Why Does So Much Heat Go Up The Chimney?

Tom_22

Why are flue temperatures from steam boilers so high? Is all that heat needed to drive the gases up the chimney? It seems like a lot of heat goes up the chimney.

Could some of that heat be used by, for instance, running the flue pipe through part of the building and letting it provide some heat to the surrounding space.

Also, does it make sense to install any kind of damper(s) to keep cold air from coming down the chimney and cooling the boiler in between heating cycles?

TomBk

Unknown

What kind

of boiler are you referring to? If it is gas and of residential proportions, it should already be equipped with an automatic vent damper. If it is oil, it will not and we would not advise doing so. This was tried back in the 70's and early 80's within the oil equipment burner and in the flue pipe and was a miserable failure. I might add that some of today's oil burners are equipped with mechanical air gates and do perform well. Hope this helps.

Glenn Stanton

Burnham Hydronics

Tom_22

I'm going to get a gas fired Burnhan.

Hi Glenn,

A steam maker. Probably 7 section. I didn't realize they had a built in damper. That sounds good. Still I wonder why so much heat goes up the chimney with most boilers instead of being captured somehow.

TomBk

Unknown

Thanks

for thinking of Burnham. I am sure you will be more than pleased with your new IN-7 Independence boiler. As far as older boilers and stack losses.......well, back in the old days fuel was much less and we were all convinced that there was an undepletable supply of it. We have since found out otherwise and everyone is conscious of efficiencies these days. If you look at the flue passageways of many boilers of old, the passageways are so wide you can put your arm or leg down through them. This is not the case any longer and water contents have decreased dramatically to improve on the efficiencies also. Thanks again!

Glenn

Tom_22

So what's the typical flue gas temperature that goes up the stack?

Unknown

On a gas boiler

flue gas net stack temperature (after deducting room temp) is 350 degrees (F) to 550 degrees (F). This is necessary on older systems in order for proper draft to be created in the flue. When you get down below 275 degrees you will start to have condensation problems.

Draft is created by delta T (temp difference) and height of the flue or chimney.

A vent damper on a gas system will reduce standby loss but will not stop running flue gas temp loss.

A combustion test will determine if your system is set up correctly. It will also insure its safe operation with low carbon monoxide below 100 PPM.

Steamhead (in transit)

\"Miserable Failure\"?

Then my V-14 must be the exception, Glenn. It still has the original Flair stack damper. The only thing I've had to do to it is add some lube to the gear case. If the boiler runs a bit on a mild morning, then doesn't run all day, it will still be warm in the evening since the damper prevents standby loss. That's efficiency!

Of course, I maintain my boiler properly. I would assume that a stack damper that gets plugged with soot will fail. Mine never does.



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thfurnitureguy_4

That is one of my next upgrades to my V905A feeding into an old coal flu Can you talk more about the possible problems?

Fred Harwood

Stack losses

You've touched on one reason to keep masonry chimneys inside of the heated space -- stack losses warm the chimney and thus the spaces around them, recovering some of the loss. Also, chimneys outside the building envelope cool down between cycles (especially here in the Northeast), exacerbating condensation and reverse-flow draft problems.

Chimney draft in winter also induces much air infiltration between heating calls. Oil burners such as the Beckett AFG contain a built-in air damper that prevents air from drafting through the boiler when off. The flue damper, however, pulls much heated air from the boiler room. Increasingly, oil boiler packages can be installed without dampers, thus saving more stack losses.

Because burning one pound of fuel oil in air also produces 1.18 pounds of water (as steam vapor), unless the boiler can condense and recover the latent energy in the steam, it also just goes up the stack, and is the main reason for chimney condensation problems (gas produces even more water per pound of fuel).

Burning any carbonacious fuel in air also produces noncondensible, hot CO2, which also passes through the boiler and out the chimney. And because air is 79 percent nitrogen, a large fraction of the heat of combustion goes into heating that inert, noncondensible gas, which also leaves via the chimney. The water vapor, CO2, and nitrogen all leave at the stack temperature, which must be high enough to prevent condensation in the chimney.

The parts that the installer can control are: chose a boiler with low breach temperature (intimate fire-side contact between the combustion gases and the boiler surfaces), ensure high combustion efficiency, prevent air from entering the boiler when off, and consider a damperless flue installation to minimize ourside air infiltration.

Of course, electric heat avoids all of these problems, at a cost.

thfurnitureguy_4

So if I understand corectly. It is better to stop the air on the supply side rather than the exhaust. My goal was to reduce the effect of an over sized chimney on combustion efficiency(flue breach pressure was unstable. flux between -.01 and +.01 In. WC). I also wanted to reduce the thru the box cooling of my boiler. My boiler room has a 1 ft. sq. duct leading to fresh air and on a cold day the boiler room is still warm from the waste heat of the flu pipe and such. I don't see a disadvantage in loosing the excess heat from the boiler room if I could keep the boiler water hot longer. Back to the burner, is there a Beckett burner with a supply side damper for these larger Industrial Burnahams? Thanks T.

bigugh_4

Hold it!

electric heat does not eliminate the stack loses, It just occurs at a different location. Even the hydro-power has is problems, There was a note about the methane produced by resivours (damed water). there is just no free lunch when it comes to heat. Somewhere, somehow, it all costs us something. Its a physics law, law of conserveration. The trick is to make heating as costless as possible. and by the way, what if we paid for the btu's instead of the gallon or watt or cu ft? The fuel co's are just like everyone else, they would all charge the same if they could. Remember, insulate, and tighten up the envelope to save fuel, but make your heating apporatus clean, efficient, and operating on it's exact line for combustion for safe warm living. JMO bigugh

heatboy

Off cycle losses.....

I believe, are more of a function of boiler design than anything else. A good tight, scotch-marine design or horizontal three pass will have very little off cycle losses compared to a atmospheric, negative pressure boiler. The same holds true of stack temps while running.

hb

ED_29

Speaking of failures,

My Riello/Pennsotti is still running ,O K. Riello damper ,works just fine! If you are looking for a burner with a damper,ccheck-out Riello.

Tom_22

How does the draft vary with the height of the chimney? How does one calculate the best flue size for a given boiler or boiler and hot water heater? My chimney is existing but may be due for a lining so there may be a little flexibility in what size liner goes into it.

Bob Harper

furniture guy needs a liner

. You know you have an oversized flue. Stop right there! A liner will size it to the appliance and minimise stack losses. The metal liner will warm up quickly for a good, stable draft. The oversized masonry flue will be damp with condensation so it will first need to dry out, then heat that cold mass of clay tile. Heck, by the time you do that, the boiler has cycled off. More condensation. It amazes me how many HVAC guys kill themselves trying to fix appliances that simply cannot breath properly.

Tom_22

I'm going to be getting a Burnham Independence Steam boiler. How does that compare for off cycle losses?

Constantin

I'm a mere homeowner...

... but the NFPA has a whole section on what flue sizes go with what equipment. Many manufacturers will also list the required flue sizes in their prospectus. The NFPA also tells you everything about how to combine flue sizes.

It may be as easy as adding up all the BTU's and choosing a flue diameter on that basis or even an additive formula that takes the two flue cross-sectional areas, adds them, and then allows you to determine the new flue diameter.

Anyway, if I were in your shoes, I would seriously consider getting an indirect water heater installed along with the boiler. It's one less piece of combustion equipment to maintain, you don't have to worry about CO-production from it, and it simplifies your flue situation at the same time. The marginal cost of going with an indirect may be lower than you think...

Joe Brix

I still want to know why

large mass cast iron boilers are still prefered over light steel low volume ones for oil fired equipment? Why is low mass/low volume design used in condensing technology only?
From a stand-by loss perspective, a damper or air intake gate on the burner cannot stop all the heat migration up a flue. Doesn't it make more sense not to fight the standby loss. Just make the boiler quicker to come up to temp with less volume? Of course this doesn't apply to gravity systems. 95% of the oil fired boilers sold are Burnham, Peerless, and W-M pinned cast iron verticle flue models.
The AFUE's go up with bigger mass but all that weight can't realy help real run-time efficencies. Am I wrong in this logic?

Unknown

Combustion Whats that?

Bob harper states:

It amazes me how many HVAC guys kill themselves trying to fix appliances that simply cannot breath properly.

Bob, I am not suprised as most of the HVAC people, contractors,installers, service techs, engineers etc I run into do not know much about combustion. They also are still in the dark ages when it comes to solving combustion problems. Their solution is to sell the customer a new package.

Constantin

Several factors coem to mind...

> large mass cast iron boilers are still prefered

> over light steel low volume ones for oil fired

> equipment? Why is low mass/low volume design

> used in condensing technology only? From a

> stand-by loss perspective, a damper or air intake

> gate on the burner cannot stop all the heat

> migration up a flue. Doesn't it make more sense

> not to fight the standby loss. Just make the

> boiler quicker to come up to temp with less

> volume? Of course this doesn't apply to gravity

> systems. 95% of the oil fired boilers sold are

> Burnham, Peerless, and W-M pinned cast iron

> verticle flue models. The AFUE's go up with

> bigger mass but all that weight can't realy help

> real run-time efficencies. Am I wrong in this

> logic?

Constantin

Several factors come to mind...

A low mass boiler only really makes sense as long as you have a lot of water flow and a modulating boiler to match the load. Otherwise, that oil burner will cycle like a banshee during the shoulder months and only be somewhat happy during the coldest days of the year.

... and that presupposes that the oil burner/boiler aren't oversized in the first place. If oversized, the short cycling will be constant and quickly lead to destruction.

By adding mass to the system, you can use the boiler as a buffer to lengthen the time between burner cycling. The disadvantage is standby losses (i.e. insulate the boiler well), as well as stack losses during idling (i.e. have a shutter up front to reduce that).

Unknown

Constantin stated:

but the NFPA has a whole section on what flue sizes go with what equipment. Many manufacturers will also list the required flue sizes in their prospectus. The NFPA also tells you everything about how to combine flue sizes.

It may be as easy as adding up all the BTU's and choosing a flue diameter on that basis or even an additive formula that takes the two flue cross-sectional areas, adds them, and then allows you to determine the new flue diameter.

My comments:

Constantin it has shown to be the case in recent years that the charts and tables in NFPA 54 and others are not adequate nor are they necessarily correct for all applications. A glowing example is the flue connections of "Fan Assisted" with "Natural Draft" equipment. The fan assisted furnace or boiler with the nat gas water heater actually creates a dangerous condition many times. At the least pilot outage problems may occur.

In order to overcome these problems it is necessary to change the design of the equipment so that they will work together. It is also necessary to add safety devices to insure safe shut down in the event of poor venting.

Another example no appliances should be flued today with less than 4" flue size.

Appliances should not be flued into chimney's exposed on three sides.

All chimneys should be lined with stainless steel high quality liners.

No single wall pipe should be used as a connector to any chimney or "B" vent. All connectors should be double wall.

A good 95% of all chimneys in the U.S. are inadequate for the equipment that is being sold today.

All air for combustion should come from outdoors. No indoor air for combustion.

The only thing a chimney may be any good for is a wood burning fireplace. Of course when it is burning you have an increased heat loss.

When we had equipment running with flue temps of 350 degrees and higher masonry chimneys could typically do their thing. With lower flue gas temps along with shutting of pilots (pilots maintained heat in the furnace and boiler and somewhat kept the chimney warm) on gas equipment we now have a problem.

Solution for the question being asked here is to side wall vent the boiler with an acceptable kit. Bring all air into the space mechanically and if possible preheat to some degree the air being brought into the combustion space.

The list could go on and on. We just simply have not brought the venting technology up to date with the equipment we are installing.

Joe Brix

Constantin

You're right, but many oil burners probably don't have nat gas for hot water, so an indirect could make for a better buffer then the boiler mass with a flue pipe stuck on it. Even in oil fired radiant or micro-load environments, a buffer tank will probably be needed. I'd rather stick my BTU's in a themos bottle. Well insulated boilers are just like well insulated gas/oil water heaters: they still have that flue pipe sucking heat out of the unit.

But like Tim said, most dealers are just going for the sale and have a warehouse full of the same boiler they bought in volume for a great discount. No one thinks outside the box.

Constantin

Thanks for the clarification!

It's equipment interactions like this that lead me down the path to the "one appliance, one flue" approach I use in my home. An indirect water heater is a simple way to reduce weird interactions and (usually) save some energy at the same time.

I also totally agree that most chimneys are not built to handle the gases we impose on them. I am glad that our chimney has all the materials you recommend for safe operation, however!

Constantin

You make a very good point!

It's a lot easier dealing with thermal loss out of a thermos bottle than a large atmospheric boiler with a flue pipe in it. On the other hand, it's also a question of exposed surface area (area of single boiler w/flue, vs. smaller boiler w/buffer tank), boiler design, etc. For example, I doubt that the stack losses during standby in the GE downfired design amounted to much.

I like the Vitola approach (which is high mass) by allowing the internal boiler temperatures to be as low as is warranted by the home heating system. This lowers the heat flux, and the Vitolas insulation is impressive as well. Plus, another factor is to what extent people actually completely insulate their heating rooms to prevent standby losses from pipes and the like.... few do!

Tom_22

Indirect water heater is not in the cards

> ... I would seriously consider

> getting an indirect water heater installed along

> with the boiler.
==========
Hi Constantin,
I want to take advantage of an offer from Keyspan Energy Gas Company wherein they are offering a boiler for about $300 to those that convert to gas. It is a Burnham Independence steam boiler and it is not available with a built in water heater. If I wanted to get a boiler that had such a thing I would have to buy it on my own and pay full price for it.

thfurnitureguy_2

Bob, the Liner is the $2000.oo fix and the damper is the $100.00 patch. But since the goal is to take some of the wind out of the blow hole, I would think there are 2 ways of doing that. One is a smaller pipe. The other is to add a pressure /vacume regulator IE a barometric damper. Yes it will cause additional air infiltration in the boiler room. Yes it will cause cooling of the outside of the somewhat insulated boiler housing,and the insulated steam pipes. All of these are losses from the addition of the damper and not installing the flue liner. The question is will these losses be more than the constant venting of the fully exposed fire box/heat exchanger which have full contact with the hot boiler water. Keep in mind my boiler room runs about 80 deg. on a 15 deg. day. My goal is to keep as much heat in the boiler so it can heat the part of the building I use. As for condensation. The bricks in my chimney stay warm to the touch throughout the season. I have no condensation running anywhere, and the chimney is on the inside of the building.