Two stage furnaces and thermostats

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[Quote] In residential, its only Heat Pump resistance heaters that are sometimes enabled/disabled off of outdoor temp. --- high efficiency gas fired furnaces are not...BUT, it is possible to modify such a system to accomplish it. [Quote]
OD temp *is* required for balance point changeover on dual fuel systems, but not for gas furnaces ... high efficiency or not.
Heat pump with resistance heat back-up will run the strips when needed during defrost cycle so it won't blow snowballs, or if the heat pump isn't keeping up with the heat loss.... no additional OD temp sensor is needed or required. FWIW, Rheem heat pumps have sensors built in that will shut down the heat pump and energize strips only when the OD temp drops below 6F.
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When Carrier air to air Heat Pumps were in their infancy, many of the installations operated with OD thermostats enabling the resistance heaters to operate in conjunction with the Outdoor Unit during normal operation . I still have the training material from that time depicting this.
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In residential, its only Heat Pump resistance heaters that are sometimes enabled/disabled off of outdoor temp. --- high efficiency gas fired furnaces are not...BUT, it is possible to modify such a system to accomplish it. The OP is asking if it would be beneficial for him to do it off of outdoor temp. since he accurately knows that first stage is good down to about 20 f. for his application ; he could very easily do it this way...but its not going to be the most economical . On your furnace....you are using a single stage Thermostat in conjuction with the 10 min. Time Delay feature of the furnace to bring in 2nd stage . In my opinion, 10 min. isnt long enough but you cant do much about that --- youd see better economy if you had a true 2 stage Thermostat operating the furnace off of true space demand and eliminating any time function . Of...if you were proficient , you could at least increase the 10 minutes to 20 minutes by wiring in an independent 20 minute Time Delay Relay for 2nd

Or if you just want to be geeky, use 2 single stage thermostats, the first between R and W1 as per normal the second between W1 and W2. Set the first a couple degrees higher than the second, so that as long as the first stage is enough to keep Tstat 1 satisfied stage 2 never runs, if stage 1 can't keep up the temp drops and the Tstat for stage 2 kicks in.
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I am no expert, but do know that some hot water furnaces do have a sensor for outside temperature and adjusts the flame based on the outside temperature. In my case i was thinking of just a switch controlled by the outside temperature . And use that to switch the furnace to high heat when it is really cold outside. That is not mentioned in the furnace manual, but seems like it would work nicely.
Dan
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snipped-for-privacy@krl.org wrote:

Hmmm, That does not make sense. Buildings have different air tightness and insulation. Monitoring outside temp. won't do much for any thing. For humidity, Yes but temp., No. Furnace will do it's own thing depending on demand from 'stat.
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