# fossil fuel systems



## 53 ford (Oct 6, 2011)

air to air heat pump and lp furnace back up ---i only want the lp to come on if it gets real cold out and if the heat pump wont keep up --what is the best way to do this ? :001_unsure: thank you


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## Houston204 (Oct 6, 2011)

The Honeywell TH8321 with the optional outdoor sensor can do this.



















The section above the highlighted area would apply...










The section above the highlighted area would apply...


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## beenthere (May 30, 2009)

The IAQ (YTH9421) is better for dual fuel systems.


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## Houston204 (Oct 6, 2011)

Looks like it does...

http://customer.honeywell.com/techlit/pdf/PackedLit/69-1816EFS.pdf


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## haleymcadams1 (Sep 5, 2011)

Well between IAQ (YTH9421) and TH8000 series which is better?


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## beenthere (May 30, 2009)

The IAQ. And since the EIM is mounted/installed at the furnace, you don't hear any clicks at the thermostat like you would with the 8000 series.


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## ironpit (Apr 10, 2011)

aprilaire has a stat that can also do this without an outside sensor.


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