ### jk96Member

Here's my shot at calculating COP. This is 1st stage with desuper turned off. I've found that medium speed on my pumps is giving me the best combination of run-times and watts used. My calcs are using total watts from the unit & pumps.
EWT = 45.7
LWT = 40.7
RAT = 71.3
SAT = 94.6
Amps = 11.7
GPM = 10

HE = 485*10*5 = 24,250
DMD = 2357 (taken from my energy monitor with .85 power factor)
HC = 24,250 + (3.412*2357) = 32,292
COP = 32,292/(3.412*2357) = 4.01

This seems to jive pretty close to my monitor on the HVAC unit. The screen shot shows a COP of 4.1. My temps on the orb unit show a different water temp than my readings at the ports but the delta is within .1
20170115_151417

10. ### jk96Member

Hi Doc,
you mentioned my LWT temps moving up and down during cycles sounded like the expansion valve was hunting pressure and said nothing unusual. Should it be doing this throughout the entire cycle? The temps never seem to settle in. They constantly fluctuate up and down from cycle start to finish. You can also see the constant change in power consumption on my monitor for each cycle on this unit. The power usage and LWT temps on my other unit hold steady.

11. ### docjenserWell-Known MemberIndustry ProfessionalForum Leader

We see power fluctuation usage within 100 watts all the time. Can't fully explain it why the other unit does not have it.

12. ### jk96Member

Is it normal for the LWT temps to be in constant movement up and down as I showed in the earlier video? Its as if the unit extracts heat for 10 seconds then stops for 10 seconds in a constant repeating cycle.

13. ### docjenserWell-Known MemberIndustry ProfessionalForum Leader

They move up and down. Again, so does the power. It is never constant.

14. ### engineerWell-Known MemberIndustry ProfessionalForum Leader

I like the hunting TXV hypothesis. It's a PITA for dialing in charge as well as energy monitoring.

15. ### docjenserWell-Known MemberIndustry ProfessionalForum Leader

Again, it is the illusion of precision. One needs to be in the ballpark.
But if you think that the dual stages are bad, wait until you see the electronic TXV valve in the 7 series. LWT fluctuates by 2.5 degrees within 30 seconds.

16. ### jk96Member

Thanks everyone. I found it odd that one unit LWT fluctuates by .2 degrees and the other by 1.5 degrees. Thanks for confirming its normal. I think I have everything dialed in pretty well now. With some additional controls on the wink app for my t-stats I'm now getting longer run times, more down time between calls, and less watts when running. I'm looking forward to seeing how things compare to last year when I get into our cooling season. Hopefully the increased run-time will help with my humidity issues I had last year.

Here is a side by side pic of my run times for heating before and after all of the changes (zone bleeds, t-state differential, blower speed, etc). Thanks everyone who has helped me get to this point.