Draft Settings

Hobbs

Home owner with a Beckett AFG forced hot air system. The problem I'm having is no matter what damper setting I use, the OF draft is always -.01 more, than the stack draft. If the stack draft is -.04 the OF draft will read -.05 if I set the OF draft to -.02 then the stack draft will read -.01. So if I'm understanding things right, my system is running under a positive draft.

STEVEusaPA

Make/model number of heating unit?
It's possible you're getting those readings, but it's not possible that's what is happening. You can't have a higher over the fire draft (negative) then at the breech, unless you are leaking products of combustion out between the heat exchanger and where you are sampling in the flue pipe.
How/where are you reading the over fire, same at the breech (flue pipe)?
What type of draft gauge/analyzer, and did let it warm up (if needed) and zero'd out?
I would have someone with another gauge/analyzer come over and do some side-by-side readings.
Maybe your test port in the flue is in the wrong spot (getting some turbulation), or your not reading correctly over the fire. I usually use a small copper tube extension to get it closer to the fire-and take the reading rather quickly!

Hobbs

Thanks Steve,
Hallmark lowboy LBD85 furnace. I recently barrowed an older Bacharach 0010-5022 Oil Burner Combustion Testing Kit in good condition. I'm using the MZF draft gauge that came with the kit with an 6'' brass tube extention and is zero'd out.

I'm taking my readings from the furnace after warmup through supplied peek port with brass bolt hole above flame in front. And from the rear, the breech pipe that's on a 45 degree angle and a previous drilled hole about 18'' up from furnace and about 12'' before damper.

Net stack temp is about 425, zero smoke

captainco

As Steve said this can't happen. The overfire draft is created by the outlet draft and can't be higher. Sounds like you are getting some kind of venturi affect. The blower could cause this if the burner airtube is not properly sealed to the combustion chamber but that is a stretch.

Hobbs

The furnace is 22 yrs old and when the furnace was installed back then the technician attached a ticket with all the readings but left the over the fire and stack readings blank for what ever reason. So when I took my reading I was kind of wondering if maybe it's been this way since then. Guess we'll never know.

The access panels gaskets to the firebox are getting worn from maintenance and such and didn't know if there would be any other reason for the odd draft readings other than a possible cracked heat exchanger or maybe leaking gaskets. Turning the blower unit on and off makes no difference to the readings. Not even any kind of waver on the meter.

I've blocked off the exhaust pipe and filled the heat exchanger compartment full of smoke and turned the blower on and peeked inside and saw no signs of any air movement anywhere. Then let the blower run for 20min with it full of smoke, but didn't smell or see anything throughout the house or at any of the vent registers.

The only thing wrong is the draft reading and that we can smell odors from time to time. Bought a co detector and a co meter awhile back and so far all is clear.

So I didn't know if that because the draft readings were off, the exhaust wasn't exiting properly and maybe that's why we get a slight exhaust like smell.

Just didn't know if anything else could cause the draft numbers to be off like this.

EBEBRATT-Ed

The only way overfire draft can be higher than the breeching draft is if your running + positive pressure in the combustion chamber. That would account for the smells you are getting.

Are you reading the draft gage right? is one reading on the + side and one on the neg side??

Just for something to try temporally to prove what is going on would be install a smaller nozzle and see which way the combustion test goes. This may tell something

Hobbs

What I'm getting is on average -.01 higher over the fire draft then whatever the stack draft is. So no matter what I set the damper to, the OTF draft will be higher than the stack draft. EX. OTF draft -.05.......stack will be -.04 If I set it the OTF to .02 then the stack draft will read -.01 The furnace calls for OTF -.01-.02 and a breech of -.03 -.04.

The furnace calls for a 0.65 80A nozzle. Would a 0.60 80A make any difference or do I need something smaller to try?

EBEBRATT-Ed

I would put a .50 in to try it just for the fun of it. have you checked the boiler to see if the heat exchanger is plugged with soot? Could it need cleaning?

I know you checked the HX for a leak but just for more fun from a cold start take draft readings quickly without the furnace blower running and then again with the blower on...see if they change. A small leak in the hx could cause blower (room air) to affect the draft readings

Hobbs

The readings are lower when first starting up and still off by the same amount. As it warms up so do the numbers go up but still not right. The blower has no effect on the draft numbers at all. There's no waver with the meter even with turning the blower on and off several times over a short period.

The heat exchanger is completely cleaned out. That's what started this whole thing. This fall the unit was smelling a little when the heat kicked on, so I did some google searches. Upon learning about oil burners and hot air systems I ran across some proper stack temps, draft measurements ect. Took a stack temp and it was over 650. Opened things up and everything looked pretty clean but I went through it all anyways. Still had upper 600 readings. Did some more research and everything I read said it's probably a plugged heat exchanger.

Open it back up and the only thing blocking my view of the complete viewing and getting to everything in the heat exchanger is a over the fire baffle assembly that basically diverts the heat to go through the heat exchanger channels more efficiently. That's held in place by a center arm suspended from the top of the fire box with a cotter pin. Pulled the pin ,dropped the baffle down and lo and behold half way up the heat exchanger is a tack weld baffle in between all the exchanger channels that was never cleaned out since the unit was installed and was loaded with crap, blocking off the top half of the exchanger. You would never know it unless you dropped the baffle assembly and obviously nobody ever has.

So ever since then I've been searching the net for info and came across this site a couple weeks back and read a lot of good info and then today I researched how to join.

EBEBRATT-Ed

Sounds like your doing a good job with this. I can't think of anything else to check. Since you tested the hx and the blower operation doesn't affect the draft I guess we can rule out a bad hx.

I would run-.02 over the fire and don't worry about the breeching as long as it is a-neg draft.

You gave us stack temp, draft and smoke. What is your Co2??

It would be normal for the draft # to be lower on start-up. The draft increases as the chimney warms up.

You might want to shine a light and use a mirror to look up the chimney. Just something to check.

Hobbs

I don't have anyway to check the Co2 readings ATM and I checked the chimney a week or so ago and all was clear. Yes the draft is lower on start up and increases with rising chimney temps.

The only way I can run a .02 OTF draft is to open the damper all the way and lock it there. With the weight all the way out I can get a max of about -.04 breech which leaves me with a OTF draft of -.05 when up to temp and that's where I've been running it.

I'll try running a new or different size nozzle tomorrow and see what happens.

captainco

Just for the fun of it check both drafts 30 seconds after the burner shuts off.

Hobbs

Draft.................................... OTF........................BREACH
furnace off cold..................-.01........................... -.01
first 30 sec ..........................-.02............................-.02
up to temp..........................-.05............................-.04
off 30 sec.............................-.05............................-.04
0ff 2 min..............................-.04........................... -.03

I tried a nozzle from HD 0.60 80A and for whatever reason it made a much larger and hotter flame. Tried adjusting the flame but within 4-5 min the stack reading were approaching 600 so I shut things down. Check everything over, tightness, electrode setting ect. re-installed it...... still was running an oversized flame and hot.
Removed it and put the old nozzle back in and I'm now back to square 1. Guess I'll have to order a couple of nozzles on line.