Why no 90+% efficiency steam boilers???

Bill Stockhausen

At this time of increasing energy costs and emphasis on maximum effieciency why doesn't anyone manufacture a gas fired steam boiler higher than 82% efficiency?
There are many hot water boilers at 90+% efficiency - this shouldn't be rocket science.

Unknown

The question is

How many customers are there?

The cost to get each model and size tested and approved would have to be recouped through sales. The numbers aren't there.

Commercial size boilers, on the other hand.....

Noel

Kal Row

what exactly is the point of a 90% AFUE

steam sys, if the whole steam concept is like "if you want light in one room, then you need to light up the whole house", clearly, the application doesnt scream for more eff than they are already giving you,

on the other hand - if you always really do, "want to light up the whole house" - then, a "gut verkin shhteeem zyz-tem" at 80% will cost you less per winter than radiant at 98% - dont ask me why - maybe "shhteem" is like radiant with just smaller radiant points, and a zero resistance factor becaused the radiant source is exposed, hence more echonomical - i'm just guessing, got to ask dan-the-man

imatellerslie

I think it's because...

a steam boiler obviously has to operate at a higher temperature than a boiler, which means it will have hotter exhaust, which means its efficiency is naturally limited.

Richard O'Connor

what dan says

paraphrased badly (and possibly not completely clearly) ....

This has to do with what it takes to turn water into steam....

1BTU raises 1gal? of water 1 degree farenheit

but to go from 212 to 215 it takes tens of btus, 100's of btus ... anyway...way more than 1 btu is required...

There's a good explanation of this by Dan in "We Got Steam Heat"

imatellerslie

This doesn't adversely affect efficiency...

The fact that energy is used to change the phase of the heat transfer medium doesn't negatively impact the efficiency.

That energy is still in the system, waiting to be used at the radiator. Storing energy in the latent heat of vaporization just dramatically reduces the mass flow rate necessary to get the energy to the radiator.

Unfortunately, boiling the water inside the boiler does necessitate higher temps than in a non-steam boiler, leading to higher exhaust temperature. This leads to more heat going out in the exhaust, and thus less efficiency.

I guess it is possible to design a steam boiler that uses that exhaust heat to warm the returning condensate, but I think it would be more complex than the low temp condensing boilers out there.

Kal Row

you know

if you have a really good stack system, that can take full condensation, then you can add a stainless water recovery coil off the acctual stack and run radiant heat in the basement with those prefab radiant wood panels - this way you lose nothing - just a thought

Fred Harwood

Heat recovery

Stack "losses" also depend on chimney design, and whether it's inside or out. My inside chimney brick is nice and toasty during the winter, and cool during the summer. It's like another radiator on the system, and returns some stack loss to the occupants.

scrook_2

it's not...

rocket science, just that the requirement to generate steam at about 1 psi (therefore ~215°F) means you cannot operate at 100°-130­°F temperatures required to cause the condensation required for efficiencies above the low 80's%. If you were making process steam where there was a continuous stream of cold water coming in you might, but the hot condensate of a residential heating system makes it impractical. Hot water systems, particularly those w/ low temperature radiant or w/ oversized CI radiators and outside reset can operate at these low temperatures much of the time.

Now if you operated your steam system w/ a vacuum pump to maintain an internal vacuum of about 26 to 28 inches Hg (about 13 to 14 psi vacuum) you could get the poiling point down to about 100°-120°F and have condensation on the fire side, but you'd have to run a pretty big vacuum pump (which would collect your condensate) and maintain a tight system!

The old coal fired systems would go to vacuum so as to continue to function a little longer as the fire died down, but at 5 inch Hg the boiling point was still about 201°F and at 10 inch Hg about 192°F -- though I don't know how low they would get.

Doug_7

Boiler Efficiency vs. Flue Temperature

The attached charts explain it. Combustion efficiency is a function of flue temperature. For overall efficiency there are also all the other heat losses, such as heating excess combustion air, standby losses when the boiler is not firing, the heat losses from the surface of the boiler etc.

The attached graph is theoretical combustion efficiency with zero excess air, which is not practical. Actual efficiency will be lower. It clearly shows where condensing boilers with low return-water and flue temperatures improve efficiency.

The attached table shows conventional boiler efficiency at several flue temperatures and levels of excess air.

With a steam boiler you are locked into the higher flue temperatures, but you can make sure you have the right amount of excess air.

john_27

I have often wondered why vacuum pumps weren't fitted to Broomell systems to recreate the effects of a coal fired system....two questions...wouldn't boiling at or near zero atmospheric create condensation, or , to put the question another way, why not set the vacuum pump to produce steam at 150 degrees(ie. above condensation).....savings from 212 to 150
should be significant..

Boilerpro_3

You are forgetting that the DunkirkQL condenses up to 180F

because it is not only relying the the water heat exchanger but is moving heat from the exhaust to the supply air. This would improve efficiency on a steam boiler. The concept is the simliar to a heat recovery ventilator or condensing hot air furnaces, except the air moving across the heat exchanger is cold outdoor air rather than warmer indoor air. Basically you have your flue gases hitting a surface that is usually near or below freezing temps, well below the condensing temp of 117F. Needless to see alot of condesate is generated and alot of heat released for use.

Boilerpro

Mark Eatherton1

There is a condensing steam boiler...

http://www.gasmaster-ind.com/newproducts.html

There are others boilers on thier site claiming 99.8% efficiency. Guess my home lab wasn't too far off eh...

ME

S Ebels

But then

You have to factor in the cost of the vac pump and its operation/maintenance. The math would probably prove it to be unfeasible.

Bill Stockhausen

Thanks for the tip. It looks like their smallest unit is 200,000 BTU. This unit is obviously in a different league than the conventional residential boiler maker, who probably can't afford a separate line for steam - too expensive and too few sales volume. Do you know ballpark how much the GasMaster goes for?

Doug_7

Gasmaster

Correct me if I am wrong, but I don't think the Gasmaster is a steam boiler. I think it is a condensing modulating water heater, that achieves its high efficiency because the unique design results in a low flue-gas exit temperature.

Problem is the tradition of calling these things boilers when they are actually heaters.

You can boost the overall efficiency of a steam boiler into the 90% range by adding a separate heat recovery unit to the flue gas. This will give an overall efficiency corresponding to the reduced exit flue gas temperature and will condense some of the water in the flue gas if the temperature is low enough.

What is your take - is the Gasmaster a boiler or a heater ?

Mark Eatherton1

They make both...

Hydronic heaters and steam heaters according to their web site.

I agree, I refer to them as natural gas powered thermal conversion units. They're not your grandpas Buick any more...

Also, I have no idear what the buckage for one is, but I'm sure they're NOT inexpensive. Bleeding edge technology never is:-)


ME

Bill Stockhausen

Heat Exchanger

Yes, the QL flue gas heat exchanger and evaporator tower is a nice feature but it is only on Dunkirk's water boiler. It would be neat if something like that (wouldn't even have to be fully condensing) were available for a steam unit. Can something be adapted?

scrook_2

you'd need...

about 22 1/2" Hg of vacuum (7.57" Hg abs pressure from the steam tables) to get a 150°F boiling point, meaning a significant expense for the vac pump and the energy to run it. Lower that temperature and the vacuum must be even higher.

Unknown

Your numbers don't mean much...

The energy from 150 to 212 WATER is not much energy at all. The energy to change water to steam, at whichever temperature is considerable, like 10 times as much, but there will be no temperature change. THAT'S the energy that consumes the fuel.

Bring a quart of water from 150 to 212. Happens pretty quick, right? Now boil the pan dry. Takes a LOT longer, doesn't it?

Noel

Kal Row

yes it takes 970 btu per lb to convert water

from water at 212 to steam at 212, however when it condenses in the radiator, it gives 970 btu per lb right back to you, thus 212 degree steam, carries a lot more energy per weight to the radiator, than water, thats why it's an old favorite for tall buildings

no: the effiency is lost by the fact that the whole show runs hotter and you lose heat out the flue

with a good radiant system, at 110f suply and 90f return, you suck every drop of heat out of the flue gases, and that where it's at!!!

another big loss is that fact that "zer ist noh vay, tzu con-trrrol zat shtufff" as the germans told dan holohan at ish, unless of course you want heat all day, and in the whole house - then, it's abso-blumin-lutly zimply ze best

Bill Stockhausen

I'm intrigued - steam @ 80% better than HW @ 98%

Did Dan say this in one of his books? How do you know?

Dave Palmer_3

because

oil is 86%

Kal Row

ask people who have switched from one to the other

and i am talking about cost, yes, i know, it's completely subjective, that’s the trouble with this kind of thing - there are just too many variables, that why wetheads don’t sell economy, rather comfort and control, you just cant pin down the numbers on economy outside the lab,

let me give you an example, i took someone from a basic one zone baseboard and an old boiler system, to a 4 zone system with primary/secondary/mixing with outdoor reset and all,

is the new system more efficient?, you betchya!, will he save money on gas?, probably not!, since in the old day, there was one thermostat on his level, so the downstairs apt froze, and the upstairs roasted, cause all the hot water just gravity migrated upstairs, and they'd just open the windows, (dan’s double hung zone valve)

now, with four zones, the downstairs calls for heat a lot, where before it couldn’t, and the upstairs people, used to opening the window, still do, and as a result, call for heat anyway, so now i have to put in wireless alarm window sensors to drop out the top zone when the window is open!

so, while i can swear up and down, that the new system is more efficient, there is no way i can tell him, that his gas bill will be lower!!!

same thing happens in our case, in nyc most of the old steam is oil, and i have yet to see a radiant powered by oil, so the new system is already more expensive, plus, with steam, the system ran mostly mornings and evenings only, but with radiant you have to keep it on 24 hrs, ... so at the end of the day you have saved nothing, however it's more comfortable, quieter, and "greener"

but even comfort varies, there is nothing like a bathroom surrounded by radiant with towel warmer and all, then again there is also nothing like the warmth you get off an old steam radiator - maybe that's why mad-dog put steam and radiant into his home

so there you have it – there is science, and there is reality – get over it

Bill Stockhausen

Thanks

Thanks for the responses on this. I guess I could help the system a little by putting a heat exchanger between the flue and the outside sourced combustion air - do you know of any that are suitable for that?