Is there a programmable thermostat (for furnace) that allows the hysteresis
to be changed from the front panel, using keys and display?
E.g. turn on at 65 degree and turn off at 66 degree
Or, maintain temp at 65 with X number of on/off cycle per hour
I have a 10 year old honeywell programmable thermostat. Although the
hysteresis can be changed, it is done with a pair of screws on the back of
the unit, and it is not continuously adjustable, just several predefined
settings to choose from, and is hard to use (I can only guess what the
current setting is since the screws have no indicators).
It worked fine for many years but last year, it started to cycle the furnace
too frequently (felt like every few minutes). Using the screws in the back I
changed to a less frequent setting, but it is too infrequent. There is no
setting in -between.
I need something more flexible and easier to use. Any suggestions?
I just installed a Honeywell RTH7400 in
my motorhome. It really works
great. I was only using it for AC and
Heat Pump. On the last trip, the
Heat Pump was cycling too often so I
checked the manual. They don't
identify it as a cycling setting, but
describe its settings for various types
of heating systems. They only show a
few numbers, however, all numbers
from 1 to 9 affect the cycling. I
adjusted mine to 5 and it seems perfect.
You are talking in terms of old mechanical 'stat. Digital programmable
'stat such as Honeywell Vision Pro series is VERY versatile. Just pick
the right model for your application. I like 8000 series, often over
kill but I like them a lot.
Hysteresis Vs. Deadband
Deadband is different from hysteresis. With hysteresis there is no dead zon
e, and so the output is always in one direction or another.[clarification n
eeded] Devices with hysteresis have memory, in that previous system states
dictate future states.[clarification needed] Examples of devices with hyste
resis are single-mode thermostats and smoke alarms.
Simple (single mode) thermostats exhibit hysteresis. The furnace in the bas
ement of a house is adjusted automatically by the thermostat to be switched
on as soon as the temperature at the thermostat falls to 18 Â°C, for exam
ple, and the furnace is switched off by the thermostat as soon as the tempe
rature at the thermostat reaches 22 Â°C. There is no temperature at which
the house is not being heated or allowed to cool (furnace on or off).
A thermostat which sets a single temperature and automatically controls bot
h heating and cooling systems without a mode change exhibits a deadband ran
ge around the target temperature. The low end of the deadband is just above
the temperature where the heating system turns on. The high end of the dea
dband is just below the temperature where the air-conditioning system start
A smoke detector is also an example of hysteresis, not deadband. The smoke
detector at the ceiling of the kitchen starts the alarm as soon as the leve
l of smoke reaches a certain starting value, x, then the smoke detector sta
ys in the alarm position until the level of smoke has been reduced to level
y, after which the smoke detector is reset automatically to "normal". The
hysteresis here is x minus y.
Johnson, Curtis D. "Process Control Instrumentation Technology", Prentice H
all (2002, 7th ed.)
"Dead Band Plus Hysteresis Estimation with ValveLink Diagnostics". Product
Bulletin. Fisher Controls International. October 2012. Retrieved 18 January
Murty, D.V.S. (2009). Transducers & Instrumentation (2nd ed.). New Delhi: P
rentice-Hall of India. pp. 15-16. ISBN 978-8120335691. Retrieved 18 January
Postlethwaite, Bruce. "On-Off Control". Introduction to Process Control. De
partment of Chemical and Process Engineering, University of Strathclyde. Re
trieved 18 January 2013.
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