vent damper on steam system?

john_27

why don't gas fired steam systems have dampers, or something to keep heat from escaping out the chimney when the system isn't running? Aren't considerable savings being lost? If one insulates steam pipes to hold temperature, why not try to keep heat from escaping from the boiler? Thoughts?

darian

vent damper

I just installed a new Weil-Mclain gas fired steam boiler model EG-50. It has an automatic vent damper that opens and closes. It also has a spill switch that will shut the boiler down if the chimney gets blocked.

Mike T., Swampeast MO

That's a question dear to my heart as it's the second question I asked here at "The Wall".

Answer: Yes, vent dampers do work and they do reduce loss up the flue when the boiler isn't firing.

Problem: They're short-lived, unreliable and generally removed or defeated after the second service call...

Brad White_160

Electronic Ignition and Vent Dampers

are quite common and do save energy by slowing the "reverse heat exchanger effect", that "radiator" that is your boiler convecting up the chimney to warm a bird's butt.

If the boiler has a pilot light, there are dampers available but these have a knock-out plug, the theory that the flue emissions from the pilot light have to have a pathway out. Even so that always troubled me at some level for you still have a leak and a potential source of CO.

But with electronic ignition, the sequence of firing does not start until the damper is open and the end-switch makes to prove it.

If your boiler has a continuous pilot, I would not mess with it at this point. On the other hand, how old is it? Might you be nearing the replacement point anyway?

Boilerpro_5

I haven't found that to be true

I now have boilers in place for over 10 years without a failure...other than the factory defects experienced by Effikal a few years ago. I bet the short service life on many dampers is due to the fact the boilers are so oversized they shortcycle or the controls are not set up right so they.....shortcycle.

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Boilerpro_5

Electronic Ignition usually a bad idea here in Midwest..

Humidity levels tend to be very high in basements in the summer so the modules tend to be short lived. In addition, the heat exchangers get heavy rust scale buildup during the summer when there is no pilot keeping them warm. Heavy scale buildup reduces heat transfer efficiency, which reduces seasonal efficiency. In my area, electronic ignation likely reduces your efficiency and increases your operating costs for both fuel and maintenance and repairs.

Boilerpro

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Steamhead (in transit)

On your LGB

I'm not sure if a damper that big was available. What is the diameter of the flue pipe coming up off each half of the boiler?

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Brad White_9

Really?

Not a challenge, just that here in Boston, what with salt air, sea breezes, rain, snow, blizzards, hurricane remnants and used weather from points West...


What does the midwest have that we are missing besides better drivers, slower more understandable speech and the proper use of the letter "R"?

Never occurred to me. Interesting though!

Unknown

Dave we find the same thing

here in New England. It has been my experience that the worst thing we do is remove the pilot from boilers. Since 1987 until now (1987 was the NAEC Act fully implemented 1992) I have found that standing pilot systems versus EI systems the level of rust and scale is much higher on the EI boilers. I have several interesting comparisions in four different three floor dwellings all in the same neighborhood. There is a mix of EI boilers and standing pilot boilers in these dwellings. Of the tweleve units 8 are EI units and the other four are standing pilot powerpile systems (EI units are all newer than the powerpile units also). The powerpile standing pilot boilers stay much drier and cleaner than the EI boilers. I also have some very interesting combustion testing results on the standing pilot versus EI before annual cleaning of the units. EI units have higher CO levels after one year of running as compared to the standing pilot versions. All of these are very close in input so it really is a nice test environment. They are not real near the ocean by the way. The basements in all of these dwellings are also very dry. Conclusion: leaving the pilot going 24/7 is just a better way to go.

A side note all eight of the EI units have had modules or other related service calls in the last nine years I have been dealing with them. None of the powerpile units have had any parts replacment or no heat calls. We do clean all of these once a year and also clean the pilots once a year.

Two of the powerpile units have the original pilot generator in them over 10 years old, I know because they were there when I took them over nine years ago.

I have many other examples of the very same thing.

Steamhead (in transit)

Tim, do you think

this is because the EI units sit all summer without firing? Do you think a boiler that feeds an indirect as well as the heat system might not have this problem, since it would operate periodically during the summer?

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Christian Egli_2

The inertial world breaking apart at the steam

Hi John,

Putting a plug on the heat that will leave up the flue makes sense. It's like picking up pennies, it always makes sense, and it gets better and better as you upgrade to picking up $100 bills, particularly since the cost of lifting a penny is the same as for a big bill.

A boiler that has it's fire shut off turns into a convector (what Brad said), a pretty efficient convector: it has it's nostrils on the ground, sniffing up the coldest air available in the whole home, and it is powered by a strong chimney draught. Once warmed the air goes up up and away.

This goes on for as long as there is something hot to cool off, or until you run out of cash.

So, how much heat is there in a boiler system? How much thermal inertia is there? If there is none, then, there's no problem. If there is only a little, we'll be all right without the flue damper; but if there is a lot of thermal inertia, then the flue damper becomes a vital part of the efficiency equation.

Let's add up this thermal inertia and see if it adds up to pennies we can waste.

Consider a radiator home heating system that contains 200 gallons of water. This, according to standard tables is good for a 175,000 BTU/h cast iron system, or 350,000 BTU/h with more slender copper fintubes. Cool, representative enough.

We know a pound of water holds on to one BTU for each degree F rise. Assuming we have an indoor temperature of 70F and a system water temperature of 170F average (the standard 180F 20 delta T) thus, we measure a heat rise of (170 - 70) 100 degree F.

Next, our 200 gallons, at about 8.4 pounds per gallon give us a total inertial mass of about 1700 pounds. Let's mix it all up:

1700 lb * 1 BTU/(lb.degF) * 100 F = 170,000 BTU.

This is approximately 1.7 therms of heat. At today's natural gas cost, you might as well say it's worth a hundred dollars. And this is the thermal inertia you'll mostly loose up the flue as your system is cooled through the efficient convector effect going on in an OFF boiler. This loss of about $2 - $4, is quickly recouped by a damper on a hot water boiler.

If we redo the math with a system running at 120F 20 delta T, the loss comes down to 68,000 BTU. Still not pennies.

The question was for a steam boiler. If you'll allow me, I'll go on with some more math.

Let's compare the exact same system as before, but just converted to steam. This is real easy to do. Steam has practically no mass, it is well known that a thimble full of water will fill an entire system once vaporized. When steam is vaporized, it occupies 1700 times the space it previously needed as liquid water.

Our previous 200 gallon system filled with about 1700 pound-volume of water, once shrunk down 1700 times, turns out to be only 1 pound of steam. 1 pound of water turned into steam will occupy the space of 1700 equal volumes and still weight 1 pound.

But steam is hot.

If we go to a steam table we can observe how much heat it contains. Taking the difference between water at 70F room temperature and full fledged steam we measure about 1100 BTU per pound.

Our total thermal inertia for this steam system is

1 lb * 1100 BTU/lb = 1100 BTU

Steam systems have no inertia, in spite of how deceptively hot they seem. Compared to a water system, steam has easily (170,000 / 1,100) 150 times less thermal inertia.

Putting a damper on a steam boiler is for picking up pennies, while for a hot water boiler you're looking at big dollar bills...

But wait, there's more...

Hot water and hot air pick up and dump heat at will, at anytime and without any discipline; that's why we need flow control (they work but not enough to stop migrant thermal flows). Steam and AC refrigerant liquids, on the other hand, move heat only one way. There is hardly ever any occasion for tramp condensate at the bottom of a radiator to turn back into steam and go condense itself in the boiler. The heat transfer only and always goes from the steam boiler to the radiator, not in reverse.

So... the 1100 BTU of thermal inertia in our steam system is actually not very likely to come back to the boiler for dumping it's heat remnants (unlike any hot water boiler)

If we postulate that 10% of the steam will condense in the boiler, in our case this would be a flue loss of 110 BTU, then we in fact, bump up the ratio between the thermal inertia of steam and that of water to 1500 times better.

It is like looking for pennies and big time dollar bills.

There is still the issue of the mass of iron loosing heat. Iron only stores a tenth of the BTUs water can hold - therefore it does not amount to much (and the difference between a steam and a hot water boiler becomes insignificant)

This all explains why modern hot water boilers go with small volumes and totally controlled flue systems. It also explains the quest for large delta T and low supply temperatures. It also explains the obvious benefits of decoupled circulation such as in primary / secondary piping.

All this while steam operates with no practical thermal inertia while, all at the same time, it hoists loads of heat around your home, at the extravagant rate of 1000 BTU per pound.

Once the fire goes out in the boiler, the steam radiators go flat. About that quick. Within the steam pipes, the very valuable heat we keep form escaping just does not stick around once the fire is out. What's there to escape now?

Isn't this neat? On steam, a damper only gives you a small bonus, but still, I'll bend down for a penny.

Steamhead (in transit)

More to it than that

With the typical chimney designed for burning coal, the incredibly strong draft can pull all the heat out of an atmospheric gas boiler between firing cycles. This makes the boiler start from scratch on every cycle. The damper might not keep the water boiling, but it will keep enough heat in the boiler that it will reach the boiling point sooner on the next cycle. That's where the fuel savings happens.

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Christian Egli_2

More words yet you mean?

Thanks everyone for reading so far.

ttekushan_3

blown away by draft

Thanks Christian and Steamhead for your comments. When you adjust burners by eye, there's really no good way to determine how much of the heat of combustion is going right out the chimney. As I got into this boiler stuff more deeply and started using a combustion analyzer with draft measurements my eyes popped out of my head with what I was seeing (and boy did that hurt. Mostly when I pushed them back in). Great combustion efficiency but so what. There's no heat getting to the water. And yes, its almost always in older homes that originally had coal furnaces and chimneys to suit. Apparently during firing an envelope of air will form around the combustion gasses and escort the heat right past the iron and onward up the chimney.

This explains why a "92%" mod/con direct vent HW boiler saves 40% or more in heating bills over an "82%" CI natural draft. For my money, I put an adjustable draft damper to control outrageous overdraft (set up with an analyzer) and lock it down. Huge savings, minimal cost. And do this to a steamer and you can afford to keep the house balmy without obscene fuel costs (unless you have an obscene home).

-Terry

P.S. Ya don't need to worry too much about that with this thoroughly modern steamer complete with gas operated pneumatic draft controls:

And what others have said I think are true about standing pilots. Every time I encounter one of these oldies in perfect condition, there's always about 4 olympic torches in there keeping everything warm!

ttekushan_3

damper

Field makes a 12" motorized damper. I think they're very helpful.

Terry

Unknown

Yes I feel that a boiler

or furnace that sits all of the off season without some source of heat has an affect on the overall condition of the equipment.

I am also a big believer in a boiler system that has a keep warm cycle of some sort operating has an advantage. I have never been one who felt "cold start" was a good thing on oil or on gas. Getting domestic hot water from the heating system will certainly help in this process. The indirect surely becomes a part of that.

A vent damper is a definite advantage to overall efficiency of the system. Any heat held in the water is a plus.

As for large oversized chimneys I encourage using a type B double wall vent inside the existing old monster of a chimney (I am not a big fan of chimney liners). The "B" vent allows proper sizing of the flue system and gives you 20% advantage over chimney liners as to actual capacity versus flue size. In other words 6" chimney liners have 20% less capacity than a 6" "B" Vent.

Remove the draft hood and put a barometric draft regulator and we have a much more efficient system. Finish that all up with an effective combustion test and adjustments to proper firing rate with low CO and good draft and things are much better.

Some might say well gas systems that did not have domestic hot water actually are cold start. Well the answer there is that in many cases just the pilot running would keep the boiler water around 90 to 100 degrees many times depending on the size of the boiler.

Add a good dehumidifier to the damp wet basement also helps.