How to combat condensation in chimney?

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Old 10-17-12, 06:02 PM
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How to combat condensation in chimney?

As many of you know, I have just completed installing a new Burnham MPO boiler. It's working great so far ... except for one hitch.

Walked in from the day job today and as I walked past the boiler I noticed something right away... it wasn't there this morning when I left for work.

There was a long white streak down the side of the flue pipe. It started just above the vent damper and traveled all the way down.

As I was doing my combustion testing after firing up for the first time, one of the things I measured was the flue gas temp going into the bottom of my manufactured 6" chimney. When the boiler water was at 180 the highest temp I saw going into the chimney was 230F. This was with the gross flue gas temp at the breech right around 360F. I knew right then that I was going to have a problem with condensation in the chimney itself.



I should probably post a full length pic of the flue pipe... maybe later... but let me describe it first.

The MPO has a 5" flue outlet. In the pic above, I've teed into 5" pipe with a cleanout at the bottom. On the way up, there is the barometric damper. Just above that is an INCREASER to 6" so that I could use my almost new OVD vent damper on the new boiler. Above that I continued with 6" into the ceiling box of my 6" HT103 manufactured chimney. I forget exactly how tall the chimney itself is, but it's in the neighborhood of 25' or so.

The MPO manual says that by removing baffles from the boiler one can increase the flue gas temp by 65F and of course this means an efficiency sacrifice so I'm not keen on doing that, but if I have to I have to.

Here's a question... or two...

Would replacing the 6" chimney with a 5" chimney raise the gas temp in the chimney? How much? Enough?

Is there ANYTHING else I can do to prevent the condensation in the chimney?
 

Last edited by NJT; 02-25-15 at 01:44 PM.
  #2  
Old 10-17-12, 10:11 PM
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How do you lose 130f from the breech to the vent damper? I assume it is dilution air from the damper but it looks closed. Pulling the baffles will not affect the efficiency that much.
Making the chimney 5" would help but with the small affect on the efficiency by removing the baffles I would wonder how long it would take to make up the difference between the price on a liner.
 
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Old 10-17-12, 10:11 PM
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going to 5" would decrease flue gas flow rate, no? the flue gas stays in the chimney longer, heat lingers for awhile longer.

perhaps also, 5" pipe has less surface area than 6" pipe, per unit length. Less area reduces heat transfer rate.
 
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Old 10-18-12, 03:37 PM
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How do you lose 130f from the breech to the vent damper?
I'm going to verify the accuracy of the barbecue thermometer that I've got stuck in at the top where the chimney starts... last time I checked it it was fairly close. The other thermometer at the breech is a good one...

I'm not measuring at the vent damper, but about 4' above that, right where the flue connects to the chimney. Note the thermometer at the top and the one on the pipe from the boiler into the tee. The 130 (110) drop is from the breech to the top. Even with foil over the baro damper there isn't much difference.

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Keep in mind that the 360 GROSS, and 230 are when the boiler is at 180 after a full heat up test run. I haven't seen much over 300 and 190 with the limited heat calls that we've had. Boiler setpoint hasn't moved from 130 yet with the still young heating season.

with the small affect on the efficiency by removing the baffles
And that's certainly the first thing I'll do. At least experimentally for now, just to see the effect.

I know the stuff is kinda pricey ( $85.00 for 4 foot section ) but I was looking at 316L 5" stainless (rigid, not flex) liner material today. I'm quite sure that I can 'line' the 6" Metalbestos chimney with that. I can probably also fit it in there with the 1/4" insulation blanket around it which would help even more.

Spoke to a rep at the factory that makes the stuff and he agreed... it could be done. I don't see any reason why it wouldn't pass inspection... ( uhhhh, inspection? )...

I can tellya one thing, with 316L 24ga stainless top to bottom, I would NEVER have to replace a hunk of flue pipe again! I've already got the stainless tee. The whole setup might run me about $500 or so... would bum me a bit if I couldn't use the almost new vent damper though. I guess I could go up to 6" as I have already, and then BACK to 5" to connect to the chimney. Or just bite the bullet and purchase a 5" model and see if I can sell this one to my neighbor.
 

Last edited by NJT; 10-18-12 at 04:09 PM.
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Old 10-18-12, 03:40 PM
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5" would decrease flue gas flow rate, no?
I'm sure... and being that the boiler is designed for 5" it would not be a problem there.

I know that regarding the 'NPP' that I installed ( band clamp holding it in, can be seen in the pics on the black pipe just above the stainless tee out of the boiler, I absolutely will have to trim it some more. I'm wondering if going to 5" might not just eliminate the need for it all together!
 
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Old 10-18-12, 04:13 PM
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I wonder if the barometric damper is opening up too wide? You might try adusting the counterweight.
 
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Old 10-18-12, 04:18 PM
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No... even with damper foiled over, same low temp at chimney.

Notice the NPP (Neutral Pressure Point) adapter in the pipe below damper which fixed the problem of too much draft in boiler and allowed proper adjustment of damper.

The damper only starts to open when the flue gas at breech reaches about 300.

I'm sure it will open wider when it gets colder out and the draft increases in the chimney.

By the way, also note the draft gauge on the wall behind the boiler.
 
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Old 10-18-12, 04:32 PM
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Maybe your recent 130 deg F setpoint is just too low. Try raising it to 150 deg.

With your automatic flue damper, flue gas will be bottled up in the flue and chimney. A regular flue connection might allow the flue gas to continue to escape, and dry out, after the burner shuts off.

I wouldn't do anything drastic until you have a chance to think this through.
 
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Old 10-18-12, 04:39 PM
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The 130 setpoint is coming from the ODR module... yeah, I could raise it, but the heating calls this time of the year end before the boiler hits setpoint anyway.

Still, on the test runs, with the boiler all the way at 180, the entering temp at the chimney was never over 230, and that is still too low. As soon as the weather gets COLD, this will turn into a big problem... I'll have condensate weeping outta that chimney all over the place!
 
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Old 10-18-12, 04:45 PM
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OK, jumper things and go to a warm-start configuration, with water set at 160 deg or so. See what happens.

I don't particularly like that the boiler never hits its setpoin.
 
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Old 10-18-12, 04:53 PM
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No way... not on your life will I ever run a warm start boiler! Not even as an experiment.

First gonna pull the baffles and re-adjust burner and see how much diff that makes.

Then, if not enough, gonna run the 5" liner...

After that... well, I just don't know!
 
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Old 10-18-12, 05:03 PM
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Whoo, OK, sorry. I never met an experiment that I didn't like!

I do tend to think there must be a simple answer to this.
 
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Old 10-18-12, 05:16 PM
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Haha! I like to experiment too... but this boiler is just too new to go messing with hacking at the controls and such. I actually DID experiment with warm start on the old boiler and found that it wasted a tremendous amount of oil. I think my 'K-factor' went from like 8-9, all the way down to like 6 or so. I was burning at LEAST 30% more oil with warm start.

Thing is, looking for a simpler answer... what could be simpler than removing two bolts, swinging the burner door open, pulling the baffles, closing the door and readjusting the burner?

Looking at my 'fire finder', even if the flue temp goes up 65 I'm only losing 1% on my combustion efficiency... from 86 to 85. I don't think that will make a huge impact on the AFUE.

I might try raising the ODR setpoint floor from 130 to 140 though... I think that's reasonable.

Also, keep in mind that this boiler is DESIGNED for a 5" flue. In a way, I'm breakin' the rulz by upping to 6", and when you think about the extra volume of 6" vs 5", while it doesn't sound like a lot, it actually IS quite a huge difference.

Even though money is tight all around, I wouldn't have a problem cuttin' into my nephew's college fund for a 5" liner! After all, think of all the money I saved by DIY'ing the install!
 
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Old 10-18-12, 08:47 PM
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Going to a five-inch stack will NOT decrease the volume of stack gases but it WILL increase their velocity. Increased velocity means there will be less time for the water to condense on the stack walls.

I don't like the idea of a NPP slide on an oil burner but...unless you want to decrease the area of the existing stack (new stack or liner) there isn't much else you can do. I would suggest that you use an oil-rated insulated stack rather than the single-wall you have. It will mean less heat heat radiated to the surrounding area and more heat in the stack.

These "baffles" that you can remove from the burner door...do you mean "retarders", twisted sheet metal that fits into fire tubes? If yes, I strongly suggest that you do NOT remove them.
 
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Old 10-18-12, 09:06 PM
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Troop did you not put a smaller nozzle in? Is that de-rating?

From your manual:

Do not de-rate the appliance. Failure to fire the
boiler at it's designed input may cause excessive
condensation upon the interior walls of the chimney.
In addition, the lower input may not create enough
draft to adequately evacuate the by-products of
combustion.


And I know you. You want those baffles in....LOL

Remove the baffles if there are any signs of
condensation in the chimney or chimney connector.


The MPO manual says that by removing baffles from the boiler one can increase the flue gas temp by 65


I read 52F in the manual. And the baffle should be in 3rd position (pass)? The 2nd pass baffle should not be there.

States this also.

MPO-IQ84 at 0.60 GPH firing rate utilizes a low fire baffle




Oh just my two cents. I am sure you know all this.
 
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Old 10-18-12, 09:48 PM
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NOT decrease the volume of stack gases
Wouldn't it decrease the amount of dilution air drawn in by the baro damper, thus a smaller volume of colder air to cool the gases?

it WILL increase their velocity... less time for the water to condense on the stack walls
I understand what you are saying... good point!

I would suggest that you use an oil-rated insulated stack rather than the single-wall you have
Yes, looking at that option also. For that short a run do you think it would make that much difference? That double wall or insulated stuff is expensive! ( more so than the 5" rigid liner (per foot)! )

These "baffles" that you can remove from the burner door...do you mean "retarders", twisted sheet metal that fits into fire tubes? If yes, I strongly suggest that you do NOT remove them.
The baffles aren't in the door, they are in the 3rd pass of this size MPO (and also in the 2nd pass of the larger models). This isn't a fire-tube design, it's a 3-pass. The baffles are stamped out stainless steel metal thingys that slide in from the front. The manual is pretty clear that it is acceptable to remove them in the event the flue gas temp needs to be raised to counter any FGC issues. Even tells you how much higher temp to expect! But yeah, I don't WANT to remove them! But if I HAVE to ... well, I have to.
 
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Old 10-18-12, 09:58 PM
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Troop did you not put a smaller nozzle in? Is that de-rating?
No, of course not... I'm not even sure you can BUY a nozzle smaller than this!

The nozzle is a 0.50 45W which at 150 PSI pump pressure gives the 0.60 GPH firing rate.

I read 52F in the manual
Yes, I misquoted. I'm afraid that 52 ain't gonna be enough. I'm probably going to have to remove the baffles AND go with the 5" liner. (may have to play tricks with the CO2 also...)

You're right, I really want the baffles in, but on the other hand, I can't have this condensation issue happening so it's going to be the lesser of two evils I'm afraid!

MPO-IQ84 at 0.60 GPH firing rate utilizes a low fire baffle
This is a baffle in the air tube of the burner. That stays in... unless I convert it to a 115 model (remove low fire rate baffle, change static plate, change nozzle). Which if I can't solve the condensation issues by any other means, I may have to do... this should definitely raise the FG temp!
 
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Old 10-18-12, 10:02 PM
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Wouldn't it decrease the amount of dilution air drawn in by the baro damper, thus a smaller volume of colder air to cool the gases?
Not significantly. The smaller diameter will still develop the same draft, maybe even more than the 6 inch. Draft is a function of the temperature differential, the volume of the exhaust gases and the height of the chimney. Your use of a NPP is artificially altering the natural draft.


Yes, looking at that option also. For that short a run do you think it would make that much difference? That double wall or insulated stuff is expensive! ( more so than the 5" rigid liner (per foot)! )
Calculate the entire area of the single wall stack and you will see it is a significant amount of radiating surface. Using insulated (double wall) stack will definitely reduce this radiated heat and therefore contain it to the flue gases.


The baffles aren't in the door, they are in the 3rd pass of this size MPO (and also in the 2nd pass of the larger models). This isn't a fire-tube design, it's a 3-pass.
Fire tube boilers can have multiple passes. Two, three and four pass fire tube boilers are common. At any rate, twisted retarders serve the purpose of giving turbulence to the gas stream and a more vigorous "wiping" of the heat exchange surface. This increases heat transfer by eliminating laminar flow and the "boundary layer" next to the heat exchanger. Sure, you CAN remove them but it WILL cause a decrease in overall efficiency.


Ideally, a smaller diameter (and insulated) stack is the preferred resolution.
 
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Old 10-18-12, 10:12 PM
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Ideally, a smaller diameter (and insulated) stack is the preferred resolution.
For the immediate need, I'm going to pull the baffles tomorrow. That will certainly help somewhat for the next couple days.

I'll look into what I would need to install the insulated stack... ( I bet I could 'make' one myself! I'm pretty handy when it comes to sheet metal! )

I do feel that the liner is also going to be necessary, for the same reason you mentioned... smaller size, higher velocity. So I will probably go ahead and order those materials Monday.

Hopefully, if all goes to plan, I will be able to reinstall the baffles after I convert to 5" and insulate the connector pipe in some way.
 
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Old 10-19-12, 07:23 AM
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Condensate

Trooper
Would a through the wall power vent work in this case?
Sid
 
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Old 10-19-12, 03:39 PM
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Would a through the wall power vent work in this case?
It might Sid... at somebody else's house perhaps. I don't know if it's recommended for this boiler though. And I don't have anywhere that I could vent one to. It would have to blow out onto the walk up to the front door... that wouldn't be good!
 
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Old 10-19-12, 03:51 PM
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I am following this thread with interest because I am gradully changing to smaller sizes of nossels , and I dont want condensation ethier. I have all the instraments, so I am proceding with caution. Also waiting for cooler weather with mixed feelings.
Sid
 
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Old 10-19-12, 04:03 PM
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I'm going to keep it going until I'm satisfied I have a cure. Keep yer eyes peeled! ( wait, on second thought, don't do that, it sounds painful! )
 
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Old 10-20-12, 01:09 PM
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This morning I pulled the baffles.

There actually was some evidence of a bit of condensation inside the HX passes of the boiler itself! Not much, but enough to catch my eye. I'm sure it's because of the short cycles that the boiler is subject to at this time of year, and probably not enough to be concerned about.

After pulling the baffles, I readjusted the burner to spec and the flue gas at the breech is in fact about 50 hotter. Up at the chimney entrance it's about the same, 50 hotter... so yeah, my combustion efficiency did take about a half percent hit by this change.

Using a mirror and bright light I was able to look up the chimney and it is obvious that it's condensing further up and running down the inside from the top.

My next step will be to fit a 'radiation shield' around the exposed pipe from the boiler to the chimney. I'll do the 6" section first, I'm going to use a section of 8" flue pipe and suspend it around the 6" to see what difference that will make in the losses between the breech and the chimney.

The final step ( I HOPE! ) will be to fit a 5" insulated liner inside the 6" UL HT103 manufactured chimney.
 
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Old 01-29-13, 05:06 PM
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UPDATE.............................

There are QUARTS of water draining out the bottom of the tee! I have a bucket underneath and a hose out a hole in the cap on the bottom. The condensate is definitely coming down all the way from the top of the chimney.

Pulling the baffles did help somewhat, but not nearly enough.

I did manage to fashion a double wall pipe (kinda temporary, kinda ugly, no photo op there!) around the 6" section and that helped a little bit also.

Since the boiler got a little dirty inside, and the baffles are pulled, a typical temp for the flue gas at the breech is around 360-380.

The temp at the point of entering the chimney has gone DOWN! I have not seen it over 200F since the really cold weather set in.

This boiler is in a basically unheated utility room. As MUCH COLDER! dilution air is pulled in by the baro damper, the effect is that the flue gas is even colder when it gets to the chimney, enough to offset the difference in pulling the baffles and adding the 'radiation shield' around the exposed 6" single wall section.

I've still got choices... attempting to install the 5" rigid stainless liner up inside the existing 6" manufactured chimney, which could be problematic trying to slide the liner and insulation section by section up from the bottom...

OR

Biting the bullet and just replacing the 6" chimney altogether with a 5" product...

Of course, I'm living with it as-is for now, but springtime I need to decide.

On the one hand, adding the 5" insulated liner will afford an extra layer of insulation, but not sure I can get it in there... so I might purchase the stuff and find it's a no-show game and I've wasted my money.

On the other hand, new chimney ain't cheap, and I AM!
 
  #26  
Old 01-29-13, 05:49 PM
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Rule Was you could up fire a boiler 10 percent.Higher stack but higher co2,sometimes a wash.Maybe you could find out about yours?
 
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Old 01-29-13, 06:06 PM
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Yes, I'm told that one recommendation would be to increase pump pressure to up-fire, and that would in fact raise the flue gas temp, but I don't believe it will be enough to cure the problem. The root cause seems to be an issue with the chimney being too large and too much dilution air.

Remember that the temp of the gases at the breech is almost 400F...
 
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Old 01-29-13, 09:27 PM
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I wonder if you can rent a camera prob that can withstand high temps ?
I'd be curious if most of that moisture is raining out right after it fires up, or evenly along the burn cycle ?
 
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Old 01-30-13, 07:51 AM
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When I have looked up the pipe (straight shot all the way to the top), it has been at the end of a burner cycle and it was dripping on the mirror... drops of water could be seen on the pipe all the way to the top... I believe that the chimney remains cold enough to support condensation througout the run cycle.
 
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Old 01-31-13, 09:00 PM
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Trooper.

Looking at the pictures I see that you installed the NPP slide below the barometric damper. This means that the barometric damper still has to allow enough room air in to the chimney to overcome the draft setting of the barometric damper. You might have far better luck if you place the NPP slide ABOVE the barometric damper. Be sure to allow for a significant flow through the barometric damper to allow for changes in the atmospheric conditions.

Note that this is just for experimentation and is NOT a fix for having too large a chimney.
 

Last edited by Furd; 01-31-13 at 09:17 PM. Reason: Additional information
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Old 02-01-13, 08:31 AM
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I did consider that Furd...

I deferred to rbeck's recommendation of below the damper...

Remember that this boiler is designed to be pressure fired, and for that reason I would be hesitant of placing the NPP above, I'm quite sure that at least for the first 10 seconds or so, until draft is established in the chimney that there there would be at least some outpouring of flue gas from the damper.

When the burner first fires, the draft gauge at the breech fitting at rear of boiler is somewhat POSITIVE. Once the draft is established in the chimney after the initial 10 seconds or so, the draft at that point settles in at very close to zero (or ever so slightly negative).

I'm going to live with it until the heating season is done and then go with one of the chimney fixes I've mentioned. I just hope that the expense of either solution fixes the problem!
 
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Old 02-24-15, 07:20 PM
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Smile condensation

NJ Trooper,
Are there any follow-ups on your condensation problem?
I have a similar problem with my boiler and am looking for solutions.

Thank you
 
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Old 02-24-15, 08:22 PM
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Condensation in the flue can be overcome by, reducing the nozzle size, reducing the smoke pipe one size from the breach size .Reducing the firing rate causes the burner to run longer which warms the entire length of the flue, ideally above the due point . Large firing rates , if they cause short cycling promote flue condensation . No mater what size nozzle is used a steady state efficiency should approach 86% . I have run 0.40 nozzles at 140 psi. with out flue condensation .
 
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Old 02-24-15, 09:25 PM
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Hi Fred,

I guess I lost track of this thread and never updated it. Here is what I ended up with:

I lined the existing 6" manufactured chimney with a 4" (NOT 5" as I earlier was thinking) INSULATED flex liner. The boiler flue outlet is 5". So, to use my vent damper, I went from 5 up to 6 for the damper, then back down to 5 up to the chimney base, then went from 5 to 4 at that point.

No joy...

Still condensing, not as badly, but still.

I've fashioned a 'funnel' for the bottom of the tee and the condensate runs into a plastic bucket.

So, no, haven't got a 'fix' for you.

Reducing the firing rate causes the burner to run longer which warms the entire length of the flue, ideally above the due point
Yeah... well... good luck with that.

Dropping the firing rate further reduces the flue gas temperature. OK, you might run longer, but not as hot. Ends up being a 'wash' in my opinion.
 

Last edited by NJT; 02-25-15 at 02:00 PM.
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Old 02-24-15, 09:40 PM
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Running smaller nozzle?
That's like heating the chimney with a match. Hold it there all day long, how do you think you'll make out.
 
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Old 02-25-15, 07:40 AM
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Most flue condensation happens on outside flues ,the solution for them is to frame them in and insulate or run an insulated liner down. I don't know why you would pounce on the idea of using a smaller nozzle .The theory of heating states that the burner should run 24 hours on the coldest day and just put in enough heat to match the heat loss . I understand that most heating systems are too big and burner cycling is the accepted norm . I have a friend who is a licenced oil burner teck. who just installed an air to air heat pump and he noticed that the air coming up the duct work is 90f for the heat pump and 150f for the oil , the thermostat remained at 72f with both sources of heat the difference was the cycling .
 
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Old 02-25-15, 08:01 AM
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I don't know why you would pounce on the idea of using a smaller nozzle
Because if you stop an think about it, it simply doesn't make any sense in the TOPIC.

Smaller nozzle, cooler flue gases. Less air moving in the chimney, longer 'dwell time' for the gases to stay IN the chimney and cool and condense that much further.

The bottom line is that modern equipment is TOO EFFICIENT for conventional chimney venting. Less heat + less flow = flue gas condensation.

Don't go off on a tangent... about fuel savings and all that, because it has nothing at all to do with the TOPIC of solving condensation in the chimney.
 
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Old 02-25-15, 08:28 AM
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Trooper I respect your vast knowledge but in my opinion and experience I have run 0.40 nozzles that provided 200f of flue gas temperature to the entrance of the flue of old outside chimney's with out producing condensation Think about it if the burner runs long enough that 200f will heat the chimney above the dew point ,nothing to do about the fuel it will also save by preventing short cycling. Another though is if the chimney has leaks, cold air could get pulled into the flue gasses to cause condensation.
 
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Old 02-25-15, 09:20 AM
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NJT
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provided 200f of flue gas temperature to the entrance of the flue of old outside chimney's with out producing condensation
How do you know? Did you run a camera down while it was running and observe?

Think about it if the burner runs long enough that 200f will heat the chimney above the dew point
I am thinking about it and can not possibly imagine that there is enough BTU output in the flue gases to heat an exposed chimney. No way, no how.

The firing rate on my system is ALREADY very low ( 0.5 nozzle at 150 PSI = 0.6 firing rate). I DID drop the liner size down to 4". This liner is actually DOUBLE insulated because it is an insulated liner INSIDE an insulated chimney. And it STILL is condensing in the chimney.

So what you're saying in theory does not work in reality... on MY system... your mileage must vary I guess!
 

Last edited by NJT; 02-25-15 at 02:01 PM.
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Old 02-25-15, 11:03 AM
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No I did not run a camera ,but when the gas temperature hits the dew point there seems to be enough condensate to show up at the base of the flue. Do you know how many btus are produced by 0.4 nozzle that are left in the volume of gas at 200f and how many are required to heat an outside flue above the dew point. The set up you have described should no way no how be condensing unless you still have very short burner on times and very long burner off times to allow the chimney material to cool to the dew point temperature .Maybe you need to install a temperature probe in the flue were you have observed with your camera were the condensation is forming ,there maybe a source of outside cold air entering the flue.
 
 

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