Efficiency and O2

mculik5

I have an oil-fired furnace. I've had it serviced every year since we bought the house in 2016 and have tried 4-5 different service companies because none of them have done a proper combustion test. I've always asked if they have a tester and they've all said, "Yes," but when they're in my basement they say "I left my tester at home; I've been doing this for 30 years and know what a good flame looks like."

So, I decided to buy my own test equipment and do the service myself. The equipment was $800 and costs about $50/year to maintain. Annual service in my area (NJ) costs ~$200, so I'll be net positive in 6 years (not considering any oil I might save from a properly set burner).

Here are my questions:

I tuned the furnace today to 13.3% CO2/3% O2, no smoke, 83.9% efficiency, 430F net stack temp, stack draft of -.04" WC.

The sticker on the appliance says 85% efficient.

1. What do you think of my numbers above?
2. Are the AFUE stickers generally an optimum number, where you expect to be slightly under, or are they a middle number, where you expect to be a little over or under depending on ambient conditions, etc.? Or a worst-case number, where you should be over?

Thanks.

heatdoc1

Looks like you will need to add some air. the co2 should be down around 12% on most oil furnaces. This will lower combustion efficiency but give you a cleaner burn.

DJD775

Combustion efficiency and AFUE are not the same.

captainco

The number is made up by adding off cycle savings. Your equipment is operating exceptionally well. The actual efficiency is lower than what is calculated because it doesn't deduct latent heat. It is important to get the CO reading also. Even at Zero smoke CO can be high, although with those readings I would be surprised.

mculik5

Thanks. As I'm reading more, seems like 4% O2 is the low-end for oil-fired equipment, so I need more air, to @heatdoc1's point.

For what it's worth, this oil-fired furnace is actually the backup to an air-source heat pump that heats down to about 20F. Since I have my own equipment now, I could easily adjust it now and then do another check when we got a cold spell and the furnace actually needs to work.

On the other hand, as I think about this, if I set it to 4% O2 now, when temps are higher, that might translate to higher O2 when temps drop and the air becomes more dense... Am I thinking about that correctly?

captainco

When it gets colder out the draft in the flue will increase. However the barometric will keep the draft at the equipment the same so the numbers should stay close. The 3% is at the bottom but if the smoke is zero it is okay. Of course if the burner gets dirty it could go to lots of smoke later.

STEVEusaPA

What’s the make/model number of the appliance and what burner?

EBEBRATT-Ed

@mculik5

The correct way to adjust the burner is to close the air until you just get barely a trace of smoke. Take a Co2 a note the reading. Then open the air until your Co2 drops about 1% or a little more. This will give you some margin when the fan gets a little dirty or the draft changes.


Your #s are good but you are close to smoking and need a little margin. Don't want to soot it up

mculik5

@captainco , @EBEBRATT-Ed

Re-tuned it tonight. 4% O2, 0ppm CO, 12.5% CO2, efficiency 82.9%. net stack temp 444F, no smoke.

I read that about the trace of smoke. What is the point of doing that vs. simply targeting numbers with the analyzer? I'm guessing it has to do with variations in fuel and combustion chamber design? So the smoke point will vary from appliance to appliance and the idea is to add extra air beyond the smoke point to lower the risk that variations over the season will cause it to soot up?

The reason I haven't done that is because there's no viewport on my furnace (just a tiny tube for over-fire draft that I have to lay on the floor to see through, and even then all I see is yellow), so getting a trace of smoke is going to take a lot of messing around and smoke tests. To get me close, at what O2 or CO2 % does smoke usually start?

Thanks.

SuperTech

The smoke test isn't done at the view port. It's taken in the stack. The CO2/O2 will vary depending on the equipment, draft and density of the air when it's done. That's why the best practice is to do it the way @EBEBRATT-Ed outlined. 4% O2 may or may not be OK. How are you determining "no smoke"? Are you using a smoke pump and testing paper?

mculik5

@SuperTech - Yes, I'm using a smoke pump and paper, and taking the measurement in the stack before the damper. My point about the viewport is I can't see the flame, so to determine trace of smoke is going to be a matter of adjust/smoke test/adjust/smoke test...

I'll go back down and do it properly today. I think I was just excited to use the analyzer yesterday.

Thanks!

STEVEusaPA

One of the main reasons to properly do a smoke test, is to determine which side of the stoichiometric curve you are on to get you to true zero smoke. Steady state, then proper draft, then smoke test to get you to true zero smoke. Then look at the numbers on the analyzer to fine tune your combustion.

HVACNUT

What's the excess air? It shouldn't be less than 25%. Any issues with combustion air to the burner? It's not in a tight closet?

mculik5

@HVACNUT - Excess air during my last tune - the one with the 4% O2 - was 26.1%.

Furnace is in my unfinished (and, fortunately/unfortunately not well air-sealed) basement.

captainco

Excess air and O2 are the same thing. Excess air is calculated almost the same as Air Free CO. Take the O2 reading, subtract it from 21 and then divide it into 21.

For years I have talked to people that said they looked at the flame. I asked how do you do that? They say they open up the inspection door and look at it. Does anyone know this changes the combustion 20% to 40%?

MikeAmann

Of course. You just basically opened a door (or window) into the combustion chamber.

I would add just a little more air to get the excess air to 30%.

EBEBRATT-Ed

nothing wrong with looking at the flame to get close before you put the analyzer on it.

but every once in a while you look at one and your eyes lie to you. the analyzer doesn't (usually)

mculik5

Thanks for all the help. I adjusted it again tonight.

Set it by trial and error to #3 smoke on the Bacharach card, which was 14.3% CO2 on the analyzer. Since #3 might be a bit more than "trace," I decided to take the CO2 down by 2.1% instead of 1% or 1.5%.

Landed on:

12.2% CO2, 4.5% O2, 27% excess air, 3ppm CO, net stack temp 444F, efficiency 83%, no smoke

captainco

You should be happy with those numbers.

mculik5

Thanks again for all your help. One more question...

I've read online that the fan for an oil-fired furnace should be set at 100 CFM per 10,000 Btus. My furnace is 0.9 GPH, which is ~104K Btus at 83% efficiency, so I need 1100 CFM. However, since this furnace is a backup for an air-source heat pump, and the heat pump (Bosch BOVA) automatically varies condenser capacity to match fan speed, I have to crank the fan speed up to 2000 CFM in order to get all 5 tons of heating capacity from the system. If I don't, I get reduced capacity, which means I have to switch to oil sooner and user more oil, which is $$$ compared to the heat pump.

So, my question is, if I leave the fan speed set to 2000 CFM, are there any issues with that other than a lower delta T at the registers (might feel drafty) and more energy being used to move air? I imagine that as long as I'm providing at least enough air to keep the heat exchanger from overheating, it really doesn't matter other than electricity usage and the fact that my wife might prefer warmer air coming out of the registers.

Thanks.

captainco

You will see a rise in the flue temperature showing you are losing efficiency.

mculik5

@captainco - You mean that increasing fan speed beyond the speed that gets me the rated temp rise on the sticker (60F in the case of this furnace) DECREASES combustion efficiency/sends more heat up the flue? How so?

captainco

It is called residence time. The same reason if you operate the blower on A/C too fast you will end up icing up. It is a fact that when your blower speed is too fast or too slow the amount of heat transfer decreases. You do lose more when you are too slow. In class we discuss how setting up the blower speed in heating for maximum heat transfer is too look for the lowest flue temperature.