Heating, Air Conditioning, Fridge, HVAC/heat pump settings

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Question
I live in Cincinnati, OH in a well-built 13-year old second floor condo in a two-story building. Our HVAC is a Goodman GSZ13 3 ton heat pump (replaced 5 years ago) and a matching air handler (A-coil replaced recently), equipped with a single-speed blower (having a High and Low taps and currently wired for High speed by my tech). It also has a 3-coil heating strip, each coil 5KW to a total of 15KW (measured amperage of each coil at about 50 Amps, or about 5KW given 110 VAC voltage, so it has been confirmed as working to spec). I control it with a Braeburn 2220 digital thermostat. I specifically picked it because it has a wide range of stage 1 and stage 2 temp differential stage 1 at .5 and 1 degree and stage 2 at .5, 1, 2, 3, 4, 5 and 6 degree). The air is circulated thru the flexible ducts and ceiling registers.
I moved in this house last year and this is my first real winter. I am trying to figure out the best and most economical ways of running heating. My observations so far are as following:
-at moderately cold outside temperature (say, around 20°F), the heat pump is still running O.K., but the Freon temp (judged by the supply copper line temp) is not very high, probably around 90-100° to touch (I can measure it more accurately though). The air temp coming out of the registers is around 85°. I set the thermostat to 1° stage 1 differential and 3° stage 2 differential and program my day times and temps so that it supposed to rise gradually, not kicking in the Aux heat (to save on electricity). By the same token, the heat pump is struggling keeping up with it and is working overtime, sometimes for hours on before it raises the temp to my set 71-72°, meanwhile our house feels kind of chilly.
-if the Aux heating strips kick in, the ceiling registers air temp picks up drastically to around 120° (which is expected), heating up the house quicker. But that does not happen often because of my thermostat stage 2 differential settings.
-so, I am wondering about this common dilemma: to run or not to run Aux heating strip. On one hand, it is very expensive. On the other hand, running just the heat pump at low outside temps causes some discomfort and long-long non-stop compressor running. Which is more economical?
-I did some “back of the napkin” calculations. My heat pump is about 2.3 KW at moderately cold temp (including both compressor and cooling fan per manufacturer’s specs). My blower fan motor is 1/3 HP or .25 KW. And assuming all three heat strips are ON when Aux is called for, they are 15 KW. My rough calculations are:
1) heat pump running only: 2.3KW+.25KW=2.55 KW
2) heat pump and Aux: 2.3KW+.25KW+15KW=17.55KW, or about 7 times higher.
As we are charged for KWHours, to break even it seems that I need to have option 2 (Aux kicking in) heat up the house to my set temp about 7 times quicker than option 1 (heat pump running alone). It is next to impossible for me to collect data over time, so I am at loss.
-hence, my question: are my calculations and assumptions at least in a ballpark? Or should I set stage 2 (Aux) differential to much smaller, say .5 or 1° which will cause the Aux heating strip to come ON much more frequently and heating the house quicker, but at higher cost per hour?
Thank you for your help.

Answer
This is a common issue with heat pumps.  There are several factors that go into how well they will heat.  Once it starts to dip into the mid to low 30's, heat pumps start to loose their ability to supply enough heat.  You did the math on the KW, but you left out the efficiency of the heat pump.  Since heat strips are in the air stream, they are nearly 100% efficient.  When you run the strips all of the heat they produce is going into the home.  When you are running the heat pump,  you are not going to get all of the heat from the energy it consumes in the home.  The efficiency gets worse as it gets colder outside.  There is a point to which the heat pump just can keep up and you are going to have to use a source of heat that is not affected by the outdoor temp.  Keeping in mind, the greater the temp. difference between the great outdoors and the inside, the more heating capacity you need.

So, with that being said...Once the temp dips down, shut off the heat pump and run only the heat strips.  You are going to have to monitor things and see where that point is.  Sometimes there is an outdoor sensor installed on the t-stat to do that automatically or a controller mounted near the outdoor unit.  I do not like two stage stats.  In some cases, it is against building code to run both the strips and a heat pump, except in defrost mode.  Set the t-stat for heat pump or emer. heat/aux heat.  The other reason not to use two stage is you do not want to use the strips when the heat pump will do just fine.  Example...You leave for the day and turn the heat setting down to 68.  It is in the mid 40's outside.  You get home and turn the heating temp to 72.  I want the heat pump to run a bit longer then have the strips come on.  

As I do not know your lilving arragemtns, you might find it bennifical to only heat the room(s) are are using.  It is cheaper to heat smaller areas, rather then the entire home.  

Craig

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Craig HVAC Expert

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I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.

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I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

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5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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