Hysteresis on the Honeywell old-style bulb thermostat

[ssinzig]

[Mark Lloyd]

[snip]

The mercury switch provides some hysteresis.

[Dave M]

Yep, I understand that the "Spring controls the position of bulb depending on temperature". That was, I thought, my intention to describe. When the spring bends far enough to tip the bulb, the weight of the mercury in the bulb swings the bulb a bit farther , requiring the temperature to cause a greater swing in the other direction to make it switch back. That's where the hysteresis comes from. That was the question from the OP, which is what I was trying to answer.

cheers, Dave M

[Tom Miller]

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

[Dave M]

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

[Jerry Peters]

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.

[Tony Hwang]

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.

[Tony Hwang]

Sounds funny, then your system will blow cool air before warm air start blowing out.

[trader_4]

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.

[Jerry Peters]

93%. It's 30 years old, no digital controls. There's a standard fan/limit thermal switch, but it only turns the blower off.

[Jerry Peters]

For perhaps 30 seconds or so, then the secondary heat exchanger starts warming the air while the primary is heating up.

[Tony Hwang]

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.

[trader_4]

They made a 93%, back in 1985? What brand and model?

[trader_4]

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