Honeywell CT3600 Thermostat---Cycle Setting Question

Roddy

I'm happy with my Honeywell CT3600 programmable thermostat, despite the fact that I long ago abandoned using the main program feature, i.e. varying temperature settings for day, night, morning, etc. , in favor of a "HOLD 68" degrees for my one pipe low pressure steam system. I'm wondering, though, in order to even out the temperature in my house a bit more, if I should change the system type setting from the suggested "1" for steam systems, to the next option of "3" meant for hot water systems. I'm pretty sure that would mean my system could cycle at a max of 3 times per hour if needed versus the current max of 1 time per hour. Any thoughts?

Jamie Hall

no harm to trying... though depending on where the cool and warm places are, it may be worse. But you can always change it back!

vaporvac

I think ChrisJ fooled around with his settings and found 2 cycles to be his sweet spot.

Mark N

Just be aware that most likely if you choose a setting of 3cph, the thermostat will most likely turn off the boiler before steam even gets to the radiators. Then because the set point wasn't reached the boiler will be cycled on.

Roddy

Thanks for your input. I'm trying the 3 cph setting, and it seems to be working okay. The rads all get heat, but they don't heat to the extent they did when it was a 1 cph setting. I wish my thermostat had the 2 cph option available for use, but it doesn't. I wonder which thermostats have that option...

KC_Jones

I actually just replaced my 3600 with a new vision pro 8000 and it can be set at anything from 1-12.

Roddy

Thanks! Is the Visionpro battery operated also?

KC_Jones

Yes similar to what you already have. I should mention I only replaced mine because the 3600 stopped functioning correctly.

Mark N

How many cycles did you observe since changing to 3cph? How long are you running on a call for heat. Have you observed any cycles where the rads barely heated and the boiler came back on in a few minutes?

PMJ

Perhaps we should start a thread about what the definition of even heat really means. One subject is about differences in how long it takes different rads to get steam from fire. The lucky ones among us have original systems piped correctly in the first place where delivery is pretty even all around without any devices at all - just properly sized pipe. Those with systems where things were added and removed which unbalanced things have a bigger problem.

A separate issue regarding even heat is present in all steam systems. That subject is the unavoidable overshooting that comes with boilers with on/off (aka high/off) burners. Mark S describes this condition very well on his site. So unless a system has a modulating burner like he does, the only way to even out the heat and reduce overshooting is to add cycles. Doing this necessarily increases call times and reduces continuous burn times. Radiators end up more like gently warm most of the time.

Think about what a "perfect" radiator would feel like to keep a room at exactly the same temperature all the time on an average day. It probably wouldn't be even 1/2 full - but it would be like that always. So any control project whose purpose is really to even out the heat will have as the goal keeping radiators partly full all the time. What it takes to do that really has more to do with time variables than pressure ones. In fact, it can't be done with just one pressure setting. And how long it takes to get new steam to the rads is more about how long it has been since the last firing than anything else. More cycles keeps that delivery time really shorter.

Come to think of it, perfectly even heat would mean the call is never satisfied - the temp is always somewhere in the deadband.

Bio

I agree with you PMJ, l like to think that if you are fully heating radiators you may very well have an oversize boiler, I could be wrong but the fact that radiators were commonly size/oversize for a "design day" it will be then when radiators would fully heat, in my case this only happened last winter when we had the polar vortex,1 CPH can give you somewhat of an overshoot and if the boiler is oversized it will be more noticeable, 3 CPH is something I'm willing to test just to see if it's possible to keep more of a constant temperature, I suspect that I'll have to let it run for one or two weeks and let the thermostat adapt to the settings before I can come to a conclusion.

PMJ

I have been at 3cph for years now. The on/off burner times are set by time only. The most the boiler ever runs is 10 min on/10 min off. Calls for heat typically go on for 1-2 hours at least - the longer the better just so the temp is slowly going up.

I think all systems have about 30% more boiler than is needed once the delivery piping is heated up. The piping doesn't cool down enough to talk about in 10 min if it is insulated properly.

Roddy

Another question...

It's been a few days since I did the switch from 1 cph to 3 cph, and so far it's working great as far as comfort of rooms is concerned as well as overall even heating. The rads heat well, and not as fully or as hot as some of them did on the 1 cph setting. Also, some of my minor pipe noises have diminished as I'm not starting from quite as cold a boiler / water temp as I did on the previous cycle setting. Overall, I'm thinking it may be a little more like the "low and slow" heat from the original coal that was likely used in my system.

One more question that comes to mind when I read the above input from Mark, PMJ, and Bio: Does a 3 cph setting actually mean a max of 3 20-minute cycles per hour as called for by the stat; or does it mean max of 3 cycles per hour each possibly having varying cycle times?

KC_Jones

I am no expert, but my interpretation has always been that it is more about how much temperature swing it allows. So at 1 CPH the thermostat will allow the temp to vary a little more because it knows it is only "allowed" to fire the boiler once per hour. The higher the CPH the tighter it keeps that temperature swing. This is why it's all about experimentation. It depends on how your system reacts (time wise) as to which settings will and won't work for you. Since this is essential a comfort based discussion it really is about personal preference.

Roddy

I understand that, KC, but I was wondering more about the factual side of what 3 cph actually means, and if it's 3 of the same cycle times per hour, or something different...or, maybe it doesn't really mean that at all...not sure.

PMJ

Roddy said:

Another question...

It's been a few days since I did the switch from 1 cph to 3 cph, and so far it's working great as far as comfort of rooms is concerned as well as overall even heating. The rads heat well, and not as fully or as hot as some of them did on the 1 cph setting. Also, some of my minor pipe noises have diminished as I'm not starting from quite as cold a boiler / water temp as I did on the previous cycle setting. Overall, I'm thinking it may be a little more like the "low and slow" heat from the original coal that was likely used in my system.

One more question that comes to mind when I read the above input from Mark, PMJ, and Bio: Does a 3 cph setting actually mean a max of 3 20-minute cycles per hour as called for by the stat; or does it mean max of 3 cycles per hour each possibly having varying cycle times?

I can't speak to how the CT3600 works - I don't have one. I run with a PLC that locks the cycles to exactly 20 minutes whenever the tstat is calling for heat. I can only speak to the improvement in performance with more cycles, as you are already seeing.

KC_Jones

Like I said my best understand is it controls the amount of temperature swing. If it's mild out it won't "force" it to fire 3 times per hour just because it still just reacts to the temperature. The way I understand it is the CPH is like an upper limit for number of calls for heat per hour. If it fires at 1 PM for 15 minutes and then the temp drops again 10 minutes later (1:25) it will fire again for say 15 minutes. If the temp drops again 10 minutes after that (1:50) it will allow it to fire again. At 1 CPH if it fires at 1 PM it might fire for 25 minutes and then shut off at 1:25 then no matter what it "can't" fire again until 2 PM so you end up with greater temperatures swings. Again that is my best interpretation of how it works, but I don't write the program code so I can only theorize based on how I have seen them work. To get it to control the actual cycle lengths like you say you would need a completely different control. A thermostat will always just react to temperature. If anything I said is out in left field please feel free to correct me.

Roddy

Thanks, KC. Good info.
Roddy

Mark N

Enter 127201 in the search box. Read the thread How Does a Digital Thermostat Really work inside. Pay attention to the post by Cid2

Roddy

Thanks, Mark. Will do.
Roddy

Captain Who

@Mark N - thanks, I've been reading and puzzling over this CPH issue and that thread really helped clear it up for me. Didn't find Cid2 though, in the one I read:

http://forum.heatinghelp.com/discussion/comment/1182912#Comment_1182912

So basically the CPH setting is just twice the lockout period where, after the setpoint is reached from a call for heat, the thermostat says I'm NOT turning back on until CPH/2 hours (the lockout period) have passed. There is also an anticipator algorithm where it turns off the call for heat some time before the setpoint is reached, to avoid overshooting too much due to that thermal mass in the system is continuing to radiate and convect. It must learn that based upon how fast the temperature is rising in the room vs time while calling for heat. The temperature the thermostat reads is not the same as a typical thermometer. It is more like our own bodies perceive warmth, which is a combination of air temperature and radiant heat we receive from the objects in the room: walls, windows, furniture, floor, etc..

Mark N

There is a link to the thread in "VisionPro Thermostat problem"

Fred

I am really puzzled by the issues expressed by several posters regarding temperature swings and I'm wondering if other factors are in play here. I have a Honeywell RTH-7000 series thermostat set at 1CPH and it is set at 66 degrees 7/24. The digital reading NEVER varies from that temp unless I change the temperature for company who may like it a little warmer. I have a second thermostat next to it (same model) used for the central air system and it also reads 66 with no variation or swing.
I have to assume the temp is not changing more than .5 degree either way between cycles, regardless of outside temp.
Which leads me to wonder if the more critical factors to temp swings might be the size of the boiler, the size of the house, the size/height of the rooms/ceilings and obviously the placement of the thermostat in the house. My house is very large and ceiling hieght is 10ft. Thermostats are mounted about half way up an inside wall, about 12 feet from the nearest radiator.
I couldn't be happier with the consistency of the temperature.

Captain Who

Honeywell more or less admitted in the manual I read that the display does not necessarily show the true temperature, even aside from roundup or round down factors. I couldn't believe it but it does say that. I read it in the new VisionPro 8000 manual online. I don't have it, I have a White Rodgers.

KC_Jones

I think this is about comfort. I think what some would like to happen is have the radiator be warm (radiating heat) as much as possible and still maintain the set point. I agree Fred mine doesn't move (though they have been proven to lie to an extent). My radiators cool down some between cycles and often will get almost cold when it's milder out. I am GUESSING what people are trying to do is keep the radiator partially warm on a more continuous basis so they feel that radiation more often. That is just my guess and again this discussion to me is more about personal preference than what is "best". What is best for me is different than what is best for someone else. Personally I like my boiler to fire up and make steam the least amount of times possible. I am never uncomfortable at 1CPH and my system seems to like it just fine. I have mine at 70 during the day and 66 for sleeping I don't set back for fuel savings it's strictly for sleeping comfort. If I save some fuel good, if not that's fine too. My reference to "swing" earlier was strictly addressing how the thermostat seems to work on my system.

KC_Jones

Captain Who said:

Honeywell more or less admitted in the manual I read that the display does not necessarily show the true temperature, even aside from roundup or round down factors. I couldn't believe it but it does say that. I read it in the new VisionPro 8000 manual online. I don't have it, I have a White Rodgers.

Ask @ChrisJ about this!

ChrisJ

Captain Who said:

Honeywell more or less admitted in the manual I read that the display does not necessarily show the true temperature, even aside from roundup or round down factors. I couldn't believe it but it does say that. I read it in the new VisionPro 8000 manual online. I don't have it, I have a White Rodgers.

I've watched my VP8000 flat out lie about the current temperature. Notice my indoor thermometer says 70.1 but the VP claims 72?

If the VP is set to 70F and you raise it to 71 seconds later the current temperature will go up to 71 even nothing changed. If you raise it to 72 like in the picture, it'll creep up to 72 and yet nothing changed.

From what I've seen it will fudge the temperature in either direction several degrees to make you think it's the set temp in the room.

And yes, it bothers me.


That said, as far as I know no one knows what CPH actually means other than 1 CPH will tend to have much larger temperature swings than 3 CPH. The higher the CPH number and the shorter the heating cycles you will have, kinda. In my experience 1 CPH is safe for steam but 2 or even 3 can be squeezed out of a well tuned system.

PMJ

Fred said:

I am really puzzled by the issues expressed by several posters regarding temperature swings and I'm wondering if other factors are in play here. I have a Honeywell RTH-7000 series thermostat set at 1CPH and it is set at 66 degrees 7/24. The digital reading NEVER varies from that temp unless I change the temperature for company who may like it a little warmer. I have a second thermostat next to it (same model) used for the central air system and it also reads 66 with no variation or swing.
I have to assume the temp is not changing more than .5 degree either way between cycles, regardless of outside temp.
Which leads me to wonder if the more critical factors to temp swings might be the size of the boiler, the size of the house, the size/height of the rooms/ceilings and obviously the placement of the thermostat in the house. My house is very large and ceiling hieght is 10ft. Thermostats are mounted about half way up an inside wall, about 12 feet from the nearest radiator.
I couldn't be happier with the consistency of the temperature.

I don't know about the others but the swings I am talking about are not just air temperature. The temp on a far wall from the rad is only part of the story for a couple of reasons. First, especially older metal lath inside walls like mine are large heat sinks and don't change temperature vary fast. Tstats mounted to them( there really isn't much choice) tend to show a dampened view of what is really happening to the air. Secondly, because so much of the heat is radiant, a person closer to a radiator that is cycling between quite hot and back down close to room temperature experiences considerably more of a swing than just the air temperature would show.

I am only fiddling with my system because I am an engineer and enjoy the tweaking. I know that logically heat would be more even if the radiator temps didn't change so much. The others can speak for themselves but that is all i am doing here. And I think I have made what was already decent a whole lot better.

KC_Jones

I have seen people mount the thermostat on stand off brackets to get it an inch away from the wall to isolate it and get more air circulation around it to theoretically get a more accurate picture of room temp.

Fred

I actually have mine mounted over a 1/2 inch insulation board because I wanted a more accurate reading of air temp than wall temp. Anyway, I am sure there is a degree (No pun intended) of inaccuracy in these thermostats and with any residential thermostat but if that margin of error isnt physically noticable, I'm ok with that until something better comes along that is compeditive price wise and when I need a replacement or it has some other feature/function I want. If mine were off by 2 or three degrees, I would notice that. One degree, not a problem.
Everybody is different though. I, for one, don't care if the radiators are hot or warm most of the time as long as I can walk through my house and feel comfortable. That is the intent of any heating system. Even at 1CPH, my rads aren't cold when the next cycle happens unless it is 45 or above outside. Then when I feel them, if they are cold, I think "Great weather, and take "Comfort" in knowing the local utility isn't in my pocket (but it is stll 66 in my house).Steam just does it best!

KC_Jones

@Fred I think my wife would divorce me at 66°. lol

Mark N

A perfect CPH setting I think would be 1.5. But it is not available. The anticipator allowed you to fine tune things. An old Honeywell T87 was a great thermostat for steam.

Fred

All the more time to "Cuddle" with that boiler KC! I wonder if anyone's wife ever divorced them for alienation of affection with the boiler being the third party

KC_Jones

lmao @Fred My wife said this exact phrase when I finally finished (using that term loosely) the boiler installation. "Does this mean I get my husband back?" I kid you not!

Captain Who

ChrisJ said:

That said, as far as I know no one knows what CPH actually means other than 1 CPH will tend to have much larger temperature swings than 3 CPH. The higher the CPH number and the shorter the heating cycles you will have, kinda. In my experience 1 CPH is safe for steam but 2 or even 3 can be squeezed out of a well tuned system.

Well after reading that thread I posted above I THOUGHT well I finally DO understand it now. The key is that it is a "lockout" setting as I said above. Someone stated that they even called it a lockout in the manual or installation guide.

Jean-David Beyer

My downstairs zone is radiant slab at grade heated by a mod-con with outdoor reset, so I have the Honeywell CT-3600 set to one cycle per hour. Since the heating is radiant, I want the thermostat to be influenced by the temperature of the (inside) wall it is mounted on. So I made no attempt to insulate it from the wall.

About the cycles per hour setting. I picked one cycle per hour as the longest available. Since the reset is adjusted to be just barely enough to make up the heat loss, the thermostat can sometimes call for heat for 8 to 18 hours straight. Today it went on a little after midnight, and was satisfied after 10 hours and 23 minutes; it is around 36F outside at the moment. Inside temperature is such that the thermometer on the thermostat read the set point the entire time. The CT-3600 does not seem to turn off the heat demand if it is not satisfied. It does not cause overshoot because the outdoor reset is so tight it would take an extremely rapid increase of outside temperature to reduce the heat loss to where the stored heat in the slab would overheat the house.

Fred

KC_Jones said:

lmao @Fred My wife said this exact phrase when I finally finished (using that term loosely) the boiler installation. "Does this mean I get my husband back?" I kid you not!

LOL

PMJ

I think it is really great that there are so many choices today about controls and so many different ways to do things. To each his own. We are just sharing ideas here right? I know that I don't have all the answers. A little back and forth with others about what they are doing and experiencing keeps my education moving along. Hopefully we are all helping to keep residential steam alive too.

Roddy

Totally agree, PMJ. As a novice, I find the whole exchange enlightening.
Roddy

Fred

I agree 100%! It is great to have this forum!

vaporvac

I have the Honeywell Wi-Fi 7-Day Programmable Thermostat - RTH6580WF. It's too sensitive to temp and I think not at all accurate for the small changes or mabye overly accurate. It does just what Fred has mentioned: once it reaches temp it never varies. I have it set for 1CPH, but it will come on for a nanosecond a few times an hour. I guess that's OK for me since I'm generally in a vacuum and it doesn't take as much to boil, but it still seems crazy as the Tstat still reads the set point. I just don't see how that little time can generate enough steam to reach the hallway rads. Anyone else have that happen? Maybe it's defective. I sure like the wi-fi capability, though. Now if only it would give me a pressure reading, too!
P.S. I don't think it's just guys groovin' on this steam thing, but probably only one person out of a couple who's a bit cuckoo for it.