combustion air requirements

Reply

  #1  
Old 01-09-10, 12:51 AM
D
Member
Thread Starter
Join Date: Jan 2010
Location: Colorado
Posts: 70
Received 0 Votes on 0 Posts
combustion air requirements

I've been chasing a yellow flame issue and may have stumbled across an obvious oversite by someone. My boiler is 96k (83k output induced draft) input sitting in a very small (approx 7'x7'x8'high) room with the only door being a weatherstipped door into the garage. One wall is adjacent to the garage, two to the living space and one (a half wall) is an outside wall. There has been no combustion air provisions provided anywhere in the room. The combustion air is, apparently, coming via infiltration from the normal leaks and openings. (Could explain my yellow tipping.)

I have been reading code requirements all night and it seems some of the openings required are huge (e.g. two 100sq.in. one high and one low, etc.)

I obviously have an issue I need to fix ASAP but I don't want to just starting cutting holes. I did read where the 2006 Uniform Plumbing Code allows for a single, high, outside opening sized a 1 sqin per 3kBTU. I'm concerned that an opening this large in the boiler room may allow the boiler room temp to drop significantly on very cold days. The other options are for two 100 sqin openings (one high and one low) into an adjacent interior rooms assuming they have adequate volume. I have this option available to me but the drawback here is now I'm pulling combustion air from inside the house causing infiltration through the doors and windows. Neither option seems ideal. Any thoughts?
 
  #2  
Old 01-09-10, 03:50 AM
S
Join Date: Jan 2010
Location: alaska
Posts: 49
Received 0 Votes on 0 Posts
Combustion Air Intake

Quick, cheap, and easy-
You will need a 4" hooded dryer vent with a bird/rodent
screen, a 4" 28-30 gauge galvanized adjustable elbow,
two 24" long 4" 28-30 gauge galvanized pipe, 5' length of
16" wide silver "bubble wrap" insulation, a small roll of self-
adhesive silver foil tape, plumbers strap, whatever screws
or anchors needed to affix pipe to wall, silicone caulk.
A 4 1/8" holesaw is handy for making a hole through the
exterior wall.

Find a place in the mechanical room, on the exterior wall
that would be suitable to make a hole to the outside, 12"
above anticipated snow level, and from that point on the
inside vertically 36-48" towards the ceiling away from any
plumbing.

Drill the hole, (dont hit a stud, wire, pipe!) remove the flapper
from the hooded vent and temporarily mount it in the hole on
the outside. Twist the elbow into a 90 degree angle and insert
it into the hood pipe, angled upwards towards the ceiling. Now
assemble the pipe, you want at least 24" or better for your
vertical riser but no closer than one pipe diameter to the
ceiling. Test fit the pipe in the elbow. If everything looks good
remove the pipe with elbow attached, tape all the seams,
wrap the pipe and elbow with insulation and tape all seams.
Remove the hood, caulk the back of it and reinstall it.
Install insulated riser, use plumbers strap and your imagination
to affix it to the wall. DONE!

Air will only come into the mech. room if it's being pulled by
the burner or draft of your heating equipment because cold
air does not like to rise by nature.
 
  #3  
Old 01-09-10, 05:04 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
First, the boiler has probably been back drafting your flue, very bad.

You can check to see if there is a combustion air kit available for your burner. The boiler crew should be able to advise on that.

You will need more than the 4" vent suggested above. Drawing air from the living space is what 90% of the combustion systems do, so not all that bad. The pros will be here soon.

Bud
 
  #4  
Old 01-09-10, 05:46 AM
D
Member
Join Date: Aug 2009
Location: USA
Posts: 4,946
Received 0 Votes on 0 Posts
It appears that there is no door to the living space from his boiler room. Only the weatherstripped door into the garage. Maybe a vent between the living space and the boiler room will do the trick, or a combination of that and a vent to the outside.
 
  #5  
Old 01-09-10, 07:26 AM
rbeck's Avatar
Member
Join Date: Jul 2007
Posts: 2,481
Received 5 Votes on 4 Posts
Follow this link to combustion air info.
Combustion_Air_Calculation
between this and the manufacturers info should be all you need.
The termination of the air intake should be at least 6" from the wall not against the outside wall. The purpose for this is air turbulence. When a wind blows against the wall the turbulence at the wall can be pretty high and may not allow the boiler to pull in as much as needed. A 6" clearance will be enough to get away from that turbulence.
 
  #6  
Old 01-09-10, 04:39 PM
D
Member
Thread Starter
Join Date: Jan 2010
Location: Colorado
Posts: 70
Received 0 Votes on 0 Posts
I improved the situation considerably. I provided a 7" directly from the outside, 12" above the ground, over the concrete stem wall and turned down to within about 12" of the floor. Using 1sqin per 3000BTU and 96k BTU input I calculated slightly over a 6" diameter required. According to a local code expert the local code allows the 1 sqin/3000BTU calculations. Bottom line is there is much more air now available than before but the flame still has yellow streaks. I do know the orifice size is #45 as it was shipped from the factory according to Burnham (I measured using hole gauges). According to Burnham this orifice was spec'd for 0 to 2000 feet above sea level. Since I'm at 5,000 the #45's may be too big.

Using a light and a mirror I was able to look up into the exchanger. The finger, while they looked like they had been exposed to heat where not overly dirty. They appeared to be red or rust colored.

I measured the gas flow and it is to spec. I have not yet measure the manifold gas pressure or the blower pressure yet but will get that done tomorrow.

I may try to get some film footage of the flame and post it so everyone can see what I'm seeing. Someone suggested the yellow may be caused by dust.

Thanks for everyone's help.
 
  #7  
Old 01-09-10, 06:37 PM
NJT's Avatar
NJT
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 3 Votes on 2 Posts
If you put a bucket under that intake pipe, with the pipe extending into the bucket, you can create a sort of 'heat trap'... the bucket will fill with cold air... and similar to what the other poster said about running the pipe up the wall, it will minimize air flow until the boiler fires up...

(plus, the bucket is a good place to chill some beers)
Beer 4U2
 
  #8  
Old 01-09-10, 08:43 PM
S
Join Date: Jan 2010
Location: alaska
Posts: 49
Received 0 Votes on 0 Posts
Cold air will siphon into the mech. room if you elbow down.
We want to barometrically trap the cold air with a vertical
riser so that air is only drawn in from the outside equal to
the amount that it is drawn or forced to the outside by the
heating appliance. Cold air is more dense than warm air,
therefore cold air falls and warm air rises. When cold air
exits the top of the pipe, it pushes the warm air towards
the floor thereby helping draft the appliance.

I helped a friend today who was having a problem with a
gas water heater. The sidewall, non-induced vent was
allowing cold air to siphon into the house past the draft
hood when at pilot. The oil drip stove and woodstove
depressurizing the house wasn't helping either. It took some
convincing to get past his "flat earth" mentality, but since
he wasn't paying for it, I got the thumbs up to install
exactly what I've been trying to explain here. Immediately
upon completion, the water heater drafted properly as
evident by the warm vent and water vapor exiting the
vent outside. The house even felt warmer because air was
now being drawn in at a control point, instead of through
draft leaks all around the house.

I live in interior Alaska. Our design tempurature is 40 below
and the wind always blows. I've installed hundreds of air
intakes in this manner, and I only install what works.
 
  #9  
Old 01-10-10, 06:28 AM
rbeck's Avatar
Member
Join Date: Jul 2007
Posts: 2,481
Received 5 Votes on 4 Posts
If you are just chasing orange steaks in the fire it may not be an issue. Streaks many times is just dust in the combustion air or dirty burners. Lack of combustion air is usually yellow tipping.
The air intake pipe I call an elephant trunk, goes down to the floor, 2 elbows to create a "U" back up the wall about 1/2 to 3/4 of the way and again 2 elbows. This traps the air and only comes in when the room goes into a vacuum.
 
  #10  
Old 01-10-10, 08:52 AM
D
Member
Thread Starter
Join Date: Jan 2010
Location: Colorado
Posts: 70
Received 0 Votes on 0 Posts
Thanks to all. NJ, I do have the bucket under the vent pipe. I don't yet have the beer in it, however. Beer 4U2

It seems like what SuperSteve and rbeck are suggesting is pretty much the same thing - taking advantage of the more dense cold air so it lays in the bottom of the vent/outside until pulled in by the boiler. I would imagine an inspector would, perhaps, frown on this given how the spec is written but then an inspector approved the installation in my house and it HAD NO COMBUSTION AIR PROVISION AT ALL!! Since it is a normal sheet metal elbow I can as easily point it up as point it down but I will lose the beer cooler if it's up. I'm going to point it up and put the beer in the 'frig.

rbeck, it may be orange steaks. Not being a pro at this stuff I really don't have much of a baseline from which to compare. It is not a hard blue flame like a saw when I installed my neighbor's new water heater a few weeks ago. About this same time I did have the burners out, brushed them off and vacuumed out the fire box. The floor in my boiler room, while I wouldn't necessarily eat off it, is not terribly dirty. It's concrete and I sweep in down when the mood strikes me. I'll trying a vacuum the floor and see if that helps. With snow all over the ground I don't expect there is as much dust in the air as there would be in middle of a hot dry summer.

I'll check the two pressures I mention below and see if I can post some fire footage to the web. I also am trying to lay my hands on a combustion analyzer which, I am assuming, will give me the quantitative data to say if things are good or bad.

Any comments about derating orifice size for altitude? A Denver orifice firm was pushing this idea but then they sell orifices for boilers.
 
  #11  
Old 01-10-10, 09:02 AM
D
Member
Thread Starter
Join Date: Jan 2010
Location: Colorado
Posts: 70
Received 0 Votes on 0 Posts
SuperSteve, when I installed my vent I didn't pay much attention to sealing the seams other than to the outside. I figured with the huge 7" round hole why would I be concerned if the sheetmetal joints were tight. You also suggest wrapping the pipe with insulation. I'm assuming that is to futher minimize the cooling affect of have the cold pipe in the room.

My vent comes out of north wall. Over the years my wind break has grown to provide a pretty decent barrier to the predominant NW winter winds. Also the portion of the wall where my vent exits is shielded to the west by a wall section that runs north and south. The local supplier only had an air vent that lays flat on the wall so I did not extend the pipe the 6 inches as rbeck suggested. This can always be done later and easily. The big thing was to get a hole in the wall without cutting a bunch of wiring in the process.

Thanks again.
 
  #12  
Old 01-10-10, 09:18 AM
NJT's Avatar
NJT
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 3 Votes on 2 Posts
It probably is not a bad idea to persue the question of the orifice change for high altitude... I would anyway...

I like the idea of the elephant trunk... I'm sure it's way more efficient at keeping out the cold than the simple bucket is... the only thing in error with Steve's original suggestion was that the 4" he recommended was not big enough. You've gone to 7" ? So that will be fine. I can't imagine how an inspector could have a problem with it... but then inspectors can be 'weird' sometimes!

(Like when he would not pass an electrical panel install because the green wire to the ground rod was 4ga... and his book said 6ga... don't inspectors know about the 'code plus' concept?)
 
  #13  
Old 01-10-10, 03:05 PM
D
Member
Thread Starter
Join Date: Jan 2010
Location: Colorado
Posts: 70
Received 0 Votes on 0 Posts
I like the elephant trunk as well but in miy little room space is at a premium.

The film was a bust. The camera won't focus on flame so things were pretty much a blur.

I took the measurements I planned, however. The power vent fan is pulling over 1" wc against a spec minimum of 0.4" so I'm good on that end. Of course pressure doesn't guarantee flow but indications are, looking at the vent outlet in the current cold temp, I'm getting pretty good volume as well.

I hooked my homemade manometer up to the manifold. My initial reading was 3"wc. I knew from a previous measurements my gas flow was 4.7 cuft per minute versus a calculated target of 4.8 (at the rated input of 96k). I tweaked the pressure up to the labeled pressure of 3.5"wc and, of course, the flow went up as well -way beyond 4.8. It was implied in the manual that flow trumps pressure so I backed the pressure back down until the flow was 4.8. (roughly where is was when I first started at 3"wc.). So my question after all of this is if flow trumps pressure and I knew my orifices were the right size, was taking a pressure reading even necessary? It seems the orifice size defines the manifold pressure and if the orifice is right and the flow is right the pressure is what it is - in my case 0.5"wc below the value stamped on the boiler's tag.

Further, if the orfices are right, the flow is right and a combustion anayzer indicates the gases are right, aren't things as good as they can get - yellow flames and all?

The reason I ask is the Burnham tech I talked to, when I explained my concern about yellow flames, was adament about having manometer readings to review. He insisted I call him back after I had a manometer hooked up. It wasn't clear if he was concerned about the power vent or the gas pressure, but our conversation was on hold until I had the meter hooked up to something and things were running.

NJ, I'll start another string about derating the orifice size for altitude.

Thanks.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: