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Old 01-21-2013, 07:07 PM   #1
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Temp to switch from heat pump to gas furnace


We have a TRANE dual heating system consisting of a XL16i heat pump and a HE gas furnace (XV95). It has performed very well with excellent efficiency. It is set to switch from heat pump to gas furnace when the outside temp falls below a set value. With gas prices falling and those for electricity being stable, the optimal performance will occur at a slightly higher set temperature to switch from heat pump to gas. I am trying to calculate the optimal set temperature

I contacted Trane to see if they had a formula or spreadsheet model to calculate the optimal temperature to switch from heat pump to gas heat but they were not helpful at all. I can possibly figure out the formula myself with the necessary inputs. I would need the efficiency curve of the heat pump showing the conversion rate of a kwh into heat as a function of outside temperature. I think that the efficiency of the gas furnace is not dependent on the outside temperature, but I do need to know the conversion rate of a volume of natural gas into a unit of heat.

Does anyone have a formula or model or the necessary inputs?

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Old 01-22-2013, 07:31 AM   #2
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Temp to switch from heat pump to gas furnace


I could be all wrong but I am disposed to believe that natural gas is always more economical than a heat pump.

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Old 01-22-2013, 09:05 AM   #3
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Temp to switch from heat pump to gas furnace


Thanks PoleCat. We live in the Pacific North West. Prevailing Temperatures are in the 40s and low 50s much of the heating season. Supposedly that range is the sweet spot for a heat pump. Electricity prices are relatively low because of proximity of hydropower facilities but gas is typically somewhat more expensive than, say, in the Mid West (where we used to live). That led me to the heat pump. As a result of another post I did get the COP curve for my heat pump. Still looking for the one for the furnace. Given those I should be able to do the calculation and see which is more economical and will report back. Stay tuned!
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Old 01-22-2013, 09:10 AM   #4
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Temp to switch from heat pump to gas furnace


Quote:
Originally Posted by PoleCat View Post
I could be all wrong but I am disposed to believe that natural gas is always more economical than a heat pump.
you can not re-use ng as it burns. heat pumps use the refrigerant in the system to both heat and cool a dwelling, it doesn't burn off so that's yours to keep, paid for once at time of install.
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Old 01-23-2013, 06:44 AM   #5
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Temp to switch from heat pump to gas furnace


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Originally Posted by Doc Holliday View Post
you can not re-use ng as it burns. heat pumps use the refrigerant in the system to both heat and cool a dwelling, it doesn't burn off so that's yours to keep, paid for once at time of install.
Around here it is the electricity that it takes to run a heat pump that gets expensive. Plus we have too many winter days that have so much moisture in the air that the unit requires a lot of defrosting.
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Old 01-23-2013, 07:31 AM   #6
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Temp to switch from heat pump to gas furnace


If you can get all the necessary info, there is certainly a way to calculate the optimal switchover temperature (sounds like a question on an Engineering exam LOL). Sorry I can't help (I wasn't the right kind of engineer) but am curious to know the answer. My home has a hybrid setup like yours, except my backup furnace is propane, which is still fairly expensive. I'm in the Eastern Panhandle of WV so winter temps are much lower than yours - as I'm typing this it's 13 degrees outside. I've programmed the system to switch to backup at 25 degrees. No particular reason for that number other than I like the hot air that comes from the furnace.

Where in the Northwest are you? We used to live in the Tri-Cities (Richland) when I worked at the nuclear plant.


Last edited by md2lgyk; 01-23-2013 at 07:34 AM.
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