Boiler efficiency cycling vs continuous run

ChrisJ

I've always heard a boiler's efficiency drops off rapidly as it starts cycling.



Why does cycling the burner drop efficiency? Is this only on steam systems or does it effect hot water as well? What about forced hot air systems?

JohnNY

Here's my stock answer on short-cycling and setback thermostats.

The bottom line is steam systems run at 0% efficiency until they are actually making steam that is actively travelling out of the boiler and into the radiators.



Consider these stages of steam heat: 1. flame on/boiler firing/no steam/pipes cold; 2. boiler producing steam; 3. steam pressure created/steam in motion/pipes absorbing heat; 4. radiators warming and releasing heat to rooms.



One definition of efficiency is the measure of how much fuel being burned actually translates to heat in the living space. In a steam system, only the 4th stage is producing measurable efficiency. That means the longer a steam boiler remains in the first 75% of its heating cycle, the less efficient it runs. That's a lot of fuel to burn before any benefit is realized.



It's like incandescent light bulbs. Something like 90% of the electricity used for light bulbs is needed to make the filament glow, the other 10% determines the intensity it glows.



By comparison, hot water heating (hydronic) systems exchange heat from the water to the living space the moment the pump turns on.

Same can be said for the fan and burners of a hot air system, the difference being the steam and hydronic systems release heat long after the whole process shut down because the radiators are still hot and release heat slowly.



With setback, or for the purposes of this conversation, short cycling, you force the system into low periods where all the residual heat from previous cycles is lost, in many cases up the chimney, and the next cycle starts from deeper within the least efficient part of the process where it remains longer.



Many steam systems were designed originally for coal, which means they were designed for a constant burn and a steady temperature throughout. Setback thermostats might have well have been space travel back then.



Another thing to consider is that when you allow your home to drop in temperature, it takes everything with it: the walls, floors, furniture, etc. All that stuff will seek to equalize with the room temperature when the thermostat goes into the high setting. That consumes energy, too.

ChrisJ

Cycling

Thank you for responding John.



Didn't have setbacks in mind when I asked the question, but I'll never turn down information.



So does this mean when a hot water boiler cycles it doesn't hurt efficiency at all?

BoBoBoB

Red herring

"All that stuff will seek to equalize with the room temperature when the thermostat goes into the high setting. That consumes energy, too."



All things being equal, it costs *more* energy to keep the walls etc at 66F all day long than it does to let them drop to 60F twice a day and heat them back up to 66F. The latter, when the solids lag the air temp, will be less comfortable, however, which may lead to choosing a higher setpoint.

JohnNY

Haha. This could be a long conversation...

BoBoBoB, the bottom line is it costs money/energy to heat anything and everything above its ambient temperature for any amount of time.





ChrisJ, no, it doesn't mean that. The part of any heating cycle that involves a continuing call for heat (via the thermostat) and the burner is off due to having reached a high limit, that is the part of the cycle where efficiency is lost. That high limit can be the aquastat setting on a hydronic boiler, or a pressuretrol on a steam system.

The ideal heating system reaches pressure/temperature just as the thermostat is satisfied.



It rarely ever happens quite that way.



That's why we vent the heck out of steam, and love our mod-cons or size our non-cons properly for hydronics.



Someone's going to take me task on this.



Bring it.

ChrisJ

Heat loss

I would think the same applies to the boiler and piping. No matter what you need to add X amount of heat back into the boiler and piping. I would assume the hotter things are, the faster they loose heat just like a house.



To me, this means having the system shut off for hours and cool down it should loose less heat than one running every hour regardless of how long it takes to heat back up.

JohnNY

Well...

...not necessarily.

When I started taking I=B=R classes back in 1992, this was a huge topic of conversation. I guess it still is.



The instructors there were a stone wall in their response that the only way setback thermostats showed any positive benefit was when there were long and deep periods, as in many days of setback at 10° or more.



They had study after study that showed multiple temperature periods in a single (24 hour) day showed no significant benefit at all. In fact, anything beyond a 4-degree temperature drop in those 24 hours often resulted in an efficiency decline.

SWEI

Boiler efficiency

is definitely higher when running longer cycles.  Also less thermal stress on the block, the pipes, the flue, etc.  We really do need some two-stage steam boiler options in residential sizes.



System efficiency is probably worthy of a debate.  Given how fast steam can heat a space, with a leaky enough building I can see setbacks making sense.  IIRC Terry T has mentioned this specifically with regard to intermittently-occupied classrooms and churches.



A mod/con running on ODR is a different matter altogether.  Lower modulation rates increase combustion efficiency and the ability to recover temps is far less given the small ∆T between the supply temp and the indoor air temp.  The savings we see when properly implemented are so dramatic that the setback argument quickly becomes uninteresting.

Jamie Hall

Well now...

Depends a lot on what you really mean by cycling If we confine our remarks to the cycling which a steam boiler which is slightly oversize will do at the near the end of a long cycle, cycling off on pressure so that the radiation can catch up to the boiler, then the answer is yes, there is a small loss in efficiency.  Emphasis on small.  If the off period is short -- typically just long enough for the burner to shut down and restart, the loss is very small, as the boiler and the piping never really cool down at all.  If the off time gets longer, the efficiency starts to suffer, as it is  necessary to bring things back up to temperature.

neilc

another angule

suppose the boiler or furnace is right sized to the smidge,

run all day at design temp,

btu's leaking out at the same rate we send them in,

isn't it less btu's burnt, at the 10* less setback, creating a savings when setback?

is the design temp full btu output btu's sufficient to then reheat the space?

and if so, how long would or should that take?

a 10* rise, at design temp, with a output on that ragged edge,,,,

is it as simple as btu output vs space delta btu use / leakage?

what is it I am missing ?

Jamie Hall

In principle...

you are basically correct.  A setback should save energy.  In practice, the amount saved is small enough to get buried in the noise, at least for reasonable setbacks on a daily basis (the actual energy used, all else being equal, should be related to the time weighted average of the upper and lower setbacks -- thus if you run 12 hours at 60, and 12 at 70, in principle you should see the same energy use as if you ran all the time at 65)(in practice, the whole thing is vastly more complicated -- if only because outdoor air temperatures aren't constant).



On the question of recovery, though... let us suppose, for the moment, that in fact the system is exactly sized to the heat loss at the design temperature, and that we are enquiring about steady state outdoor conditions (never happens, but what the heck) and recovery from a ten degree setback.  Now the rate of temperature rise will be proportional to the excess available heat, which in turn is proportional to the difference between our target indoor temperature and the current indoor temperature.



If we do a little mathematical magic with that -- which we needn't put down here, if only because I haven't any idea how to write the equations in a comment box! (hmm... come to think of it... the basic equation is dT/dt = K(Tdesign - T), where dT/dt is the rate of change of temperature with time, K is a constant depending on the structure, Tdesign is the design or target temperature and T is the actual temperature and time t) -- we find that the time required to reach the indoor design temperature is... infinite.  That is, we will never quite get there.

JohnNY

Which design temp?

Outdoor at 10°, indoor at 70°?

Outdoor at 10°, indoor at 60°?



Which is your fictional boiler sized "to the smidge" for?

neilc

title posted

johnny,

it's your pick,

my smidge could be -20 outdoors,,,

it was / is theoretics,

and it's been bothering me for a while.

Jamie,

are you suggesting I never get back to setpoint if OA stays at design ?

( let's say 70 / 0 )

Jamie Hall

Exactly, Neil

If the boiler is sized to exactly maintain the setpoint temperature difference -- say 70 inside, 0 outside -- and something happens to drop the inside temperature -- say someone sets the thermostat to 60 for a day -- the boiler cannot bring the temperature back to the setpoint.  Close, yes.  Give it enough time you might not see the difference.  But actually get there?  No.



Mark S has an excellent point on the efficiency loss from heating the steam pipes and radiation (not to mention the boiler and the water) after the burner has been off.  This is why I disagree with the theory of "let the latent heat do its work".  If you are dealing with the typically slightly oversize boiler, and are at the point where the thing is cycling on pressure, you want the off part of the cycle to be as short as possible, to minimize these losses.



A related question is the question of how many cycles per hour do you want to have.  That's really system dependent, but even in very cold weather most systems will have cooled enough in a half hour or so that there is no real additional loss in going out to an hour.



And yes, Mark, the thermodynamics involved is messy! 

ChrisJ

Pressure

Jamie,



What confuses me is you are paying money to develop the pressure you are cycling on, by maintaining the higher pressure for a longer time you are burning more fuel. You are also keeping the piping and boiler hotter for longer therefore losing more heat from those items as well.





I think I'm going to pickup a copy of this.

http://www.amazon.com/Thermodynamics-For-Dummies-Mike-Pauken/dp/1118002911

Jamie Hall

I like to look at it this way...

You need to deliver a certain number of BTUs to your heated space.  To minimize parasitic losses -- heat lost from the steam mains, lost by radiation from the boiler, etc. -- you'd like to do that in as short a time as possible.  However, the radiation can only deliver heat at a certain maximum rate -- so there is a minimum time which the radiation must be at steam temperature to do the job.  The problem arises when the boiler is delivering steam faster than the radiation can condense it.  Then you have to shut the boiler off long enough for the radiation to catch up -- but no longer.  Then run just long enough to fill all the radiation again, and so on.



The last thing you want is for the system to build pressure beyond what is needed to gett the steam to the radiators -- a pound or so at the very most in almost all cases (a few ounces for vapour systems!).  An ideal system would modulate the heat output of the burner to maintain just enough pressure to keep the steam moving; that's what the old coal burners did (but not very efficiently!) by varying the draught.  Instead, almost all residential systems modulate the heat output of the burner by turning it off and on -- and in most systems which are running well, the off time is limited by the burner itself!  Many power burners and oil burners have pre and post purge cycles, which are often plenty long for the pressure to drop to the point where the vapourstat or pressuretrol will cut back in.



It's not that intuitively obvious...

BoBoBoB

How come?

Did those studies explain that observation? I have no doubt that real life introduces a whole host of complications that the physics of ideality doesn't explain.



I buy that many heating systems, and especially laggy/high-mass ones like steam, are bad at efficiently recovering from setbacks. But purely in terms of the amount of energy the house loses, and thus how much needs to be replaced, houses with setbacks lose less energy than those without setbacks. Again--drops in boiler efficiency may more than offset any reduced heat loss, but there's no world where a house at 66F loses less heat than one that alternates between 66F and 56F over a 24hr period.

PMJ

What Losses?

I keep seeing comments about "losses" from mains and boilers that are cooling down (a very small amount) during the flame off part of a cycle. These things are inside the house no? Any heat that leaves them goes where - somewhere else in the house. It is not lost at all. Same goes for the heat required to warm up the boiler to the point that it can produce steam. It too is inside the house. This heat did not get "lost" like it went up the chimney or something. It is being talked about here like any time that a boiler runs without steam actually flowing into radiators at the ends of the supply lines is completely lost. Clearly it is not. 



And to me it is a small matter anyway because the time we most care about efficiency is when we actually need heat - as in when it is cold outside. During these times the delivery system is always hot. I run 10 min on/10 min off cycles when it is cold and the cooling of boiler or mains in 10 minutes is negligible. On next fire my farthest radiator sees new steam in 90 seconds and most of them in 30-45 seconds. Once again though, the small amount of heat on this cycle that went into heating back up mains is inside my house and heating it just as much as heat that came through the radiators.



And for some reason I never see the electric bill discussed for running pumps in water systems or the fan in forced air ones. On a percentage basis of the total heat bill this electric part is significant.

ChrisJ

Lost

PMJ, I had thought about the same thing, even with my drafty crawl spaces I have to assume the majority of the heat dissipated from the piping goes up into the house. There is of course a pretty good amount of heat that gets sucked up the stack, but I have no idea how much it is.





As far as pumps and blowers, personally I don't like forced hot air and I worked on one hot water system and as soon as it was time to bleed all of the convectors I was done with hot water.

BoBoBoB

Depends on the house

If the boiler room and areas surrounding steam piping are finished, inhabited, and insulated/sealed as well as the rest of the house, you're right. But there are lots of houses where that's not the case. As long as my boiler room stays warm enough to keep the pipes from freezing, I'm happy. And I have steam piping in exterior walls--most of that heat gets lost to the exterior.



Now, the main that runs under the floor of my living room and in the ceiling of the fished basement? I happily leave that uninsulated, getting a nice warm, radiant-like floor in the living room and a comfortable room in the basement without needing supplementary heat.

PMJ

Even uninsulated spaces

like walls and crawl spaces that have their temps raised by a supply line in turn slow down the transfer of heat through that wall from the inside space. My point is that the burner off condition of these spaces is not different than the burner on one as far as any losses are concerned.

Jamie Hall

Of course

you all are quite correct -- so long as the pipes and the boiler itself are within the more or less heated envelope, the "loss" isn't actually a loss in terms of overall heating efficiency.  How much these heat transfers -- to avoid the term loss! -- contribute to the comfort of the house is another matter, and one completely dependent on the structure.  This is one of the many reasons why trying to pin down the "most efficient" way to operate a heating system is so darn difficult!



The only real total loss is the heat in the stack gas, and even then only to the extent that the stack is not part of the heated space.



Same thing is true, of course, of the water heater and the hot water pipes -- in any climate where heating is used much of the time, the "losses" from the water heater and the pipes isn't really a net loss.



As for the pumps and fans -- quite right there.  They do use a surprising amount of energy, which is conveniently overlooked by the folks (including the government boffins) pushing high efficiency hydronics, radiant, and particularly hot air.



All of which is why, although steam is sometimes given a bad rap (the best we can do without heat recovery heat exchangers on the stack gas being in the high 80s) the overall efficiency of pretty much any of the various heating systems is within a very small number of percentage points, provided they are all maintained equally well -- a rather big if.

Joe V_2

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Boiler Jacket Loss

The International Energy Conservation Code addresses the controversial subject.

By Ray Wohlfarth

March 15, 2013