Hysteresis on the Honeywell old-style bulb thermostat

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That depends on the furnace. MY blower comes on as soon as the burner ignites, it's a high efficiency furnace, so the blower must start to cool the secondary heat exchanger.
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Jerry Peters wrote:

How high efficiency? Ours are 96% 2 stage one. There is built-in delay for blower to come one. Off delay is adjustable by dip switches. Inducer blower purges vent already before ingnition comes on.
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On Sunday, December 27, 2015 at 5:08:30 PM UTC-5, Tony Hwang wrote:

Same here with 93%. I'd have to measure it, but the air handler blower comes on about 30 secs or a min after the burner lights.
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trader_4 wrote:

Exactly, there is one logical step B4 blower starts. Flame sensor has to sense the flame is on steady B4 letting the blower starts. Furnace operation is single line logical sequence.
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93%. It's 30 years old, no digital controls. There's a standard fan/limit thermal switch, but it only turns the blower off.
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On Monday, December 28, 2015 at 3:39:49 PM UTC-5, Jerry Peters wrote:

They made a 93%, back in 1985? What brand and model?
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Jerry Peters wrote:

Sounds funny, then your system will blow cool air before warm air start blowing out.
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For perhaps 30 seconds or so, then the secondary heat exchanger starts warming the air while the primary is heating up.
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On Monday, December 28, 2015 at 3:41:47 PM UTC-5, Jerry Peters wrote:

Which makes no sense. How is the secondary going to get hot faster than the primary?
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Dave M wrote:

I forgot to add that, in more direct response to the OP's question, that the hysteresis (temperature difference required to switch the HVAC unit from off to on, and back to off) is created by the temperature characteristics of the bimetal spring, the weight of the mercury ball (both unchangeable by the user), and modified by the anticipator resistor.
More Cheers, Dave M
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Kirk Landaur wrote:

Have you read the instructions for the thermostat?
<https://customer.honeywell.com/resources/techlit/TechLitDocuments/60-0000s/60-0830.pdf
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On 12/26/2015 5:37 PM, Michael A. Terrell wrote:

Men never read instructions. Nor ask travel directions. It's part of the male code. We can also pretty much always find a place to go potty, even if it's behind second base in a crowded baseball theatre. No guy will ever comment on another guy who is.... no comment.
When I was doing AC installs, we'd jump R to W with AC ammeter, and then use that figure to set the slider in the old style round stat.
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On 12/26/2015 02:37 PM, Kirk Landaur wrote:
[snip]

The mercury switch provides some hysteresis.

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That's the main reason it is so important to level the t-stat.
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