Thermostat CPH setting: The Goldilocks question

nicatniter

I find that especially with this recent subzero cold, a 1 CPH (cycle per hour) thermostat setting causes a lot of temperature fluctuation and maxes out the radiators each cycle (i.e. they get really really hot).

As an experiment I changed the setting to 3 CPH (the next higher option), and this has evened out the room temp fluctuation considerably, and results in less extreme radiator temps. With 3 CPH, I measure on average 23-26 minutes of down time between cycles, and 6-8 minutes of firing (making steam after ~2 minutes) per cycle.

But as some of the radiators never really cool down between cycles, I'm wondering if 2 CPH would be optimal. Anybody know an inexpensive T'stat that provides this option? Any other comments on what I'm trying to accomplish? This is a single family home with a 1929 Webster 2-pipe steam system (new-ish gas fired boiler), in-wall convectors downstairs & (non-covered) radiators upstairs. The house is as well-insulated as it's going to be, and the windows/storms are decently tight.

Happy new year all,
Jon

EBEBRATT-Ed

Sounds like the 3 cycles/hour fixed it. Your boiler may be slightly oversized. Sounds like it's working ok now.

If your water line is stable while running, your not building pressure over 1.5-2 lbs and heating evenly then is sounds ok

Fred

I have a Honeywell RTH7400. It has 2 cycles per hour.

Jamie Hall

The more I hear about these new-fangled gadgets the better I like my mercury T87's with anticipators...

Gordo

We have also found that Honeywell's recommendation of 1 cph for steam systems is too aggressive. It appears that setting would work if there was no main venting, no main pipe insulation, etc etc.

As the OP discovered, 3 cph seems to work most of the time for the better maintained steam systems and especially for 2-pipe steam.

Terran

Personally I don't really like the CPH-set type thermostats (I'm looking at you Honeywell). I prefer to set the hysteresis/swing in degrees. If it's a warm day it'll cycle less often and on a cold day it'll cycle more often, maintaining the same temperature variation. I have never met anybody who cared about cycles per hour but lots of people care about how much the temperature varies.

At home I'm running a Radio Thermostat Company of America model with internet, which is adjustable from 0.5 to 3 degrees of hysteresis. I have it set on 0.5 and it's running between 1 and 2 CPH on my 1-pipe steam depending how cold it gets outside (it's been getting down to -20F this week).

Gsmith

@Terran: What model Radio Thermostat are you using on your 1 pipe steam system?

Gordo

I personally have found the Honeywell cph system to be better and much easier to adjust than the "swing" settings on other brands.

I set the cph correctly for the system and the complaints of uneven heating go away.

If the customers have a brand that uses swing, I usually advise customers to get the Honeywell brands, and the complaints of uneven heating go away.

If it don't mean a thing if it ain't got that swing, and that works for you, that's good, too.

Terran

@Gary Smith It's a CT50. bought it directly from the manufacturer at radiothermostat.com. I chose that manufacturer because they have a documented API that you can program yourself. I originally got it to monitor the house when I was away on vacation, but now I also use it to ramp up the temperature gradually in the mornings after a setback. I need that because I have a house with several incorrectly sized radiators which I've balanced with the venting, so if I ever get to the point where it's cutting out on pressure, the balance gets all off.

@Gordo Interesting to hear that you're having better experiences with CPH! Do you think it's because regular cycling provides better balance with the rooms away from the thermostat? Good thing we have a choice of products.

Fizz

Agree with 3cph setting. Since moving to that setting heat is much more steady. System is Richardson(11 rads from original install of 1917, with 3 add-ons in20's which are with traps). The system has been in vacuum for 2yrs with very good results. Haven't added water since, and gauge water is crystal clear! Also have 2nd house, much smaller one pipe working-off old vertical mercury Honewell t-stat, changed setting to .225 with good results. Jamie is right-on with his comments.

PMJ

nicatniter said:

I find that especially with this recent subzero cold, a 1 CPH (cycle per hour) thermostat setting causes a lot of temperature fluctuation and maxes out the radiators each cycle (i.e. they get really really hot).

As an experiment I changed the setting to 3 CPH (the next higher option), and this has evened out the room temp fluctuation considerably, and results in less extreme radiator temps. With 3 CPH, I measure on average 23-26 minutes of down time between cycles, and 6-8 minutes of firing (making steam after ~2 minutes) per cycle.

But as some of the radiators never really cool down between cycles, I'm wondering if 2 CPH would be optimal. Anybody know an inexpensive T'stat that provides this option? Any other comments on what I'm trying to accomplish? This is a single family home with a 1929 Webster 2-pipe steam system (new-ish gas fired boiler), in-wall convectors downstairs & (non-covered) radiators upstairs. The house is as well-insulated as it's going to be, and the windows/storms are decently tight.

Happy new year all,
Jon

If I am reading this correctly your 6 min fire/23 minute wait is a 29 min cycle so you are at 2CPH now(having set the control on 3).
Who knows what their algorithm really is. And you don't want the rads to cool down between cycles if you really want even heat. You want them always warm and a little less full than you have now.

Actually netting out 3CPH somehow will be more even heat yet. Since you have 2 pipe consider trying not letting the air back in between those cycles. Your time to steam on each refire will be just 15 seconds or less and your heat will be more even yet.

The Steam Whisperer

I believe the CFH setting is targeted for 50% load, it will vary from when you move from that load. If you are running 6 min per 29 min cycle at subzero temps, either your boiler and/ or system is grossly oversized (assuming you are maintaining comfortable interior temps)....thats only about 20% on time! That would mean you are 5 times oversized. On the typical existing boilers we see ( 60 to 80% oversized), it is typical that we have better results with a 2 cph setting, instead of 1 CPH. We also have much better luck with the Honeywell control logic rather than swing.

PMJ

The Steam Whisperer said:

I believe the CFH setting is targeted for 50% load, it will vary from when you move from that load. If you are running 6 min per 29 min cycle at subzero temps, either your boiler and/ or system is grossly oversized (assuming you are maintaining comfortable interior temps)....thats only about 20% on time! That would mean you are 5 times oversized. On the typical existing boilers we see ( 60 to 80% oversized), it is typical that we have better results with a 2 cph setting, instead of 1 CPH. We also have much better luck with the Honeywell control logic rather than swing.

About being oversized - when you think about it the original design must have had a really oversized coal fired boiler. To be able to modulate the fire as they did you wouldn't be operating at the very max end of the range requiring a raging fire to heat on design day. Practically speaking the coal boiler must have been capable of doing way more than it was ever asked to - even in the bitter cold or I think not very controllable.

Another point - to make any use of natural vacuum the boiler absolutely needs to be considerably oversized. If the output required gets anywhere close to the capacity of the boiler then in the bitter cold it needs to run a very high percentage of the time. That means no time for vacuum - certainly not natural vacuum anyway.

Currently with temps in the single digits I am running as continuously as I possibly can arrange at 6-7 minute burns and 13-14 minute waits. So the boiler more than three times what is really needed in these conditions. But I wouldn't have it any other way. I find the vacuum time to be too valuable.

mikeg2015

. > @Jamie Hall said:
> The more I hear about these new-fangled gadgets the better I like my mercury T87's with anticipators...


The new faned device also showed me that my boiler wasn’t running when I was 3 hours away from home about to fly to somewhere 800 miles away for 4 days. I was able to adjust my heat pump balance points to keep the house over 45f while I got my contractor there to check it out. It was a failed vent damper. He put it in service mode in the open position and I’m good for now.

On my drive home from the airport, I’ll raise the temp so it’s warm when I get there.

Funny how these things fail on a Friday when your going out of town and there’s an extreme cold weather system commingled. If it was only calling for average daily temps around 15f+ I would have just left it in he heat pumps until I got home since they can deliver a 15-18f net heat gain at hose temps. But an average daily temp of 5f for 2 days is something else.

Jamie Hall

Oh I'm not saying that the new contraptions don't have a place! They certainly do, and your example neatly shows one of the advantages. They do new and different things; they don't always do the same old things any better, that's all.

nicatniter

The boiler is ~40% oversized (contractors didn't do their homework & I was ignorant at the time). @steamwhisperer I am not sure where you get 5 times oversized (500%?).

With 3 CPH the system stays in vacuum till next cycle which is great (I replaced the vent tree with a check valve a couple years ago). I imagine this is adding to the efficiency of the system. I would need to get a wider ranging vacuum gauge to measure the negative pressure since the one I doesn't give me accurate measurement beyond -2 inches H2O.

Overall I am really liking the even heating; I'm still tweaking individual rooms a bit with inlet valve settings but it's an enormous improvement in comfort and quiet. I can't even tell if the cycle is on unless I head for the basement stairs!

Will have to see how it performs with more moderate cold.

I'm kind of curious to try a different cycling strategy as mentioned by @terran. But I realize I have little understanding of how the thermostat works. My Focuspro 5000 gives me no information about temperature changes, it just sits on 69 degree readout. So just how accurately does a thermostat measure the ambient temperature above or below its setting (if at all). Or, does it only register a binary "at or above set temp" vs "below set temp"? This would imply that a heating cycle overshoots the set temp and thwn the temperature decays over a period of time related to how fast heat is lost (i.e. how cold it is outside) until the device says "I'm below set temp" and fires the boiler (if the CPH timeout has passed).

-Jon

dgn

In response to an inquiry about CPH, someone over at hvac-talk.com suggested that CPH only works when the room temperature is within a very small (+/- 1-deg.) of the T-stat set temperature -- similar to the old anticipators. Otherwise, the call for heat always overrides the CPH. With my programmable Honeywell TH6000 it seems the CPH didn't work when responding after a 3-deg. overnight setback.

Can anyone here confirm this about CPH, at least on a Honeywell?

Fred

The "Cycles per hour" Thermostat does not actually control the number of cycles per hour. I think Honeywell must have wrestled with some nomenclature that the general homeowner might tend to understand, since it is typically set one time and forgotten about. Basically each "Cycle" just tightens up the temp swing an additional fraction of a degree. As an example (I don't know the actual programming) set on 1 cycle per hour allows the temp to swing by 1 degree, set at 2 cycles, tightens that swing to maybe 3/4 of a degree, 3 cycles tightens it to 1/2 degree swing, etc. Those swings may be different by manufacturer and they are pre-programed. Don't have the flexibility that the old heat anticipator use to have but generally the same approach.

Jamie Hall

Funny thing about the old anticipators. If they are properly set (very rarely -- way too much hassle) they can limit the temperature swing to very small amounts -- in principle, to almost zero. But that's rare...

ratio

re: Honeywell CPS, I found this some time back about a Honeywell 8000 stat, the pertinent info is:

Cycle rate limits the maximum number of times the system can cycle in a 1 hour period measured at a 50% load. For example, when set to 3 CPH, at a 50% load, the most the system will cycle is 3 times per hour (10 minutes on, 10 minutes off). The system cycles less often when load conditions are less than or greater than a 50% load.

nicatniter

@ratio that would explain why my 3 CPH is actually closer to 2 CPH -- I'm only firing for 6 out of the 10 minutes required for a 50% load.