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"Heating cycles per hour" options for gas or oil furnaces

34997 Views 14 Replies 4 Participants Last post by  rb624
For Honeywell RTH111 Thermostat,

there's a System Setup page in the manual, where it gives two "Heating cycles per hour" options for gas or oil furnaces: 4 cycles of 15 minutes or 5 cycles of 12 minutes. Please tell me what the considerations are for choosing one or the other.

Also on the same page, there's a 5-minute time delay option (On or OFF) to avoid A/C compressors restarts too soon after shutdown. Is this important on all compressors, older and newer?

Thx for your help.
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For Honeywell RTH111 Thermostat,

there's a System Setup page in the manual, where it gives two "Heating cycles per hour" options for gas or oil furnaces: 4 cycles of 15 minutes or 5 cycles of 12 minutes. Please tell me what the considerations are for choosing one or the other. How cold do you like to get between cycles. Shorter cycles cause more wear and tear and are less efficient but more comfortable.

Also on the same page, there's a 5-minute time delay option (On or OFF) to avoid A/C compressors restarts too soon after shutdown. Is this important on all compressors, older and newer? Yes. Allows the freon pressures to equalize so it is easier to start the compressor.

Thx for your help.

Thx for your reply yuri.

Never thought about this before, when I posted my questions, but I thought a furnace would run until room temp satisfied the Set temp, then shut off. The furnace won't just keep running, if for example windows were left open?
If the windows are left open and the heat can escape thru them faster than the furnace can produce it then it won't stop. If the furnace can produce more BTU's of heat/hr than your windows/house lose them then it will shut off.
So when do these Heating Cycles come into play? Doesn't the thermostat always control how long heat is on and off, according to how well the space is insulated?

Thermostat runs till room reaches Set temp. Room temp gradually falls, depending on how well room is insulated. Thermostat brings room temperature back up to Set temperature again. What does the mentioned Heating Cycles have to do with this process?

Doesn't the thermostat keep the furnace on continously till the room reaches Set temp.?
It gets real complicated and only the real old school heating guys understand what a heat anticipator does. The old mercury tstats had a resistance heater to add some "false heat" to shut off the tstat a bit before the setpoint as the furnace blower blows heat from the heat exchanger once the burner shuts off and in the ole days this would cause the house temp to overshoot the setpoint. Electronic tstats simulate this with voodoo magic/hysteresis or some such a thing. Try the different settings until you feel comfortable is what I would say. Or google some of those terms to try find more info.
Thx yuri - I do understand a heat anticipator, in detail, but I don't see how these options for Heating Cycles Per Hour relate. I will google as you suggest, but I'll also add the following in attempt to better illustrate what I don't understand, in case anyone does happen to understand these options (when/how they come into play I mean). I'm not trying to decide which option to choose, just want to learn exactly when/how they come into play.

The two options for gas & oil furnaces, shown in the thermostat manual's System Setup page: 4 cycles of 15 minutes or 5 cycles of 12 minutes.

"Heating Cycles Per Hour" gives the impression of ON & OFF (a beginning and ending point of a cycle). If a tstat stays on continuously till SET temp is satisfied, when/how does one of these (ON/OFF) Heating Cycles Per Hour come into play, especially if the total run time of either option/cycle adds up to 60 minutes per hour (implies continuous running per hour, to me).

An atypical example: What if the windows were left open - would it not run continuously as long as it didn't overheat and get shut off by the limit switch? If it would run continuously, then where's the (ON/OFF) Heating Cycles Per Hour?

If anyone's answer is that it would not run continuously - one of those 4 or 5 heating cycles per hour would shut it off, what's it going to do then (after it shuts it off), restart another heating cycle immediately?...cause there are 4 or 5 heating cycles per hour, which both (either one) add up to 60 minutes/hour, which implies continuous running per hour (which leaves no room for ON/OFF time per hour).

Please forgive any over-simplification, but just trying to better illustrate what/why I don't understand. I called Honeywell, but the call center rep couldn't answer it.

My apologies if I'm missing something simple.
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Those cycles per hour are based on a normal house with no open windows and average insulation. The old oil furnaces and gas furnaces had a heavier mass and more metal in the heat exchanger which held heat when the bi-metal fan control turned off the fan. Enough to build up to 150 deg F and restart the fan and heat up the house more and overshoot the setpoint. Hence the heat anticipator. Carrier uses a timed on algorithm to determine a method to get longer cycles with their circuit boards and staging and Honeywell probably engineered it by averaging out 8 hours of running and counting and timing the length of cycles for a average 1000 sq ft house with an oil furnace, hot water boiler, medium efficiency furnace and high efficiency furnace to see how long the cycles take etc and built in a timing feature to try even out the cycles. I usually set it for max cycles so the elderly folks who sit around don't start to feel chilly B4 the furnace starts. One of my electrical engineer customers says they use a "fuzzy logic" circuit. Google that for more info.
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Thx yuri for elaborating, and for your patience.

What I'm trying to do is to get someone to answer in relation to the context of my specific questions and circumstances posed. I haven't been presenting my question(s) effectively, but I think I'm on the right track now, so I'll continue at the risk of annoying everyone further.

The following is Honeywell's reply to my first e-mailed question (a second one was just sent to them - I'll include it below and you will probably have the answer). But first, here's the reply (a truncated excerpt from their reply) to the first email sent to them. I've left out parts about how the cycles feature minimizes temperature swing and how the cycles are designed - that's not what interests me, but I like to learn and am grateful for your input on that also.

Honeywell's first reply:
A cycle rate is the ideal number of times a heating system will run, in an
hour, to maintain temperature within one degree. For instance, gas or oil
forced air high efficiency systems have a recommended cycle rate of 3. With
a cycle rate of 3, the heating system, at a 50% load, will cycle on, off and
on again 3 times per hour. This results in cycling of approximately 20
minutes on, 20 minutes off and 20 minutes on again. The time that the
heating system is on and off will vary depending on the type of home, size
of the heating system, and the difference between the outside temperature...

My question today, in the second email sent to Honeywell:
Please consider the following example, with default option "5" ( heating
cycles per hour) selected (thermostat model RTH111), which is 5 heating
cycles per hour of 12 minutes each. From your reply (description of 20
minute cycles/recommended setting "3" for high efficiency gas furnaces), I
gather this means the RTH111 will run this example furnace for 12 minutes
on, shut off for 12 minutes, back on for 12 minutes, and continue this
cycling 5 times in an hour, then repeat.

The example:

It's 40 degrees inside the home. Thermostat is turned up to 80 degrees.
Let's assume for this example, due to this home's heat loss factors, that it
would take longer than 12 minutes of continuous furnace on-time for the room
temp to rise from 40 to 80 degrees (just using this as an example - maybe
taking longer than 12 minutes to raise the temp 40 degrees, from room
temperature of 40 degrees, would be normal for any home - don't know, but no matter).

After furnace turns on, will it run continuously (without cycling on & off)
until room temp reaches 80 degrees, or is it going to run for 12 minutes,
shut off for 12 minutes, on again for 12 minutes, etc., until reaching room
temperature of 80 degrees?

Will be posting Honeywell's second reply, and I expect that to be soon - they got back to me quickly yesterday (same day).
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In response to the example (last post before this one) and question sent to Honeywell, their reply was:

"It is correct to understand that when a furnace turns on, it will run constantly to achieve the set temperature. And after achieving it will cycle on and off to maintain it."
sorry if I'm reviving an old post, but it is relevant to my situation

Sorry to be reviving an old thread. It just seems very relevant to my situation.

I'm just as confused as justplumducky was. I didn't see how <cycles per hour> and <target temp and swing differential> can work together. They seem like they are settings that would compete with each other.

However maybe the fact that they do work together makes my situation a little more explicable. I have an HVAC system that works directly off the <target temp and swing> setting on the stat. As long as my swing differential is set at 1.0 or higher the furnace/ac will always adhere to the stat's commands immediately. Everything is precise. If the swing is reached the furnace turns on; if the target is reached the furnace turns off. The stat actually has NO cycle per hour setting (which is fine with me), and the system cycles as necessary to keep up with the <target temp and swing> settings on the stat.

HOWEVER, once I switch the swing to .5 all hell breaks loose. The furnace goes into a quandry and has no idea how to operate. The ignition, blower, and motor all go on and off in random patterns. After turning on and off a few times, the furnace will finally start to work properly and runs until the target temp is reached. But once the stat reaches the swing setting again, the whole convoluted start-up process starts all over again.

With the ac its slightly less wacky. The ac will go on and blow for about thirty seconds and then cut-off. Then after waiting the four minutes that the stat's compressor protection requires, the ac turns on and runs properly until it reaches the target. However once the stat reaches the swing setting again, the same on/off process starts again.

The fact that other stats have <cycles per hour> settings at least explains to me how a furnace/ac can seemingly ignore a the <target temp and swing diff>. I was under the impression that every system was supposed to adhere directly to the <target & swing>.

Anyway, it doesn't make me understand why .5 is giving my system so much confusion. But it does explain that it's not crazy that my furnace and stat do not always go on and off together.

If anyone can venture to guess a general cause to my problem, I'd be happy to hear it. My aim is to have as steady of a temp as possible. A 1.0 swing is too much. I once posted on an HVAC board which was not DIY, and they tried to give me hints but I couldn't follow them. They mentioned things about trying it with windows open and blower door open, but it was all going over my head, especially since I was stuck believing that a furnace was directly controlled by the stat's target&swing. Some also specualted that a common wire (which I do not have set up) would fix this problem. If there are any other ideas, I'd appreciate them.

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Probably a defect in your thermostat.

Get a honey well with CPH, you'll be far more comfortable.
Probably a defect in your thermostat.

Get a honey well with CPH, you'll be far more comfortable.
CPH did pique my interest as a way to get a precise temperature, but I think swing differential has better potential. I just need to find a good one. I've seen a couple that you can set in .25 increments, and I'll probably get one and give it a go.

Even if CPH turns out to be better than swing-settings, that wouldn't help me in the summer anyway, as the CPH is only for heat.
Honeywell thermostats have a CPH setting for heat and a separate one for cooling mode.
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Honeywell thermostats have a CPH setting for heat and a separate one for cooling mode.
I had a Honeywell laying around and had checked its manual and some manuals online and hadn't found one with that option. But, upon your advice, I've looked harder and found some. Thanks for the info.

I got the .5 to work, as its supposed to after three years of confusion. My installer had set up the common wire as a second heat wire, but my furnace doesn't have a second heat wire. The stat communicates to both stages of the furnace with just the W1 wire.

However, although the common wire now has the .5 setting working as it is supposed to, it still is not the answer. The furnace obeys the stat, which is what I wanted, but the setting is so precise that the temp sometimes drops by .5 as the furnace is cooling down. Though I don't mind constant cycling, having the stat tell the furnace to fire up while it's cooling down is going a little too far.

So I'll be trying HW with heat/cool CPH next. Thanks a lot for the info.
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