Soot Coming Out of Furnace

mculik5

We have a new (installed October 2018) Armstrong oil furnace. Our primary source of heat is a Bosch heat pump, and we switch to oil automatically when the heat pump can't keep up.

Towards the end of the 2018/2019 heating season, I noticed a bit of soot on the floor and horizontal surfaces around the furnace, but didn't do anything about it because we were almost done with heat for the season.

When we had the technician out for annual maintenance this fall, he said we had a lot of soot build-up in the exhaust pipe for just one season of use. He cleaned it all out and also vacuumed up what was around the furnace.

The soot is now back, although when I look through the damper, it's not caked on the exhaust pipe.

Both the installer (who we were very happy with) and the service tech used Testo instruments to set the burner. The service tech noted that the air adjustment screw was loose and said this could have caused the mixture to go off due to the vibrations of the motor over the course of the first season. I watched him tighten in, so I don't think the mixture has moved from what he set.

I think the soot blows back into the house when the chimney pressurizes, but the question is, what is causing so much soot in the first place? We didn't have this problem with our old furnace.

Any ideas?

One thing to note - because of the automatic heat pump/oil switching and the overall thermostat logic, the oil heat runs for relatively short periods of time (sometimes just 3-4 minutes). Thinking that the burner needs to run for a bit to heat up and clean up, I've started setting the thermostat to oil-only on colder nights. We haven't had enough cold nights for me to say whether this is helping. That said, I have a diesel heater in my workshop that cleans up almost immediately after the first (very minor) puff of smoke when it starts, so I'm not sure if the short run times make that much of a difference.

Lastly, for what it's worth, I add HOT 4-1 to the oil in my tank.

Again, any ideas about the soot? Thanks.

STEVEusaPA

What were the combustion numbers from the tech that set up the burner with the analyzer.

HVACNUT

Yeah, knowing the combustion test results will help.
What model burner?
What's providing DHW? Oil fired or electric?
Did the tech do a smoke test and check/adjust draft?

Obviously if theres soot, there's incomplete combustion.

The tech needs to brush and vacuum the HX, flue pipe and check/clean the chimney base. Check to make sure it's the correct nozzle and the correct pump pressure.

Adjust for 0 smoke and proper draft. Test at steady state. Make adjustments if necessary. Check smoke and draft again.

Steamhead

@mculik5 , check to see if the chimney is partially or completely blocked. A chimney should never "pressurize" as you say in your first post. There should always be a slight suction, called "draft", which is what pulls the exhaust out of the house. If there is no draft, or not enough, the burner won't be able to move air into the firing zone, which will cause soot from incomplete combustion.

You'll probably need a chimney contractor if this is the problem. They will likely install a stainless-steel liner in the chimney. Once this is properly done, it will last longer than we will. You'll need to have the burner tuned again once the chimney work is done.

SlamDunk

We have an armstrong oil furnace that was installed at the same time. Nice furnace. Uses a Beckett NX burner. You need to check chimney for blockage as @Steamhead said. Especially now that you found a soot problem. Then, Make sure there is enough outside air infiltrating basement for combustion.

Our installer was shady and didn't use a combustion analyzer for start up. When I flew up, I had to dig the furnace, stack and base of chimney out of soot.

If you have the same burner, it is a joy to start up-provided everything else is in order.

mculik5

Thanks for the responses.

The furnace is an Armstrong L85BR1V104/118F20. Burner is a Beckett NX. Hot water is electric.

The tech didn't do a smoke test or test pump pressure. Not sure if he checked draft. The combustion numbers were:

- 5.7% O2 / 11.43% CO2
- 718.8 F exhaust gas temp

I watched the tech clean the HX, entire flue and the base of the chimney. We also had a chimney sweep at the house a few weeks prior to the tech and he said the chimney was in good shape.

Regarding the chimney pressurizing, I'm not talking about when the furnace is running - I'm talking about if the pressure in the chimney exceeds the pressure in the basement (such as when the wind blows and depressurizes the house) so that air flows down the chimney, taking the soot with it and blowing it out the gaps in the damper.

Lastly, our house was built in the 80s and the construction is average, so I'd be VERY surprised if the basement is not leaky enough to allow for sufficient combustion.

Is there any issue with the burner cycling on/off a lot and only running for a short period of time?

STEVEusaPA

Burner not set up properly. Flue temp is way too high.
Draft and excess air probably too high also.
Sounds like your tech is not very familiar with setting up a burner properly, and less familiar with an NX.

SuperTech

Stack temperature is way too high. 5.7% O2 may be ok, but it depends on the draft and smoke number. I don't see the CO measurement. PPM of CO is a crucial measurement. Needs to be as low as possible. Pump pressure and nozzle selection is critical in setting up the NX burner. If it's not right the unit will become full of soot rapidly.

STEVEusaPA

Also the zeroing out the head and having it properly adjusted makes all the difference. 1/8" takes it from perfect to bad.

Jamie Hall

You mention that the pressure in the chimney sometimes exceeds the pressure in the basement -- I presume you are aware of this because you get a downdraught through the chimney into the basement?

No. That is not good. A properly constructed chimney should never downdraught, unless there are some big fans in the house. Is it too close to some tall trees? Is it to short and below the ridge of the house or the roof near it? Something is wrong with the chimney, and even though it may be clear, it isn't right. A really good chimney person should be able to identify the problem -- it's in the placement and height of the chimney -- and be able to fix it.

SlamDunk

I have the same model except "UF". Below is a cut and paste from a discussion I had with this group. These are the before and after combustion numbers and if you wish, you can review the entire thread here:

https://forum.heatinghelp.com/discussion/169605/smoke-pump-question#latest

But here are the numbers cut and pasted:

As found numbers: stack temp 700*; O2 3.5%; CO 4ppm; CO2 11.2% ; Excess air 27%. Efficiency 78%; draft +0.008inwc. Burner air setting found at 3.

As left numbers: stack temp 503* ; O2 5.5%; CO 1ppm; CO2 11.8%; Excess air 30%; efficiency 83.3%.; draft -0.02 to -0.05inwc.
Burner air setting left at 4.25

Note, the stack temp was after a long run while tuning. During a typical cycle, it is in the low 400's.

Hope it helps- and one more thing. I forgot to drive the head completely forward to click into detent. I have read this is done at factory but it is on my list of things to check when I go back up. There is a good Beckett training video on you tube.

I would say yes, there is a problem with short cycling. Mine is on 15-20 minutes and off twice that time and to date, we burn an average of 115gallons/month in NYC.

mculik5

Thanks again for all the responses.

One note on the chimney pressurizing - I've never "seen" this happen, but I don't know how else (or from where else) soot would get into the house. The only physical opening to the basement between the combustion chamber and the chimney is the damper.

That said, I have a hard time believing it's a chimney problem because we never had soot with our old furnace. The chimney hasn't been touched; the only differences between our old and new furnace are configuration (old = highboy, new = lowboy w/ coil on top) and output (old = 125K Btu, new = 118K Btu).

Steamhead

Have the chimney checked- that way you know for sure.

HVACNUT

You will get a change in pressure on ignition. You might notice the draft regulator swing on ignition.
Regardless, there shouldn't be any soot anywhere.

Delayed ignition, that's a different story.

SlamDunk

This burner/furnace will make soot like snow machines make snow. We realized we had a problem when we found a large pile of soot on the floor beneath the damper. I opened the heat exchanger clean outs and soot spilled out. That is probably why your stack temp is 700- no heat transfer, soot is an insulator.

It blew my mind. I thought the chimney would have to be lined because it lacked draft. A reasonable conclusion because it is three stories tall, unlined with a more efficient furnace. But, luckily, it turned out to be a burner tuning issue. you still have to look inside the chimney , stack, and clean outs. The stack temp says you do.

SlamDunk

Here is what i found before tuning. Stuck phone into damper.

SlamDunk

This is temp gage two months later. Stack was as clean as I left it. The temperature is telling you something.