Doc, this agrees with what several of us here have noticed (even with a single tank setup), namely better DSH results with fewer CPH (and longer run times.)

Before hanging my hat on CPH alone, I would want to see data from customer A at 3 CPH and customer B at 1 CPH

OTOH given that refrigerant charging instructions call for waiting 15-20 minutes of run time for a system to fully stabilize in steady state before using readings to ascertain charge, I'm not surprised to hear of additional evidence that short cycling is really inefficient.

Customer A (3 CPH) http://welserver.com/WEL0336/ Customer B (1 CPH) http://welserver.com/WEL0288/ Now comes the question: How do you define short cycling. Is 3 cycles/hour shotcycling? At the end of the day, that is the factory setting for the hydron supplied thermostats. We did not change anything.

Doc, any idea on the cooling mode buffer tank temperatures on these units in the midst of cooling season?

Regarding cycles - as a homeowner interested in unit efficiency (and unit longevity), I like our units to run for longer lengths of time with fewer cycles as opposed to running more cycles for shorter lengths of time. My understanding is that this is more efficient for the unit, including DSH operation, and have observed this with our DSH's. In any given system, I'd define short cycling as running more cycles than necessary for comfort (assuming a system that is properly designed, sized, installed, etc.)

P.S.  We have our thermostats set for 1 CPH for first stage, and 1 CPH for second stage.  This gives us 2 CPH total per my observations in first stage.  Different thermostats may behave differently.  I assume this would allow only 1 CPH in second stage even if if conditions warranted more.

WEL 288 the DSH never turned on in the cooling season. It is one of those second generation R-410 units, which did not allow the DSH pump to turn on unless the discharge temp reached 125 degrees F. The EWT never got above 65 degrees F. That is the reason I had an issue with Climatemaster's claim that "The HWG is active throughout the year, providing virtually free hot water when the heat pump operates in the cooling mode..." They now changed the algorithm on the control board. WEL 336 is getting up to 90 degrees during the (mild) cooling season. It's a hydron unit with different controls.

Posted By docjenser on 09 Nov 2010 10:26 AM
I continue to be surprised how much difference equipment cycling makes. I have a customer doing one longer cycle per hour, with the buffer tank up to 145 degrees, and one with 3 cycles per hour, buffer tank around 90 degrees. Both 2 person households, same equipment size, same total run time.

Does seem counterintuitive. Any other contributing factors? 1 open loop one closed....different early season EWT's since one family likes more AC......lengthy process pipe runs.....different buffers......
Seems like there should be an obvious distinction between the 2 (besides run time).
Joe

No, the systems are very comparable. Similar run time, one is vertical, one horizontal, same EWTs right now. Same run time in first stage, same size buffer. Buffer tanks sit both right beside the heatpump. The only real difference is the cycles per hour!
Dramatic impact on DSH performance.

Posted By docjenser on 10 Nov 2010 12:52 AM
Now comes the question: How do you define short cycling. Is 3 cycles/hour shotcycling?

I came across this Comfort Alert flash code the other day regarding Short Cycling - "Compressor run time of less than 3 minutes on 4 consecutive cycles"

Personally, I like my definition (previously mentioned in this thread) better.  Of course, the system can't, currently, tell when a person is comfortable.