Furnace Run Time

[HowAboutThis]

Our furnace is a Rheem 80 series (I believe is what I recall). One speed fan I believe. So, 80% efficient. Insulation on home is, eh... Decent but not great. 1990 build and probably decent for that time period. When it's 20° to 40° the gas furnace was turning on and blowing heat for about 5-6 minutes, shutting off for about 12-15 minutes (total cycle time about 15-20 minutes or 3-4 cycles per hour). It just seemed weird to me that the thermostat is constantly at 68 and never drops before kicking on again. I checked the settings on the thermostat and it was set to a CPH of 5 for heating. Which is in the recommended range from the thermostat manual for a furnace less than 90% efficient.

I'd prefer that it drops a degree or two before kicking on so I lowered the CPH to 2 on the t-stat and I've been timing the cycles this morning. It's about as cold as it gets right now and rarely gets lower than the 20's for lows and often in the 30's for lows and mid-40's for highs.

Instead of running for 5-6 minutes, the last couple times it's been running for 7-9 minutes and then off for about 17 minutes. So about 2-3 cycles per hour.

Any harm to the furnace setting the CPH to 2 with these run times? Is it easier or harder on a furnace to run a few minutes longer but fewer times per hour? I've read it's easier on the furnace to run about 10-15 minutes at a time because it's the starting up that wastes the most energy and causes the most wear and tear. Is this also true on a single speed, 80% furnace?

Thanks if you have knowledge of HVAC.

 

[HonorAndHonesty]

Its easier on the furnace to cycle less. We have a 2 stage 90 plus gas furnace and its minus 5 out here today, it has been running in stage one non stop maintaining temperature, less expanding and contracting of the heat exchanger so that should help it last longer.

On some thermostats you can set deadband as well, so if deadband is set to 1.0 and t stat setpoint is 68 then it will kick on at 67 and off at 69 degrees.

 

[SHOZ]

There should be a way to set the differential. Older mechanical ones had a slider were as the digital ones it's just a menu item. 1 degree would be a minimum but if it's cold enough that may get skewed trying to keep up. 2 degrees and it gets noticeable temp wise.

 

[Nukeman7]

Our furnace is a Rheem 80 series (I believe is what I recall). One speed fan I believe. So, 80% efficient. Insulation on home is, eh... Decent but not great. 1990 build and probably decent for that time period. When it's 20° to 40° the gas furnace was turning on and blowing heat for about 5-6 minutes, shutting off for about 12-15 minutes (total cycle time about 15-20 minutes or 3-4 cycles per hour). It just seemed weird to me that the thermostat is constantly at 68 and never drops before kicking on again. I checked the settings on the thermostat and it was set to a CPH of 5 for heating. Which is in the recommended range from the thermostat manual for a furnace less than 90% efficient.

Instead of running for 5-6 minutes, the last couple times it's been running for 7-9 minutes and then off for about 17 minutes. So about 2-3 cycles per hour.


Thanks if you have knowledge of HVAC.

It sounds like it may be short cycling to me. Has it always displayed this behavior? If not, I suggest that you shut off the main A.C. power switch to the furnace, let it cool off for 15 minutes, then clean the flame sensor as shown in the video below. A dirty flame sensor can cause short cycling of a gas furnace.

 

[dnewton3]

You do get into the theory of end-of-life component cycles versus run time.

If your CpH is at 5, then you're cycling the components more often. 5x an hour the motors would start, the gas valve would cycle, the ignitor would cycle, etc.

Versus CpH at 3; that would (in theory) be a 40% reduction in start cycles.

Generally, it's easier for a motor to run, than it is to start. It would be less wear on a gas valve to stay open versus cycle open/closed. And so on ...


The HVAC companies figure on "X" cycles in their projected lifetime calculations, based on an average of "Y" cycles per season, multiplied by "Z" seasons of life. That way they can project "our furnace should last 15 years ..." or whatever target they want to set.

If your equipment cycles less, but runs longer, it may add some life to the unit. In theory.

 

[HonorAndHonesty]

How many BTUs is the furnace and how many Square Feet is the house?

 

[HowAboutThis]

It sounds like it may be short cycling to me. Has it always displayed this behavior? If not, I suggest that you shut off the main A.C. power switch to the furnace, let it cool off for 15 minutes, then clean the flame sensor as shown in the video below. A dirty flame sensor can cause short cycling of a gas furnace.

It gets up to temp. We just moved here in June so this is the first winter with it. Had a furnace tech do a fall check up on it in November and he said things were good. It's 6 or 7 years old.

The way I understand short cycling is that it doesn't reach the temp you have it set to. I don't have that issue.

I feel like (but am not certain) our issue is with a higher CPH is the thermostat has a small temp differential built in (Honeywell so we can change cph but not the temp differential or whatever) so it kicks on when the temp has barely dropped, runs for 5, gets back to 68°, kicks off, etc. Whereas with a lower CPH I'm trying to force it to let the temp drop more before kicking on.

I wouldn't be shocked our furnace it oversized, either. At night we set it to 62° and when we're home I set it to 68°. So from 62-68 it takes about 20 minutes to raise 6°.

So, yeah, I've read about short cycling and I do believe the cycles are "short" but not because the furnace is shutting down because it's broken but because it's reached the temp we have it set at. So it's working "too well" (oversized?)

Does that make sense?

 

[Warstud]

It sounds like it may be short cycling to me. Has it always displayed this behavior? If not, I suggest that you shut off the main A.C. power switch to the furnace, let it cool off for 15 minutes, then clean the flame sensor as shown in the video below. A dirty flame sensor can cause short cycling of a gas furnace.

I prefer to use a $100 bill

 

[Dave9]

edit: Nevermind, more info posted made this reply irrelevant

One simple thing you can do is close the register(s) nearer the thermostat, and/or put a cover over it so it is sensing more of the wall temperature than the immediate ambient air temp change.

 

[HowAboutThis]

How many BTUs is the furnace and how many Square Feet is the house?

Yeah..need to change the filter and was going to look at the model number and see. 1230 sq ft house so I'm guessing it's too big. I think I read 30-50 BTU's per square foot is a rule of thumb. With 80% efficiency I'd venture we'd probably be best suited to 50-60k BTU's. I'd venture we have more than that but will soon check on it...

 

[Nukeman7]

It gets up to temp. We just moved here in June so this is the first winter with it. Had a furnace tech do a fall check up on it in November and he said things were good. It's 6 or 7 years old.

The way I understand short cycling is that it doesn't reach the temp you have it set to. I don't have that issue.

So, yeah, I've read about short cycling and I do believe the cycles are "short" but not because the furnace is shutting down because it's broken but because it's reached the temp we have it set at. So it's working "too well" (oversized?)

Does that make sense?

Yes...it may be that the furnace is oversized and or the thermostat's on/off threshold is too narrow causing the short run times. It is not common, but I have experienced one flame sensor that started failing by short cycling every few minutes (not seconds) after the sensor came up to temp.

 

[HowAboutThis]

There should be a way to set the differential. Older mechanical ones had a slider were as the digital ones it's just a menu item. 1 degree would be a minimum but if it's cold enough that may get skewed trying to keep up. 2 degrees and it gets noticeable temp wise.

Yeah, Honeywell install manual I think only shows CPH and not the temp drop/diff before kicking on. Google seems to say Honeywell has about a 1.0 degree differential before calling for heat/cool. I I'm using the CPH to, essentially, create the temp diff...I think???

 

[HowAboutThis]

Yes...it may be that the furnace is oversized and or the thermostat's on/off threshold is too narrow causing the short run times. It is not common, but I have experienced one flame sensor that started failing by short cycling every few minutes (not seconds) after the sensor came up to temp.

Our old furnace in the Midwest did that. It was a complicated, variable speed, super high efficiency one. It was blinking a "flame sensor" code and cleaning it fixed that problem. I don't think this is the same problem on this furnace right now.

Edit: to add, I cleaned it myself but then thought "I should have someone check it, too, and they found a recall they fixed for free of some pressure thingy was cracked that could cause CO to be leaked into our house. So it was good I dropped that $100 (whoever wrote that comment).

 

[HowAboutThis]

edit: Nevermind, more info posted made this reply irrelevant

One simple thing you can do is close the register(s) nearer the thermostat, and/or put a cover over it so it is sensing more of the wall temperature than the immediate ambient air temp change.

Did that in the hall bathroom. The cold air intake is in the ceiling in the hallway where the t-stat is (about ten feet) so I think the hot air gets drawn down that hallway past the t-stat. However, the t-stat reads 68 and the 3 standalone thermometers in our home read 67.5 to 68.2 in various rooms. So it seems to be reasonably and evenly heating things.

 

[LazyDog]

I did something similar to what you are doing. I have a 75K high efficiency furnace and to get better/ longer run times in the advance setting of the thermostat I change it to 3. The furnace runs ~ 12 minutes on and 20 minutes off at 32F degrees outside.

I like longer run times to dry the combustion chamber from moisture, my furnace is on the basement.

 

[HowAboutThis]

I did something similar to what you are doing. I have a 75K high efficiency furnace and to get better/ longer run times in the advance setting of the thermostat I change it to 3. The furnace runs ~ 12 minutes on and 20 minutes off at 32F degrees outside.

I like longer run times to dry the combustion chamber from moisture, my furnace is on the basement.

Ours is in the garage with the water heater. Weird West Coast placement. Currently 40° in the garage.

 

[Dave9]

Did that in the hall bathroom. The cold air intake is in the ceiling in the hallway where the t-stat is (about ten feet) so I think the hot air gets drawn down that hallway past the t-stat. However, the t-stat reads 68 and the 3 standalone thermometers in our home read 67.5 to 68.2 in various rooms. So it seems to be reasonably and evenly heating things.

But if it is not covered, you are still reading immediate ambient air temp.

Objects with mass, take longer than the air around them, to warm up to the air temperature, and longer to cool down as well. They exhibit a longer cycle than the air does.

 

[HowAboutThis]

But if it is not covered, you are still reading immediate ambient air temp.

Objects with mass, take longer than the air around them, to warm up to the air temperature, and longer to cool down as well. They exhibit a longer cycle than the air does.

Yes, I've never seen a thermostat covered in a house or business before. But a potential idea. However, then I'd venture our air temp would be way above 68°. Which would be a similar problem but in that opposite direction.

 

[doitmyself]

For heating, can someone explain how the CPH works when the Honeywell has a non-adjustable temperature differential of +/- 1 degree? I'm not quite getting it. How, or does the CPH over ride the temperature differential? For instance, if it is very cold out and the temp drops 3 degrees below the setpoint but the furnace has already cycled 2 times in less than an hour, what happens?

 

[totegoat]

What's your gas bill run? My last one was $207 staying warm.