New boiler, moisture/soot mix running in basement

[pit monster]

We bought a 2nd home recently (built in 1923) that my wife uses during the week, empty on weekends. New Oil boiler was installed shortly after we bought the home, New Yorker that the manufacturer states is 86% efficient.

The boiler ties into the homes original masonary flue, I do not think it is lined with clay but I'm not positive. I would guess the prior owners, an old couple, did little to no maintenance on the flue, and it certainly seems from the basement that there was a lot of soot at the bottom of the flue.

It was rather cold this past weekend and when my wife returned to the home on Monday evening there was clearly a leakage of "stuff" on the basement floor. My heating company originally was going to put in a gas boiler, including a metal liner, but we were forced to stay with oil and when we changed the unit out for an oil boiler they felt the flue did not need to be lined.

From my brief research it seems as if what has leaked onto the floor is a result of condensation accumlation inside the flue falling back down and getting dirty with soot along the way. Manufacturer literature states the minimum flue size is 8"x8" rectangular or 6" circular by 15'. The 15' is not a problem and I think it probably meets the 8"x8" inside dimension but I'm not positive.

The heating company is coming back out tomorrow to see, what would the Pro's suggest I discuss with them? Thanks.

[jadnashua]

As I understand it, the problem is more likely the flue might be too large, which cools off the exhaust too much. Exhaust gas condensates are quite acidic, and can eat a chimney. You'd need to know the actual size and run that by the manufacturer's instructions.

[pit monster]

As I understand it, the problem is more likely the flue might be too large, which cools off the exhaust too much. Exhaust gas condensates are quite acidic, and can eat a chimney. You'd need to know the actual size and run that by the manufacturer's instructions.

The instruction book only talks about minimums but does not discuss any maximum sizes.

Perhaps a good cleaning of the flue is all it needs or do the experts here think a metal liner (stainless?) would be the best solution?

[Dana]

The instruction book only talks about minimums but does not discuss any maximum sizes.

Perhaps a good cleaning of the flue is all it needs or do the experts here think a metal liner (stainless?) would be the best solution?

Cleaning an oversized flue still leaves you with an oversized flue.

Obstructions aren't likely to cause this problem, but for the record, do you have a large raptor nesting in/on it to stay warm?

What size is the existing flue? 2x the minimum cross-sectional area can be problematic, more than 3x is a problem for sure. Flue tiles are measure to their outer dimensions, so their inner cross section is considerably smaller see:

http://superiorclay.com/flue-liners.php

An 8x8 liner has an interior cross section of 42square inches. Measure yours, if you can. 8x10 on the interior dimension or bigger could be an issue- more so if the chimney is on an exterior wall instead of running up the center of the house through the attic.

You can also run into flue condensation problems if the boiler is being operated at too low a temp resulting in low stack temps. The boiler's high/low limit adjustment or circulator pump controls have to be set correctly to preclude this from happening. (If temps are low enough it'll condense inside the boiler, destroying it in a few short seasons. Cold-start or no, the return temps from the radiators need to be 140F or higher where it enters the boiler to be re-heated.)

A stuck atmospheric damper on the exhaust vent could also result in flue condensation, since it's function is to dilute the exhaust to a lower it's CO2 concentration, reducing the dew-point to something well-under 100F. (Undiluted oil exhaust will condense at temps north of 125F, and the top of your flue isn't likely to be anywhere near that hot in normal operation.)

No matter what the cause turns out to be, it's the installers responsibility to get it right. If the existing flue was too large, that should have been noted & corrected before firing it up. If it needs a narrower flue liner it's reasonable for you to pay something toward that (and it's not cheap), but it's a bit unprofessional to have installed the boiler without it in the first place, if that's what's going on. (In some areas the code inspection on the installation should have caught it as well.)

[pit monster]

My heating contractor looked at things and concluded the following:

It is definitely a condensation issue.

Flue size opening is appropriate - the contractor and I got up on the roof and viewed it from the top - it meets the 8"x8" minimum and may be a bit larger.

The boiler is operating properly, they made a minor adjustment on the damper, allowing it to send more of the basement air up the flue when the system is not running (hoping that will keep interior of flue warmer)

Another minor adjustment was made on the mixing valve, they either opened or closed it all the way (not sure which way-but it made sense when they explained it)

They asked me to only set back to 60 degrees (vs my current 55 degrees) on the weekend when the property is unoccupied.

They did not feel me setting up a more gradual warm up on Monday would be of any value, but for some reason I still think it might. (ie - step the temp up in 4 degree increments vs 8 degrees)

Ultimately they said it may be the only way to completely eliminate the condensation potential is to install a stainless liner. They said they did one recently and it was a $2,000 job (used a 35' kit).

[jadnashua]

If you warm up in steps, that implies a gap in the heat run. This will allow the flue to cool off, starting the condensation all over again when it restarts. Any heating device tends to be least efficient upon startup...they take varying amounts of time to reach 100%, so each time you start, you aren't starting at 100%, but something, sometimes a lot less. Your car works better once it warms up, too, and it can take awhile before you don't see the vapor trail out the exhaust. Prior to that, it will be drenched in condensation. Do that often enough, and it rots out quickly. Lasts a lot longer when used on long hauls, or long cycles...shorten them, and things fail sooner.

[Doherty Plumbing]

My heating contractor looked at things and concluded the following:

It is definitely a condensation issue.

Flue size opening is appropriate - the contractor and I got up on the roof and viewed it from the top - it meets the 8"x8" minimum and may be a bit larger.

The boiler is operating properly, they made a minor adjustment on the damper, allowing it to send more of the basement air up the flue when the system is not running (hoping that will keep interior of flue warmer)

Another minor adjustment was made on the mixing valve, they either opened or closed it all the way (not sure which way-but it made sense when they explained it)

They asked me to only set back to 60 degrees (vs my current 55 degrees) on the weekend when the property is unoccupied.

They did not feel me setting up a more gradual warm up on Monday would be of any value, but for some reason I still think it might. (ie - step the temp up in 4 degree increments vs 8 degrees)

Ultimately they said it may be the only way to completely eliminate the condensation potential is to install a stainless liner. They said they did one recently and it was a $2,000 job (used a 35' kit).

Sounds like you had return temps coming back into the boiler too cold and they products of combustion were cooling too much as they crossed the heat exchanger.

They most likely opened up the mixing valve to allow warmer water to come back to the boiler.

On a natural draft boiler you need about 140*F min. coming back to the boiler. I usually shoot for a bit hotter just to be safe, about 145*F.

I would suggest you installed a motorized 3-way valve so you can automatically adjust the mixing valve to bring back the correct temperatures.

[jadnashua]

The higher the efficiency of the boiler, the more chance of condensation problems you can have if the flue isn't working properly. If you're getting condensation on a regular basis in a ceramic flue, it won't last long. The boiler gets those higher efficiencies by pulling more heat out of the flame, which means the exhaust is cooler and it doesn't keep the moisture created during combustion in vapor form unless it's fully warmed up. At the stated efficiency, it's on the edge of requiring SS, from what I understand. you might want to call the manufacturer and get their recommendations and discuss your conditions with them. They have a lot more experience with that unit than a typical installer.