# Thread: Design Temperature

1. Regular Guest
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Several times on this site the term "design temperature" has been used. It has been defined as the temperature at which a furnace should run all the time and is specific to the city/area where the furnace is installed. My question: With a 2 stage furnace, would the furnace be running all the time at first stage or second stage? Also, running all the time to keep the house at what temperature? It would be quite different to keep the house at 63 degrees vs. say 72 degrees by having the furnace run all the time. Thanks

2. Heat load calcultations use both an indoor design temp and outdoor design temp. If the load calc is done using a 72 degree inside design temp and the design day is 10 degrees then the furnace would run constantly to maintain 72 degrees on this 10 degree day if it were sized exactly to that load.

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Ray,
A properly designed and sized system should run constantly on second(high) stage at outdoor design temp to keep the indoor temp at designated indoor design temp.. That being said,,, a properly designed 2stage system will run almost constantly on low cycling to high as needed then back to low well above the design temp.. This is the reason a good 2stage t-stat is needed to properly control the unit..

Any other q's just ask..
J

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Thanks for the replies. That's pretty clear. I guess a person hopes that the furnace doesn't end up having to run very long at the design temp, just from the standpoint of expense. I calculated that my furnace uses almost a dollar's worth of gas/hour at second stage. That could get spendy pretty fast!

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Ray,
If the furnace is designed/installed/set-up correctly the 2nd stage will run constantly at design outdoor temp/indoor temp.. It will cost no more to run than a oversized furnace that short cycles at the design temp and will have a more comfortable temp in the house due to the more constant running... An oversized furnace consumes more gas in a shorter amount of time so it will use the same amount of gas but give less comfortable feel in home than a properly sized unit that constantly runs at design temps..

Also a oversized furnace that cycles at design temp is very oversized 90% of the time that the outdoor temp is not at design temp which means even more short cycles and less comfortable temps in home..

Hope this helps..

If you want cheaper operational costs check into dual fuel heat pump system with gas back up.. I personally like the Carrier or bryant two stage TS heat pump with 90I gas back-up with evolution/infinity control.. Cream of the crop..

J

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Thanks. I'd been thinking about a heat pump. Can I use the Carrier or bryant two stage TS heat pump with my new Trane furnace, or should I stick with the same manufacturer as my furnace?

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You can only use the Infinity (Evolution) control with Carrier (Bryant) top of the line furnaces and air handlers. So it won't work with a Trane. And without Infinity, there's nothing so incredible about the Carrier HPs that would make you want to cross brands.

I'd suggest just using a Trane heat pump of your choice. With a relatively new furnace, you should be able to get a match that's ARI listed. As always with dual fuel, make sure to get a heat pump with demand defrost, not time-temp defrost (this isn't much of a problem with Trane).

In lieu of the Infinity, I'd recommend the Carrier dual fuel thermostat or Thermidistat for controlling dual fuel systems, especially if there is any multistage equipment involved. The dual fuel stat will handle three heating stages, and the thermidistat will handle four (two speed HP, two speed furnace). The VisionPro is popular, looks cool, and is said to have some dual fuel capability, but at the moment the capability is very limited with respect to staging and also fuel selection logic.

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