Do all steam boilers eventually short cycle?

jim_14

What is true short cycling?

I mean, once the steam reaches all the radiators (at the same time) and heat starts to flow from them - the pressure starts to build inside the boiler.. Then, at least in my case, the boiler shuts down on pressure before restarting again- over and over until the thermostat is satisfied.

In my case its usually a 2 to 3 degree raise in temp that I am requesting. And I have my pressuretrol to cut out at about 10 ounces and cut back in at about 3 ounces.

So my boiler never shuts down on pressure before I get heat from the radiators, its only after all my rads are giving off heat that it shut down in cycles until the thermostat is satisfied. It never seems to run "perfectly" like Ive read about where other peoples boilers seem to run on zero pressure until the thermostat is satisfied... Is that even realistic??

I mean i could change the pressuretrol settings? Any suggestions or I am I fine?

Hap_Hazzard

How many times does it usually cycle on pressure before the thermostat is satisfied?

To answer your question, no. If the EDR of the boiler is very closely matched to the radiant load, the radiators will keep converting the steam into water as fast as the boiler can produce it, and the pressure will never exceed the amount necessary to move the steam to the radiators aganst the frictional resistance of the piping, which is—or should be—well below the cut-out point of the pressure-limiting control.

Jamie Hall

What is happening -- and most, but not all, steam systems will do it -- is that the boiler is producing steam faster than the radiators can condense it. The critical question is -- how much faster. If you had some magic way of measuring the output -- condensing rate -- of every radiator and all the piping, you would find that right at the beginning of a cycle both the piping and the radiators will be condensing at their maximum rate. Fairly soon the piping will be hot, and the rate at which it condenses will drop off (particularly if it is insulated, as it should be). At this point most -- but not all -- systems will show a very slight pressure rise (say to 2 or 3 ounces per square inch) which reflects the frictional resistance of the steam moving through the pipes. At this point, the radiators are beginning to warm up -- more or less uniformly. As various radiators become hot all the way across -- which they won't usually do all at the same time -- the steam pressure will rise -- just slightly -- as it redirects to the radiators which aren't quite there yet. But... there will come a time when all the radiators are as full of steam condensing as they can be, and all the vents or traps are closed.

This may take quite some time -- which is why in a well balanced system with the boiler and radiator well matched, it simply won't happen except of very long runs -- such as your instance of a 2 or 3 degree rise. Some happy few systems have the boiler and the radiation so well matched that under normal conditions they never quite get there. More often, the boiler is somewhat too large, and the system will reach that point sooner.

Changing pressure control settings has astonishingly little impact on the nature of the cycling. However, there is simply no point in running the pressure up above the point where the radiation is filled or very nearly so.

Fred

If the short cycling only happens when you raise the thermostat setting 2 or 3 degrees and that concerns you, then don't do that. Set it at a temperature that is comfortable for you and leave it there. Anytime you up the temperature or use a set-back (when not home or at night) the boiler has to run longer on the first heating cycle, trying to recover and get back to the higher setting. It not only has to increase the air temp but it also has to increase the temp of everything in the house, furniture and all. That may be a long cycle and, if the boiler is somewhat over-sized, it is going to produce more steam than the radiators can condense. It probably isn't even saving you any money on fuel cost.
Also make sure you have very good main venting. Delaying the build up of pressure until the steam gets to the radiators will delay when the boiler starts to short cycle.
Finally, I assume you have a Vaporstat and not a Pressuretrol?? You can't adjust a Pressuretrol down to the ounces.

Neild5

My system no longer short cycles, it holds right at 4 ozs. Prior to downfireing what was a ridiculously oversized system it could get up to 8 psi and cycled a lot. The heat is more even and the gas bill is down 25%.

EBEBRATT-Ed

@jin_14

Weather the short cycling you are having is "normal" or not depends.

The correct way to size a steam boiler is to match the boiler output to the total radiator edr. There is also a safety factor of 1.33 built into the boiler rating for warming up the piping and radiators which makes the boiler "oversized" in some situations

I have seen many residential jobs where the boiler never builds pressure and that is a ideal situation.

If you boiler cycles 2-3 times while coming out of set back I would consider that normal.

If it cycles more than once or twice on a normal call for heat I would call that excessive

jim_14

No if I don’t touch the thermostat it doesn’t cycle much if at all. Sometimes it shuts off before steam even hits the radiators at all - I think that’s just the thermostat being finicky. It’s one of those digital WiFi enabled stats. Sensei by emerson. Nothing fancy like the Nest. Cost me $50 with the Con Ed rebate. Or I would have kept the Honeywell round.

I guess I could tweak the settings a bit?

ethicalpaul

I'd try SLOW Heat Cycle Rate

jim_14

Ok I will try that... thanks!

ethicalpaul

See if it makes too much of a temperature swing for your comfort. If so, just go back to medium

Fred

That thermostat is designed for "Forced Air" only. It will run at 5 cycles per hour, the standard for forced air and not long enough for steam to get to all of the radiators. It assumes air is being directed to each room at the same time the blower is operating. That's not the case with Steam. Get a thermostat that is programmable for Steam, Hot Water, Forced air, and Heat Pump and program it for either Steam (1 cph) or Hot Water (2cph).

jim_14

Are you sure about that? It’s the ST-55 and I remember when I set it up it had a few heating options. I just looked it up on Home Depot and it appears to be usable for more then just forced air

Fred

jim_14 said:

Are you sure about that? It’s the ST-55 and I remember when I set it up it had a few heating options. I just looked it up on Home Depot and it appears to be usable for more then just forced air

jim_14 said:

Are you sure about that? It’s the ST-55 and I remember when I set it up it had a few heating options. I just looked it up on Home Depot and it appears to be usable for more then just forced air

No, I'm not sure but from the options you posted above, that was my take-away. I looked this model up and the owner's manual does say boiler but it also says "Do not use this manual independently of the installation app that must be downloaded".
Did you download that app and program it accordingly?

jim_14

Yes I did set it up with the app... It was a few months ago..I do remember boiler, and I think steam was an option...Im pretty sure but not 100% sure

jumper

Why aren't multistage burners more common? Usually burner is sized for unusually cold day plus pick-up. In the seventies fashion was to replace worn out boiler with multiple little boilers. Except for mornings and evenings only one boiler would be on. Building might be a little cool during day but so what?

jim_14

One point to make is that my radiators are all copper fin tubing. I wonder if that makes a difference. It’s a shame the builder went this route, they heat up quickly but also lose heat quickly. They don’t stay warm for long periods of time like the older cast free standing radiators do.

Jamie Hall

jim_14 said:

One point to make is that my radiators are all copper fin tubing. I wonder if that makes a difference. It’s a shame the builder went this route, they heat up quickly but also lose heat quickly. They don’t stay warm for long periods of time like the older cast free standing radiators do.

And because they do heat and cool quickly, you may want to try using more cycles per hour -- just as an experiment -- unless you can get your outdoor reset dialed in well enough that you always have circulation.

ChicagoCooperator

Yes, convectors make a difference - you'll need more cycles per hour.

Hap_Hazzard

jim_14 said:

One point to make is that my radiators are all copper fin tubing. I wonder if that makes a difference. It’s a shame the builder went this route, they heat up quickly but also lose heat quickly. They don’t stay warm for long periods of time like the older cast free standing radiators do.

I hear you. They chose aluminum because it absorbs and radiates heat rapidly, but the trouble is, it radiates heat rapidly!

The best thing you can do is match the boiler EDR to the radiation as closely as you can and set the swing as low as possible on the thermostat. Your basic "simmer all winter" scenario.

jim_14

So I guess my heat cycle rate on this brand in thermostat should be set to fast?

Hap_Hazzard

CPH and swing aren't the same, but I guess setting it to fast would allow it to maintain a more constant heat. ¯\_(ツ)_/¯

It just seems like a no brainer to set the temperature swing as low as possible if you're noticing large temperature swings, and if CPH is your only way of controlling the swing, then I gues that means set it to fast, but I have to admit mu understanding of CPH is less than crystal clear.

Jamie Hall

"but I have to admit mu understanding of CPH is less than crystal clear." Oh good, @Hap_Hazzard -- I'm not alone.

Hap_Hazzard

Jamie Hall said:

"but I have to admit mu understanding of CPH is less than crystal clear." Oh good, @Hap_Hazzard -- I'm not alone.

Yeah, doesn't it seem like a bass-ackwards concept? It seems a lot more intuitive to say, "turn on the heat when it gets to X degrees and turn it off at X+1 degrees." I don't care how many cycles it does in an hour, but the colder it is, the more it's probably going to have to do, so that seems like the wrong parameter to limit. Long, slow cycles are best, IMO, but the way to achieve that is by corrrectly matching capacity to load, not by saying, "don't cycle more than once per hour."

jim_14

I am finding it better to leave the thermostat at a constant 68 let’s say instead of turning it way down to 60 during the day to save on gas usage when I’m at work all day. The run times are shorter and I get heat rather quickly.

It’s only when I try to go from 64,65 room temp to 69,69 or 70 that it’s starts to cycle over and over and then that when i would just manually lower the thermostat to stop the cycling.

The only draw back is like to feel the warmth coming from the rads sometimes when I’m cold and the copper tubing doesn’t keep that lasting too long.

The other thing is I don’t like the heat coming on a night - I find that it gets to stuffy so I will set it back at night to avoid waking up stuffy. But it’s been a mild winter here in nyc so far so I have not had an issue with that so far.

If and when we get a cold snap then that’s another story.

PMJ

Hap_Hazzard said:

Jamie Hall said:

"but I have to admit mu understanding of CPH is less than crystal clear." Oh good, @Hap_Hazzard -- I'm not alone.

Yeah, doesn't it seem like a bass-ackwards concept? It seems a lot more intuitive to say, "turn on the heat when it gets to X degrees and turn it off at X+1 degrees." I don't care how many cycles it does in an hour, but the colder it is, the more it's probably going to have to do, so that seems like the wrong parameter to limit. Long, slow cycles are best, IMO, but the way to achieve that is by corrrectly matching capacity to load, not by saying, "don't cycle more than once per hour."

The thing to think about is how long it takes to do this. The less time it takes from call to satisfaction the less even the heat is (the more the swing). Even heat requires moving very slowly from cut in temp to cut out temp. So how even the heat is can be related directly to how long the calls are. The longer the call, the more even the heat. Perfectly even heat would have an endless call wouldn't it?

So spreading out the required burn time needed to satisfy the demand with a higher CPH is the only way to extend the call time on average days and even out the heat. Just replacing that single straight run with somewhere 2-3 CPH makes a big difference.

Unfortunately all these electronic thermostats have some mystery algorithm inside that you don't get to know exactly how it works.

jim_14

On second thought.... I left the thermostat at 68 today and the temp was in the 40s. Looking at the app at 5pm I had 3 hours of run time. Now granted that’s from midnight to 5pm.

I have to wonder is it better to save on run time (gas) since it’s empty house and just run it when I get home (from set back)...

What do you guys think?

Fred

What is the run time when you turn it on when you get home? I bet it's at least that much. That will tell you all you need to know.

jim_14

Oh it’s not 3 hours that’s for sure. I would say at least 20 to 30 minutes before I get the radiators to give off heat. Then after that it depends on the rise in temperature that’s needed.

I have never timed it. But now I will.

Will leaving it off all day contribute to rust? I did have my first burnham fail after 16 years with rust above the water line. You can see how where the water line ended and how rust free it was. At least that’s what I think it is

Hap_Hazzard

Did that leak develop near one of the risers?

jim_14

I am not sure.. i used both tappings with drop headers. Only have one riser

Fred

Hard to say. Many boilers seem to have a 10 to 15 year average life, especially the Burnham IN series. Could be water quality, poor castings, thinner walls in the block, leaks elsewhere in the system that causes high water use, etc.

Hap_Hazzard

I meant the riser tapping on the boiler.

jim_14

No leaks. I manually filled. Kept good care of it. The last few years I did feel that the water level didn’t “last” as long as it used to in the past. The last 2 seasons i definitely noticed I needed to add water more frequently then ever before but nothing crazy like every day.

(Small house, returns are above ground, all radiators are quiet and when they hissed I took care of it. )

I now believe that was the beginning of pinhole leaks allowing some steam to escape before it became very noticeable at the end of this past December....

Hap_Hazzard

I'm talking about the rusted hole in the boiler in the picture. Did ths hole form near the tapping on top of the boiler where the riser was attached.

jim_14

I am not sure. I didn’t watch them take it apart. They only showed me the section afterwards and I didn’t think to ask

Kjmass1

In short, because our boilers are massively oversized, because our radiators were sized before insulation and improved windows. I have a 50 year old 200k boiler but my heat loss is down to 45k/hr, probably close to 30k/hr if I didn’t heat the basement. Kinda like trying to fit a square peg in to a round hole.

For comparison, I am running 15 minute cycles every 1.25hr with temps in the 30s. That’s roughly 12 minutes per hour per day on average.

EBEBRATT-Ed

That's 2.48 hours of run time in a 24 hour span or 6.2 min/hour Does not seem bad to me

gfrbrookline

I wish my visonpro app showed me the daily run time, that's a really nice feature. I only get a monthly report.

My system with large cast iron radiators that are vented according to the Gill/Pajek formula that includes the run outs takes about 1 hour to recover from a 3 degree set back.

I have it set to 2 cph which keeps the heat nice and even. I like 2 cph because is allows the pipes to stay warm so I am not wasting money reheating them each cycle. I also have them well insulated. I think if the weren't insulated 1 cph would be the way to go.

Given your Tstat only has a fast or slow cycle as I think anything higher than 2 cph with steam wont let the boiler make enough steam to reach the furthest radiators.