Adjustable cycle rate differential thermostats
Posted on February 26, 2020
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.
In other words more accurate temperature control. This is done electronically by adjusting the cycle rate(ON+Off times ) of HVAC system proportionate to sum of output of P and I controller.
The PI controller is subset of more sophisticated PID controller where D stands for derivative. The I part alone of controller can make system unstable (Toggling and never narrowing down to set point). So a derivative function is added. It is anticipatory meaning it anticipates error and is used to limit over and under shooting of parameter in high noise or fast varying process. Since house thermostat are ON/OFF, rather than continuous type controllers D is generally not required.
Dead Band or Noise margin.
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 unwantedly 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)
The two dotted green lines are the differential temperature limits while red dotted lines is the set point. Output A is the switching on/off when there is no temperature margins. Output B is when the temperature margin in used (Differential limits is used).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 adjustment
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
You can check manual of the model here before buying – honeywellmanual.com
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.
Robertshaw offer many programmable and non programmable thermostat. All comes with differential setting of 0.5 to 3 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.
Two good models of Robertshaw programmable thermostat are
4.) Venstar thermostat
Venstart 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 is referenced as dead band in its manuals.
It has dead band of 1-6 degree for first stage and 0-10 for second and third stage with adjustable timer between stages.
5.)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.
Emerson offers two 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.
White rogers 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.
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