# Thread: Proper run times (and thermostat CPH)

1. Ryan, there is no harm of moving it to 5. I know I've changed my cousin's to 3 and he said the house felt better, and less on and off. You can always move it back to 5 if you feel it over heats the house.... I think it may let you set it to 4.

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For a furnace, if you want to hold a differential temperature in the room, say 2 deg, then you have NO control of the CPH. Under these conditions, the CPH depends on the heat capacity of the house, the house leakage and the furnace output.
For example, say it takes 20 minutes for the house to lose 2 deg when the OT is 30 deg, then the furnace "OFF" time is fixed, in this case is 20.
You get the CPH from the calculations:
CPH=60/(ON+OFF)
So continuing the calculation, if the Toutside is 30 deg and the design temp = 0 deg, and the indoor design temp is 70 deg, I get
ON/(ON + 20)=(70-30)/(70-0) and this becomes
ON= 26.6 minutes of furnace "ON" time

And therefore in this example the CPH is =60/(26.6+20)= 1.06 CPH. For more leaky homes the, The CPH would go higher. Obviously, the higher the CPH the lower the differential, but it isn't cost effective to go too high., just
more comfortable.
So, to specify a CPH arbitrarily might lead to very different comfort levels depending on the tightness of the houses.

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still not sure if I understand why CPH is set at the thermostat. Is this an indirect way to adjust the temperature drop between cycles. What tells the thermostat to come on after a half degree, 2 degree, or 5 degree drop from the set point.

Does a set CPH equal a maximum for cycles? In other words if you set at 3, does that mean the furnace will run, less than or equal to 3 on-off cycles, but never 4?

I'm more interested in homes with 2stage w/ VS or modulating furnaces (90+ condensing). If you walked into the home and the furnace was set to 5CPH and actually running 5 cycles then I can't imagine that it's running more than 5-6 minutes. Is this ever a good thing when the outside temp is moderate? or is very cold?

4. It takes the stat a while to learn the house.
It will shorten its on time to regulate the CPH to what you have set.

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Beenthere: So essentially the thermostat will do its best to meet the CPH set?

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Originally Posted by mike08054
still not sure if I understand why CPH is set at the thermostat. Is this an indirect way to adjust the temperature drop between cycles. What tells the thermostat to come on after a half degree, 2 degree, or 5 degree drop from the set point.

Does a set CPH equal a maximum for cycles? In other words if you set at 3, does that mean the furnace will run, less than or equal to 3 on-off cycles, but never 4?

I'm more interested in homes with 2stage w/ VS or modulating furnaces (90+ condensing). If you walked into the home and the furnace was set to 5CPH and actually running 5 cycles then I can't imagine that it's running more than 5-6 minutes. Is this ever a good thing when the outside temp is moderate? or is very cold?
CPH is a value from which an "intelligent" thermostat derives temperature differential and anticipation. It does not apply when the temperature is far below the setpoint.

Output is determined by the duty cycle. Startup and shutdown losses aside (it might take ten minutes for a furnace to reach peak output, while the same amount of fuel is being consumed), changing the CPH setting does not affect total runtime.

Example:

100,000BTU/Hr output
Heat loss = 70,000BTU/Hr.

70% Duty cycle @ 6CPH = ON 7 minutes, off 3 minutes.
70% duty cycle @ 3CPH = 14 minutes on, 6 minutes off

If you're furnace or A/C is running that means you are spending money via gas or electricity being used. Being off is better in my opinion and on my pocket book. Insulate properly and your house should hold the heat or hold out the heat whatever that particular season may be. If you're not losing or gaining heat that fast, then your unit shouldn't run. Running on low speed or fire is still using energy.
Simple logic check required:

Total energy consumed = Power (energy consumed instantaneously) x Time

Furnace oversized by 100% = (Power x 2) x 1/2 runtime

Theoretically, the product will always be the same.

Once efficiency during startup and shutdown is considered, an oversized furnace will always be less efficient than a properly size one. No exceptions.

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With a 80&#37; gas furnace, is there any problem with having the thermostat at 5 CPH? The place in concern is a two-story condo on Maryland. The home, facing near the sun, gets a lot of heat gain as it is. My understanding is the CPH really applies to when a place is trying to maintain a temperature within a 1 degree margin (i.e. thermostat at 70 with 5 cycles per hour), whereas if there were a 2 or 3 degree margin, the CPH would not really apply until it gets within a 1 degree differential of the setpoint. Am I totally off on this? My real question is should I have my relative's CPH at 5 or 3 on a VisionPRO 8000 for an 80% gas furnace.

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There's no real problem with 5CPH but lower cycle rates reduce wear and improve efficiency.

CPH applies whenever HVAC equipment is maintaining a setpoint. The actual temperature differential varies depending on equipment size, the CPH setting, and heat loss. You will usually never have a differential above 2F on an electronic thermostat.

Some thermostats with adaptive "intelligent" recovery cycle equipment during recovery.

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Why does the Honeywell 8321 thermostat recommend 3CPH for a furnaces >90% efficient and 5CPH for a <90% furnaces?

What would occur if I set my 80% furnace to 3CPH?

10. Originally Posted by deejoe
wow! just what we all need.....a furnace that runs CONTINUOUSLY at design temperature.
I still say having an "slightly" oversized furnace is best.
Everyone needs a rest at some time or another, including a furnace
Unless the temperature is down to the design temperature, every furnace "is" oversized. To oversize a furnace beyond it's already oversizing because of normal conditions is the worst thing you could do to a furnace.

Every time a furnace needs to start up if operates less efficient and causes more wear to all electrical components such as ignitor, blower motor and all relays controlling gas valve, induced draft blower, air blower etc.

The worst thing we can do to a motor is to start it. Therefore, the longer the furnace operates, the less amount of times it is going to have to start, the longer it will last.

Oversized furnaces never last as long as properly or undersized furnaces do.

11. Originally Posted by RyanHughes
Beenthere: So essentially the thermostat will do its best to meet the CPH set?
Yes, it will alter the off and run time to maintain the set CPH.
And this of course only applies when the stat is maintaining temp, not in recovery, or if you bump the stat up a couple degrees.

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[QUOTE=RoBoTeq;
Every time a furnace needs to start up if operates less efficient and causes more wear to all electrical components such as ignitor, blower motor and all relays controlling gas valve, induced draft blower, air blower etc.

The worst thing we can do to a motor is to start it. Therefore, the longer the furnace operates, the less amount of times it is going to have to start, the longer it will last.
************************************************** ********
BINGO!!! your above quotes back up my theory that an oversized furnace is best. As I 've always said 1 or 2 6 minute burner cycles per hour will mean less wear and tear on the parts, etc. .and now ,you Hobo tech have just
backed it up.

As many other jesters on this site have said, 4,5,6. cph's per hour is the norm......
duh,. gee, I wonder if that is hard on the parts etc. (snicker)

I'll take my 1 or 2 cycles per hour (even at design or lower) any day.

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Hmm, varying opinions. I'm thinking about moving it from 5 to 3 for a York Diamond 80&#37; AFUE gas furnace. Any thoughts? Keep it? Switch it?

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