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Condensing temperatures

The punctuation made me think the opposite 20 off/5 on. Makes sense now.

Primary-Secondary is a piping method whereby the radiation distribution circuit is distinct and separate from the boiler circuit. Boiler is kept hotter by about 10*F than the radiation requirements. Hot boiler water is injected into the radiation loop based on what temperature is required at a given time. This radiation loop temperature is dictated by the heating curve which is derived by outdoor temperature. P/S piping allows you to set back radiation loop temperature quite a bit yet still keep your boiler hotter. The boiler can be kept at a constant temperature (say 180) and the radiation loop "sips" hot water in as needed. But for more savings you can key in a parallel temperature for the boiler, say 10 degrees more than the radiation needs at a given time but flatline it at 140 F for boiler protection.

Tekmar controls has some neat essays to enhance your understanding of this.

Boiler safety High Limit is normally set about 10-20 degrees hotter than your normal operating high limit, so if you need 180 water for operation then 200 would be the HL setpoint. If you run 160 then set it at 180. The reality is you can set it at 200 to be safe from boiling point operation.

Comments

  • Holycack
    Holycack Member Posts: 12


    Reading on Tekmar site they say a boiler temp of 140*F is hot enough to prevent flue gas condensation, this is where my 363 tekmar is set at as a boiler min. Now reading on this site it is said return temp has to be above 140*F. My boiler normally runs between 120*F and 160*F return line temp. My question is who is right? I have read an article saying oil condenses at 116*F where gas condensates at 138*F. I am getting confused. Will this harm the boiler? It is a burnham v84.
    BTW I have a pumped bypass and a 4-way actuated mixing valve already installed. The only time my return drops so low is when my barn (which is attached to the same boiler via insulated 1"piping)calls for heat. The thermostat is set for 43*F so the return is cold due to the slab being operated so cold. The boiler will run for approx 20 min off for 5min back on for 20min and off again before demand is met. When the house calls for heat temps are alot higher on the return but boiler only runs for around 5 mins at a time.

    Thanks in advance
  • Brad White_34
    Brad White_34 Member Posts: 18
    Nothing stated is incorrect

    The 140 degree rule is a safe one, some may drop the return temperature another 5-10 degrees and still be OK, but that is risky in my book.

    When one source says "140" and the other one says "above 140" they are both correct in that 140 is a minimum in their independent views. The semantics may get you before the cold snap does.

    That gas flue gasses condense at 138 should tell you that 140 is safer. But the dewpoint varies with the CO2 content of the gas, so may be up or down a few degrees.


    Now, when you WANT to promote condensing such as in a condensing boiler, it is often recommended to impose return water 10 degrees colder than the dewpoint of the exhaust gas... but when you want to PREVENT it, you keep it higher. Remember, hard as we try, we are not going to keep return water temperatures controlled that tightly unless you have a bypass valve dedicated to that practice. You are striving for a general principle and tendency to have the HWR above gas dewpoint.

    Remember your return water will drop below that occasionally such as during a cold start, but this also means "don't make a habit of it".

    The 4-way mixing valve is great for boiler protection but when you have a sudden imposition of a high mass load (your barn? You gave the cow her own thermostat? How cool are you!) you might consider primary-secondary such that the boiler will not see that slug of cold water.

    Sounds as if your boiler is over sized maybe? You sait it runs 5 minutes at a time with 20 minute breaks. Seems P/S might make sense for the barn. It may even be glycol off a heat exchanger but that can impose a chilly load on your boiler even with a 4-way.
  • Holycack
    Holycack Member Posts: 12
    primary secondary?

    The boiler actually runs 20 min. on and 5 min off. What is meant by primary secondary? As well at what temp should my boiler overtemp relay be set at? currently it is at 180*F. In order to get my return temp to my boiler at 140*F I have to run my boiler min at 160*F and this span of only 20*F is not enough to suffeciently run my boiler.
  • Jim Davis_3
    Jim Davis_3 Member Posts: 578


    The vapor in flue gases is steam. I always thought steam changed to water at 212 degrees. Some say it is based on how much dilution air is mixed in and can absorb the moisture. In the 80's I sold the original Recuperative Furnaces and they condensed at 160,190, 210 & 230 degrees. Guess they didn't read any books. Condensation is a function of flue gas temperature and flue pressure. Keep the gase hot and the flue pressure negative for the proper period of time and they will not condense. Water heaters bring in a lot colder water than 120 degrees(45-50 degrees from the street). I have taught contractors to operate boilers as low as 105 degree return with no condensation problems ever. Condensation has more to do with combsution and venting than it does with water temperature.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    The vapor in flue gases is steam.

    Not a correct statement. Steam is PURE water vapor--nothing else.

    The water in flue gas is in the form of vapor. Just like water vapor contained in the air surrounding us, water vapor in flue gas has a dewpoint temperature. Cool the flue gas below the dewpoint and water that was previously in vapor form condenses into a liquid. When burning natural gas that happens at around 138°F

    Confuses me as well--particularly when you think of so-called "vapor" heating systems which actually produce steam--but at lower than atmospheric pressure.

    Even more confusing when a common definition for "saturated steam" is "steam holding all the moisture it can hold and still remain a vapor". ARGGH!!!

    The best way I attempt to understand is that "steam" is a pure substance while "vapor" can coexist with other gasses.



  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Water Heaters

    indeed bring in cold water, below the dewpoint of the flue gasses. Which is why the combustion tubes rot out among other hazards.

    Mike T. is absolutely correct in that what goes out of a gas combusting device is not steam but water vapor (and CO2, some CO and other miscellaneous gasses in the mixture.) But to get water vapor to condense it has to be below it's dewpoint (obviously). This within flue gas is about 138F and does vary with CO2 composition.

    When I run my condensing boiler to high limit, condensation stops while the flue gasses are about 150-160 F. When the drop below the 138 F range -voila!- let the dripping begin.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    Here's a goofy question.

    What if pure hydrogen is the fuel? Is the combustion product steam or water vapor? Perhaps it could only be steam if you somehow supplied only pure oxygen in the exact amount required?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Das Hindenburg

    I would suspect water vapor.

    I would submit that it would be called steam if above it's boiling point and under pressure (steam being invisible and what most folks call steam is really water vapor in visible form). Interesting discussion. Never really thought about it this way.

    I think that like any other chemical reaction in impure states (O2 in air being about 21%, Nitrogen being 78% and trace gasses the rest, the Hydrogen will make as much H20 as the O2 levels allow. Any other products would be, geez, NH3, ammonia, I have no idea...

  • jp_2
    jp_2 Member Posts: 1,935
    right jim

    you are right jim, what you speak of is part of the 'ideal gas law'. honesty you should take a college chemistry course, you'd have fun. its all in there, hard part is figuring out how it all fits together, that they really do not teach.
    also the absolute detail is not there either, its a starting point.
  • jp_2
    jp_2 Member Posts: 1,935
    hmmmm?

    not sure i understand you on the direction thing.

    radios and tv's(rabbit ears) transmit as well as recieve.

    yep, I think noone knows anything for sure, we just have ideas that seem to fit by way of experiments. and if you ever saw how lots of eqautions came to be, you'll wonder why anything makes sense. the 'oil drop' experiment comes to mind.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    how it all fits together, that they really do not teach

    My question is, "Do 'they' know how it all fits together?"

    I'm still completely perplexed by "radiation" in the form of infrared heat. Actual LAWS say such ONLY occurs bi-directionally.

    Infrared heat is just a tiny portion of the electromagnetic spectrum yet nobody seems to consider bi-directionality for radio, visible light, ultraviolet, x-ray, gamma, alpha, beta...
  • Ron Schroeder
    Ron Schroeder Member Posts: 998
    Bidirectioal Radio Waves

    Yes radio waves are bidirectional by nature its just that most people use radios as a local comunication device and the bidirectionality of them does not matter that much however as a Ham Radio operater I must decide it which direction to setup my antenna so to talk to other places in the world ie tal to Africa must run ends of antenna NE to SW, So Pacific I run antenna ends SE to NE. SO America E to W etc. Also Radio Waves Light Waves, Electrical Waves are all seine waves which gives them there Bidirectional nature, Ocean Waves are also bidirectional think of them whe thinking of all the rest
  • Holycack
    Holycack Member Posts: 12


    How long does a boiler have to run at above 140*F return to be safe? Mine seems to just hit the 140*F mark and the boiler is up to temp and shuts down. It may run 1 min or so above the 140*F mark.
  • Brad White_34
    Brad White_34 Member Posts: 18
    Normally most boilers start

    from a cold start well below condensing then rise as fast as the combination of fuel input and heat output will allow. Most boilers warm up in five minutes or so, faster if low water content.

    I think if the boiler runs for about two minutes above the 140 mark that indicates stable combustion and enough time to dry off/re-evaporate any condensation. Unfortunately it is the same re-evaporation that deposits acids and salts onto the metal, the root of corrosion...
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    Over.

    Even though you have to give the "over" message with amateur radio to change "direction" do you think that the receiver is in some sort of two-way connection with the transmitter?
  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    Now you too can play the transformer tune

    Such is life, MikeT, the convenient step input is actually quite hard to find in real life, most all things affecting us come in the shape of snaking sinusoidal waves. Adam and Eve might have been the first ones to see it; we'll simply need to hold on real hard to follow along. Even the weather is one giant self-canceling bidirectional wave pattern.

    Everyday electricity comes in bidirectional waves, all electricians know the averaged 110V we believe we see at the plug is something quite different in actuality, fuses are rated for a different voltage that has many more mysterious zeroes, but it just follows the peaks on the wave.

    People who tune pianos by the ear know about the bidirectionality of sound waves, they count on frequencies canceling each other out to get the right harmony. A piano follows the range of the human ear and starts at about 27.5 Hz, the frequency goes up to past 4000 Hz (although some, mainly the young, can hear a few octave further up)

    All the other things you mentioned, including light visible to us, and the microwave oven are just a frequency somewhere else beyond the keys of a piano.

    Electricity that is so special to us comes and goes at the frequency of 60 beats per second, we can all recognize the familiar hum of a transformer, can't we? On the piano you'll nearly find that particular pitch at the second B from the left, ninth white key (61.74Hz)

    Play any two notes, listen attentively, and you'll hear the bidirectional sounds adding and deducting on each other like the disharmonious frequencies of two separate orange blinking lights that warn you at construction sites on the highway. If you put two transformers together in a chorus, they'll never do that, they have perfect pitch. Harmony.

    That's the secret skill of the guy who tunes the piano. Noise cancelling headsets share the same tric, so do LCD watches display.

    Thanks Bruce for the neat Ham radio demonstration and the right hand rule. Africa is a long hitchhiker's ride away. Have you ever been to the Dayton Hamvention? Antennae point this way.

    Sorry Holycack, this has nothing to do with your question I think... put me down in the group that says steam and water vapor are strictly equivalent. English, but not all languages, has this special steam word for water vapor. We never qualify "water steam", but we could have "gasoline vapor".
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    Not just a simple, snaking sinusoidal wave.

    TWO waves at direct (90 degree) opposition to one another that [seem] to be one wave...
  • Mark Hunt
    Mark Hunt Member Posts: 4,908
    Curious


    How do vapor/vac steam systems work?

    Dew point vs. pressure?? Lower the pressure and the dew point follows??

    No?? Yes???

    As I understand it, steam turbines are kept in a "vacuum" to prevent gaseous water from condensing on the turbine blades thus causing an imbalance and catastrophic disintegration of said blades. Is this not true?

    0 to 2000 feet above sea level, water has a boiling point of about 212 degrees. Go to Boulder and what is the boiling point?

    I seem to recall something about "adequate positive pressure" required to keep a liquid in it's liquid state at a given temperature. So I can heat liquid water to 3000 degrees F and keep it liquid as long as I have "adequate positive pressure" to keep it from turning into a gas. If I reduce the pressure, the liquid will become a gas at a lower temperature. Yes? No?

    So it seems to me that Mr. Davis's statement makes sense. Temperature up, pressure low = gas. Liquid can't exist.

    If that is not true, explain to me how vapor/vac systems worked.

    Respectfully,

    Mark H






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  • Mark Hunt
    Mark Hunt Member Posts: 4,908
    Also


    Please explain how combustion products contain nothing but PURE H2O.

    This would be true with 100% complete combustion, but that is not possible on any planet in our solar system.

    Respectfully,

    Mark H

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  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    Pulling out the 3D vision

    Well, electric and magnetic waves are related that way, orthogonally that is, polarized light falls into this and the relation that appears is in 3D. What happens for sound waves in a single plane, is graphically the same thing, just 2D. Polarization is the reason we can see LCD displays.

    Not simple but, I figured getting a handle on 2D was the first step before jumping to 3D, that was my thinking at least, hope it was helpful.

    Look up Lissajous perhaps
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    electric and magnetic waves are related that way

    Aren't radio, infraret, et.al. elements of the electromagnetic spectrum?

    Am I supposed to believe that electricity and magnetism are different than electromagnetism?

  • Jim Davis_3
    Jim Davis_3 Member Posts: 578


    Trying to keep it simple, flue gases condense because of improper venting and underfired equipment not low return water temperatures as has been often taughted. Again I sold furnaces that condensed at 230 degrees. The exception is on a cold start where the heat exchanger is cold. If a water heater has rusted baffles it is because it is not venting or firing properly. Doesn't really matter if water vapor at 400 degrees is vapor or steam it is certainly above boiling point.
  • jp_2
    jp_2 Member Posts: 1,935
    h20

    2H + 02 = 2H20

    I suppose if C02 were in the right place, heat of cobustion, might dislodge an oxygen from co2 and then you have CO, but thats only a guess.
  • jp_2
    jp_2 Member Posts: 1,935
    ideal gas law

    pv = nrt

    pressure x volume = # of molecules, x a constant, x temperature.

    google 'gas law', some easy to understand web pages out there.

    this is pretty much what you and jim davis are saying.

    of course there more to it but this is a start.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    Vapor systems worked by controlling how air entered and exited the system. When a radiator filled with steam the air was liberated to the atmosphere. When that steam condensed, the system would allow some air to re-enter the radiator, but not necessarily as much as had left. This produced a partical vacuum in the system. The partial vacuum lowered the boiling point of the water so steam (yes, real 100% steam) was produced at lower temp than possible in the normal atmosphere.

    Vacuum systems do essentially the same thing, but they use a vacuum pump to remove air.

    It hasn't lowered the dewpoint, it's lowered the boiling point. Steam (don't forget that steam is 100% H20 in gasseous form) doesn't really have a dewpoint. Instead once the steam has given up its latent heat (the energy above boiling point required to induce the change in state) it condenses back into liquid water. Every reference to and definiton of "dewpoint" I can find has to do with a gas mix--not a pure gas.

    When you burn natural gas or oil with ordinary air you do not get pure H20 in gasseous form. You get a mix of gasses--not only from the combustion process, but from the combustion air itself. Depending on the fuel composition and the amount of excess air mixed with the fuel when it burns, the resulting gas has a fixed amount of water it can hold in vapor form--thus there is a definite dewpoint where the vapor condenses into a liquid--and it's not the boiling point of water because the flue gas is not pure H20 (steam).

    I remember a chemistry experiment where we separated water into hydrogen and oxygen via hydrolysis (DC electricity). We then combined the hydrogen and oxygen we collected and added a spark. Heat and liquid water were produced. Not heat and steam.

  • Constantin
    Constantin Member Posts: 3,796
    Too true...

    IIRC, the evaporated deposits are actually more corrosive than aqeuous ones?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Turbine Vacuum

    Actually turbines are usually under a positive pressure but the steam used is superheated (well above it's saturation point) so it retains it's gaseous state through the blades and into (as one example) the multi-port orifice valve where it can go to district steam heating or back to a condenser for recirculation.

    And if you go to Boulder (well, Fort Collins at least in my experience at 5000 ft.) and adiabatic cooling aside, the barometric pressure is about 12.1 psia versus 14.696 at sea level. Steam boils/condenses at about 202 degrees. In Boulder at about 6000 ft., the barometric pressure is about 11.9 psia so steam state-change would be about 198 degrees. In Estes Park at about 8000 ft. about 195.5 degrees. Nothing sadder than mule deer and elk sobbing over cold coffee....
  • jp_2
    jp_2 Member Posts: 1,935
    chem experiment

    I remember that exeriment too.

    but at the temperatures of that little combustion, the product surely must be gas that quickly condenses, I would think it needs to go through that phase change?

    we did it in an upside down test tube.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    We did electrolysis of water

    similarly but used sodium hypochlorite as a catalyst I believe, and 120 volts (screw the battery)...

    Fortunately nobody got hurt... But it did take out a window when the weather ballon they pumped the gas into ignited....
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