Hello Everyone,

I had the Tranquility 27 2 Stage Geothermal system installed in June 2009.  My electric bills were high in the winter of 2009, however, I had no more oil bills (I live in Massachusetts) so I did save a lot of money.  Anyway, to the issue or rather question I have.  Often times, I notice that when the unit is either heating or cooling, it goes into the heating 2 or cooling 2 stage.  Is this the second stage?  If so, what makes the unit go into the second stage?  I understand the heating has 3 stages and the cooling has 2 stages.  Could it be that my unit is not getting enough water from my well and that is why it goes into the second stage?  I had my well tested to ensure the well was sufficient enough to handle a geothermal unit.

I have a 3500 square foot home in Massachusetts.  It is a geodesic dome so I guess I like the geo's since I have a geothermal system and a geodesic home!

The system does heat and cool very well, I just want to make sure I understand what the system is doing and why.

Thank you in advance for sharing your experience or addressing my questions.

Michael

Systems can be functioning normally and use all available stages depending on conditions and user inputs.

Typically, Honeywell thermostats call for second stage when running at 90% of capacity in first stage. Not sure what type of thermostat you have. It might be in the manual as to when higher stages engage. Manual thermostat adjustments can make the system go into higher stages more quickly so as to satisfy user input more quickly than programmed changes with a "smart/intelligent recovery" feature.

I'm unclear on how a thermostat calculates or otherwise determines "% of capacity" A zone board, on the other hand, will activate 2nd stage depending on % or number of zones calling.

Thermostats typically call for stage 2 based on temperature differential - actual vs setpoint. My Honeywell 8000 series typically call for 2nd stage if delta-T is 2 or more. Some may activate 2nd stage if 1st stage has been running longer than 'x' minutes.

The electrical strip heaters comprising stage 3 of heating may have a system of sequencers bringing in yet more strip heat every 5 minutes or so if a stage 3 call persists.

This is from the Honeywell VisionPRO 8000 manual: "Second Stage Heat and Cool Control
While maintaining setpoint, several factors affect when 2nd stage energizes such as load conditions, environmental conditions, P+I control, and home insulation. The second stage energizes when the thermostat senses 1st stage is running at 90% capacity. This operation is droopless control."

I previously asked here about how this might be calculated with no replies that I recall. Honeywell sites proprietary information, so no luck there either. They just said that higher stages engage when they are needed. :-) The only thing that comes to mind, as far as how the 90% is calculated, is run time, just my opinion.

Posted By geome on 03 May 2010 07:22 AM
  The second stage energizes when the thermostat senses 1st stage is running at 90% capacity. This operation is droopless control."

I see this as pure marketing bull crap.

"Adaptive recovery" logs data and makes changes based on it, I could see the same microprocessor calculating stage usage.
Not that it is anymore useful than a timed or differential system....... as a matter of fact I always disable this feature confident that I know our weather better. We're in a season now where folks are running heating and cooling....let's see "intelligent" thermostat sort that one out!
j

Automatic mode?

Automatic mode is of course available, but not a favorite around here as folks often let the house get a little warmer during the day so as not to have to heat it at night (or heat it as much). My point was shoulder seasons create a target in flux that "smart" technologies will have a hard time keeping up with.
J

Posted By geome on 03 May 2010 07:22 AM
The only thing that comes to mind, as far as how the
90% is calculated, is run time, just my opinion.

The t-stat doesn't know anything about the heat pump.
The only information it has is: setpoint temperature,
actual temperature, (maybe) outdoor temperature, and
time.

The problem with calculations based only on run time
is that run time is a lagging indicator. At best, it can
tell you about "% capacity" as of an hour or so ago.

I'd expect rate of change of temperature (dT/dt) to
be a much more useful predictor, but I've seen no
hint of that approach in Honeywell's marketing hype.

Interesting that they bang on about P&I control when in fact the third letter generally associated with multimode control, "D" is the one needed to react to rate of change (DT/dt) you cite. Nothing is stopping them from implementing full PID control in a thermostat, but I gotta figure it is overkill when the control element is merely tri-state (off, 1 or 2)

When and if fully variable load compressors make their way into our corner of the HVAC world, full PID control might make sense

Posted By engineer on 03 May 2010 06:44 PM
... gotta figure it is overkill when the control element is merely tri-state (off, 1 or 2)

Well, three in Florida, but it's four in my neck o' the woods (off, 1, 2, aux$$).

Sure would be nice to add the 'D' for some predictive capability to avoid
going to aux$ unnecessarily ('P' and 'I' see only the present and past).

Would also be nice to be able to experiment with larger setbacks without
awakening the aux monster. If recovery turns out to be unacceptably slow,
no harm done -- try a different setting. It's my heat pump, not Honeywell's.

In principle, it would be very easy to add yet another (half) stage that would
keep the compressor running in stage2, but kill the DSH -- without calling
for aux$. Of course, in practice, this would require the cooperation of both
thermostat and heat pump manufacturers.

All good points. It has been my (Florida) experience that DSH diversion drops during really cold weather since the pretank gets so warm. Ultimately the DSH pump goes out on high EWT, though I've never made it to that point.

Assuming you have an electric water heater, dropping DSH to avoid aux use is a Peter-Paul situation - non-use of DSH shifts load to resistance water heating. Viewing the house space and water heating loads as a system shows that you'd merely move resistance power use from air to water.

Posted By engineer on 04 May 2010 10:49 PM
...dropping DSH to avoid aux use is a Peter-Paul situation ...

Good point. With an electric water heater (as in my system)
it would just add complexity for little/no benefit.

Yes, the system is in Automatic mode.