Proper run times (and thermostat CPH)

[mike08054]

I often read about "longer run time" when the furnace is properly sized. Is there a target "ideal" run time that you get when the furnace is properly sized?

If there is an ideal, or target run time, what is the average time between cycles.

Also, how to cycle times relate to the cycle per hour settings on the thermostat.

Thanks!

[hvactech1jm]

it all depends on the load the system has on it. like windows doors insulation concrete floors sky lights appliances

[skippedover]

A 1-stage furnace, properly sized, will run continuously at design temperature. In other words, if the design temperature is +5*, then when the outdoor temperature gets down to +5*, the furnace is maintaining the indoor design temperature but never shutting off. If the OAT goes down to +4*, the indoor temp. drops 1* but the furnace is still running.

Now when the OAT is 10*, the furnace is actually 'oversized' for the existing conditions, meaning it's delivering more Btu's/hour than the house needs. So if on a +5* OAT day the house needs 60,000 Btu's of heat every hour and as we've stated that's the output from the furnace. When the OAT is +10*, the furnace is still delivering 60,000 Btu's/hour but the house only needs 50,000 Btu's per hour. Thus, the furnace must turn off for some period of time so that it only puts a total of 50,000 Btu's into the home each hour to prevent over heating or over shooting the t-stat set point. This is the reason a 2-stage furnace is more comfortable. In reality, because the 1st stage heat is significantly smaller than the home needs in very cold weather, it's like having 2-deisgn temperatures. As the OAT gets closer to the warmer of the 2-deisgn temps, the furnace runs longer and longer cycles until, just like the +5* example above, the 1st stage is running constantly and any lower temperature will begin to cycle the 2nd stage heat on and off. As it gets colder and colder, the 2nd stage will be on longer and longer, the 1st stage will be on constantly until the OAT gets down to +5* in our example, then 2nd stage is on constantly, just like the 1-stage furnace.

[deejoe]

wow! just what we all need.....a furnace that runs CONTINUOUSLY at design temperature.:rolleyes:
I still say having an "slightly" oversized furnace is best.:p
Everyone needs a rest at some time or another, including a furnace:)

[bmathews]

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.

[super_tech_1]

a furnace was built to run i would rather wear out a gas bill that ill have regardless then wear out a furnace because its oversized. But thats just me.

[mobileguy]

5 cycles per hour is what i've been taught as rule of thumb, if the furnace is the proper one for the house, and typically is what I find to be true. Let me know if I'm out to lunch on this one!!!

[fxb80]

The thermostat CPH is the maximum number of cycles per hour, and 5 or 6 is a typical setting for a properly sized furnace. A fewer number of cycles can result in complaints of the space temperature being too warm then too cold. Increasing the number of cycles can result in more even space temperature, but at the expense of more wear on the furnace (HSI, relay points, etc.), and a need to increase the CPH would suggest the furnace is oversized. A non-condensing furnace needs to run a minimum time to heat the vent enough to prevent condensation drain back into the furnace. What that minimum time is depends on the length and construction of the vent. The idea that a furnace should run continuously at design temperature is true, but assumes that the ductwork is either in the conditioned space or is well insulated and free of leakage. A system with duct leakage that permits outside air to be drawn in, or warm air to be leaked out of the conditioned space (that will cause outside air to be drawn in through the structure as the space depressurizes) could be more efficient if the furnace doesn't run all the time.

[deejoe]

5 cycles per hour is what i've been taught as rule of thumb, if the furnace is the proper one for the house, and typically is what I find to be true. Let me know if I'm out to lunch on this one!!!
__________________________________________________ ____________ha ha ha.... next joke, please
btw, you can stick that rule of thumb on your lunch plate and eat it.

[beenthere]

A high number of CPH(5+) will keep the temp very close to set point(within a few tenthes). It will also increase operating cost and wear parts out quicker.

3 CPH will handle most residencial applications, very few houses need 4 CPH let alone 5 or 6.

[mobileguy]

__________________________________________________ ____________ha ha ha.... next joke, please
btw, you can stick that rule of thumb on your lunch plate and eat it.

Good to see you(Deejoe) don't agree, even better to see others do.

Perhaps you can post some insight. Til then I'll follow the 5x hr, my customers are comfortable, and the furnaces are working as they were designed too.

[mayguy]

5CPH is too hard on the equipment.. most of all the newer ones.. 3CPH is bes settings, and I've had a few people come back saying they notice an increese in comfort..

Some times 5CPH isn't a long enough run time to get the air warmed up in a long run of ductwork.

You will only see the xCPH happing when the system is at 50% load. Will be less when the heating load is higher, and less when load is light.

[RyanHughes]

A relative of mine has a York Diamond 80% AFUE furnace. The VisionPRO manual recommended 5 CPH for standard fossil fuel forced air systems and 3 for high efficiency fossiel fuel forced air systems. I wish it allowed for a setting of 4. Right now it's at 5. This is a maximum setting for the VisionPRO, correct? I'm still debating whether I should go back and move it down to 3 or not. When I was there I noticed the furnace ran for short periods of time (I don't think it was too cold outside). As it gets colder outside perhaps it will run longer cycles. Is this something that you would be concerned about? I'd appreciate input on this. Thanks.

[mayguy]

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.

[sskzekeman]

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)
ON/(ON+OFF) =load/furnace output=(Troom-Toutside)/(Tr-Tdesign)
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.

[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?

[beenthere]

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.

[RyanHughes]

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

[amd]

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.

[RyanHughes]

With a 80% 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.

[amd]

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.

[jechow]

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?

[RoBoTeq]

wow! just what we all need.....a furnace that runs CONTINUOUSLY at design temperature.:rolleyes:
I still say having an "slightly" oversized furnace is best.:p
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.

[beenthere]

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.

[deejoe]

[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.

[RyanHughes]

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?

[beenthere]

Switch it to 3

[cmajerus]

I set all furnaces with circuit boards to 3, 5 cycles is too much. they turn on as soon as they shut off, is the complaint I get from leaving them at 5.

[RyanHughes]

Thanks. It will have to wait until this weekend most likely, but I think it can last another week. It's been like this for a bit of time anyway. Thanks again - I think 3 is a more reasonable setting.

[kitra]

Our 2-story 2670 square foot home had a 1989 Rheem 80% 150,000 BTU furnace. The thing short-cycled like crazy but still worked. New Comfortaire 92.3% furnace installed with 75,000 BTU input, 69,000+ output. Gas bill basically cut by 40%. We had 3 bids but only one contractor spent 2+ hours with a laptop computer entering data like window square feet, 2 x 4 walls, R-value in ceiling, cubic feet of area, etc. He did say we have insufficient return air (one 16" return) but couldn't figure a way to increase it as the plenum it ran through couldn't be increased. We were concerned about going to such a small furnace but couldn't be happier. We probably should have sprung for the 2-stage furnace, however. Anyway, the contractor said to heat our home for a differential of 50 degrees, outside versus inside, it would take 71,400 BTU's so he selected the 69K BTU furnace. Out here on North-West Coast, the winters are quite mild and 20 degrees outside is rare (70 degree inside assumption). Anyway, we feel we had a great contractor and kudos to you HVAC Guys and Gals that take the time to do calculations and explain it to the customer.

[Freezeking2000]

Our 2-story 2670 square foot home had a 1989 Rheem 80% 150,000 BTU furnace. The thing short-cycled like crazy but still worked. New Comfortaire 92.3% furnace installed with 75,000 BTU input, 69,000+ output. Gas bill basically cut by 40%. We had 3 bids but only one contractor spent 2+ hours with a laptop computer entering data like window square feet, 2 x 4 walls, R-value in ceiling, cubic feet of area, etc. He did say we have insufficient return air (one 16" return) but couldn't figure a way to increase it as the plenum it ran through couldn't be increased. We were concerned about going to such a small furnace but couldn't be happier. We probably should have sprung for the 2-stage furnace, however. Anyway, the contractor said to heat our home for a differential of 50 degrees, outside versus inside, it would take 71,400 BTU's so he selected the 69K BTU furnace. Out here on North-West Coast, the winters are quite mild and 20 degrees outside is rare (70 degree inside assumption). Anyway, we feel we had a great contractor and kudos to you HVAC Guys and Gals that take the time to do calculations and explain it to the customer.

Sounds like you found a good contractor. Most furnaces are 50% or more to large for the load.

[kenney t]

I set all furnaces with circuit boards to 3, 5 cycles is too much. they turn on as soon as they shut off, is the complaint I get from leaving them at 5.

:ditto:

[WillieOH]

I have a 90+ furnace fully modulating burners and blower with a fully modulating communicating t'stat that is currently running at 3cph (measured) with a total swing of 1.25 (plus minus 0.625) degrees at the t'stat. Outside temps have been around 30 to 45 degrees. Anticipator setting set to 'slow'.

It runs about 9 minutes at varying outputs then off for about 9 minutes. This is plotted on a chart so it is easy to see.

Will this temp swing increase as ODT decreases closer to design temp while cph stays constant, or will it just cycle more often with the same temp swing? There is no temp swing adj on the t'stat. This is all new installation, so I don't have much history yet.