Min Stack temp for Oil fired boilers?

knkreb

First, great job on being such a great resource for the heating community! I love your podcast of the tales of old.

I have two Bryan RV550 boilers in a school. These boilers were selected to heat full future additions to the building to be built in phases. I'm not really sure if the full design load has been met, but additions were made along through the years.

Now, since coming on board in the last two years, I've had the pleasure of getting the systems to work and keep temperature nicely. In fact, some of the staff have mentioned "wow, we had heat all year..." Sometimes, you just don't know how low the bar is set to impress someone.

We've fixed all the fuel supply issues, foot valve problems, sludge in the tank etc etc. We are on our way to a happy school year.

These are fired by #2 fuel oil. There is a boiler reset control that calls for the boilers to run. I could not find in any of the boiler manufacturer literature about the stack temp. The stack temperature gauges appear to be working, but they seem awful low. The burner is firing at low fire and the water temperature is 145°F. The stack temperature gauge hardly moves off the bottom of the temperature scale. I can run the burners up to a higher firing range and the gauges respond. (which I neglected to take the numbers with me when I left last time). These are fully modulating burners as well.

So my question is: what would be a minimum acceptable stack temperature on a boiler like this? Potentially if it may run a great deal of time at low fire?

Thanks for your help!

EdTheHeaterMan

Unless the boiler is made of some non-ferrous metal, you should be operating with a minimum of about 300° in order to prevent condensation of flue gases that can destroy some venting system components.
Another factor is the return water temperature to the boiler. Keeping that above 130° is also a normal rule of thumb.

Your boiler manufacturer may have different specifications that are allowed. Have you tried contacting Bryan's tech support?

EBEBRATT-Ed

I agree with @EdTheHeaterMan around 300 is the minimum stack temp if this is not a condensing boiler. Contacting the boiler mFG is your best bet

STEVEusaPA

At the base of the chimney, not gross.

random12345

I don't mean to hijack the discussion, but I have been wondering the same thing. Our net stack temperature on a new megasteam was 248, gross 337. Lateral vent connector is 12-13 ft long, 4 elbows, external chimney facing northwest is 35 ft tall. I also read NFPA 31 that says you need a draft fan for anything over 10 ft lateral. They also specify minimum firing rates in Annex E.5 that with our chimney we would need a minimum of 0.85-1 GPH. If 300 at the base of the chimney is the minimum, what should I do? And how is it that everyone with tank water heaters is not having flue gas condensation problems?

STEVEusaPA

@random12345 Gas and oil-fired water heaters run at higher stack temperatures.
In your case, low draft and low excess air, and underfired lead to lower stack temperatures.
Yes 10' is the max by code, but there's another issue.
The connector's (flue pipe) total equivalent pipe length (TEPL) shouldn't exceed 75% of the chimneys height.
So with 35' Chimney the maximum TEPL of the flue pipe should be 26.25 feet max.
-90° elbow = 11' of TEPL X 4 = 44'
-12' of flue pipe=12'
So you have 56' of TEPL, way over.
You'll need to get your chimney lined with a SS liner before you destroy the masonry.
And a power venter or draft inducer. But you still need to maintain 0 to -.03 draft at the breech.

You may also need to remove some baffles, based on the model #. But you can't even attempt that without all the proper combustion instruments, and proper knowledge. This will increase draft and stack temperature, but efficiency will suffer.

There's never a good reason to underfire a steamer. Which model #, 396?
Can you post a picture showing all your flue pipe?

random12345

Thank you, I didn't know any of this. The SS liner will be installed in one month. Yes it is a 396. Installer is coming back on Monday to fix something unrelated so I will ask them about all this then as well. Here is initial combustion numbers at installation:

Tstack 248.4 °F Eff 90.0 %
O₂ 5.4 % ExAir 32.4 %
CO 33 ppm CO₂ 11.65 %
CO AF 44 ppm Tamb 89.1 °F

Before this boiler, we had a Burnham V74-T. I believe the stack temp on that was in the 400s. So maybe that's why the clay tiles we have in our chimney now although flaking are still not completely deteriorated even though they are original to the house which was built in 1938.

STEVEusaPA

I didn't even account for the T, 38' of TEPL.
Improper rise, too, but at least 24 gauge before the SS.
Can you powervent directly to the outside?

random12345

I would rather avoid powerventing. Our neighbors are close and we would literally be venting directly into the wind. I'm worried the exhaust fumes would blow back into the house through nearby windows as well. You don't think a draft inducer would be enough?

EdTheHeaterMan

@random12345, You can eliminate 1 and 1/2 of the elbows in this vent connector by turning one elbow about 80° and running the vertical pipe to a slight angle. you will also need to twist the vertical so the barometric dravt control is on a plumb face of the pipe.



This will still be over the connector allowable horizontal dimension, but it will go from Way, way over to just Way over... and that might make just enough of a difference.

Although it is incorrect, It looks clean so maybe it is ok. Sometimes these poorly engineered systems still work in real world applications. How long has this been in operation?

Robert O'Brien

What's the OF draft now? That L vent can handle lower exhaust temps, is there any sign of condensing?

random12345

@EdTheHeaterMan Thank you for that great drawing! I'm definitely going to show the techs this on Monday. What an ingenious solution. I hope the barometric draft control can still be made level like it is in your drawing. It was installed at the end of July, so 3+ months. I'm just outside Boston, and we haven't hit those really low temps yet. Thermostat has been set to 55 at night so far. We have a tankless coil, so when it fires at night, it would just be to keep the water temperature at 170.

@Robert O'Brien, what is OF draft? I haven't checked for condensation. What should I look for? Would the condensing be visible or do I have to stick my arm in there and feel around?

EdTheHeaterMan

OF draft means draft over the fire.

There are three holes for test instruments from back in the early days. the hole on the vent connector (smoke pipe) within a few inches of the boiler or furnace for the smoke, draft, and CO2 measurement. A second hole not far from there in the vent connector to place the stack thermometer (because you leave it there while you take the smoke sample, draft reading, and CO2 sample. the third hole is at the flame inspection door, this may be provided by the manufacturer or you may need to drill a hole there and then seal it back up after the test is completed.

Today, with electronic combustion analyzers, you only need one hole at the "breach" ( the hole in the first few inches of the vent connector) and one OF opening

As far as condensation of flue gas, you wont be able to see it, you will only be able to see the effects of it as time goes by. The vent connector pipe (usually called the smoke pipe) will start to show signs of water dripping from the seams of the pipe connections. The further away from the boiler you go, the lower the flue gas temperature is. a thermometer positioned on the exit above the roof is the only way to determine if there is a low enough temperature to cause chimney deterioration right now. Most contractors will not do that because if the risk involved of climbing up on the roof. By measuring the temperature at the breach, you will know by experience if there is going to be a problem. So you need an experienced oil heat technician to be able to tell if there may be a problem. (don't ask for the chimney exit temperature)

Just keep an eye on your vent connector pipe (Smoke Pipe) near the boiler over the upcoming heating season. If there is excessive water stains in May, Then you want an explanation while still under warranty. You may need a stainless steel chimney liner (if the installer did not already include one)

random12345

Ok thanks. I will make sure to ask the techs about this.

EdTheHeaterMan

As far as getting the Barometric on a plumb & level portion of the angled pipe, if is very easy to get it right, and just as easy to get it wrong. I have updated my illustration below.

captainco

According to the picture the barometric is partially open which means there is draft. Don't know if it is set correctly but draft appears to be adequate,

Oil flames are 600 to 800 degrees hotter than gas flames. This means that the flue temperatures of oil equipment should always be higher than gas equipment. On commercial boilers the minimum flue temperature should be at least 245 degree higher than the water temperature and never below 275 degrees up to 375 degrees, These are normal ranges, with the 275 barely acceptable, 300 better.

Like it or not, it will cost 100 of gallons of excess oil usage if you operate the boiler in any firing rate than high fire. For years people had multi-firing range equipment to keep it running because they were afraid it wouldn't relight. Adjusting the differential control can minimize the modulation. This also will minimize any chance of condensation and messing up the flue. I remember a school in Indiana years ago that by adjusting the burner to what I just stated, ;lowered their oil usage $27,000.

As SteveusaPA said, underfiring is bad and that is exactly what modulation does.

Also make sure the differential on the reset control is at least 20 degrees to assure proper run time.

random12345

@captainco Thanks. I don't know what differential control is though or reset control. This is an oil-fired Burnham Megasteam.

Steamhead

random12345 said:

@captainco Thanks. I don't know what differential control is though or reset control. This is an oil-fired Burnham Megasteam.

I believe the Captain was referring to hot-water boiler controls.

random12345

EdTheHeaterMan said:

As far as getting the Barometric on a plumb & level portion of the angled pipe, if is very easy to get it right, and just as easy to get it wrong. I have updated my illustration below.

@EdTheHeaterMan we finally got around to following your suggestion. Here is the final result:


We also went from a 6" to a 7" draft regulator. When our oil tech measured the draft at the pressure tapping on the back of the Megasteam, it was getting as high as -.06. The Megasteam manual says no more than -.03 is ok. We were using a Field Controls RC model. Their reference guide on page 34 states that on a chimney that is 16 ft or higher, for a 6" breach/vent pipe, you need a 7" draft control. I wonder how many boilers out there have a draft regulator that is too small...I would not be surprised if this is a common problem. I might get a Dwyer 460 to check on it every once in a while now. But anyway, our venting has been fixed. Thanks again for your help.

For any other Megasteam owners who read this in the future, the proper location to measure draft is at the pressure tapping on the back of the boiler. It says so in the manual on page 32. There was soot on the turbulator inside the burner after only 6 months. Our installer did the combustion analysis from the vent pipe several feet away from the flue collar. I think this could be part of why this happened. The MST288, 396, and 513 all ship with a 6" draft regulator. But that only works for chimneys that are 15 ft or less, and where I Iive, those are not very common. All other houses, the chimney is more like 30 ft. or more and needs to go up a size to a 7" or even 8" draft regulator (for the MST613). We also installed a 5" liner and that helped too. NFPA 31 Appendix E has some great info on liner sizing, and NFPA 54 is useful as well.