Heating, Air Conditioning, Fridge, HVAC/heat pump and thermostat settings
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: Hi Alex, The first thing I would do is move the blower speed to medium. That might give you a few more degrees. Your blower motor should be a 3 or 4 speed motor. It's also possible it a variable speed motor. Can you send me the model number on the furnace?
I have never seen 110 VAC heat strip. Most likely it's 240 VAC. If one heat strip is pulling 50 AMPs then that's a problem. A 5KW strip will pull about 22 AMPs at 240 VAC.
Sounds like you heat pump is working properly. 85* supply at 20* outside temp is normal. I would rather have the heat pump run for several hours rather bring the heat strips on. I have a 2* diff. set on my stat. Your much better off just running the heat pump. I usually switch my stat to emergency heat when it gets below 20* outside.
Let me know. JIM
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Thank you so much for a reply.
You are absolutely right: my heat strip is 240VAC and each coil is pulling 20 Amps, so that's settled and working properly.
I can send you my blower motor info, but my HVAC tech assured me that it has 2 taps for High and Low speed and it is absolutely not a variable speed. Originally it was wired for Low, but he switched 2 wires and now it is on High. We talked about that: according to his logic, on Low it blows less air, but the A-coil stays hotter and the registers air temp is higher. At High it blows more air, but its temp is a little lower due to the A-coil cooling effect by the rushing air. So, it is a wash and he suggested High for some reason. I can easily switch it back to Low, just explain your reasoning, please.
And I will heed your advise and set my Aux diff to 2 degrees.
What is the advantage of manually switching to Emergency at below 20 outside? To allow the heat pump to take it easy (shut off)? Why not just let it run and go to a defrost cycle as needed by itself? Or, it will not do much at such low temps, struggling in vein and the forced Emergency will save the day?
Alex, I have never seen a 2 speed blower in a furnace. I think what the tech might have said the control board has only 2 blower terminals. One would be labeled heat the other cool. If you look at the control board you will see terminals marked Park. Those are the other speeds for the motor. I hope this makes sense.
The only way I would run it on high speed is in cooling mode.
Let me know about the furnace model number, JIM.