"Normal" Heating Cycle times...

pbennett45

So, I'm working my way through "We got steam heat" and I'm about halfway done. Let me disclose that I'm an engineer and like to know how everything works. And I'm trying to understand the heating cycle... perhaps my answers are in the part of the book I haven't gotten to or perhaps I need the "contractor" book to satisfy the engineer in me. At any rate, here's my question.



You size your boiler based on the EDR or output capacity of the entire radiation load... let's say for argument, you could measure everything perfectly, every radiator, every pipe,etc... Now let's say you can order your boiler in infinitely precise increments, so you could perfectly match the output BTUH to the entire load - no pickup factor, perfect measurements instead... just for argument. In this "ideal" situation, wouldn't there be only one cycle every call for heat? There would be a call for heat, the boiler would kick on, it would put out heat at exactly the same rate it could be output from the system, no pressure would ever build up and it would never hit the cut out pressure. The only thing that would end the heating cycle would be the thermostat. Under this non-realistic "ideal" condition, is my logic correct?



Obviously that's not reality.. instead you measure the radiators, multiply by the 1.33 piping pickup factor, and buy the next size boiler that your manufacturer of choice offers. So in reality, every boiler is slightly "oversized". There's really no such thing as perfect measurements, and hence no such thing as correctly sized, everything is slightly oversized if done correctly. So instead, the system turns off at the cut out pressure and back on at the cut in pressure if it's still not up to temp. Is my understanding of reality correct here?



So, here's the real question. With everything else in the system working perfect, how oversized is too oversized? With the thermostat cranked to infinity, and normal cut in/out pressures, what should the cycle times be? At what point do shorter cycle times (i.e. an oversized boiler) significantly hurt fuel efficiency. What % oversize would cause a system to hit that "too short" cycle time?



And like I said, maybe my answer is yet to come in the second half of the book. But so far it's said the boiler should be properly sized, then it gives a lengthy explanation of why undersizing is bad, but hasn't spoken much about oversizing. Which got me thinking, since simply the finite number of offerings by manufacturers implies that every boiler will be at least a LITTLE oversized, how oversized is oversized?



Thanks for the edumacation :-)

Charlie from wmass

Yes

The truth is in theory you are correct but boilers come in incremental sizes and Delta T changes with the weather, literally.

pbennett45

Right...

...Which I thought was what I said in the later part of the original posting, my apologies if the guts of my question wasn't clear. While the thermostat is calling for heat (i.e. - if it was bone chillingly cold out and you had it set to 85 - so the boiler is cycling only on pressure cut out/in), what is a "normal" cycle time? How short would be considered "short cycling" due to boiler oversizing?



Thanks!

Charlie from wmass

Best we can do is

Size as close to the load as we have availability. Some times the smallest boiler made is twice as large as the load.

nicholas bonham-carter

length of cycle

if you have the perfect system, completely matched to the load, and it is firing on a design temp day; it will never build much pressure. if the same boiler is firing on a warmer than design temp day, it will build some pressure, and may cycle on the pressure control. if there were no main vents in the system, then the back-pressure resulting from the air being evicted through the radiator vents, assisted by the gas company, would cause the boiler to cycle on pressure.

it is for these variables, that the ideal boiler would contain a modulating burner, controlled by pressure. at the start of firing, the burner would be on high, and as the pressure rose to 3 ounces,  would then switch to low. as the temperature fell outside, the pressure would begin to fall in the system, and the burner would switch to high in order to maintain the pressure.

therefore, how over-sized could the boiler be assuming the presence of a modulating burner? my guess is 50% as the low burn rate is about 60% of the high burn. also do a search for boilerpro's excellent treatise on boiler sizing.--nbc

pbennett45

Thanks

Thanks for the reply.. but I'm confused on your cold day/warm day analysis. My understanding was that this would be entirely driven by the BTU output of the boiler, and the radiation capacity of the radiators. The radiator capacity being only a function of the effective surface area of the radiator (fixed regardless of outside temp) and the temperature differential (room temp, which is assumed to be 70 degrees as the radiators are rated, but at any rate, is relatively constant if the thermostat is working. and the steam temp, which is between 212-214 between 0 and 1 psi). A difference in those two numbers, energy in and energy out, will cause pressure to build up. I understand that on a cold day, it may never cycle if perfectly matched, but in that case, on the warm day it's cycling because of the thermostat, not pressure build up. No?

Charlie from wmass

The condensation rate

of the steam is faster when the heat is leaving the building envelope faster. The room temp is not a constant 70 or the thermostat would not be calling for heat.

pbennett45

Thanks, but still confused on that last thing....

Fine, 70 degrees plus or minus a few... 3 degrees only changes delta-T in the heat transfer equation by 2%, for the purposes of how much heat can possibly flow out of the radiator when filled with the steam, it's pretty much a constant 70. The condensation rate at any given point in time can only equal the amount of heat the radiators can give off, which is governed by the heat transfer equation, which only includes the surface area of the radiators, the specific heat of the material of the radiators, and the difference in temperature... Thanks to both, as my original question was pretty much answered, that there's a lot more leighway on oversizing a boiler than there is on undersizing... but I remain confused about what the outdoor temperature has to do with cycling based on pressure.

Jamie Hall

Well... um... yes and no

There are two cycles to consider, not one, and if you get them confused you'll have problems.



Assuming we're talking steam here (the principles are similar, but the controls quite different for other heating systems).



Cycle 1 is related to the steam output of the boiler vs. the steam demand of the system.  That's the cycle that's controlled by the pressuretrol or vapourstat.  The steam demand of the system is set by the size of the radiation (and, as Charlie notes, to a lesser extent by the space temperature -- but the effect is noticeable).  If you had the ideal situation, on the design day the steam demand of the radiation, at the temperature assumed by the ideal day, would exactly equal the steam output of the boiler -- and the boiler would run forever.  On any day with less demand from the radiation, if the boiler is asked to run long enough by the thermostat (cycle 2, coming up) the steam output of the boiler will be greater than the demand from the radiation, and the pressure will, eventually, build.  At which point, since we are afflicted with on/off systems, the vapourstat or pressuretrol will shut down the boiler, to give time for the system demand to catch up to the boiler supply.  That's cycle1.



Cycle 2 is related to the heat output of the radiation vs. the heat loss of the space.  Again, on our ideal demand day, the heat loss of the space will exactly equal the heat output of the radiation, and once the thermostat calls for heat, the boiler will run forever (if cycle 1 is perfect too!).  But on anything less demanding, the heat output of the radiation is greater than the heat loss of the space, and eventually the thermostat senses a higher temperature and turns the system off, to allow the heat lost by the space to catch up to the heat output of the radiation.



The two cycles can interact in some weird and wonderful ways, but the end result is that the steam output of the boiler should match the heat loss of the space... on the average.



Now you asked about time.  Since you are an engineer, perhaps it is safe for me to state that both cycles can be treated like digital servomechanisms, and the cycle timing for both cycles will be controlled by the dead bands on the controllers and the inertia of the two systems.  In a perfect world, both systems would be analogue, with the gains adjusted to avoid instability.  The world isn't perfect.

Mike Kusiak_2

Undersizing may not always be bad..

Here is the link to the article mentioned above on an alternative view of steam boiler sizing.





http://www.heatinghelp.com/article/11/Hot-Tech-Tips/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell





If the venting can be worked out so that the system heats evenly, there may be advantages to sizing the boiler so that the system never cycles on pressure.

butlermog

More Cycle Time Questions

This question might be a little off from the original topic, but I believe it applies to the desire for many of us (perhaps its just us engineering types) to understand the normal cycles of our systems. (Sorry to pbennett45 if you feel that I am hijacking your post).

I have been reading as much as I can about my boiler and my Mouat vapor system (2-pipe vapor in 2,600 sq-ft house, ~840 ft^2 EDR, WM EGH-85 boiler with 349,000 btuh input). And have really been struck by how short many people's cycles seem to be compared to mine. Now, I know that I'm in an old drafty house, but I still think that something is wrong. Specifically, while reading one of the boiler manuals, I found the attached chart that states "Designed full capacity steaming time of modern boilers is 10 minutes." Does this mean that since my boiler is designed to produce 845 sq-ft of steam, it is supposed to do that in 10 minutes - given it is burning the right amount of fuel and the boiler is well maintained?

If that is true, and if the system is well vented (and correctly sized), I should expect to have all radiators filled with steam, and the system beginning to pressurize in 10 minutes? Which is exactly why a vaporstat for my boiler would be ideal - since it should only take that amount of time to start building pressure.

This makes sense to me considering what I read from BobC about his 'normal' cycle a few weeks ago: "It's just below 20 with a light wet wind right now. My steam boiler comes on about every two hours and runs for 6-7 minutes to get my radiator vents hissing (Gorton #! on the 15 ft steam main), the system cycles off on pressure initially about 3 minutes later. It cycles on and off a few more times till the thermostat is satisfied 15 minutes after the initial call for heat." Now, I don't know how large a system or house BobC has, but that seems to match up with the "designed full capacity in 10 minutes".

So, here is why I ask all of this: My boiler (currently with a pressuretrol set to 0.5/1.5) has never cut out on pressure. I have also never had all of my radiators completely full of steam (hot all the way across) even after consistently burning for 1hour+. Which leads me to believe that if my boiler is producing the amount of steam that I need to fill my radiators (assumed from the above), the steam isn't getting to where it needs to - which probably means I need more main vents, right?

Is it as simple as that - adding a few Gorton #2s and watching the steam get to my rads in 10 minutes? Currently I have 1 Hoffman 75 at the end of my main, and 2 Hoffman 75s on the return (as seen in the second attachment).

Thanks for any insight.

Jamie Hall

Each system is different

with regard to how long -- and if -- it takes to develop enough pressure to trip a vapourstat or pressuretrol.  Some -- the system in the building I supervise, for instance, just barely get there after 45 minutes of steaming.  Some get there much more quickly.  Some never get there.  This is the cycle 1 I referred to in an earlier post, and it depends completely on the relationship between boiler steam generation rate and system steam condensing rate.



With regard to Buffermog's query, though, it sounds as though your boiler may be slightly undersized or underfired for your system (assumng, that is, that all your mains are insulated -- that can make a huge difference; equivalent to a 10% change in firing rate, at least).  The question I would ask here is -- do your radiators all see steam at more or less the same time?  The rough rule of thumb which I use for an initial pass on adequate venting is that it should not take more than 1 minute for the heat to move 15 feet along a steam main -- that is, if you have a 75 foot main, it shouldn't take more than 5 minutes from the time it is hot at one end to the time it is hot at the other (again, assuming it is insulated -- if it's not, it can take hours, regardless of your venting).  If that's OK, then if there is a radiator which isn't heating across at the end of a really long cycle, check and make sure that its valve is open, or open it more if it's partly closed.  Sounds silly, but... it happens.

butlermog

Undersized/Underfired

Jamie,

First of all, please allow me to thank you for the time and effort you put into answering questions on this site. I have been reading the site for a month or so now, and have many times found your answers to be very helpful.



As for my boiler being underfired, I don't believe this is the case. The boiler manual gives a formula to use to verify that the gas usage is accurate to the input btuh (300,000) by taking 3,600,000 divided by the number of seconds it takes to use 1 ft^3 of gas. My boiler uses 1ft^3 of gas every 10 seconds, which puts my usage at 360,000/hour - slightly higher than the 300,000 input of the boiler. So doesn't that mean that my boiler is 'overfired' if anything?



My radiators do seem to get steam to the top of the supply pipe at about the same time. This is somewhere between 7-10 minutes after the call for heat from the thermostat. The radiators on the second floor might take a bit longer, but i think that is normal. Also, the near boiler piping is insulated with fiberglass, and the mains are asbestos all the way around the basement (as seen in the attached pictures).



All of my radiator valves are fully open (well, all of the ones that we want to be open), and they do all heat, eventually - i was just struck by the boiler manufacturer's claim that the entire volume of steam should be produced in about 10 minutes, and where that steam actually is inside my system. But I guess from your statements that my understanding of what I read in that manual was incorrect, since it is OK for a system to take up to 45 minutes to fill up.

pbennett45

Thanks

Thanks Jamie for the explanation of Cycle 1 and Cycle 2. So is it fair to say that the more "oversized" the boiler is, the more extreme the Cycle 1 cycling is going to be? Dan's book seems to indicate that oversizing OR undersizing will give you worse fuel efficiency.. but the only implication of oversizing I've seen is more cycle 1 switching. That would imply that your system is more efficient on cold "design days" than it is on warmer days too, because then there is more cycle 2 switching. No matter what is turning on and off the boiler, thermostat or pressuretrol, it should have the same effect on efficiency, no?

Mark N

Boiler Input and Output

Butlermog



You say in one post that the input to your boiler is 350,000btuh than the next that it is 300,000btuh. I looked up your boiler the input is 349,000btuh. So you clocked it at 360,000btuh, that is pretty close. The gross output of your boiler is 280,000btuh, the net is 210,000btuh. Take that 210,000 and divide by 240 and you will get 875sqft. Take the 210,000 and multiply by the 1.33 pick-up factor and you will get 280,000 which is the gross output. All the numbers are based on 1 hour of time. The output of your boiler after losses is 280,000btuh. All of that output is not available to your rads, The first 70,000 btus, the difference between the gross and the net is used to boil the water and heat the pipes. The gross output of your boiler is 4667 btu per minute. So there is no way you can generate 875 sqft of steam in 10 minutes, it is more like 195 sqft. It takes your boiler 45 minutes to output 875sqft of steam. Multiply 4667 by 45 and you will get 210,000 btus. You say you have 840sqft worth of radiation. You system is pretty much able to condense all the steam this boiler makes. You probably might not generate more than a couples of ounces of pressure in 1 hour.

Charlie from wmass

Not really

Turning off by thermostat stops the boiler at what ever the pressure is at that moment even if it is 0 inches water column. If you turn off on pressure you have used enough fuel to raise the pressure in the system to the cut off pressure. This is why low pressure is so important to save when the boiler is oversized but a vapourstat saves little on a properly sized boiler. It is the fuel used to increase the pressure above what is needed for the function of the system that you quit using. Slight oversize is needed it you want every radiator to fill fully with steam. I have one system I had a supply house size the boiler for. It never shuts off on pressure and one radiator always has 2 cold sections no matter the set back or thermostat setting. The house heats evenly and quietly. The fuel use is low for the size of the home. Had I sized the boiler it would have been one section longer and that radiator would have heated all the way across. it also would have been 83% efficient instead of 82% as it would be a lower relative firing rate.

If a boiler is undersized it sends much of the heat dollars up the flue as it keeps running to match the load.

pbennett45

Feel free...

Feel free to stop replying to me at any point if I get too annoying... but you're saying, if you're constantly shutting off on pressure (cycle 1 per Jamie's explanation) instead of thermostat shut off (cycle 2), which is what would happen with an grossly oversized boiler, you're less fuel efficient because you're running at a higher pressure (closer to the cutoff pressure) and it takes more fuel to get up to that pressure?



Assuming I understand that right, I got one more.. and like I said.. feel free to stop replying to me at anytime. At that higher pressure, you have a proportional amount more steam and hence more latent heat in the system. I'm no chemist, but I remember high school and the Ideal gas law, Pv = nrt says it's all linearly proportional. So with V (volume) being constant, r is a constant, and t (temperature) is relatively constant (the different between boiling point at 0.5psi and 1psi is small), doubling your pressure doubles n, the amount of steam in the system. Hence it also doubles the latent heat in the system (each molecule of steam contains a certain amount of latent heat). Granted, you had to put that latent heat INTO twice a much H2O to pack twice as much steam into the system in the first place, but you'll get it back when it condenses. The only inefficiency I can come up with is that at the higher pressure the boiling point is a little higher, so the extra energy used to go bring the water up to the higher boiling point is lost, but all the extra latent heat you put in should be recovered via a longer off-cycle because there's twice as much steam in the system at the higher pressure, which takes longer to condense and gives off heat longer before it drops to the cut in pressure. Just trying to figure out where exactly the inefficiency is... all the energy you burn has to go somewhere, either into the water, into the steam or up the flue - it can't just disappear per simple conservation of energy. So the only other option I see is that somehow the boiler doesn't operate as efficiently at the higher pressure and more is going up the flue?

butlermog

You're Right

Mark,

You are right. I thought that the boiler was 300,000 but later saw that I was wrong. Sorry for any confusion. What you are saying makes much more sense, that it takes an hour to create 875 ft^2 of steam and not 10 minutes. I was just confused by the before-attached table in the WM manual.



I guess what I am learning is that my system is actually performing pretty well (even if that means expensively). If my burn rate is what the boiler says it should be, and my radiators all get steam at the same time I don't think that there is much more modifying that I can do - right?

Charlie from wmass

Eureka! I think he has it

as the pressure increases the flow of heat from the combustion gas to the media (boiler water) slows and decreases the efficiency of the burning fuel as more heat goes up the chimney. Also the extra heat is not needed so the open window method is employed by tenants, if it is an apartment building, or children of the home, if it is a single family home to get rid of the excess heat. also the excess heat creates a larger delta T so it sends the energy faster to the outdoors. This further wastes the heat. Answering questions is no bother. Fighting over the answer does get boring at times. The great thing with a forum like this is often you only need to answer the question once then just reference the answer from then on.

Dave in QCA

Well umm.....

Hey Charlie,  I completely get what you are saying and agree.  I think you said, the efficiency of a steam boiler is greater at very low pressure because the temperature of the boiler is slightly lower, and therefore, the heat transfer is greater.  Kick me if I got it wrong.



But, if I am tracking with you correctly, and I think I am, this argument would also support the idea that vacuum heating would be even more efficient.  If you could boil water at 160 F, wouldn't your heat transfer be greater?



I am still stuck on the vacuum theory.  I have come across a number of pieces of information that refer to vacuum operation with both oil and gas fired boilers.  It seems to me that perhaps vacuum began to be dismissed about 30-40 years ago in the same way that the hydronic industry tries to dismiss steam heat today.  It just seems to me like it just might be as great as the vacuum dead men say it was.

nicholas bonham-carter

new vacuum operation

maybe we need a new burner design, which like coal would never really shut off completely. in between major cycles, it would simmer.

the controls would have to make allowance for this, because steam would be established in a few seconds!--nbc

Mike Kusiak_2

Taking it further

If you had a tight enough system and could get the vacuum down to a few inches of mercury, then condensing boiler operation would theoretically be possible. You can easily boill water at 100F with a good enough vacuum. Of course there are practical problems in maintaining this degree of vacuum, especially with old systems, but it seems perfectly feasible with a newly built system with modern components.

Mike Kusiak_2

Cycling on pressure

The problem with cycling on pressure ( cycle 1 ) is that is occurs repetitively and is of short duration. A burner runs most efficiently during a long burn. Short cycling, with cycles a couple of minutes long or less, is less efficient as the burner never gets to its steady state firing condition.



On the other hand, cycling by the thermostat (cycle 2 ) involves firing periods of much longer duration, and therefore allows the burner to reach its cleanest and most efficient steady state combustion, and to remain in that state for relatively long periods.

haaljo

Air is the heat thief per this article

http://www.heatinghelp.com/files/articles/1310/128.pdf

(Hoffman 1927 pamphlet.)

I have no idea why vacuum was abandoned but suspect you need to be a "home handyman" to maintain proper operation. As Mike notes: "You can easily boill water at 100F with a good enough vacuum" which is what going negative on pressure is all about.

Here in Boston, there is an engineer that has been working on a modern vacuum system which requires retrofitting radiators with vacumn vents (so to speak) and an elegant control strategy. Makes sense to me but I'm just a layman when it comes to heating. My concern would be having to maintain the vents/valves because if one fails - there goes your vacumn. But I have seen his setup working and seen the pressure go negative and he can maintain a :"vacumn" for several hours.

FJL

Low Pressure vs High Pressure

I hope I can ask this question clearly. I am trying to understand how running a boiler on lower pressure will reduce shorty cycling if that is true. Setting a v-stat at a lower cut out pressure will cause the boiler to turn off due to pressure sooner than if the cut out was set higher. Does lowering the cut out also cause the boiler to run longer between cycles, between the times when the boiler cycles on an off due to pressure? If so, is that because the pressure in the boiler never gets a chance to build to the point where it repeatedly needs to shot off the boiler?

Mike Kusiak_2

Cutout setting

Reducing the cutout setting will not reduce the short cycling. Actually it will tend to increase it. Short cycling is caused by the boiler producing more steam than the radiators can condense. At a higher pressure, the radiators reach a higher temperature, so they can actually dissipate more heat, and therefore condense more steam. So at a higher pressure setting, the cycling will be reduced. Unfortunately, this is not an efficient way to run the system.



The most practical and efficient way to reduce the cycling is to lower the firing rate of the boiler so it produces less steam.

haaljo

And don't be surprised if you are clocking above the minimum...

for your system. That's what I found when tech came to clean and tune-up.

FJL

Confused

Then what is the rational for running a boiler at "low pressure" (below 1.5 psi) as many on this board advocate?

Mike Kusiak_2

Low pressure

The best way to accomplish "low pressure" operation is to set the steam production rate at a level balanced by the condensation rate. The pressure will never rise too high because too much steam will not be generated.



Low pressure operation is beneficial in many ways as far as proper operation of vents and efficiency is concerned. It is how the low pressure operation is accomplished that is important. Limiting the pressure by constantly cycling the pressuretrol is not the best way to achieve this goal. Rather, proper boiler sizing and selection of firing rate, which results in steam generation within the limits of the radiation's ability to accept it is a better way to implement operation at low pressure.

FJL

Pressure Level

OK.  Now I understand.  High pressure is not the answer to every steam problem.  Don't raise the pressure to solve a problem that can be solved with more venting.