In modern days where automation has reached the residential and is no longer the norm of only industries and factories there are certain terms or technical jargon that people still struggle with.
Basically automation is part of a subject called control systems. The idea is that you have a control parameter(in our case temperature) and and the set point we want the temperature to be. The work of thermostat is to check the difference between the measured temperature and set point and control the fan or your heating and cooling system to adjust the temperature.
The thermostat is analogous to P+I (in parallel) controller used in commercial temperature controllers. P stands for proportionate I stands for integrate. P part of controller increases output directly proportionate to the difference between set point and actual temperature, while I affect the rate of change of its output proportional to its input by taking into account time duration of error signal(Set point – Actual temperature) to adjust the next switch on and off time accordingly and basically prevent offset error(final range of temperature swing in stable condition). Offset error is high if only P control is used.
The PI controller is subset of more sophisticated PID controller where D stands for derivative. P alone produce large offset or steady state error. It also increases maximum overshoot of system. So I part which produces output rate of change proportional to error is used which reduces this steady state error and bring the system to exact set point but I alone of controller can make system unstable and slow (Toggling for longer time.)
So a derivative function is added to make it stable and improve the transient response(reduce overshoot and undershoot and response time). It is anticipatory meaning it anticipates error and is used to limit over and under shooting of parameter. The main disadvantage is that it never reduce steady state error and produce saturation effect and amplify noise signals. Also since house thermostat are ON/OFF, rather than continuous type controllers D is generally not required.
Note-PID control for is overkill and is not used for house thermostat as it provides continuous or analog time changing (modulation) of heating or cooling capabilities.
Dead Band or Noise margin.
It is a differential also called hysteresis which is included in control system to account for noise or small temperature swing.
The above loop is shows there is +T and -T margin for temperature to swing beyond or below which the output changes. This is done to account for noise and other factors not in control that may change sensed temperature of thermostat. The work of thermostat is to adjust cph to keep temperature in this range of temperature near set point. Higher differential will result in less cph and vice versa.
That is why you may not notice HVAC instruments switching on or off in decimal temperature changes because your thermostat is +-1,2 or 3 differential temperature rated.
The noise may look like below(Graphical representation)
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As you can see using margin for noise etc leads to less unwanted toggling(On/Off) of HVAC instruments.
Why does your thermostat over cycle or cycles frequently.
First understand what is cycling. Cycling is a process of thermostat(temperature controller) to keep the HVAC instrument(ex-AC in summer, Furnace in winter) on and then off for a period of time so as to maximize efficiency. One such period is one cycle.
Why Frequent cycling should be avoided. Frequent switching is damaging to furnace or AC(specially compressor of AC) components due to constant wear and tear in the process. Also it may cost you money because a furnace or AC is more efficient if it is running continuously. It requires some time to build up that efficiency. So if it is switching on/off most of the time it is running predominantly on low efficiency.
Frequent cycling may be because of two reasons.
1.)Your thermostat is sensing some noise signal, like from an air coming through the hole behind the thermostat or air vents, windows, doors etc. To remove that make sure the hole behind the thermostat is properly sealed and no air draft is affecting the thermostat.
2.)Second is because of your boiler or ac system not responding as quickly as desired. So taking into account the “time error” the I part of thermostat controller increase output which in turn increases cycle rate to keep temperature at required set point. Another reason may be that your boiler or Ac is over efficient and delivers heat or cooling faster than required resulting in frequent over shooting as can seen in above diagram. To tackle this you must install a proper sized furnace or AC in the first place after consulting your HVAC person.
If no such option are left you can opt for an adjustable cycle rate differential thermostat.
1.)Honeywell thermostat cycle rate adjustment9
Honeywell thermostats do not come with adjustable temperature differential. It is fixed to +- 1 degree F but with some of the models you can adjust the number of cycles for heating(Winter) or cooling(Summer). After you change the cph(cycles per hour) it may take few hours to adjust because the thermostat has to learn the heating or cooling times so that the temperature is as close to set point as possible.
If you refer to Honeywell thermostat manual it states how you can change the cycles per hour rate using program buttons. It also tells how many cph is recommended for various type of system. Here is a picture of what to find it in the manual.
Some Honeywell thermostat that have adjustable cycle per hour are
HONEYWELL RTH8560D T5 touch screen
and
You can check manual of the model here before buying – honeywellmanual.com
2.)Wyze smart thermostat
If you want a smart thermostat with adjustable differential temperature at an affordable price this thermostat is for you. It comes smart features like AI learning of schedules, smart remote sensors and other Wyze devices like Wyze camera, lock, plug etc compatiblity, geofencing only at a fraction of the cost compared to other smart thermostat in about 70 dollars.
It has a default temperature differential of +-1 degree. But you can change from +-0.5 to +- 2 degrees by going to Settings>System>Differential Temp.
It also has a Minimum Runtime feature that prevent short cycling of compressor by waiting a minimum amount of time before restarting. The default is 5 minutes but can be changed from 2 to 8 minutes.by going to Settings>System>Minimum Runtime. The thermostat will show Cool on for AC or Heat on if you have heat pump during the brief waiting period.
Check out more details,price,reviews and ratings of Wyze smart thermostat
3.)Lux thermostat-
If you looking for a cheap thermostat because you want to control your hvac short cycling issue. All lux thermostat have a differential setting upto +-3 with increments of 0.25 degree F. In these type of thermostat if you system is short cycling just increase the differential temperature and you would be ok.
4.)Robertshaw thermostat-
Robertshaw offer many programmable and non programmable thermostat. All comes with differential setting of 0.2 to 2 degree F for first stage of system. There is also a second stage differential. When first stage cannot keep the desired set point for more than set timer(20 minutes default) and the temperature difference is more than sum of first and second stage differential the second stage system gets on to lower the load on first stage.
For ex if set differential is 0.5 and set temperature is 70 degree and the first stage heating will come at 69.5 degree second heating will come at 69. If there is a third stage it will come at 68.5 degree.
It also has adjustable minimum time( maximum 5 minutes) delay for compressor ON/OFF to prevent short cycling in cooling mode.
Two good models of Robertshaw programmable thermostat are
Robertshaw RS10420T Pro Series 7-Day Wi-Fi Programmable Touchscreen Thermostat
Robertshaw RS9320T Pro Series Programmable Thermostat, Multi-Stage, 3 Heat, 2 Cool
5.) Venstar thermostat
Venstar designs beautiful color screen wifi thermostats. But main feature is that it allows differential temperature of upto 0-6 degree F. Also if the thermostat is designed for two or more stage heating/cooling the it allows differential temperature changes for second stage as well. A timer or time lag is introduced between activation of first and second stage systems and similarly for higher stages. The differential setting is referenced as dead band in its manuals.
You can also limit the cycles per hour in Venstar thermostat which will limit the number of time your HVAC system.
Venstar T1050 5/2 Day Programmable thermostat
Venstar T1700 1-Day programmable thermostat
It has dead band of 1-6 degree for first stage and 0-10 for second and third stage with adjustable timer between stages.
6.)Ecobee smart thermostat-
If you want to go for smart thermostat though i don’t really recommend unless you need it for other than the frequent cycle differential settings. Ecobee is the only thermostat that offers differential spread of 1-3 degree F. All other factors considered it is excellent investment in terms of ts smart features and operation like geofencing and auto adjust using humidity and temperature sensor that you can place any where across the room.
Check out ecobee3 lite Smart Thermostat on amazon.
7.)Emerson Thermostat
Emerson offers thermostat with adjustable cycle rate
Emerson thermostat do not comes with differential spread but offer to change cycle rate as slow , medium or fast from the app.
Slow is suited for hot water boiler steam heating
Medium for Ducted heating/cooling and for most accurate control choose fast.
Emerson 1F83C-11PR Conventional programmable thermostat
8.)White rogers-
White rogers which a brand owned by Emerson also has a two option SL anf FA for slow and fast cycles per hour for heating only.If after changing the cph to FA or SL optimum cph is not achieved contact local service person. Cooling cycle rate is not adjustable and is fixed.
9.)Pro1 IAQ thermostats
This is another brand that have adjustable temperature differential. They offer adjustable differential of +-0.2 to +-2 degreeF with default setting of 0.5 for cooling and 0.4 for heating.
To set the differential select heat or cool slide switch and press both + and – buttons for three seconds and press+- key again to increase or decrease differential in 0.2 degree increment or decrements.
It also has 5 minutes time delay like Robertshaw for compressor ON/OFF to prevent short cycling of compressor.
This company offers thermostat of all types including programmable,set back, wifi, with or without humidity control, line voltage, thermostats with remote sensors, single and multistage thermostats etc.
Check out Pro1 Iaq T701 Digital Non-Programmable Thermostat (1H/1C)
One of the feature offered by company is free private label badge that removes it’s logo if you want.
Many brands that do not have a dedicated site are probably selling rebranded models from sellers like Pro1IAQ or other whole sale sellers in China.
We say so because they all sell almost identical models with almost identical features and functions.
So you can buy from any of them as they all are more or less the same.
They have cheap programmable and non programmable models for single and multi stage as well but their non programmable models usually sells the most.
Few such brands on Amazon are
Heagstat
Electeck
Suuwer
All of the above three brands offers identical models with same differential setting of +-0.2 to +-2 degree F along with 5 minutes compressor delay ON/OF protection and same procedure of setting them.
Check out
Heagstat H721 Non Programmable Heat Pump Thermostat
ELECTECK Non-Programmable thermostat
Suuwer non programable thermostat
Aowel and Garystat
All feature are same except that it has 3 minute rather than 5 compressor ON/OFF delay protection.
Check out
Recommended CPH for various furnaces in well insulated new homes with low heat loss.
1.)Heat pumps-1.5 to 2 cph
2.)Electric furnace-3-5 cph
3.)Gas furnace-3-4 cph
4.)Oil furnace-3 cph