Back to basics
CPH (Cycles Per Hour) function is provided to
Originally Posted by jeepgrady
minimize the number of on/off cycles
when there are low to medium load conditions.
The thermostat determines the actual length of time required to heat/cool the house based on satisfying the temperature set-point within a given tolerance.
Last edited by dan sw fl; 02-04-2008 at 06:48 AM.
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Color me as another confused one for CPH. I always thought it was Cycles Per Hour. In the cooling portion of the setup manual, it calls it that. Or is it different for cooling?
What I see, is good temperature control (temperature is always at setpoint) but the furnace appears to cycle more than I'd like. I'd rather see a longer run time than numerous stops and starts. I don't mind a droop of a degree or two (I'm not that sensitive to variations). I set mine for 2 CPH but it doesn't appear to make a difference. I really hate to think of the one CPH as my only option.
Most thermostats go by a differential in temperature to cycle the furnace. That is if the temp goes below x degrees below (heat) or above (cool) then the unit starts. Cycles per hour, to me, means that the number of starts per hour will not exceed the number set (2 in my case). Yes/No?
Any other options besides replacement?
Also, what determines when second stage kicks in? There is no mention of when it does this in the manuals. All I can see is a CPH setting there as well.
This is my second VisionPro (the first one died with an EE error code). I originally thought the issue was a defective tstat. But the second stat does the same.
Over sized furnaces and A/C's can also be the cause of short on cycles.
Beenthere- Thanks for that detailed explanation. It was very helpful and informative
Thanks for your help......I have a better understanding on this subject
Originally Posted by beenthere
I agree, sounds like it's oversized if it's doing the on and off often on 2CPH. The stat is a great stat, and with a wrong equipment in the home, the stat is not going to work as you'd like it do.
Definitely not over sized. My Robertshaw/Invensys Insight 2-stage thermostat works it just fine. My thoughts are that it just has a very small dead-band since it tries to maintain a constant temp (i.e. less than 1 degree). It turns on even when the display is at setpoint. Hence more cycles. If I could get it to drop 1 degree before it turns on, I'd be happy. Why should the furnace come on when it is at setpoint? Certainly not an over sizing issue.
Over sized means it would shut off right away, which it does not. It runs for a while after it starts. Just no noticeable droop or dead-band. What this thermostat does is it makes the furnace comes on too early making for more starts per hour or counts or whatever.
Many digital thermostats don't display any temp drop when they start the furnace. So that is not an indication that the furnace isn't over sized.
How often does it go to second stage stage.
So your saying the display on the thermostat is incorrect. The temperature it displays is right at setpoint, granted it could be a few tenths low. According to you, it could be less. Not argumentative, just looking for an answers. My Robertshaw would drop to about 1.5 degrees below setpoint before starting the furnace. This was visual at the stat. It displays tenths of a degree.
Originally Posted by beenthere
Are your saying that the size of the furnace determines when the furnace will start? I thought the size would determine how long it runs.
During a normal cycle (i.e. not recovery), I cannot tell by visual or audible sounds if it hits second stage. I'd have to open it up and watch it closely or put a meter on the W2 terminal. Again, what determines when it hits second stage?
This AM, I am near the furnace. It shut off and restarted around ten minutes later (did not catch time so it could have been 15 minutes). I don't think it hit second stage as it ran for around 15 minutes. It restarted 15 minutes later and ran for 15 minutes again. So that appears to be 2 CPH.
I just want to learn about how it works and not have to replace my furnace prematurely due to excessive cycles. Thanks for bearing with me on this.
First: It is Cycles Per Hour.
Currently you seem to be getting your 2 CPH that you set it for.
The display is most likely correct. Honywells stats don't show tenthes of a degree.
If the stat starts senses its is not maintaining temp, it will bring second stage on.
If a furnace is over sized, it will cause the furnace to short cycle no matter how good the stat is. So yes, an over sized furnace could cause the stat to start the furnace early to meet its CPH.
What temp was it outside this morning when you noticed how long the furnace was on, and off.
What is the design temp for your area.( Lowest normal winter OD temps ) What size is your furnace, and what is the sq ft of your house.
Average temp for this time of year is 20 - 25. Min temps are in the middle teens. It was warmer this morning, around 30.
1600 sq foot ranch built in the 60s with average insulation for the vintage.
75k btu furnace. This is the same size recommended by 3 separate contractors using load calcs.
The CPH was referred to as counts in a previous post. I did confirm this AM it was doing 2 cycles an hour. I may give 1 CPH a try and see how it does, just for giggles.
So with a milder day, it still met the 2 CPH. A colder day would have netted a longer runtime, yes?
Does the stat *have* to do 2 cycles per hour? Or is that a maximum number of cycles.
We use these stats and if people take the time to learn how to use them they are very nice. Most people are intimidated by them or technology challanged and don't want to mess with it.
Hence, I ask and want to know how they work. But I don't want to shorten the lifespan of my furnace due to excessive cycles. The quest continues.
Originally Posted by tomwr1958
Not intimidated at all and not afraid to experiment. CPH is now 1.
36 degrees this AM. Furnace just shut down. Will see when next start is. If it is over sized furnace syndrome, it will show here.