Once again - cph setting for steam

Alex265

I know it should be set to 1, it's all over the Internet. :)



However, has anyone actually measured  fuel consumption (boiler ON time) with different setting of cph for steam? I am asking because I have been doing this for the last several weeks, and I conclude that the actual ON time of my boiler is slightly less with cph = 5 than with cph = 3 or 1. I run with the same setting for a week, then take this week's HDD data from degreedays.com and calculate the amount of consumed gas per degree day. I ran 3 weeks with cph = 5, 2 weeks with cph = 3 and this week I am running with cph = 1. Outside temperature range has been pretty wide during this time. It's really hard to make this a 'clean' experiment, but averaging per week gives it at least some sense. For what it's worth, each week with cph = 5 shows less gas consumption per degree day than each week with cph = 3, and the last four days with cph = 1. The difference is small, but it does look better for higher cph.



Any thoughts on that? My house is poorly insulated overall, piping is insulated pretty well. Thermostat is VisionPro TH8110U1003.

Mark N

Run Times

Alex



Have you kept track on how the long the boiler runs to satisfy the tstat at the different cph settings? My tstat is set for 1cph and the boiler consistently runs 20 to 25 minutes to satisfy the tstat. This gives good steam distribution to my rads. When you say set at a cph=5 does the boiler run a nice long cycle to get good steam distribution or does in short cycle on the tstat.



Mark

Alex265

Hi Mark,

Yes, I do keep track of that, but I don't have any logging devices so it's not very valuable. With cph = 5 boiler still goes on averagely 3 times an hour, one time for 15-20 minutes, and couple of times for 3-4 minutes, that just keeps the pipes warm. This naturally depends on the outside temperature, but the pattern is the same - one longer time that would slightly overshoot the set temperature, and 2-3 shorter times. I wouldn't call this short-cycling. Note that this is the algorithm chosen by the thermostat, which seems kind of smart to me this season. Getting from 3-degree setback in the morning takes 25-30 minutes run, again depending on the temperature outside, pressure usually does not reach cut out or even start to rise noticeably.



With cph = 3 pattern is about the same, but naturally run times are little longer as thermostat needs to overshoot more, and it's 1-2 short runs per hour instead of 2-3. With cph = 1 runs are longer (40 minutes from setback) which results in cycling on pressure at the end of the cycle (sometimes) and 2 degree overshoot is normal. Didn't notice short runs.

Mark N

Run Times

Alex



I don't have any logging equipment either. I installed a new tstat this year its a Honeywell Focuspro 5000. I set the cph to 1 as per the instructions. Since the middle of Nov the tstat has been set to 70. I don't use set backs. The weather has been cold over the past few weeks and this is what I observed. The tstat turns the boiler on it will run between 20 to 25 minutes then turn off on the tstat. I've observed no overshoot. I have thermometers in all my rooms. Temp is very uniform. The last few nights have been in the teens. The boiler then stays off for about 2 hours. Then will run the same 20 to 25 minute cycle. When the weather was milder it would stay off for 3 hrs.



Mark

Alex265

Mark, this sounds like

your house is very well insulated, unfortunately not the case with my house. When house is well insulated, there is no need for the thermostat to overshoot in order to be able to maintain cph = 1, and thermostats can learn by adjusting their algorithm. In this case cph = 1 must be optimal. In my case it needs to overshoot, because otherwise it'll have to start more than once an hour to maintain the set temperature.



In other words, it seems in a poorly insulated house it may turn out to be cheaper to start more often to maintain water and piping warm.

Mark N

Insulation

Alex



Actually only parts of the house are insulated. Rooms that have been renovated have been insulated. The attic is insulated, and I have have thermal doors and windows. I'm not sure how the 1cph works. I assume that it causes the boiler to run longer because steam systems have a lag time between turn on and heat arrival. Then it stays off longer because the rads take awhile to cool back down. You state that your boiler will kick on for 2 short cycles a few minutes in duration. These are wasting fuel. They are not long enough to give you any heat.



Mark

Mike Kusiak_2

Short cycling

I have to agree with Mark. Those short cycles are not really heating your whole house. If the room with the thermostat is close to the boiler, it may be receiving steam first and producing the response. You are sacrificing uniformity of heating the other rooms since there is not enough time to completely fill the system with steam during the short cycle.



You may actually be saving fuel with the short cycles because the whole house may not be heated up to the thermostat setpoint.

Alex265

Mike

the short cycles don't heat the house, but they do heat up the water in the boiler several degrees, so that the next long cycle will start producing steam faster, and so more of the long cycle will be spent actually heating the house than bringing the water to boil. So I don't think the energy is completely wasted during these short cycles.



Your explanation of why I may be seeing fuel saving with cph = 5 is a good point and might be true. But it does not feel any more comfortable when I'm using cph = 3, and subjectively, cph = 1 provides much less comfort than cph = 5 because of the serious temperature swing.



So, the bottom line seems to be: in my house, cph = 5 provides more comfort and uses less fuel than cph = 1. I know this is wrong and was wondering if anyone has some numbers that would show how cph = 1 uses less fuel than higher setting in their house.

jpf321

there was a great...

thread last year about this .. where I even found a technical Honeywell doc that explained how they work this all out ..



as I hear more about CPH and I have been learning more about Heat Timer, perhaps the concepts are similar and maybe you can learn something by reading an old HT manual: http://www.heat-timer.com/enFiles/ProductDocument/PN-059134.pdf



i'm not sure that my t-stat (a big box Ritetemp) has CPH, or if it does, it's set factory high .. for the most part, it's strictly temperature controlled it never undershoots or overshoots .. perhaps I'm luck in having it mounted in a good location.



if there is determined to be some real value/savings with going for a $100+ t-stat vs my $30 unit, i'd be happy to investigate and run some data .. if anyone wants to loan me one for data collecting, i'm happy to borrow it and try it out for a few days/weeks.

Alex265

Thanks for the link

I will definitely read it.



I also have somewhere an old Honeywell patent that describes their Intelligent Recovery algorithm and how thermostat adjusts its running time. Somehow I think not much has changes in this algorithm over the years.

Mike Kusiak_2

Honeywell Manual

Here is a Honeywell engineering manual that goes in to a lot of detail on building temperature control. I think this might be the document jpf321 is referring to.



The chapter on control fundamentals is especially interesting as it relates to the fundamental algorithms which could be used in current digital thermostats.

jpf321

yep looks like it

yep looks like it

jpf321

last year's thread RE: CPH

Here ya go .. http://www.heatinghelp.com/forum-thread/128816/VisionPro-Thermostat-Problem

Mark N

CPH=2

Alex



I set the cph on my tstat to 2 from 1 to see if it would make a difference. With the cph set to 2, I got much shorter boiler run times. Boiler would run 10 to 15 minutes. Instead of 20 to 25 minutes. With the shorter run times the rads were barely heating. I put the setting back to 1.



Mark

Alex265

CPH = 2

Mark, thanks for making this experiment!



I have a question - while the rads were barely heating - was the house still at the set temperature? Because if it wasn't - then this is a different question - with any CPH thermostat should be able to achieve its goal, and it does in my case.



But if it was - the goal was achieved even with the rads barely warm and this sounds like some fuel saving.

Mark N

Temp Setting

Alex



The temp setting on the tstat never changed, but the room temp did because the thermometer I have showed it . The boiler then stayed off for a short time then kicked back on and ran the same short cycle.



Mark

jpf321

it was my understanding...

from last year's thread .. that the CPH dictates how often the t-stat should try to run .. but that it shouldn't truncate a run .. perhaps this is incorrect based on Mark's truncated runs. 

Alex265

As far as I know,

CPH does influence both how OFTEN and how LONG thermostat calls for heat, and an intelligent thermostat will adjust its algorithm based on the actual system performance and a set of hard-coded assumptions (like outside wind and temperature).