Thermostat Adjustment Question
Hi, everyone. I have a question about my thermostat.
Last summer I purchased a brand new electric AC/heating system. It is a 13 seer, 3.5 ton, Rheem. The installer also put in a brand new Honeywell FocusPro TH5000 series thermostat.
Since the colder weather has arrived, I have noticed that the furnace is running 3 minutes on, 5 minutes off.
I did some researching and from what I can tell it could be that the thermostat's heat cycle rate might need an adjustment.
The factory default is 9 cycles per hour, but it also offers alternative cycles of 2,4,6,7,8,10,11, or 12. It shows setting 1 is for a gravity system, 3 is for a hot water system, and 5 is for gas or oil furnaces with less than 90% efficiency.
It does not allow you adjust the temperature variation (like up/down 1 degree) - only the cycles per hour.
I have seen several similiar posts where people recommend switching it to 3 cycles per hour.
So my questions are:
1. Do you think changing the cycles per hour to 3 is right, okay, better for the system?
2. Do I need to adjust any other settings? The emergency heat cycle also defaults to 9. Should this one be adjusted to 3 as well?
I can call the installer (the unit is under warranty), but if it is just an adjustment I can make to the thermostat, I won't have to waste his time.
I appreciate your professional opinions!
this should have been set up correctly for a heat pump with aux heat when installed. It sounds like something else might be going on with the units refrigerent charge or something. I recommend calling the contractor out to check it out.
Originally Posted by ksdallas
I do not have a heat pump. Just a regular furnace/ac.
I'd recommend changing it to 3 cycles per hour if it's a high efficiency furnace (plastic venting). Or 5 cycles per hour for a mid.
This will result in longer cycles, which make your system more efficient. As with driving: town/city vs. highway driving. If you can get your furnace into "cruise control" with longer cycles, you can save a few bucks and save your system some wear and tear.
Dallas, for your Electric Furnace honeywell recommends 9 cycles per hour. Electric resistance heat is 100% efficient, so there is really no run time required to reach steady state operation (like with a gas furnace, or a heat pump). The higher cycle rate will keep you closer to your setpoint.
If the rapid cycling is annoying you, setting to a lower CPH will slow that down, but will take you farther away from your setpoint.
I can't give you diy advice on how to do it, but know that some thermostat setup functions can cause problems or sometimes damage to your system if not set properly.
It is quite easy to change the CPH setting in TH5000 Focus Pro by following the installation guide. Dont change the other parameters if you dont understand the meanings.
Your current CPH is 3, meaning every 20 minutes the thermostat checks whether it is necessary to start heating or cooling system, and your sympton indicates the CPH may be low. Increase your heating CPH to 6. See whether it improves.
Normally for Heatpump, the CPH is recommended to be 3 in both heating and cooling. Compressor will be damaged if too many times on & off in a short period.
Originally Posted by leoxxl
OP, Set your CPH to 3. I don't care what Honeywell recommends for electric resistance heat, 9 is too many cycles per hour.
Originally Posted by beenthere
Could you kindly explain how CPH works?
CPH, is the digital equivalent to the old heat anticipators that actually heated the thermostat.
In Honeywell thermostats its just an algorithm based on previous on and off times, to attempt to maintain an X amount of temp differential/deadband/droop, by increasing or decreasing the on and off times. And is only accurate when the homes heat loss or gain is at 50% of the equipments heating/cooling capacity. The size of the equipment will determine when the thermostat can maintain its set CPH. And as the outdoor temp varies, the actual CPH will also vary, and the duration of the on and the off time will change accordingly with one increasing and the other decreasing.
A thermostat set for 3 CPH, will have a continuous run time when the outdoor temps and indoor set temp are at design conditions, and the heating or cooling equipment is sized for those design conditions. If the outdoor temp rises 5 degrees above outdoor design in winter, even with the CPH set to 6, the system will be cycling at 1.5 or 2 cycles per hour. If it rises another 5 degrees, the stat may begin to have 1 or 2 minute off times, which becomes very annoying and hard on the equipment.
Temp over rides CPH setting. The thermostat continually monitors temp, And recalculates when it should bring on the heat or A/C. And how long it should run it. And then when it brings on the equipment, in monitors the progress and recalculates how long it should keep on running the equipment. Along with how long it should keep it off once it shuts the equipment off. It is constantly recalculating the on and off time by monitoring the temp. Temp over rides CPH setting.
A higher CPH means more on and off times an hour, so the equipment will short cycle. But maintain a very tight temp control of the hose. Very comfortable, but hard on the equipment.
A lower setting will allow a wider temp swing in the house, and be much easier on the equipment. The temp swing increase will only be a few tenths of a degree.
Some where on this forum, I have a much better explanation, but I don't know where it is.
Thanks a lot for the info. The algorithm is more complicated than I thought. Never done any research on whether Honeywell thermostat logic is based on PI control. If it is a PI control loop, the CPH value may be a combination of Pband/deadband/intergral time and delay time. Looks like its more than a simple cycle rate.
Originally Posted by beenthere
I think, if CPH is set to 6, the system cycles 6 times per hour maximum. If its like what you said 1 or 2 minute off times, there still will be high risks short cycling the compressor even though CHP is set to 3 or 2.
Thank you everyone for replying.
I do not have a problem with the fact that it is cycling 9 times an hour (current/factory setting), but I am concerned about the wear on the machine as well as my electric bill.
I know how to adjust the setting, I really just want to make sure it is the right thing to do. I would hate to mess up a brand new system!
Short off times will only occur when outdoor temp is near design temp(on properly sized systems).
Originally Posted by leoxxl
It is PI control.
I called the installer this morning and he recommended switching the CPH to a 4. I made the adjustment and it solved the problem instantly. I appreciate everyone's input!