Outdoor combustion air kit question

[Theodore]

Another outdoor combustion air question. Comments appreciated.

My 9year old Burnham MPO147 (147,000 BTU, located in lower New York State) has an optional "fresh air intake kit" (part #611280031 Burnham Air Intake) that consists of a 4" dia metal duct piped to the burner.

I'm making a boiler room enclosure and considering this.

I've read some places that very cold outdoor air temps (i.e. night in the middle of winter) piped directly to the burner in this manner can cause flue gas condensation. My boiler guy's opinion is not strong in either direction (to install vs. not to).

Is this a bad idea to do? Any experience appreciated.
Thanks
Theodore

 

[Dana]

If you have a stainless flue liner don't sweat it. If you are venting into a right-sized chimney it's not much of an issue either.

Do you happen to know if the baffles that turn it into a 3- pass are installed? (See page 19 of the manual.)

 

[Sponsor]

 

[Theodore]

Hi Dana,
Boiler vents to clay tile chimney (with brick exterior that is exposed on 3sides) that has never had a problem in the 9years we've had this installed; and boiler ALWAYS serviced/cleaned annually. Chimney has two independent smoke conduits: one for oil boiler that is clay tile only, and one for fireplace that is lined with stainless steel pipe. Chimney inspected annually, but oil side is rarely cleaned because the chimney sweep says it looks fine.

I know that the 3-pass baffles are definitely installed. I see them in place every time it's cleaned.

My alternative to a fresh air kit is to leave 2 louvered "windows" in the boiler room curtain wall that open into the main basement room, so that it acts as if it's one big room.

All comments greatly appreciated.
Theodore

 

[Dana]

How big is the tile-lined flue for the oil burner? How far is it from where the boiler vent enters the flue, to the very top, approximately? (I suspect it's about the right size, if the annual inspection doesn't show any spalling or condensate erosion.)

Having the chimney on an exterior wall, isn't in your favor.

Got a ZIP code (for weather data purposes)?

If on a regular fill-up service, is there a "K-factor" stamped on the billing slips? If yes, what are the K-factors for mid to late winter fill-ups?

The primary factors for flue condensation are the temperature of the flue, the temperature of the exhaust, and the amount of dilution air in the exhaust.

The dew point of the raw oil combustion exhaust is pretty high, since a major component of the combustion product is water. Diluting it with much much drier (in absolute terms) air from the conditioned space lowers dew point of what's going up the flue to more manageable temperature. The dilution air is typically introduced with a barometric damper on the vent piping between the boiler and the chimney, and the amount of that dilution air is somewhat tweakable if you see evidence of condensation happening.

If the chimney is located somewhere in the interior of the house, not on an exterior wall, the flue doesn't need to be heated up as much to be above the dew point of the exhaust, which makes it inherently lower risk than an exterior chimney that cools off quickly between burn cycles. The smaller the cross section of the flue, the less cold surface area is in contact with the exhaust, which means it heats up more quickly, and the higher the stack velocity. Ideally the flue liner would be just barely big enough for the burner size, and not 2x.

The duty cycle of the burner also matters, since the higher the duty cycle, the higher the average temp of the flue liner, which lowers condensation risk. If the boiler output is no more than 1.5x the amount need to cover the heat loss at the 99th percentile outdoor temperature bin (the 99% outside design temperature) the duty cycle would be over 65% whenever it was really cold out. That would usually be enough to keep a terra-cotta lined flue warm enough to avoid major condensation if the flue is right-sized for the burner. But a duty cycle of more than 30-50% isn't very likely (even on the coldest days) unless yours is a very large or very leaky house: The MPO147 has a DOE output of 129,000 BTU/hr which would require an outdoor temp of -180F before it started to lose ground. Even if your heat load is 2x mine, it would still have a duty cycle less than 60%. But if the other risk factors align that's not a big deal. With the K-factor and a ZIP code it's possible to estimate the duty cycle it would be experiencing during the coldest hours of the year.

In my area installing an ~87% AFUE oil boiler without a right-sized stainless flue liner would be a code violation. It's not a cheap solution, but it solves a lot of issues: By right-sizing the flue the stack velocity is maximized, and the thermal mass of stainless steel is a fraction that of terra cotta liners, so it heats up very quickly during a burn cycle. Both the high stack velocity and low thermal mass minimizes flue condensation risk, but more importantly, (nearly) completely mitigates damage to the integrity of the chimney when condensation does occur, since it is condensing on stainless steel, which can handle the high acidity rather than soaking into terra cotta and mortar, both of which become degrade by the acidity.

 

[Theodore]

Hi Dana,

• Last time I was up there, I vaguely recall it was around 8" square, while my vent pipe (from the boiler and the chimney) is 8" dia round.
• From the boiler to the wall, it's a diagonal 6ft (4ft vertical, 5ft transverse) . Then it's 27ft vertical to the chimney cap.
• Zipcode = 10579
• The k-factor listed on every bill I can find (over the past 6+ years) is 0.000. I usually pay c.o.d. and purchase oil infrequently because I have two 220gal tanks; and I also supplement with a wood stove insert from time to time.
• I definitely have a barometric damper on the vent pipe (between the boiler and the chimney). The boiler is in the basmenet, on the exterior wall. The chimney runs up the same exterior wall.
• I completely understand the theory and value of a stainless liner. Given all the information I provided, I hope that you or others can offer a direction, but I would also understand if you that a fresh-air kit would be a "try-and-see-if-it-works-for-you" kind of thing. If the latter, I might say it's not worth the risk and just leave two open "windows" into the boiler room wall so that basment air from the main room can get to the boiler, as is the current condition.

All comments appreciated.
Thanks
Theodore

 

[Dana]

A K-factor would only appear on slips from a regular fill-up service. Wintertime fill up dates & quantities would deliver the same information, as long as the dates are EXACT, and the tank is actually filled rather than "give me 100 gallons, no more". If you burn through less than a cord of wood per year we can almost ignore that, but if it's 2 cords or more it's likely to be displacing quite a bit of oil use, so we'd have to factor that in. The as-used efficiency of a wood burning insert in an exterior chimney like that is well below it's manufacturer-tested EPA effciency rating due to the losses out the exposed side of the chimney, so we'd have to come up with reasonable fudge-factors on that.

The 99% outside design temp in Putnam Valley is about +10F, give or take a couple. (Poughkeepsie's is +6F, Newburgh's, +10F). This will become relevant in the boiler-sizing estimation once we know how many BTUs you burned through per heating degree-day, as well as the duty cycle to expect out of the burner. That's not a scary-low outdoor air temp

In Figure 16, p27 of the manual it indicates 8"x8" x 15' as the smallest allowable masonry chimney, and if 8 x 8 x 27' is indeed what you have, you're in pretty good shape as long as the duty cycle is reasonable. The cross sectional flue area of an 8 x 8 terra cotta lining is about 60-62 square inches (not 64 due to the rounding of the corners. The cross sectional area of an 8" round flue pipe is about 50 square inches. Is it really that big right at the boiler? (The vent output of the newer MPO-IQ147 is 6" round according to the manual, or about 28 square inches, less than half that of the terra cotta liner.) With 27' of height (almost 2x the minimum) you'll have better than the bare-minimum of stack draw, which helps.

So, your only real risk factor is the fact that it's an uninsulated exterior chimney, in which case I WOULD recommend trying it, but keep an eye on it. If in practice you see flue condenstation, the solution would be to either insulate the exterior of the chimney (complicated to do right), or pop out the 3 pass baffle. There would be an efficiency hit from removing the baffle, but it's not a huge hit.

Leaving windows open to the boiler room is the last thing you'd want to do, since that increases the heat load in your house The (probably ridiculously) oversized boiler would still be able to keep up, but burning 1.2-1.5x the amount of oil every year would be nuts, and not cheap.

Have you considered the concentric direct vent kit option (part number 102130-02 ) , and abandoning/sealing up the masonry flue? That would have the least condensation risk, and the maximum efficiency and would lower the net heat load, since it wouldn't have the parasitic stack effect losses. See Figure 2B, p.9 of the MPO-IQ manual (essentially the same boiler under the sheet metal, but different controls). As long as it terminates above the reasonable anticipated snow depth it's almost a no-brainer. With the direct vent kit the exhaust is side-vented without dilution with a barometric damper, and the 4" air duct comes off the concentric terminal, which pre-heats the combustion air slightly with the heat of the exhaust. Being stainless it runs ~$450 (from multiple online vendors), but it would pay for itself in short years compared to the increased load of open windows. Verify with Burhnam (or a competent tech) that it's still legit to use with the non-IQ MPO boilers, but I'd be surprised if it were a problem.

 

[Theodore]

Wait... what I meant by "open windows" is that I'd leave to openings in the curtain wall around the boiler room so that the air in the main room of the finished basement can get to the boiler room I'm making. This would be roughly the same as not having created a boiler room. I definitely will NEVER leave an open window to outside of the house.

 

[Dana]

Ahh, that makes more sense, assuming you really mean "partition wall", not "curtain wall" (<< an architectural term for an exterior wall that is not the structural support for the roof, common in multi-story commercial buildings.)

Even if you do nothing to the venting scheme, it's worth figuring out the oversizing factor on the boiler using oil use against heating degree-days to measure the heat load at +10F. If it's 1.7x it should hit it's AFUE numbers, even 2x oversized is probably fine, at a factor anywhere near 3x or more (I think yours probably is, just like most 120K+ output boilers), modifications such as retrofit heat purging controls are VERY cost effective, cutting fuel use by a double-digit percentage. The "IQ" in "MPO-IQ" refers to the smarter controls of the newer versions of the boiler, that use heat purging and other techniques for reducing standby loss, reducing the total number of burn cycles by lengthening burn cycles (better for reducing flue condensation too, BTW), which has a significant improvement on fuel use and wear & tear when the boiler is oversized. Pretty-good retrofit smart controls can be had for ~$200 or less, and most are DIY-able for those with electrician skills.

 

[Theodore]

Dana, thanks. Yes. I've decided that a fresh air kit isn't worth the potential for a problem. Even the manual says:

"Alternate method for boiler located within confined space. Use indoor air if two permanent openings communicate directly with additional space(s) of sufficient volume such that combined volume of all spaces meet criteria for unconfined space. Size each opening for minimum free area of 1 square inch per 1,000 BTU per hour input of all equipment in spaces, but not less than 100 square inches"

So I will install two louvers (hi/low) in the wall separating my boiler room from the rest of the basement. I will make sure that the free area opening of each louver satisfies the requirements. (side note: it's a little ridiculous that the building code requires 5/8" fire rated sheetrock and a fire rated door, only to then install 2 louvers opening directly into the room).

My MPO is the model before the IQ series. I don't mean to change the subject of the thread, but:
1. Is there a detailed guideline or tutorial I can follow along to calculate the oversizing factor you write about? I have all my years of oil bills. Note, I also have an indirect water heater connected to my boiler, in case that matters in the calcs.
2. Can you recommend a few examples of the "retrofit smart controls" you write about? I assume you're talking about something more/other than an outdoor reset control.

Thanks

 

[Dana]

A few months ago I wrote this bit o' bloggery that covers the methodology on how to use the boiler as a measuring instrument for calculating the heat load. Scroll down the boldfaced header that reads "You can calculate a building’s heat load in 15 minutes" if you want to cut to the chase.

The example case used therms of natural gas, but the methods are the same. The nameplate AFUE on that boiler is 87%. Assume a gallon of heating oil contains about 138,000 BTU/gallon of source-fuel energy, so net supplied to the heating system would be 0.87 x 138,000 BTU = ~ 120,000 BTU/ per gallon. Use only mid to late winter fill-ups, since winter is when the duty cycle is highest, lowering the error introduced by standby losses, and less solar gain error, and the impact of the hot water use is much smaller, etc. If it's a 2x4 framed house kept between 65-72F most of the time, use base 65F heating degree day data from a nearby weatherstation on degreedays.net. Get the daily HDD, preferably over at least a 2 month period. Use +10F as the outside design temp (even though we know it gets a lot colder than that in a typcial year, it is fewer than 87 hours per year on average- the definition of the 99th percentile temperture bin.) If you design EXACTLY to the 99% load you would still stay comfortable- it wouldn't lose enough ground to matter except during the rarest of polar-vortex weather patterns.

What you end up with is really an upper bound on the actual heat load, since efficiency on a boiler that old probably isn't really 87% anymore ( it could still be 85%), and the heating of the boiler room with standby losses increases the actual heat load.

Using the load at 10F as 99% load as the design load, you can then figure out your oversizing factor. A typical 2x4 framed house would have a heat load at +10F of about 13-15 BTU/hr per square foot of conditioned space (not counting the basement.) So a 2000' house with a full basement should come in around 26,000-30,000 BTU/hr, but if it's an uninsulated basement with a lot of above grade exposure on the outside it could come in as high as 40K (and even at this year's price of oil it's worth insulating the foundation). If it's over that it usually means the house leaks air like a sieve, has a lot of single-pane window area sans-storm window, or other low hanging fruit on the building envelope efficiency front.