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High-Efficiency Steam boiler thought experiment

1. I estimate my 25 yr old Peerless 1.2 mmbtuh steam boiler to be running at about 50% efficiency on my one-pipe system.

2. Most of the lost heat goes up the flue.

3. What if I could decommission the boiler and plug the flue, turning the old boiler into an "unfired steam generator".

4. What if I used a condensing boiler to heat the water in my new steam generator?

5. Would the condensing boiler still condense at that high temperature (about 195F)?

6. I'd use the old tappings for the former sidearm domestic hot water heater to recirculate the water in the steam generator through a heat exchanger that's in turn heated with the new boiler.

I searched pretty hard and couldn't find information on such a conversion, so there is probably a reason that it's never done. Anyone know why? ("because it's never been done before" doesn't count).

It's also not a trivial question, because the simple payback would be under 4 years.
Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Condensing with 195F hot water?

    If your condensing boiler puts out hot water hot enough to boil water in the other (now unfired) boiler, do you think that the return water temperature would be as low as 195F? And granting that it would be, how would you get that down significantly below 130F to get useful condensing from the condensing boiler? A perpetual motion machine perhaps? Or would you have the condensing boiler run at 130F output temperature and run that water over the evaporator of a heat pump and use the condenser of the heat pump to boil the water in the unfired boiler? Sounds like a seriously losing proposition to me.
  • Nuclear Power?

    When I was trying to mentally put together what you were considering the first thought that came to mind is this sounds like the way they do atomic generating plants. With that thought it would seem a better configuration would be to forget the condensing boiler and use an atomic pile!

    Seriously though I see a lot of problems with efficiency and controlling the system. For example- Condensing boilers are for water and how would you stop them flashing over into steam? You need pumps and heat exchangers which consume energy and have energy losses. I think you'd  be better off to "bulldoze" and start from scratch.

    You mentioned you have "a 25 year old Peerless" running at "50% efficiency."

    If that is the case, you have some serious problems which should be corrected.

    Do you have Dan Holohan's books on steam heating? If not they are available at the "Store" at the top of this page. They will tell you how to get the most out of your steam system. 

    (See "A Steamy Deal" - http://www.heatinghelp.com/products/Super-Deals/14/129/A-Steamy-Deal )

    - Rod
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Pretty High temps for hydronics


    Thanks for the help.

    I think you may be closing in on the basic problem. At Denver's altitude, the condensing boiler won't condense at temperatures above 195F. Assuming the steam generator is then running at 185F, will it be generating enough steam? Who has a feel for steam boiler temps? I'll check the steady state pressure and temperatures and report back.

    What's the efficiency of, say, a Munchkin, running at 195F?
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Water boils in Denver at 202

    The standard atmospheric pressure in Denver I believe  is 12.1 PSI which is about 24.63 . With 29.92 being the standard sea level barometric reading that would give you a boiling point of water in Denver at around 202 degrees F.

    I've attached a table that might be of some help to you.

    - Rod
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    old boiler eficiency

    i don't think there have been many major changes in design of that size/type of boiler for many years, so i expect you are doing better than 50% with the old boiler, unless:

    the sections are full of mud, which acts as an insulator.

    or the burner is so badly adjusted that you are not getting all the therms out of your fuel.

    or the rest of the system suffers from deferred maintainance [pressure too high, venting inadequate, bad piping, etc.]

    i suppose some sort of mechanical flue damper could keep the flue from syphoning the heat out in between firing cycles.--nbc 
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    condensing above 195F

    The idea of condensing over 130F is not usually practical, much less 195F.

    While you might get some condensing at any temperature below the boiling point in your area, to get it with the exhaust gas flow rates you might expect in a condensing boiler is out of the question. Face it, the hot water you use to boil the water in the other boiler would have to be hotter than the boiling point in your area. So it would have to be under greater pressure than the boiling point too.

    You gotta remember, heat moves from a hotter place to a cooler place.
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Hydronic boilers often operate above 212F (202 in Denver)

    It depends on the pressure rating of the components, but water won't boil when the pressure is higher, just choose an operating point. The hydronic efficiency suffers when you aren't condensing, yes, but by how much?
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • ttekushan_3
    ttekushan_3 Member Posts: 958
    Reply (long, no drawings yet)

    You have to remember that it is the latent heat of evaporation of water that is the primary heat transfer medium. So for every pound of water from and at 212F about 970 BTU (at sea level) are required to turn it into steam at 212F. The sensible heat remaining is only 1 degree F per pound. This is why steam moves heat and power so efficiently. It is extremely light, low resistance, fast and has an enormous heat capacity. But steam's temperature intensity is also its achilles heel in condensing tech as stack temps are always over the saturated steam temp and the heat carrying capacity of return water is miniscule if it remains in the liquid state.

    If you used an economizer, or as you suggest, using returning condensate as your heat sink, you can only infuse that water with a few BTU's before it is up to 212 F water. You still have to provide it another 970 BTU before it steams.

    This is why an economizer for steam usually provides a number of non steam fluid streams to absorb waste heat from the stack. A cool enough source can condense flue gasses, but you are not going to generate any steam with it unless you are extracting heat from a high pressure steam boiler such that there is enough waste heat to provide a modest amount of low pressure steam from the exhaust gas heat. This is where the unfired steam generator /waste heat recovery boiler (WHRB) is used.

    Even with the condensing boiler, you would only be able to condense a minute amount of exhaust gasses because the total heat you could introduce into the system would be the difference between the return water sensible heat and about 120F with decreasing flue gas condensing above that. Then you have to introduce 970 times that heat to water already at 212F to produce steam. You wouldn't get any steam with the proposed arrangement unless you fire the original boiler. And considering the minute rise in efficiency by warming the return water in a residential heating application, the losses incurred by running the condensing boiler would probably increase your total energy consumption especially if compared to waste heat extraction with a traditional stack economizer.

    This is not to suggest that we can't consider a fairly practical means of extracting waste heat from a steam boiler, which at its best may condense a little, or at its worst allow the exiting heat to be little more than the saturated steam temp. At this point we are adopting the philosophy of water boiler manufacturers like System 2000 or perhaps Thermal Solutions non condensing boilers that run at about 88% efficiency (not thermal efficiency) in the interests of longevity and product reliability.

    I'll get into the deeper specifics of this alternative idea another time, but this is the basic idea: using your low efficiency high mass boiler as a WHRB remains as you originally proposed -only you fire with a decent efficiency steamer next to it, not a condensing water boiler. The fundamental design is a CI steam (primary) boiler exhausted without a draft hood into a power venter and down into the waste heat (secondary) boiler's flue outlet. The waste heat boiler exhausts out the bottom and up into a side wall vent or lined chimney. There are no burners in the secondary boiler. The secondary/waste heat boiler would be set up as a Mestek Ray where the combustion gasses move downward, cooling as they reach the bottom. Should there be any condensation of flue gasses water will drop into a bottom tray where the burner would ordinarily be on a regular CI steamer. If there is condensation, let the CI rot. It might still last 30 years.

    The secondary "boiler" should have a side steam outlet rather than a top and here's why. Stay with me here. Picture the two boilers side by side, with the exhaust gasses of the active boiler being blown downward through the secondary, unfired boiler. The bottom is the exhaust on the secondary boiler, remember. They look like boilers piped in tandem through two hartfords but are interconnected THROUGH THE TOP OUTLETS ONLY. The bottoms are not piped together at all. System return / fill water enters the secondary boiler at the bottom and completely fills it to the side outlet tappings where water cascades into the hartford of the primary boiler. The top of the boilers are piped in parallel. In other words, the steam side is connected such that the boilers are in parallel and the water side is connected with the boilers in series.

    As return water enters the bottom of the secondary "boiler" it rises as it is heated by the exhaust gas flow from the primary boiler in counterflow. The water warms, rises within the secondary boiler and slowly feeds preheated water into the primary boiler by overflowing into its hartford connection. At start up and beginning of the cycle, flue gas condensing may occur. Under long run times and full system saturation, the secondary boiler may actually produce steam. This steam will be ADDED to the output of the primary boiler by virtue of the header piping arrangement. In this way, we will always have somewhere to put the heat recovered from the stack. In this worst case scenario, the flue temp will be little more than the saturated steam temp.

    I am confident that this method would provide overall efficiency numbers that approach those of high efficiency non condensing hot water boilers.

    Due to steam's unique circulation and energy transfer characteristics, the boosted efficiency of this arrangement should result in total fuel conservation that meets or exceeds many competitive modern installations while retaining the comfort of steam heat.

    I'll post a few diagrams of this proposed system when I have a bit more time.


    P.S. If Steamhead, Gordo, Gerry, or Steve Pajek (though not exclusively!) are reading this, this is the most practical short term solution to my condensing steam boiler design project of this summer. The condensing steam boiler is currently beyond my $ supply and has tested my engineering abilities, but suffice it to say that it has just enough higher maintenance components that its additional cost may be a difficult sell. It works in theory, and the estimated numbers look good in addition to doing away with the concept of a traditional boiler entirely. Scale sized system subset tests are encouraging, but introduce nearly as many questions as they answer! Its electrical power consumption is on the high side of normal for off the shelf components, but at least the heat generated therefrom gets directly applied to the steam output. It should theoretically condense under all heating loads, but is confined to low pressure applications. I'll give it a try again next year.
  • Condensing steamer are already being made...

    Look up the Super Boiler Project  being developed in the Chicago area by the Institute of Gas Research, Cleaver Brooks, the Dept of Energy and others.  Condensing steam is possible when taking advantage of stack economizers and using the exhaust to preheat combustion air.  The Dunkirk Quamtum Leap boiler did this (no longer in production), which I have in my home.  I see combustion test efficiency of over 90% with 160F return water and about 99.5% at 110F return. 

    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Got my Data

    The Peerless makes its steam at 202-209F. The Pressuretrol is set to 0.5, but its deadband drives the max pressure to 3.5psi. There isn't a LWCO on this thing, and that scares me a little.

    ttekushan, I haven't quite fully digested your post, but this data makes me think that I just need to find a direct venting 88% boiler that loves to run at 220F and hook it up. (suggestions?) Don't worry, I'll make sure the City signs off on anything I do, and I'll get a PE to stamp it.

    The money I should spend for all the maintenance and safety upgrades on the old boiler could be thrown at the new one, reducing the net cost of it.

    In fact, this scenario would be a money saver for all the older steam boilers out there. I just think it's too much work to retrofit 24 apartments with a return line and go hot water.

    There would definitely be an opportunity to recover heat from the exhaust gases for DHW, but that's another research project.

    Surprisingly, another look at the label does show that the steady state efficiency of this boiler should be 80%. My estimate of 50% above comes from comparing gas usage between two identical buildings, one hydronic, one steam. Steady state is way different from the real conditions, which usually see the entire boiler drop to room temperature between firings.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • ttekushan_3
    ttekushan_3 Member Posts: 958
    Yow. 50% more?

    OK, nevermind all that nifty heat extraction stuff for now. There's a lot of low fruit on the tree to harvest first! You have a point about steady state efficiency and all, but the cast iron of the boiler can't absorb 50% of the heat intended for heating a building and make it go somewhere. Were that true, think what we could do with such a compact thermal storage unit!

    Do you know how that boiler is sized with regard to the existing load?

    Is the boiler firing at its ratings? Is excess air out of control?

    Is the chimney sized for a coal boiler and sucking the burner's heat right through the passages? This should be checked with a Manometer or analyzer. Baffle the flue pipe until draft is within proper range, -.02 to -.04" W.C. (for gas burners).

    What are the combustion numbers from when it was last serviced? Please tell me it gets serviced and that a combustion analysis is done.

    If there is no LWCO, what else is missing or incorrect? Is there something about the way it is piped and/or vented that is could cause this trouble? [remember a LWCO can be incorporated into a boiler feed pump control if you have that arrangement.]

    I recommend a carefully sized and carefully chosen boiler, but are there other symptoms that we should know about? For example, does one portion of the building have to get very hot radiators before others begin to heat?

    Take a typical apartment and cross reference the radiators and determine their EDR and figure how that relates to the size of the apartment and other heat loss considerations. Does it seem disproportionately high for your location?

    What is the condition of the rest of the system, like traps and vents?

    Do you actually have a vapor system whose operation is being ruined by excessive steam pressure or excessive boiler size? Do some radiators have "special" looking valves and traps but others do not, as though they've been replaced?

    Does the steam main run through the basements with upfeed risers or through an uninsulated attic space with downfeed risers? If downfeed, what is the condition of its insulation? Someone must actually look! Record breaking icicles can be a good indicator. Once troublesome building I tend to has just this problem. The insulation has fallen from the overhead mains. This place is right on lake Erie and the winter winds exhaust tremendous amounts of heat from that space. As a matter of fact, if gentle breezes come from certain directions and lock the heat up there, top floor apartments will run about 78F with the radiators closed. Yes, they have the biggest, most dangerous icicles in town.

    Conversely, does the boiler run all the time just to keep the place comfortable at all? An undersized boiler in a large unbalanced system can begin to run continuously without ever reaching a temp setpoint. I'm guessing you don't have this problem given the pressures you mention.

    What I'm getting at is that even a new boiler can be sized and piped into a system with various issues such that it will still consume far too much energy. I've got a number of clients with steam heated buildings of the 24 to 28 apartment size. Most fall into a fuel consumption range that I would expect for a building of that type.

    However, there are a few others that just suck up natural gas like you wouldn't believe. The outdoor temperatures have only a marginal effect on usage, a sure sign of large heating system inefficiencies. These are the type of situations that prove that combustion efficiency and fuel-to-steam efficiency can be quite different.

    Sometimes the systems are just grossly oversized. One of my gas guzzling buildings has a design temp of -45F. Thats not a typo. They don't want to spend the money rectifying the problem. $8,000 a month on gas for a 24 unit building is OK tho. It should be half that. These things often have a 2 month pay back period, sometimes less and more obviously if the boiler is being replaced as well. But I digress.

    I'm a big advocate of using radiator orifice valves or inlet orifices to control the rate of steam being fed into the radiators. Here's the basic idea:

    • Determine actual heat losses based on a modern heat loss model and design temp, etc;

    • Determine the EDR necessary;

    • Determine your operating pressure (low! and control it pretty tightly);

    • Cross reference desired EDR to be heated to supply pressure;

    • Adjust adjustable orifice valve or drill orifice to the proper number;

    • Size any new boiler to what I call the "Virtual EDR" plus a modest pick up factor since the piping is still the same.

    • I strongly suggest some form of staged firing, with the lowest stage able to maintain steam pressure under minimum load. With two stage firing the lower stage should be used to remove the pick-up factor and hold the steam pressure steady or very slowly rising under minimum load. The reason we don't want the steam pressure to drop as soon as the burner drops out of high fire is that radiators that are not yet properly heated will have their air content expand to fill the void left by the condensing steam and in the absence of enough steam output to fully replenish the steam's share of the radiator. This can cause radiators at the end of the line to run cold even with boiler running. It is a form of steam stall.

    • For heaven's sake pipe the boiler properly and control the draft.

    Finally, I'm trying to find an interesting study that shows that converting the average steam heating system (meaning one in decent running order, not the guzzler problem systems) has a payback period of 35 to 50 years. What a bargain.

  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    No exotic problems

    A couple years ago I did a little tuning that was successful. It purrs like a kitten now. For my particular system, a Honeywell thermostat with a steam setting installed in the correct apartment was the most important tweak. I even dialed in some moderate day and night setback with no ill effects.

    The near boiler pipe insulation also helped. With the long firing cycle that I have now, the most remote apartments get plenty of heat, and I don't have any banging. The orifices you speak of don't sound helpful on a one pipe system, but I'll read up on that.

    Next week a qualified service guy will do some service, and I hope he can do a combustion analysis. We'll check if the flue/chimney is oversized.

    Based on measured heat loss in a comparable building, the boiler size is in the ballpark. Design heat loss at -5F is about 400,000 btuh, and the boiler label says 1.05 mmbtuh input, and 840,000 btuh output.

    My hydronic building with a new Munchkin consumed 550 therms/month at 41F average outdoor temperature, but the steam building consumed 870 therms. Definitely room for improvement.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • ttekushan_3
    ttekushan_3 Member Posts: 958
    inlet orifice doesn't work on single pipe but

    you get the same effect with proportional venting of the radiators. Gorton has a good chart, as does Jacobus Maid-o-Mist. Also look at balancing a steam system with proportional venting by Gill and Pajek, here on heatinghelp.
  • Steamhead
    Steamhead Member Posts: 16,443
    Another question-

    are your two buildings of similar size, same insulation etc? You see where this is going- are we really comparing apples to apples?

    P.S.- Terry, keep us posted!
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Heat loss

    Yes, the Manual J heat loss calculations are within 10% of each other.

    Thanks to everyone who pitched in here, and help me think it all through.

    I just realized that if I can replace the gigantic steam boiler and some of the piping with a Munchkin, I can re-commission about 800 NRSF (net rentable square feet). Another rentable apartment is worth at least $600/month, even more than the energy savings of about $200/month. As much as I like the elegant simplicity of steam, the rental business is about today's bottom line, not yesterday's technology.

    Time to start running those return pipes!
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Steamhead
    Steamhead Member Posts: 16,443
    edited October 2009
    I still wouldn't do that

    if your old steam pipes or radiators leak or don't heat the building adequately, as is often the case if converted to hot-water, you might be facing a bunch of angry tenants and rabid lawyers. Is a few more square feet (as compared to a better steam boiler) really worth that risk?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Cold tenants don't sue, they just move

    Yes, it should be totally worth it. But it's not a done deal, I have a ton of due diligence to do before I can make the switch.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    sometimes both

    sometimes the tennants do both-move and sue especially if they are in subsidised housing!

    i suspect that if your peerless has no lwco, then its installation would have other faults as well, which would account for the higher gas bills when compared to the hot-water system of the other similar building. hot-water heat has 2 advantages: the ability to suck more heat out of the burner before it goes up the flue, and the possibility of zoning sections of the building which need more or less heat

    if i were in your shoes, i would correct the installation of the present boiler, [lower pressure, more main venting, lwco replacement]. i would want to begin saving 25-30% on my gas bills imediately instead of waiting. then i would add up the total of the building radiation and compare that to the boiler edr to see if it is over-sized. if it is i would put a peerless "mod-u-pack" 2-stage "HI-LO-HI" burner on with a second vaporstat to drop to low fire at 3 oz. pressure.

    i doubt that steam would have continued for as long as it has, if there were such a great difference in fuel use between hot-water and steam. trying to reduce heating costs is not a new phenomonon!--nbc
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Great ideas, NBC

    Speaking of due diligence, I will check out your suggestions before converting.  When it was installed, a LWCO wasn't required by the code, so I don't suspect a bad original design. 

    The vaporstat and burner tweaking you suggest make sense to help me approach the 80% design efficiency of this rock-solid durable boiler.  

    There's evidence that 10-15% of the radiators in the building have been removed, and I have turned off all the hallway radiators, which is another 10%.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • BobWalsh
    BobWalsh Member Posts: 1
    Condensing Boiler

    A high efficiency condensing boiler works on the principle of recovering as much as possible of the waste heat which is normally wasted from the flue of a conventional (non-condensing) boiler. High efficiency condensing boilers convert 86% or more of their fuel into heat, compared to 65% for old G rated boilers. The current lifespan of a boiler is around 15 years and fitting an A rated high efficiency condensing boilers with correct heating controls can make a huge difference to your heating bills over time.


    Greentech are specialists in a range of Condensing-Boilers Services.
  • gerry gill
    gerry gill Member Posts: 3,076
    i think if you want high efficiency steam

    one needs look no furthur than iron firemans select temp style systems..the boiler may be the same boiler but 230 degree steam moving faster than you can get to the other end of the building is certainly efficient.
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588

    Gerry, thanks for mentioning that, I hadn't heard of Iron Fireman before.

    What was the rated boiler efficiency?
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,311
    If you want condensing on steam I think

    one would need to push the flue gases through a heat exchanger that is submersed in the return water. Like a small receiving tank at the back of the boiler. Have it with unions so when it rots out of sludges up it can be removed from the main block for cleaning and replacement. Power burners only need apply for this service.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
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