Do setbacks make sense for an oversized system?

Thanks to past help from members on this forum with my severely oversized geo system I now have a system that is working well and efficiently for us. As we enter the main part of a new heating season I'm wondering about utilizing setbacks while we are away from home with an oversized system. I know that typically setbacks are not recommended however I have my aux heat turned off and my units set so they only use stage one of a two stage compressor. Even with those settings it only takes about 40 minutes of run time for a 2 degree temperature rise in the house. Thoughts anyone? Am I correct in assuming that one long runtime as the house comes back up to temp would not deplete the loop field anymore than several shorter run times during the day? Any other drawbacks I'm not thinking of if utilizing setbacks?

Edit: Checked today's runtimes using my energy monitor and I am averaging 25 minute runtimes with 2 degree temp rise from my south unit and 35 minute runtimes with 2 degree temp rise from my north unit. This is with outside temps at 7 degrees today and two stage compressor running in 1st stage only.

 

There is really no depletion of the loopfield with daily setback strategies. During cold spans the systems are usually running full out in highest compressor stage, sometimes for days. Not mach drawback in your scenario her then a colder house for a while when you come back.

 

Thanks for the reply Doc. I have an additional questions if you or others don't mind in regards to our pumping strategy. Currently we are running two units, both 4 tons each. Pumps are Grundfos 230V - 26-99 3/Spd Pumps. We have a 4 pump flow center and are currently running 2 pumps in medium speed for each unit. One of the changes I made after my first post back in 2015 was to lock out the compressors 2nd stage on both units. We now run stage 1 only. Given this I'm wondering if I need to be running 4 pumps? What info is needed to find out?

 

It depends a bit on how much flow you have through your heat pumps, which partially depends on the pressure drop of your piping and your loop field design. Do you have the gpm for each heatpump when they are both running? Or at least the difference in temperature between incoming and outgoing water temperature?

 

I don't know the gpm of each heat pump when running. I did measure both of my units. Here's my numbers. These are with desuperheater running.

North Unit - 17 min runtime
EWT = 47.3
LWT = 43.4
Return Air = 69.96
Supply Air = 95

South Unit - 15 min runtime
EWT = 47.2
LWT = 43.5
Return Air = 70.6
Supply Air = 93.5

I also found the following chart for our unit.

 

Yes, you are over pumping likely due to first stage only operation. You could just disconnect the wiring for the second pumps....Not too bad, you are just paying for pumping you do not need.

 

Thanks. I'm assuming unit would just go into low pressure lockout if one pump was not providing enough flow? I'll move to one pump and see what my numbers look like. If one pump is providing enough flow it would save a few watts and would also provide me with a working backup pump should one ever go down.

 

Here's my new numbers. Switched from 2 pumps at medium setting to single pump per unit at high setting. According to my energy monitor it looks like I've reduced HVAC wattage by roughly 160 watts per unit. Thoughts on below numbers?

North Unit - 17 min runtime
EWT = 46
LWT = 41.6
Return Air = 70.8
Supply Air = 92.9

South Unit - 15 min runtime
EWT = 45.2
LWT = 41.1
Return Air = 71.9
Supply Air = 94.2

 

Still plenty (actually still somewhat overpumped) of loop water flow based on low waterside delta-Ts

 

Should I drop the pumps to medium speed and recheck numbers?

Edit: Looks like only about 18 watts difference between high and medium so not sure it makes much difference from an energy standpoint.

 

So try "low", but don't rob Peter (compressor wattage) to pay Paul (pump Wattage)

 

Got it. Each unit + pump is on a combined circuit with an electric usage monitor so should be pretty easy to figure out which setting is using less total watts. I'll re-check water and air deltas once I figure that out. Thanks.

 

Here are my new numbers for the south unit with pumps moved to low settings. My total wattage dropped another 78 watts at this setting. I've now gone from roughly 2830 watts per unit with both pumps running to 2600 watts with a single pump in low setting. Am I correct that my flow looks ok with a delta of about 6.5 on the water side?

North Unit - 15 min runtime
EWT = 47.9
LWT = 41.4
Return Air = 70.3
Supply Air = 94.4

South Unit - 15 min runtime
EWT = 48.2
LWT = 41.9
Return Air = 71.3
Supply Air = 91.3

 

The manufacturers sometimes stick to the randomly picked number of 3 gpm, since the ratings do not account for high pumping power, and their equipment looks good by the numbers at 3 gpm.

However, you as the customer pays for the pumps running as well.

YOU JUST DISCOVERED THE HOLY GRAIL OF GEO HEATPUMP SYSTEM EFFICIENCY! Congratulation! You now know more than many designers, engineers and manufacturers together. You just made your entire geo system 8.1% more efficient for life, just by a simple trick. Now on to the final challenge. The blower setting....

 

We can't be sure that your system is performing to spec without an "HE" (heat of extraction) calculation, but my hunch is that the reasonable delta-Ts on both water and air sides indicate that your systems are tickety-boo.

2600 Watts all in seems reasonable for 4 tonners locked in low gear. If their heat output is in the range of 35kBtuh that works out to a COP approaching 4...very good.

Amazing how oversized and overpumped this system apparently was...changed from 2 pumps at medium speed to a single pump at low speed. There are some who would fret about potentially perilously low Reynolds number values (fear of the laminar flow bogeyman!), but for now both systems and their pumps seem to be just loafing along.

As to setbacks, they only save significant energy on the load side if the building cools significantly. As long as the recovery is timely handled by the systems in their hobbled state (locked in low stage) then there may be minor additional savings by reducing on-off cycles - systems go dormant for hours during setbacks and then recover using a single long running on cycle. It might be useful to check waterside temps 2 or 3 hours into the continuous "burn" incurred during setback recovery. As long as they don't drop more than 6-8 degrees during the course of recovery you are probably good to go.

Of course, life can be simpler and a lot more comfortable if you simply "set it and forget it" at comfy temps and let your newly efficiency bolstered geos just do their thing.

 

Now I'm curious to hear Doc's advice as to blower settings...your 20+ airside DTs sound good to me, but he's the heating pro...

 

Your milage might vary. Attached is an example of a 4 ton 5-series humming away in first stage with 30.4 F EWT, 70.7 F EAT, using a total of 2080 watts. What bugs me is the 236 pumping power from the single 26-99, it should be 40 watts. But we will get there. The key is the blower running at 36 watts.
The message is simple: Make the ductwork efficient, make the loop efficient and supply it with half way warm water, reduce the energy used by the blower and the pump, and the system can run extremely efficient. More efficient than rated on a seasonal average.
This is not meant to brag here, this is just to drive home the message and show evidence that this is possible. Who says that 20+airside is optimal, ...it is not...but it is good for the rating...slow down the blower, see what happens.

 

Engineer as to your statement about how oversized my system is, here is an example. I am using sensi tstats with wink app. I have the app telling the tstats to turn off when temp gets above 71 degrees for 5 min or longer. Turn back on when temp gets below 71 degrees for longer than 15 minutes. 2 degrees + 15 minutes in order to keep the units from constantly cycling on and off. Zone bleeds per Docs suggestion have also worked wonderfully. On Jan 7th we hit overnight temps of minus 7 degrees. My units locked in stage one with a 2 degree recovery still cycled on and off about 4 times per unit overnight.

Doc I see your post about slowing down the blower. Current default cfm is 1123. I can reduce cfm on the digital controller down to 933. I have done this and my air delta jumps to 28.5. EAT = 71.7 LAT = 100.2 I'm not sure if the higher air temp will offset potential longer run times with reduced cfm. Will have to monitor and see. It looks like I am getting to the point where the last of the low hanging fruit may be hard for a lay person like me to dial in. I do know that my unit in 1st stage has a COP rating of 4.3. According to engineer it looks like I may be getting close to that.

Edit: After checking several times my units seem to be getting a consistent 26 degree air delta at 933 cfm. Slightly lower than the 28.5 I posted initially. This is with desuper running.

 

How is your energy usage at that setting compared to before?

 

It looks like around -3% (1033 cfm) seems to be the sweet spot as far as total watts and run-times. I did purchase a pressure gauge. I am showing approx 8 gal per minute on low setting. Does anyone have a COP equation they can link to? I've found a couple online and they seem to be slightly different in how they calculate COP. Going to test one last time to verify my finale numbers and will post.