High Efficiency Steam Boiler?

DanT

I thought I read that one can't use a high efficiency boiler for a steam system.  Is this true?  What's the most efficient type of boiler for a one pipe steam system?  It would be a ideal if it was not vented through the chimney.  My main concern is back drafting. 



Thanks,

Dan

jpf321

gas or oil?

inquiring minds need to know

nicholas bonham-carter

most efficient steam boiler

the most efficient boiler is one which has been properly installed by someone who can read the  manufacturer's instruction manual.

some boilers are a bit more efficient than others, but in general it is the proper installation, and piping which is most important.--nbc

Big-Al_2

Not Condensing

The boilers and furnaces with the highest combustion efficiency achieve it by extracting so much heat out of the combustion gasses that the water vapor in them condenses.  In order to do this, they obviously need to get the gasses below the boiling point.  If you think about it, a steam boiler operates above the boiling point, so it's not possible to run it as a condensing unit.  This limits the combustion efficiency to about 86%.



Like NBC alluded to, combustion efficiency is only part of the overall economy.  For example, a high efficiency furnace connected to ductwork with a lot of heat losses will still use a lot of fuel.  Conversely, a properly installed, well maintained steam system might actually use less fuel.  It's the whole system that matters.



I'm sure someone here can recommend an induced draft boiler that has good efficiency.

The Steam Whisperer (Formerly Boilerpro)

Nearly All Steam Boilers are High Efficiency!

This may sound nuts, but a more thorough look at efficiency of the heating unit is quite revealing.

The basic test for AFUE numbers is how much fuel in versus how much heat out, and then corrected for seasonal use and typicaly installation patterns (oversizing, etc).

Looking at this carefully, some flaws become immediately visible.

First off, electrical use is not measured and taken into account in the efficiency number.  Typical Gas Steam boilers use virtually no electricity.  Hot water boilers use 10 to 20 times or more to run pumps and combustion blowers on high efficiency units.  Hot air furnaces require about 100 times as much electricity to run large blower motors and combustion fans. 

Also, there does not appear to be anyplace where the heat generated by running these blowers is taken out of the heat output of the unit to calculate a real btu output number from the burning of fuel only.  So the combustion efficiencies are artificially high.  This extra heat produced by electricty bumps the efficiency by about 2% or more from my preliminary numbers.

Once all this gets totaled, if you use the current standards for hot air furnaces, that typical 80% steam boiler will have the same input costs as about a 90% efficient furnace.

And with more up to date technology, 90 to 92% combustion efficiency steam boilers are a reality.  I am eagerly a waiting another manufacturer that may be coming out with smaller models soon that would be used in light commercial and large homes.

Big-Al_2

Stack Temperature

It really boils down to stack temperature. (Pun intended.) It's conservation of energy.  Whatever heat goes up the stack doesn't go into the heated space, A high efficiency furnace or boiler sends it's combustion products out at less than 200 F, but even an "efficient" steam boiler might have a stack temperature of 300 F or more.  The hotter the flue gasses are, the more heat that's carried out with them and wasted.  On a big commercial steam boiler, some of the stack heat can be recovered through an economizer to pre-heat the feedwater, but in a smaller application, that heat is just wasted.  Given a similar fuel/air mix, if the stack temperature is higher, more heat is wasted, and the unit is less efficient.  Period. Can't defy the laws of physics.  .    

The Steam Whisperer (Formerly Boilerpro)

I agree and disagree...

When comparing boilers that are basically the same design, stack temperature is all that matters.  However, when comparing a power burner boiler to a standard atmospheric  things begin to go sideways.  The power burner is now using additional fuel (electricity) and is adding a small amount of heat to the boiler. 

The real problem comes in when comparing a steam boiler to a hot air furnace pr even a hot water boiler.  This is what many (most) consumers are doing.  Electrical usage is a huge factor in the fuel usage of a hot air furnace and almost nonexistant for most steam boilers. However, the efficeincy ratings they see do not take this into account.   Furnaces end up with grossly overinflated efficiencies and steam boilers get hammered.  They see an 80% efficiency rating on a steam boiler and automaticallly assume that the steam boiler is less efficient than a 90% efficiency furnace.  Steam must be old and inefficient heat and we need to tear it out and put in a new 90% efficiency furnace.....this is what they are thinking.  You and I know better, but every thing else tells them overwise.  We're talking about using 1200 watts of power to run fans to gain just 7% in fuel conversion to heat efficiency.  This is a 0 or negative gain when you consider the cost and exceptionally low efficiency of electrical generation and transmission ( 30 to 50% or so)

WE NEED TO CONTERACT THIS HUGE MISCONCEPTION.   One of the ways of changing the "steam is inefficient conception" is to get a ratings system that takes into account all energy used by the heating unit, not just fuel. This still leaves out system efficiency issues, but it is at least a start, 

Big-Al_2

Yes, agree and disagree

The electrical energy used by a fan or a pump doesn't disappear. It ultimately turns into heat.  The energy used by a circulator in a hot water system, or the energy from a fan used to move hot air out of a forced air furnace ultimately winds up as heat in the house. Granted, at least in Wisconsin, electrical energy is about twice the cost of the equivalent amount of gas, so that part of the heat costs more.  But in rating a fuel burning appliance for energy efficiency, I'm sure the electrical energy use is factored into the overall rating.



Think of it this way.  If you compare a 100,000 BTU (input) furnace with 8,000 BTU of energy going up the stack . . . to . . . a 100,000 BTU (input) boiler with 14,000 BTU going up the stack (higher temperature) obviously the one with less heat going up the stack is putting more heat into the house for the same amount of fuel. 



Also consider that the rated efficiency of a heating unit is for steady-state conditions.  When first you fire up a hot-air furnace or hot water boiler, the heat almost immediately goes into the heated space.  A steam boiler can take quite a few minutes to start steaming, and all that time the burner is on and no heat is entering the conditioned space.  When the call for heat is over, a furnace fan runs for a little while to extract the rest of the heat from the heat exchanger.  A boiler stops steaming very quickly, and all the heat left in all that water and iron starts to dissipate . . . into the basement, boiler room, or up the stack.  Is most of that heat wasted?  Probably.



In a large commercial or industrial situation, where a modulating boiler can run just about all the time, steam can be pretty efficient.  In a residential setting, with an on-off boiler, the efficiency really suffers, especially in the shoulder months when the boiler only runs a small fraction of the time.

Jean-David Beyer

Not in my house.

"The electrical energy used by a fan or a pump doesn't disappear. It

ultimately turns into heat.  The energy used by a circulator in a hot

water system, or the energy from fan used to move hot air around a house

ultimately winds up as heat in the house."



In my house, that is not so. The boiler and the circulators are in the unheated garage. If I wanted to heat the garage, I suppose I could put baseboard in there, but it would do no good because code requires me to have two vents in there, each a little over a foot square, one near the ceiling, and one safely over the height of the snow line. (Actually, it says within a foot of the floor, but the snow is sometimes (rarely) over 5 feet deep, and often over 2 feet deep.



But since I do not wish to heat my garage at natural gas prices, all the heat lost from the near-boiler piping, the circulators, and the boiler outside shell, are lost. I have insulated those pipes as well as I can, to reduce the losses, but I have not insulated the circulators, the Flow Check valve, the air extractor, and things like that.

jpf321

i like....

the discussion .. but I think we need a few diagrams to depict the different ins and outs of energy ... i'll see what I can do .. does anyone have any numbers to attach?



Big-Als point about what happens to the energy after a boiler stops steaming is one argument for having a well insulated boiler .. since insulation will retain the heat within the boiler for as long as possible (and yes, some goes stackwise) .. but by retaining it as best as possible within the boiler, it's netted out on the next cycle. yes there are arguments that if it leaves the boiler, it ends up in the basement  and thence into the house (unless you're boiler is not in the house) .. but i'd rather hold it in the boiler :-)

The Steam Whisperer (Formerly Boilerpro)

AFUE ratings

It has been awhile since I looked at the AFUE rating procedures, but I don't believe there is any accounting for the electrical usage of a heating unit.  The efficiency rating is based solely on heating fuel use and btu's out, with corrections for typical heating season cycling and typical oversizing.  ( IIRC..they assume hot water boilers are 60% oversized for the application, not sure on steam).  I'll need to look up the procedures again to be sure.

I have begun researching the electrical use of furnaces and so far even the newer ECM motored furnaces have a electrical draw of 800 watts when running at full output. ECM motors use about 1/2 the power of standard high efficiency motors when at full load, so the typical large hot air furnace probably pulls 1500 watts at full load.  Since few are actually running this way, 1200 watts is probably closer to reality.  1200 watts output = 3480 btu.   If you assume an efficiency of the generating staion and distribution at about 35%, then that is an input of 10,440 btu.  Now even a gas network does not work at 100% efficiency, so the gas supplied to the unit uses energry before it arrives at the heater.  Lets assume a gas network efficiency of 85% ( need to confirm this).

For a large 150,000 btu/hr furnace rated at 90% AFUE, the total input is:

150,000 / .85 = 176,470 btu

Electrical usage +10,440 btu

= 186,910 btu



150,000 x 90% = 135,000 output

Now putting the two together you have 135,000 btu out / 186,910 btu in for a total efficiency of 72.2%

Now, looking at a steam boiler,  assuming the same grid efficiencies,

150,000btu input / .85 = 176,470 btu

Electrical usage (40 VA transformer at 1/2 load)

20 watts @ 35% efficiency = 57 watts x 3413 btu/kilowatt = 195 btu

Total energy input of the steamer =

176, 470 btu + 195 btu = 176,665 btu

output of steamer = 150,000 x 80% = 120,000 btu

Overall efficiency of steamer =

(Total output) 120,000 btu /  (total ouput)176,665 btu =  67.9 %.



Pretty close, aren't they.  



I am still digging into this, but this provides the basis behind my idea that steam boilers are much closer in efficiency to condensing furnaces than most believe.

jpf321

looks like ...

it'll cost someone $69 to get the AFUE standards document...but I'm sure someone here already has it.



I did find an online book about some of the AFUE assumptions (page 59 & 60) .. http://is.gd/i0H0V



1) Boiler is assumed to be in a heated space where heat directly emitted by boiler is considered useful (If not installed in heated space, the actual efficiency would be LESS than AFUE rating)



2) AFUE assumes boilers are 70% oversized .. if they are better matched to actual required size, the efficiency would be GREATER than AFUE



It goes on to say that true efficiency will always be affected by the system and building where it is installed and AFUE should only be used as a relative comparison between boilers.

Mike Kusiak_2

AFUE Testing

Here is some info I found a while back... There is no mention of any electrical energy being taken into consideration, only the heatibg value of the fuel minus what goes up the flue.



AFUE testing

I did a little searching, and found the DOE regulations for testing

AFUE. Their tests are almost completely based on the ASHRAE 103

Standard. Although not all the info is there, I was able to get some

details. As far as I can tell, the AFUE is based on an analysis of flue

gas to determine steady state efficiency, so it is basically combustion

efficiency test. The boiler is operated at rated input, and the input

BTUs are calculated from the gas flow rate and the higher heating value

of the gas used. Water is circulated through the boiler at 140 F out,

120 F return, plus or minus a few degrees. There is no mention of

measuring the water flow rate, only that you want to maintain the 20 F

delta T. Thermocouples are placed in the flue pipe to measure gas temp,

and flue gas mass flow rate and CO2 are measured. These measurements

are plugged into a complex formula to determine heat loss up the vent

pipe, and the heat loss up the vent is calculated. The flue heat loss

is subtracted from the BTUs in to give the output of the boiler. There

is no mention of actually measuring the flow rate of the boiler water

and delta T, and calculating how much heat gets to the water. I appears

that AFUE considers anything that doesnt go up the flue useful output,

jacket losses included.

This is a simplification because many other factors are taken into

consideration, cycling, standby heat loss, pilot energy use, etc, The

formulas for calculation are complicated, and many assumptions are

made.

All in all, It appears that AFUE is basically a combustion efficiency

test, not a boiler efficiency test, and therefore doesnt necessarily

compare to what you will get from water delta T times flow rate.

The Steam Whisperer (Formerly Boilerpro)

Mike....

do you have any info on testing of hot air furnaces?

DanT

it's gas

It's a gas boiler.  Thanks.  I want the most efficient unit that makes sense for steam and that has a very low chance of carbon monoxide in the basement.  What are the possibilities since mod-con is out?



Thanks a lot,

Dan

DanT

induced draft

What's an induced draft boiler?  What's the equivalent of a power vented hot water heater?  (e.g. it has an extremely low chance of carbon monoxide poisoning and is high efficiency).



Is there an article that talks about the different kinds of boilers?



Is there a forum concensus on the type of boilers or the best couple types (not brands but how they work)? 



Thanks guys.  I have been trying to search but haven't had much luck.



Dan

Mike Kusiak_2

ASHRAE Standard 103

No, I don't have any info on furnaces, but I just looked in the ASHRAE catalog of publications and found that Standard 103-1993 covers both boilers and furnaces " Testing for AFUE of Residential Boilers and Furnaces". So it sounds like this standard covers both. My catalog is from 2008, but the price listed is only $19 ($15 for members) so it might be worth getting. Remember that this is the standard that the DOE tests are based on. The actual DOE standard is pretty useless as it continually references 103 procedures, so you really need a copy of 103 to make any sense out of it.

jpf321

ASHRAE 103-2007 link...

I thought I linked to it above .. I guess I forgot .. I found ASHRAE 103-2007 for $69: http://is.gd/i2Wur and http://is.gd/i2WFH (table of contents displayed) and http://is.gd/i2WIE



Perhaps members can get it cheaper .. I figured someone may already have it.

SBoston

Electric Use

I am just a home owner, but to do a quick test, we hooked a voltage meeter to a brand new high efficiency furnace and were drawing less then 60 watts on average. In my mind this is a minimal electrical cost.



I think that it's a great point that so much energy is waisted to bring the water temperature to the point of generating steam and then wasted again after the boiler is shut off.



The point that was made about poorly insulated ducts can be said for poorly insulated mains.



As a steam heat owner, i don't dispute the level of comfort created by steam heat, but the fact is, it's just not efficient to change water into steam. At least not using the current technology.

Jean-David Beyer

Puzzled..

A volt meter does not measure watts.  Watts are the product of the voltage (usually 120 volts in USA) and the in-phase component of the current, measured in Amperes. I doubt you have 60 volts, but the chances of your drawing 60 amperes is very low unless you are heating a large warehouse or something.



My house is heated with hot water. This requires a boiler circulator and a circulator for the zone being heated. The usual case is heating one zone. That requires 170 watts, and a bit more for the draft inducer, so guess 225 Watts. Now if we want to be fussy, there may be 30 watts or so to run the thermostat relays and 20 watts for the low water cutoff. Add 25 more watts for things I did not think of, and we get 300 watts.



For a hot air furnace, you should not require the 190 watts for the circulators and the LWCO, but that leaves about 100 watts for the other stuff. And one more thing: a blower to get the air moved from the furnace to the rooms and back -- sort of a circulator, but for air. I have another building with a low-efficiency gas furnace (125,000 BTU/hr). It requires a motor that runs a squirrel-cage blower. I do not know the power for sure, but I would guess between 1/3 and 1/2 horsepower. So that would be 240 to 375 watts, more than the two circulators for my hot water system.

SBoston

With just a little math

To calculate Watts using a multimeter you just multiply Volts x Amps.

Mike Kusiak_2

Not Always

Volts times amps doesn't always work for AC loads. With AC you also have to take into account the phase angle between the voltage and current. For example, if you connect a resistor to a AC source, then voltage and current are in phase so V x A = W  works. With reactive loads this formula does not work. If your load was a pure inductive or capacitive load, the the voltage and current are 90 degrees out of phase and no power is actually dissipated, so V x A = zero Watts.



Real world loads are a combination of resistance and reactance, so the actual power dissipated by the load depends on the ratio between them. You really need a wattmeter such as the Kill-O-Watt, which takes phase angle into account to measure AC power. accurately.

Jean-David Beyer

volts times amps

The O.P. said he had a voltmeter, not a multimeter. So he did not have Amps.



Furthermore, volts x amps gives watts only if the volts and amps are in-phase, which is not likely with things like motors. (Power factor typically not equal to 1.) It is true for things like plain electrical heaters, incandescent light bulbs, kitchen stoves. Not likely to be true for microwave ovens (but I do not know that one for sure).



My thought was that actually producing the heat is pretty much the same for all the usual heat sources (forced air, forced hot water, steam), but that the costs of distributing it around varies considerably. 0 for steam, moderate for hot water, high for forced air. It may be that getting water to 180F (if that is what you use for hot water) and getting it to boil at 212F costs more (and you have to count the latent heat of vaporization of the water, about 540 calories per gram).