Honeywell CPH behavior?

ChrisJ

So I have a Honeywell VP 8110 hooked to my single pipe steam system (cast iron rads) and I've had it set to 1 CPH for over a year now.



Tonight is our first normal winter night since I installed the system and we are currently at 11F outside.



I'm wondering if what I think is going on, is actually going on or not. It seems like the colder it gets out, the wider the temperature swing I get which I guess makes sense if it wants to stay within 1 cycle per hour. I'm not sure if this is simply a side effect of a drafty house, or how the thermostat is designed to behave.



I'm tempted to see how 2 cycles per hour performs in this weather, but at the same time I do like how the system behaves overall (tonight and in mild weather) and am hesitant to mess with a good thing.  Even though I'm venting fast and get steam to the 2nd floor fast now I don't know if its good enough for 2CPH.

Mark N

CPH

Chris



How long is your boiler running on average at 1cph? If you had low mass emitters like convectors I would give it a try. I tried this a few years back and was very dissatisfied with the results. Short run times led to poor heat distribution.

ChrisJ

Run times

I wish I could tell you, I've been too lazy to time it. All but one of my rads is large tube cast iron, one is thin tube.



If I had to guess I'd say right now my run times are probably 30-40 minutes easy. I'd say around 5 minutes after it lights for me to get steam to the radiators.



On a mild day its of course a lot less,





BTW, even with 1CPH my temperature swing is still less than my T35 was even after tweaking the anticipator so I'm not really complaining, just observing.

Mark N

Run Times

I run 20 to 25 minutes to maintain 70 in the house. When I tried 2 cph the run times were like 12 to 15 minutes. I was just starting to get steam circulating and the boiler would turn off.

ChrisJ

Experimenting

Being I just fine-tuned all of my vents and installed TRVs and what not, I decided to try 2CPH.



What concerns me is the system fired up and just as steam was going to the radiators it shut off.  Steam never reached even the radiator in the room with the T-STAT. 

Is because the VP8110 is learning the system again and over a few cycles it will work it self out?  Or, is it just because 2CPH isn't going to work for me?  I don't see how it could make sense no matter what the CPH setting is to fire the system up and shut it off before any temperature increased.  Its dark out, temperatures are dropping out so the room did NOT go up in temperature no way no how during this.



My reason for wanting to try 2CPH is to limit overshoot and maintain a more constant temperature,  I'm thinking it can do it even with large tube cast iron radiators simply because of how fast I'm venting everything and maintaining near perfect balance. 

Mark N

2 CPH

Ok Chris how long did it actually run? Also since it didn't reach the set point, how long did it stay off before it refired? I would think it should have been about 15 minutes. How long did it run after it run on the second cycle?

ChrisJ

Yikes

I feel unprepared!



I didn't keep track, it was a pretty short time before it fired again, I could agree with 15 minutes.



The second cycle seemed longer, and did heat several sections of all radiators. If I could expect it to always work like this I'll be happy.



My fear is, the system will cool down, pipes will cool down and its going to fire up again expecting an immediate response and not get it.



What do you think Mark?

MTC

Sounds to me

like your messing with a system that's working.



If its going to take 20-25 min cycles on an hour to give adequate heat at reasonable balance and consistency, and 2 15 min cycles to get about the same, all you're doing is short cycling the system and increasing your fuel bill.



Iron holds a lot of heat for that 30-40 mins of off cycle, you shouldn't get THAT much of an evenness gain by doing 2cph...

ChrisJ

Yeah

I think you're right.



Ever heard of someone that works on something until its broke?

Mark N

2 cph

The best thing to do is let it run and document how the system behaves. Keep track of how the long the boiler run on a call for heat. How long it stays off in between calls for heat. How many sections of the rads heat. Are you maintaining temps in all the rooms. Remember steam with cast iron rads is a high mass system and takes time to respond. I myself find that 1 cph and the 20 to 25 minutes cycles I get with it is the best way to keep my house very comfortable. I experience no overshoot.

ChrisJ

Overshoot

Hi Mark,



My overshoot increases the colder it gets out. This, makes sense to me as its the only way it could maintain its 1 cycle per hour schedule.



When it was around 4F outside I was seeing around a 2 degree overshoot in the thermostat room. Again, this is perfectly reasonable I'm just always trying to improve things.



Ever since installing the TRVs in the two larger bedrooms and adjusting venting there my 2nd floor has stayed comfortable regardless of outside temperature. I used to see a chilly bedroom in mild weather while they would turn into ovens in cold weather. Not anymore though, I love TRVs.