Hello Grady,
Can you please educate this poor DIYer?
What is the procedure for testing stack temperature?
What I have been listing is the absolute temp. Room Temp is 62*
Here is the data:
Cycle time / Temp *F
0:00 = 120 (had just ended a cycle, it's 25*F @ 25 MPH outside)
0:30 = 300
1:00 = 355
2:00 = 394
3:00 = 416
3:42 = 435 = Fan start
4:00 = 440
5:00 = 449
6:00 = 462
7:00 = 474
8:00 = 486
9:00 = 495
10:00 = 500 Burner then off
32:17 = 120 = Fan off

The heat rise flattens out after the fan starts but will continue to rise over time as will the temp at the registers. Fan start = 120*F burner off = 150*F
If the stack temp and register temp continue to rise, am I over-firing the furnace? I know we have to consider the thermal mass of the furnace and ductwork and the fan speed but since it takes the fan 3 times longer to extract the energy the burner imparts, the stack & heat exchanger temp will always rise during the burn. I am at a loss as to what to list as the "stack temp."

Thanks,
Your humble padawan, Don

P.S. There is a sticker from a furnace repair guy dated 1984
Warmup time = 7 min
stack temp = 600
CO2 = 8%
Smoke spot number = 2
Draft = .06
Steady state Efficiency = 71 3/4

 

At 10 minutes, your stack temp should be pretty well stabilized. When figuring efficiency, you want to use NET stack temperature which is the measured stack temp less the room temp.
I would expect different results with your modified burner. You want the overfire draft to be around -.01 to -.03" W.C., zero to no more than a #1 smoke, & CO2 of +/- 10%. Efficiency drops rapidly as stack temp increases.

 

Don, You should put a draft regulator on the pipe and bring it down to -.o4.

 

Grady,
Here is a chart of the data so far. It's pretty easy to see that the stack temp just continues to rise even after 20 minutes. So my question is; what value do I select when it is continually rising.

I must be really confused.

 

I understand your frustration but you seem to be overthinking it a bit. Believe me, I'm super guilty of the same thing.

On series 3, it looks like the temp virtually levels off after about 18 minutes. In series 2, the temp was, for all practical purposes, flat after 10 minutes. The difference in temp in the 10 extra minutes is not significant.

There are a lot of things which can affect stack temp. Likely one of the biggest is draft. The longer the burner runs, the warmer the chimney gets, the more draft is created, the higher the stack temp goes, the warmer the chimney gets ...
Thus, as guyold pointed out, the need for a barometric damper (draft regulator).

 

Hello Guys,
I just ran another series and the stack temp continued to rise past the 20 minute mark 2.6*F per min until it cycled off at 38 minutes with a 575 stack temp and 163* register temp. So the slope you see between 10 & 20 minutes continued.

I have thought about a draft control, in fact, I asked about installing one quite a while ago in this thread. I suppose it's the next step. One thing I'm sure of, it's a heck of a lot more efficient than what that 1984 sticker shows. Back then, when I bought the house, it used 2 times the oil than it does now. Yes, I have records from that long ago showing the results of my fine tuning to save some money. That first winter was "sticker shock" to say the least.

The furnace has been running so good these last days except for Tuesday. I came home from work to find the warm up time at 6:00 min w/ stack temp at 570*F. The anticipator was shutting down the burner BEFORE the fan came on. I looked at it and couldn't figure out what went wrong. Everything looked good but I adjusted the air down to 30% (just for you guys) and it settled back to 3:40 warm up time which it has stayed at ever since. So temperamental it seems which is why I'm tracking cycle times and temps today.

I'll order a draft control and get it installed ASAP.
Any suggestions for brand / type?

Thanks,
Don

 

Just curious, Don, how & where are you measuring stack temp & what is happening to the ambient air temp at the burner air inlet at the same time?

Remember, we are talking about net stack temperature.

 

Hi Grady,
Ambient air temp is almost static temp wise, basement air @ 62*F. I'm measuring the temp with an ACT 7000 digital pyrometer good to 1000*F. I'm sampling the stack at the breaching hole 5" from the furnace outlet. I include the furnace room in the return air path to utilize the waste heat from the furnace so the ambient air temp doesn't rise that much during a burn. It's only 64* upstairs anyway. Outside air temp is now 27*F at 16 mph (better than yesterday). I get it though, I should plot the NET stack temp.

Checking just now, burner inlet air at cycle start is 61*F, at burn cycle end (10 min) it's 62*F

 

OK, the intake air is a non-issue.
The reason I asked about how & where you were measuring the temp is a lot of folks these days are using the lasers. They aren't worth a plugged nickel for what we are trying to do. The equipment & location of your sampling are quite good.

 

Hi Grady,
Been thinking about this darn data, please check my reasoning here.
1. In general there is an optimal burn time that produces the best efficiency.
Too short and it cycles too often doesn't get enough energy into the house it
goes up the stack.
Too long and the high stack temp lowers efficiency and again the waste goes
up the chimney.
2. Since there must be a calculable point of diminishing returns, it would be based on the thermal mass of the furnace, the best firing rate afforded the design / size. The thermal loss of the house at normal seasonal temps giving me the heat input the house requires from the furnace. Am I safe to assume short cycles that end with lower stack temp will be more efficient?

3. A complex calculation but the crossing point can be determined by the layman by measuring the burn times per day over the average outside temp per day. Increasing the firing rate (shorter cycles) then decreasing (longer cycles) and compare.

My mother-in-law's house has a Becket retention burner fired at .5 gph. Her house is about the same size as mine. The furnace burns for 40 minutes per cycle in this weather and it takes a long time to heat the house. It doesn't seem all that efficient and burns ~500 gal / season. The variables of insulation and greater heat loss, being located out in the east county with the incessant east wind coming out of the Columbia gorge, not being accounted for. By comparison, my house burned 275 gallons last year.


Looking at my data, with outside temps ~ 45*F the furnace burns 6 min / hr.
Today at 28*F it's burning 12 min / hr (8 min / 40 min cycle). That is ~ 4 gallons a day at this temp. Although, the average for last month (same temp range) was 2 gallons a day. When temps are in the upper 40's low 50's it averages 1 gallon a day.

Setback:
It takes a 40 min burn to raise the house up to the evening temp from the setback. That is essentially 7 hours of normal temp maintaining burns. So it makes no sense to use the lower setback temp if the time is not appreciably longer. Say 12 to 18 hours. 8 hours during sleep would be a wash.

I guess I am still thinking about this, maybe over-thinking again.

Don

 

On the surface your reasoning seems sound but the "experts" say short cycling is the least efficient way to run a furnace. In my 30 or so years I've been in the trade, I've always have been taught the most efficient way to operate is a steady run.
According to these experts, a properly sized furnace will run constantly on a "design day". A design day is one when the outdoor temperature matches that for which your furnace is sized to maintain the home at the indoor design temperature (usually 70*). In my area, the outdoor winter design temp is 18*, which in my estimation is way too high. I have experienced an entire week during which the temps never hit double digits.
Here is a link to the ACCA Manual J which is THE standard for residential heating & cooling load calculation. https://www.acca.org/wp-content/uplo...Conditions.pdf

Something engineers have done in recent years "to increase efficiency" is to bring the fan on much sooner & have it stay on longer. This is supposed to transfer more of the heat from the heat exchanger to the space being heated. By doing so they have reduced heat loss up the chimney by maintaing a lower heat exchanger temperature. The downside is customers complain about lower air temperatures from the registers.

 

Hi Grady,
One of the first things I did was lower the fan's "off temp" to 80* and "on temp" to 120* My fan control has 2 speed capability and whomever set it up for the previous owner really missed the mark. Especially since it had a one speed motor. A high off temp means a too hot basement at my house.

I read that too some time ago so I tried a low firing of my furnace. It didn't work out very well, although, I ended up somewhere in between that and where the previous owner had it. It could be that because the stack temp gets pretty high on a long burn that my situation does indeed work better cycled. I would have to under fire the furnace significantly to have a stable long burn stack temp. As I said the burner fired at .85 gph puts in 3 times more energy in than the blower fan can extract over any period of time. The heat exchanger would require extensive modifications to obtain that goal. That being said, I have thought of raising the cfm of the blower to try and level off the exchanger / stack temp or at least flatten the curve more.

The experts are more than likely talking about an "ideal" situation with a modern furnace (or better). A small fire with a continuous burn with a low thermal mass and high surface area heat exchanger. Some of the ones I've seen are not continuously fired, they are pulsed to give it a duty cycle. Unfortunately my pulses have to be longer and further apart to optimize the use of high thermal mass and the limitations of the exchanger to transfer heat to the air. I am looking for the optimal settings for my furnace and my ideas / analysis are based on only my experience with these two oil furnaces. It's a calculus problem of finding the zero point so to speak. I've tried the extremes and they are both inefficient so now I founder around on middle ground looking for nirvana. :-)

I dunno, I doubt everything and double check everything I or anyone else says or thinks. This furnace is making me crazy so my wife says. My low firing test were done with the old burner head so maybe I should revisit the idea. I'd have to buy some smaller NS nozzles maybe .65 gph coupled with a higher cfm fan speed. Then maybe.....................

 

These things will make you crazy & can be addictive. NJTrooper, over in the Boilers forum, came here several years ago with a problem with his oil fired boiler. Look at his post count now. Talk about an addiction.

You are absolutely correct about the thermal mass of your furnace vs. the new ones and what the supposed experts have to say. Few, if any, of them live in the real world.

I know how to reduce your stack temp but your basement would get way too hot.

Sometimes I just have to remind myself of the Serentiy Prayer simply to keep what little sanity I have left.

 

Fields draft regulator is the one to get. They have cheaper ones but very low quality.
When you install it get as close to the chimney as you can so you have more controlled draft.

As far as where you take your readings for temp and draft etc, they recommend 18" from furnace if possible, if not then a couple inches before draft reg. With the draft reg you'll see the stack temp level off.
I believe why yours keeps rising is you've got wild draft and the more the chimney hears the more draft is created andthe more it will suck the heat out of the furnace. Controlling the draft is part of controlling the stack ttemp.

One thing I noticed you said was your off fan temp of 80°.
Your body temp being 98°, anything lower than that feels cold to skin. Would that be counter productive after heating up the air to cool it back down.
Just a thought.

Good Luck,

 

Hello Spott,
Since I'm only heating the air to 66* an 80* turn off is high energy wise but the chill factor for moving air makes it somewhat neutral feeling unless you just stepped out of the shower. As Grady said, it's a compromise between efficiency and comfort. Body temp is 98.6* but skin temp is much lower.
When they were younger it took a while to get the kids to layer clothing during the winter. I'd come home from work and my son would be in his basketball tank top and shorts and my daughter in boxers and a wife beater T with the heat set to 75*. Well, I was at my daughters house last week and she had a picture of me on her thermostat. "That's to remind me to turn it down" she said. Nothing like having to pay the bills to make a believer out of her.

Thanks for the Fields brand tip.
Also I will need a draft gauge, any good yet cheap recommendations there?

I still need to borrow that analyzer from work............

Don

 

Do yourself a favor and get a smoke pipe tee with your 8inch reg..Cut that in.

 

Don,
Bacharach makes draft gauges. There is one that's included in the kit but they also make a less expensive pocket one that's handy and probably all you need.
If you go on eBay and type in draft gauge you can see examples.

Fields makes a couple different regulators. The one to get is the RC model. You can compare and you'll see the difference.

If you have access to a combustion analyzer it probably is included.

My compliments on all your work

 

Thanks for the advice everyone,
Fields 8-RC on order. I also threw in a Dwyer draft tester. What the hey, not too expensive. Kinda reminds me of a old school PCV valve tester or a Uni-syn.

Anyone know at what tip speed forward curved centrifugal fan blowers create too much noise?

Don

 

Hello Everyone,
I decided to do some more documenting before I install the draft control. I did two 20 minute burns and documented the stack and plenum (above the heat exchanger) temps. It taught me a lot about how it works. For these series I increased the circulating fan speed by 25% = 1390 ft/min tip speed. The increase in air flow lowered the max register temp but did nothing to flatten the stack temp curve when running lean. Running rich, however, the temp rise was nearly flat. All readings are absolute temp, they are NOT net stack temps.

In this first chart you can see the original series from last week compared to a leaner burn and a richer burn (black tips of flames above the chamber). There is a little stack temp rise at the end of the rich burn when I opened the inspection door to document the flame's appearance. My impatience screwing with the conditions of the test.





In this chart you can see the plenum temps from the lean vs rich burn. It's interesting that the lean burn imparts more energy into the exchanger for the first 5 minutes, however, the rich burn imparts more in a long burn. This has to be the effect of excessive draft build up after 5 minutes. BUT, it also suggests that a leaner burn with controlled draft would be more efficient. It also seems to verify everything I've read. The rich burn is less efficient early on because the chamber stays cold longer. The leaner burn concentrates the heat lower in the chamber so it heats up faster and thus burns more efficiently quicker. Since historically my furnace has been cycled at 6 minutes per burn, the leaner burn becomes overall more efficient. However, on long burns during cycles heating up the house from the setback temp, the richer setting is better.
Now I'm looking forward to getting that 8"-RC draft control installed. and optimizing all of these settings.



Don

 

Hi Don, bumping you up to 771/4 eff. for all the improvements you done, nice work.Put that reg. in and will go 78+. Seal off the leaks around burner to combustion chamber,closer to 80%. Good luck and keep us posted.

 

I just had a crazy idea, suppose I put a solenoid on the burner air damper. Have it activate at a certain stack temp or possibly by the circulation blower control. Make it adjustable so the furnace initially fires up lean until the chamber is hot then richens up by the solenoid activating and rotating the damper. Maybe 10% less air. Best of both worlds................

 

Might help but if you get too rich, your short term efficiency will go up but long term will go in the crapper due to soot build up.

 

Hello Guys,
You'll need to explain this to me.
Draft test results:
minute/over fire/stack
2/.055/.03
4/.055/.04
10/.065/.045
13/.07/.05
16/.065/.05

No I do not have those figures reversed, the over fire draft is greater than the stack draft. I double checked for venturi effect by reading the over fire draft as I turned off the burner and it read the same then dropped as it cooled off. I tested at an opening between the chamber and the blast tube and by swinging the inspection door open a bit (didn't matter how far) and sticking the tube in. Different tube angles over fire and in the stack did not vary the readings.

OK, so now I'm really confused. Stack temperature rises during the burn but the draft essentially does not?

On Edit: I suppose it's possible that the additional draft over the fire is from the contracting gasses as they cool. Sure easy to see how opening the inspection door causes a 50*F rise in the stack temp.

I also checked the static pressure in the blast tube at the present adjustment.
.25 WC or about 50% of what Becket says they run their retention burners at.
With the air at 100% it's .4 WC. I'm wondering now if I could fit a retention tube to my burner and it would work.

 

I presume the fan was on with all readings?
I wonder what kind of readings you would get if you held the fan off. I realize the furnace would hit limit pretty quickly but you might get enough run time to see if the fan had any influence on draft.

 

The first two readings were with the fan off. Warm up time is around 3:40 to 4:00.

I just checked at the end of the blower cycle (16 min after burn), stack temp at 120*. Over fire = .007 / stack = .004. Yes, the over fire is still greater than the stack. Turning the blower on and off does not change the readings with the furnace NOT in the return air path. There is a drop for about 2 ~ 3 seconds if the furnace is in the return air path, presumably while the pressure in the furnace room stabilizes.

ON Edit: Might this be why my furnace was not equipped with a draft control when installed?

 

You have me stumped. Maybe guyold will jump in with an idea or three.

 

My guess is too much air/lean. Try to prop the observation door,cut out cardboard over sized to opening and stick tube thru deep as it goes. See what you get.

 

Hi Guys,
More perplexing results today. I installed the RC-8 draft control and with it set for a draft of .02 the Dwyer gauge reads .04 in the stack and .05 over fire. I added some weight to the RC-8 so I can adjust to a lower draft. I set it for .03 according to the Dwyer which gave it .04 over fire. I then ran a thermal test and there was NO DIFFERENCE in stack temperature rise or the temp rise in the heat exchanger.

All the time and expense gave me no net improvement. Yes the draft stays steady and you can see the draft control opening further as the burn continues but it hasn't helped according to this first test. The fire actually looks like it's a richer mixture with less draft although the stack temp mirrored the lean series I ran previously. Such contradictions in the data, the further I get into it the behinder I feel.

I'll run some more tests and monitor it's cycling for a few days. Time to install that larger medicine cabinet in the bathroom.


On Edit: I ended up adjusting back to .04 WC for stack draft. It seems the fire becomes smokey when the draft is too low suggesting that combustion gasses are blanketing the fire a bit. Running more draft allows a lower air setting. More mutually exclusive adjustments to fine tune I guess.

 

Don,
From what I can see the unit is old and big.
My guess is there's not much baffeling inside the furnace and is wide open. Because of this I don't think you will ever get that much difference between the pipe and the over the fire draft.
A wide open furnace like that is pretty much a straight shot from the chamber to the breach.

The only good thing even though it may not seem it is by installing the regulator it's at least now being controlled and not wild draft from the chimney.
As you said, by adjusting the regulator you get different readings.

 

Hello Spott,
Turns out this thing never had wild draft, it only reached .05 after 20 minutes according to the Dwyer.
The furnace is internally laid out with the chamber at the bottom and the gasses having to circle the upper chamber in a passage which can be blocked on either side to switch which way they go and which breach is being used. That block plate is 1/4" steel and intact. So it's not quite a straight shot and I do get in there and vacuum the passage clean every other season.

 

Darn it all. I can't understand, to save my hide, why you are getting more draft overfire than in the breech. It just makes no sense.

I know it's a stupid question but I have to asK. All of these draft readings are negative pressure are they not?

Kind of a follow up to guyold's comment about measuring the overfire draft:
I don't think I've seen a picture of the front of the furnace, especially the fame observation door. You could be getting enough leakage around that door to get a higher overfire than breech draft. Can you seal around the door with silicone & drill a hole in it just large enough for the draft probe?

 

Grady,
A little off point but I'm glad you asked. I've been following this and that's one thing that would make sense but after everything Don's done I didn't have the nerve to mention it but I was curious. In my heart though I new it couldn't happen.
Don
Despite what your gauge says, if you put your hand in front of the door do you feel any heat.

 

Hello Guy's,
Grady, yes it is negative pressure or air going into the furnace. The inspection cover is a tight spring loaded thick cast part with a glass window (you can see it in the video). There are no leaks around it and no holes through it. I've not tried Guyold's cardboard trick yet to seal it off while measuring. Even after the furnace has cooled to 100*F after a cycle (fan off) it still shows a hair more draft in the chamber than at the breach (-.004 vs -.007). I don't get it, something is amiss. Even if the Dwyer is calibrated wrong the relative measurements don't jive. I can move the tube around inside the chamber and the readings don't vary. When I withdraw the tube the gauge reads higher as I pass the opening (venturi effect). I did put the Dwyer tube in along side of the blast tube, sealed around with insulation and it gave the same reading.

So, the draft is currently set to -.04 WC / -.05 over fire.

 

Thanks Don. Sorry but I had to ask. The only way I can think of where the overfire draft is higher than that of the breech is if there is an air leak somewhere causing air to leak into the chamber. If the observation door is removable, try guy's suggestion. Be sure to get a good seal from cardboard, sheet metal, or whatever you choose to use and the furnace as well as around the probe.

 

Hello,
I tried Guyold's cardboard suggestion and got the same readings, about -.01 WC greater over-fire. I tested at the 10 minute mark.........ouch that got hot fast!
If you don't mind me asking, how would air leaking into the chamber cause the over-fire draft to be greater? Cooling the gasses?

After my bold statement about the inspection cover I decided to double check. Of course there was a problem, And I thought that soot was from the earlier pulsating. Things were slightly warped after 60 years of service so I machined the cover flat and had to use a shim to get the spring loaded device to hold it tight around the circumference. It made a "slight" difference in stack temp but the draft was the same.

 

The air leakage wouldn't cause a true increase in draft, I don't think, but a measured increase due to a venturi effect around or past the probe. Never having experienced this, I'm speculating.

 

Nice job on reg. It will make a difference on cold windy days.Someone did get a -.06 right? What cooks faster a pot with a lid or without? Plug up around blast tube we were watching your flame there. You will get better c02 and a hotter flame. I would try two things after you put up your mirror. You could up jet to overwhelm some of your chamber space and heat exchanger. You might be too under fired.This would give you a hotter breech but better co2. A wash if your lucky eff. wise. Or I might restrict that passage in the chamber by the plate with part of a brick. Monitor the drafts well for a time. Oh yeah,always get your over fire draft first. Like Grady says a -.01 0r -.02. then hopefully the draft at the breech will be higher. I think these are things can be done , easier then reworking the chamber smaller.

 

Morning Grady,
The Dwyer probe has a 90* bend in it so I can manipulate it around inside the test area. When I do so the reading does not change. I kinda thought that negates the possibility of Venturi effect. Facing the end of the probe directly into the flow of gasses reads the same as facing away in the stack for example.

 

Morning Guyold,
The blast tube is sealed to the chamber. Any flames you see are through the glass window in the inspection cover. The guy that left the sticker from 1984 fired the furnace at 1 gph (Monarch 1.00 80* AR). He was also the guy that left the thermostat and fan controls messed up. I had trouble getting to sleep last night thinking about this stuff. How I long for the early days when the furnace would run for two seasons then only need the nozzle cleaned and the filters replaced, ignorant bliss for 25 years. :-)

 

A old man much once gave us straight extension tubes for over fire drafts. Went in about a foot into chamber just above fire. He was a educator and trouble shooter. Wanted me to do his grunt work. Maybe should had taken him up on it. Might have learned something.