flue gas condensation question

[Deleted User]

flue gas condensation, is it something visible or audible,(like a waterheater on cold start does) or is it something that just IS taking place when the return water temp is lower than the dew point of the flue gas, but we cant see it.

Jim Davis

flue gas condensation

Condensation is visible when it runs out the pipe on to the floor. It definitely doesn't make noise. It is cause by improper firing of equipment (usually underfiring) and/or poor venting. Has nothing to do with return water temperature, unless one believes two wrong make a right.

Ray Landry

Cooler water temps means cooler heat exchangers. This is why the two coincide. if the hx gets below 136 degrees (when burning nat. gas) flue gases will condense.

[Deleted User]

ohhhh, my brain hurts,

i'm soooo confused..

Jim Davis

Why can water heaters bring in 45 degree street water and not condense? Most water heaters run at 120 degree water temperature all the time-136 degrees?. In 25 years I have not seen a boiler condense in a flue that is tile lined, outside chimney, if it is fired correctly and venting correctly. You can run at 180 degrees and still condense if those two things are wrong!

Mike T., Swampeast MO

Brain Hurts--Welcome--So Does Mine Over This

It seems that if you extract so much heat from the combustion process that the flue gasses are produced at a temperature below the dewpoint of the HX (NOT the water--return or otherwise) you are forced to deal with the acidic nature of the condensate. If this occurs in the flue itself it's too late and you only have rapid flue "rot" to await.

This is one of my crazy ideas but the [natural gas] condensing "bonus" doesn't seem to come so much from the condensate itself but from producing the combustion product below its dewpoint in the first place...

The vapor doesn't have to be vaporized because it's already condensed or some such s($% ?

I have WAY too much work to do but can't get this out of my mind. Stupid dream where I kept waking when the non-combustibles in the process left at the same temp that they arrived with all of the heat produced by combustion being absorbed by some ideal HX...

Boilerpro_3

Here's some more confusion, Gerry

Dunkirk places that number at 117F heat exchanger temp, not necesarily water temp. Cast iron is a pretty good insulator, you know, so both sides are not even close to the same temp. Still need to take one of Jim Davis class to see how you don't condense at low temps. Heck even the Germans haven't got this figured out on the Viessmans.

Boilerpro

Jim Davis

Condensation is from extracting heat

The flame and gas temperatures in high efficiency equipment are or should be the same as conventional efficiency. But by adding additional heat exchanger surface the gases are lowered to dew point and latent heat is recovered. The furnaces of the 80's (Recuperative) condensed at outlet temperatures over 200 degrees on some models.

Dan M

Im on my second boiler in six years due to this issue
My first weil died after 6 years and was a classic case of condesation problems. The boiler had some problems from the factory but Poor piping was the main culprit, cold return water was dropping straight into the boiler the moment it fired (1st Pro). When my second Weil Gv5 was piped wronge ( By second Pro) it had a pretty good trickle of water coming from the drain . Enter "Boiler Pro" , BP repiped my "At boiler piping" for primary and secondary and the boiler now is running properly. With the boiler piped right no condensation comes from the drain. One other indicator of a problem is to check the drain trap at the boiler for metal flakes. I would find alot of metal when my boiler was piped wronge. I am sure this boilers life expectancy will be shorted by its initial installation but thanks to Boiler Pro the damage has been stopped

Gordy

This whole topic makes me scratch my head also.Because with my WM CGM 7 boiler which has been running 12 years now.I do not experiance any sign of condensation.The present configuration is cold return temps. 75* piping is simple bypass piping with a mixing valve for low temp. radiant. The boiler is grossly over sized could this play a role in no condensate? I have an auto damper could this help? My boiler flue does not go straight up and out of the roof it goes 90 degrees and out to the clay tile chimney,does this play a roll? What about the ambient air temp the combustion chamber recieves? Whats confusing me is the table pounding on condensate,and my system should be a candidate for a repipe to prevent this, but if it is not condensing why bother.Personally I think that the boiler being oversized plays a big role,but only guessing.

Kal Row

ok people here we go again...

When fuel (hydrogen and carbon) combines with air (oxygen) it produces, water (H2O), carbon dioxide (CO2), and sulfuric acid (which was in the fuel to begin with) and Oxides of nitrogen (NX , from burning the nitrogen in the air if the flame is hot enough)
Fyi – if there is not enough air, you get h20 and co (carbon monoxide) and if there is still less air, you get h20 and C (carbon as in soot on everything) – you always get your h20, cause oxygen and hydrogen stick like glue

Now, at 136 degrees (cold startup or if your boiler is not set to return the water at a min of 140), the combustion byproducts will start to turn back into liquid before getting out the chimney.

If that happens in the boiler, and it’s cast iron, it will eat it up, with time – how much time, depends on the temp, hot-cold cycles, amount of garbage in the fuel to begin with – etc. – not an exact science

What this means to you is – that if the load Is a high temp load , (most convective devices like baseboards etc), you want to get the boiler to 140+ a quickly as possible and leave it there,

If you want lower water temp in the system, then you need a bypass loop with mixing valve, or primary /secondary/injection etc - so that the boiler can be run at one temp – and load at another – FYI the efficiency rating of cast iron boilers are all based on running it above 140

Now we come to condensing boilers – which uses an aluminum or stainless steel heat exchanger – and the cooled return water meets the flue gases furthest away from the flame, and discharges from the hottest part of the heat exchanger, so we are now sucking every drop of heat out of the flue gases, in fact if the return water is 90F the combustion outlet pipe feels cool to the touch and lots of acid water draining out,

In fact the flue gases can get so cool that they could actually suck heat out of the return water – (that’s another reason why the combustion fan must be modulated down as well as the flame as the water heats up)

Now with a slab radiant heat with a supply of 110 and a return of 90, these boiler can rightly claim 98 percent efficiency since the are taking all the heat out of the flue gases, the 2% loss, is the fuel that didn’t actually burn (you always get some of that)

jim sokolovic

Wondering about CO2...

Kal, is there anything about the CO2 that could be directly correlated to the amount of fuel that didn't actually burn? For instance - with Natural gas, that has a maximum of 12% achievable, and a burner that is producing a 9% reading (which is 3/4 of the maximum), could we say the burner efficiency is only 75%? Just wondering how this all works into the final efficiency number.

Boilerpro_3

Are your cast iron sections rusty....

Do you have to vacumn or blow out rust from the burners or comsution chamber floor? If you do you are seeing the results of boiler condensation. If the boiler gets hot enough on a burner cycle, it will evaporate off. A stack damper probably helps this, because on very low water content boilers like the CG, the heat exchanger heat soaks at the end of the cycle, getting it nice and warm, driving off condensation.

Boilerpro

Boilerpro_3

Thanks Dan, for the compliment! NM

Mike T., Swampeast MO

The more I look at this picture

The more I wonder about the temp of the gasses NOT involved in the combustion process.

It would [seem] the the purpose is to prevent as much heat as possible from entering those gasses in the first place instead of recovering it later in the HX.

Gordy

So basically Kal what you are saying in my situation see post above, is that the condensation is taking place from cold start to the 136* then after that the boiler modulates between 140* and 170* High limit setting until heat call is satisfied.So if I understand correctly during call for heat even though the return water to the boiler is cold until well into the cycle, in my situation it is not condensing, only in the initial start.So there for if the boiler happens to be at 136* or more from last heating cycle until the next call for heat then I would be okay or atleast better off than a cold start.

hr

Might check that clay chimney

It could be condensing somewhere along the outside run. A lot depends on how long the boiler fires to warm the entire flue run above the dew point. Could it be condensing in the clay, and you don't know it. Maybe an inspection from the top down?

hot rod

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Gordy

I do inspect yearly on the chimney and have boiler checked. Chimney appears to be in good shape for 52 years old.Some very light tile spalling near bottom.I know what you are saying though so I do check it.The chimney is an exterior one about 20' high and 7' wide and 3' deep. accomadates a fireplace in seperate flue upstairs also.There is alot of mass to heat up I doubt my flue temps are doing much heating once they get above ground level"boiler in basement".

Unknown

To keep this simple

I will state that if the flue gasses are condensing within the boiler only, you may or may not see any visible signs of this until the boiler is completely cleaned or until the passageways have possibly plugged up. If they are condensing in the metal flue piping from the boiler to the chimney, you will more than likely see this as a whitish or yellowish substance oozing out of the joints in the pipe or inside the pipe. If they are condensing within the vertical chimney, this may show up as white or yellow streaking at the mortar joints depending on the chimney construction and integrity of the liner. Hope this helps. I will e-mail a couple of photos of what boilers will look like if they are severely condensing.

Glenn Stanton

Burnham Hydronics

Kal Row

While 9%

of a possible 12% is 75% but only for that value, we used co2 for years because it was the easiest to test, but using one gas to analyze, is a compromise at best, because there is so many, factors, speed, impingement, non laminar flow, localized mixture changes – and the list goes on
The best thing, is to use a 4gas analyzer (hc,co,o2,nx) and we try to get all of them as low as possible – well with o2, you want 1% left over to guarantee, your not making CO, do to lack of O2, anything more than that, is cooling it down – but typically HC most important to measure unburned fuel

I suspect that the burner efficiency is the smallest part of a boilers overall rating – and that what the, Burnham engineer, that your Joe Cappola has giving the local IBR course, also said – the whole show is designed to be pretty good in the 140-180

Kal Row

G.Kaske and hot rod are right ...

anywhere along the run - we use 140 instead of 136 to get us out the top of most two storie homes - even if the boiler is above 140 the upper reaches may not be, and even the boiler may still take a cold hit, if a cold zone opens up and dumps into the boiler

Einsiedler_2

wtr htrs

Water Heaters do condense when you fill them with cold water & fire em up. Just peek at the drip trying to put out the pilot on your next new startup.

Once thermostat comes into play, the temp usually doesn't get much more than 25F below setpoint.

EIN

Gordy

So what is probably happening with "my" particular chimney arrangement since the boiler flue exits out the foundation side wall about 4' up. Any condensate will be contained in that particular area of the chimney, and not run back down boiler flue.So my concern should be with the say the upper 2/3s of the chimney and not so much with the boiler.Line it.

Kal Row

a cgm7 need to be repiped

with primary, secondary, and mixing or injection to get it right in a radiant install, cause even with a simple bypass, it will still short-cycle itself to death- as it's considered a high temp device - and radiant is considered a low temp terminal, (can up to 150 in an underneath the rafter staple up job with carpets above the floors - a total waste!!! - or a huge slab are that requires rapid response) - but if you have enough loops so that the flow through each can be real slow, and have good insulation below it- then the target temps of 110 supply and 90 return is way outside your boiler's range with simple mixing - one of weil's ULTRA's would be a much better fit - you will save a bundle of gas!!!!

i know, boilers are supposed to be a 30year item but in the last seven years, the tecnology has expolded, as has the gas prices, so you need to rethink it now - sorry

jim sokolovic

More details please, Kal...

I could try guessing - but could you explain what "hc" refers to? A "4 gas analyzer" usually measures CO, O2, NO, and NO2 (or SO2). CO can be high at rich or lean mixtures, so I can't equuate it to any particular percentage of fuel being burned. O2 can be directly correlated to CO2, so I don't see what difference it makes which is used (O2 is directly measured by most analyzers and then converted to CO2 by the instrument to display a value). What are you referring to about "1 % leftover O2 to guarantee that CO is not made"? Thanks for indulging me.

Kal Row

HC

Is Hydro Carbon (all fossil fuels) – what you want is a lean as possible but not so lean that the o2 molecules and fuel molecules don’t connect with the heat and it leaves you with unburned fuel or hc – (known as the lean miss in the auto world)

As for the lowest o2 or the highest co2 – we aim for a little off peak – ie a drop too much air – and you won’t produce carbon monoxide – so we give back a little in efficiency, for safety’s sake,

For smog producing oxides of nitrogen, we slow down the combustion processes so that we can burn completely at cooler temps – the lower the temp, the less atmospheric nitrogen burned into Nx,

That’s why they uses those ceramic flame holders, and down burners, on the new fangled boilers – to completely mix the fuel air mixture and connect all the molecules with heat slowly, and also connect it with the heat exchanger in a glide along fashion instead of hitting it head on where you get “bounce back” and no heat conduction, we also want to burn, not detonate,

Having said all that I still like to install a good old slant-fin with the burner box filled with stainless steel flame rails – give a nice smooth flame – and real easy to maintain – least grief, but alas everybody wants the new stuff now – they buy efficiency ratings – instead of comfort and control – those are the only two guaranteed deliverable items, in a non laboratory environment , efficiency – is not an exact science – it’s one of those “all other things being equal” sort of thing – well, “all other things” are never equal in the real world

– you buy good quality stuff, pipe it right, match heat source to the terminal load and efficiency takes care of itself, take oil for instance – real cheap, but all you need, is one drop of sticky gunk, in the angular slots, cut in the threads of the nozzle insert leading to the swirl chamber under the spay hole, and the fuel wont spin up right, and just spray out in streams instead of mist, and it’s the end of your oil system, you get a Smokey mess, that coats the inside of your heat ex and chimney, a once a season nozzle change, is not enough InMyUnHmbleOpinion (hc’s up the wazooo)

jim sokolovic

I see \"hc\" as an sensor option...

on a Bacharach analyzer now. Honestly, I had never heard of this offered on other units or in discussion, before now. Thank you for the explanation.

Gordy

Well Kal I agree the condensing unit is the way to go when new boiler is needed down the road for my application, "ceiling radiant and inslab". but my curiosity is in that the old boiler lasted 43 years under identical parameters.So what gives, are the newer boilers junk or is the inefficiency of the older ones to their advantage higher flue temps say in this particular scenerio.The new boiler was over sized "by about 70000 Btu because installer never did a heat load calc. Just matched the old one, and the old one used to heat another building. Which was fitted with its own boiler later after it was found the load was to great for the boiler in the house.

Gordy

I guess what I have learned on this issue is a system can condense in the boiler or in the chimney or both.The chimney condensing will always be an issue depending on the size of the chimney and the flue gas temp. With that said I would think it would be hard to keep the chimney temp above 140* at least in the upper part of a masonary chimney,maybe on warmer days but when outside temps start to fall it will happen so I guess s.s lining in the chimney is the answer there. Maybe I'm interpolating this wrong.

Kal Row

in order to get them up to temp faster

the new boilers have much less iron mass, to little in fact - in the steam world it is a nightmare, it's getting to the point, where you cant buy a decent replacement steam boiler, if you put in a new, well piped, steam system, then it’s ok, but for an old system, heaven help you, the horror stories abound,

now in your radiant application the oversized boiler exacerbates an already bad situation
I don’t see how you can get away without piping changes – you need that they call “boiler protection”
Go to http://www.tekmarcontrols.com/literature.html and read the “E” Essays
They are easy to read and understand – there is lots of good stuff on that site and It’s all free

Kal Row

before you line...

if you are going to get a condensing boiler and run it into the same chimney, you need stainless coax piping - so decide first, if your chiminey is exposesd on three sides and might be leaking flue gases - why bother - just get a few co detectors for you home and have'a'nice'day - if it aint broke... unless you can do this yourself cheaply, spend your money on boiler piping first - you need to line if, the chiminey is patially burried in the house and leaking flue gases are likely to get in through walls

i'd be more worried about a good fresh air supply than condensation - your cgm7 + a hot water heater needs 26square inches of fresh are vent - (1"sq per 10,000btu min) is that there? - do you have a co detector?

jerry scharf

about that burner

Mike,

There are some "smart cookie" engineers and one came up with this. The problem with fluid flow (combustion gas is a fluid) heat exchangers is that the heat stored is a function of volume and the heat transfer is a function of surface area. So the ideal HX has very thin passages with a slightly roughened surface. Unfortunatly, this is the maximum drag situation, so you have to make a compromise. With this burner, the idea is to couple as mcuh energy as possible to the HX via radiation, and thus avoid the volume/surface/drag dilemma as much as possible. I would also guess it gives a much more stable combustion profile.

jerry

[Deleted User]

thanks glenn,

simple is good..

Mike T., Swampeast MO

So is it fair to say that the gasses are leaving this burner at a significantly lower temperature because radiation "stole" the heat before it could be transferred to the gas?

Kal Row

The point of the whole contraption

is to fire it slowly so that the fuel can be burned completely at a temp low enough, that you don’t form NX (oxides of nitrogen) and get all the BTUs out of it into the heat exchanger by conduction and radiation

jerry scharf

both are true

Mike,

I see it as a push rather than a pull, but that's it. The combustion heats the wire which radiates to cool itself. My broiler has the same thing.

Kal,

You're right about the lower temperature producing less NOx. I'm not sure about burning slower. It's burning in a more dispersed pattern with signifigant radiation output, so the nitrogen that's along for the ride stays much cooler that the peak heat in a normal flame.

jerry

Joe_30

When we needed a liner put in on a steeper roof we contacted the local climbing store and asked for an impoverished climber who would work for $20 /hour. The guy who answered said"That's me" I got a 32 year old Phd in anthropology who climbed like a monkey, all roped off safely, and capable of learning what was needed. Had a forhead light and worked into the night. Interesting talk, as well.

Alan_5

condensing boilers

With a condensing system, where does the condensate go?

Also, is it possible to add a heat exchanger to the flue? The system I have is a direct vent out the wall. the pipe slopes downward on the outside of the wall. If I condense the gases just before they pass out of the house, the condensate would just run outside. What is the acidity of this stuff, would it be neutralized by the ground, or could it cause a problem at the foundation?

Kal Row

by slower i mean

locally slower flow - so that all the parcels of fuel, air, and heat, have a chance to connect - you see a lot of that, a small tube with a combustion fan - where the fuel is introduced, and then the mixture is expanded into a much larger area to slow the velocity for complete burn, and that area usually has some sort of holding device to retain the flame, after which it is forced back into a smaller area for rapid exhaust - keeps the exhaust gas and condensate in suspension and gets it outside – at least that’s the plan - there is of course always the problem of mixture cooling as it expands (expansion is one of the 5 refrigeration cycles) – but, the heated mesh helps a little for that, by imparting some heat back into the mixture – this of course, costs you some of the wonderful infra-red energy,
the boiler is great in the it’s maximizing the usage of infra-red energy that doesn’t go up the flue, but without efficient combustion, you wouldn’t have the efficient radiant heat to begin with, this IS rocket science – literally

by the way what boiler is that??

Gordy

Boiler pro A little light surface rust. Nothing heavy you can blow the dust out of the burner pan at the end of the season.The tech from while said a little surface rust is a good thing.I have seen a few bad boiler flue way condensation pictures on the wall nothing like those. Is there suppose to be absolutley no rust.Just asking

Boilerpro_3

What I 've found....

That light rust is probably due to condensation that usually gets burnt off near the end of each heating cycle. Also,it could be summer condensation from moist air if the boiler is cool in the humid summer months (typically when there is no standing pilot). I find that most steam boilers and hot water boilers with standing pilots with return temperature protections have very little or no rust accumulation from seasos to season, mainly just dust and ash. In typicaly baseboard systems with no return temp protection, there is usually a little rust due to the time required for the boiler to warm up on each cycle. A little bit of condensation at start up and then it cooks off, creating a small amount of corrosion.
I like my boilers to have no rust as I suspedt the reust does create some insulation between the hot flue gases and the heat exchanger.

Boilerpro