Chimneys & Vents

Tim McElwain

Chimneys & Vents

 

It has been stated and is a fact that water vapor is a product of burning natural gas in air. With atmospheric equipment as long as it remains in this vapor state and is removed by the venting system to the outside atmosphere it is not a problem.

 

I have had numerous questions about venting and in particular the issue of masonry chimneys and the requirement to be lined. I would like to present some of the things we must be aware of when talking about venting equipment today. The modulating/condensing equipment can typically be side wall vented or the existing chimney be used as a chase for the Mod/Con vent. That then is not so big an issue as condensing is what it is supposed to do.

 

What are the facts concerning this water vapor and how do we handle it? We stated that water vapor is a product of burning natural gas in air. For every pound of natural gas burned, about 2.25 pounds of water are produced. In slightly different terms, burning 25 cu ft of natural gas produces about 2.25 pounds of water. A 100,000 Btu/hr gas input rate furnace burns about 100 cu ft of gas per hour of continuous opera­tion. This furnace would produce slightly more than one gallon of water if all the water vapor was allowed to condense into liquid form.

 

The moisture produced by combustion is normally in the vapor state; it is in the form of steam not liquid. As with moisture in air, there is a tempera­ture at which point the steam begins to condense and form water droplets. That temperature is called the dew point temperature. (Morning dew on the grass is a result of a cool enough surface tem­perature to condense moisture in the air.) Prob­lems may develop when the water vapor in vent gases condenses, as the condensate can become very corrosive. It can attack the mortar in brick chimneys or rust out vent connectors and furnace heat exchangers. Condensation must either be avoided or controlled.

 

Flue temperatures should be proportional to what we are heating and in the case of 80% furnaces plenum air should be at least 130 degrees. We use a 170 degree added to air water or steam temperature on 80% equipment. Mid efficiency furnaces 300 to 400 degrees would be the maximum ranges for 80% furnaces. Under 300 you are under fired over 400 you probably have poor heat transfer caused by airflow problems. These are high fire temperatures. On two-stage if the flue temperature gets much below 250 degrees some of the flue gases will probably cool and condense before they get out. Boilers would run higher. Forget the DOE chart!! We want to maintain at least 275° F as a minimum flue gas temperature at all times. I try to stay at 300°F just to be safe.



I will add some more to this as we go along and if you have any concerns, comments or questions fire away.

 

JStar

RE

Where is all of the corrosive vapor going when it's vented / drained outside? Does it dissipate and break up? Are we one day going to start having the flesh rot off our bones because of all the acid vapor?

Tim McElwain

Most of what is diluted

hopefully as condensate by neutralization at its source becomes harmless. The vapor which is airborne along with CO2, nitrous oxides, carbon monoxide is spilled into the air at the exit of the vent. Those who are concerned about the environment tell us this will eventually come back to haunts us if that isn't already the case.



What affect it has had or is having on the ozone layer is disputed among the scientific world, the question then is who do you believe. The number one polluter of the air with carbon dioxide CO2 is mother nature for example with all the rotting vegetation, some of those pretty leaves falling on the ground right now for example.

Tim McElwain

Some more on Chimneys & Vents

The amount of heat the vent gases lose as they flow up and out the vent is important for two reasons: If the vent gases lose too much heat

1) They may cool below the dew point and condensation would occur: and

2) Draft will be reduced and venting power will be lost. 



 

As a comparison, consider this example: A 45.000 Btu/hr gas-fired water heater is vented into a single wall, 8-in diameter, metal vent 15 ft high. The connector is 6 ft long and 3-in. in diameter. The vent gas temperature at the vent connector would be about 230° F. assuming 15 cu ft of dilution air at the draft hood. As the high tempera­ture gases move up the vent, much heat is lost. At the top of the vent, the vent gases would only be at 145° F and the inside wall surface temperature of the vent would be 94° F. The dew point temperature is 99° F. under these conditions and condensation would occur.



 

In our next example, a double wall Type "B" gas vent is used. The heat loss is substan­tially reduced and the leaving vent gas temperature is 166° F. The vent wall sur­face temperature (inside) is 124° F - well above the dew point and no condensation develops.



 

These two examples dramatize another im­portant point: An insulated vent will main­tain a higher vent gas temperature and provide more venting power. The average vent gas temperature for the single wall vent is 187° F. while in the double wall Type "B" gas vent it is almost 200° F.



 

It is important to remember that with more modern equipment with higher efficiency the flue loss is reduced and the flue gas temperature may not be able to power the existing flue even if it is “B” vent. Draft remember is created by Delta “T” (temperature difference) and the height of the chimney or vent.



 

With water heaters in order to overcome these short comings and also if there is no rise off the top of the water heater then increasing one size on the connector and making it a double wall connector can help assist the drafting condition. In extreme conditions the draft hood may need to be replaced with a barometric and the drat hood eliminated entirely.

 

 

Tim McElwain

What about chimneys?

 

The "excessive" heat loss problem is intensified when brick chimneys are used to vent gas appliances. With normal on-off operation, it is important that the flue passage be quickly warmed. Without a warm flue, venting action is delayed and condensation may occur. While it is a fact that all vents have some "wet time" - that is, cold surfaces and some visible condensation ­good vents warm up quickly and minimize wet time.





 

For example: A brick chimney on an outside wall (3 sides exposed) exposed to outdoor tem­peratures may represent a worse case scenario. First, a brick chimney weighs about 180 pounds per foot. It takes only a little over 300 Btu to heat the liner from 0° to 120° F, but a hefty 4,260 Btu per foot to heat the bricks to 100° F if bricks are in contact with liner (no air space provided). More than 50,000 Btu would be needed to warm a 15 ft high chimney. As a comparison, a Type ''B'' gas vent requires but 135 Btu. Even an inside brick chimney would require over 25,000 Btu. With that much warm-up required, the vent action is much delayed and wet time is extended with the potential for corrosion.





 

As you can see this is with a lined chimney, the requirements are drastically increased with an unlined chimney. This is why there are no appliances allowed today to be installed in an unlined chimney.





 

There is another problem with chimneys today and it has to do with the removal of an atmospheric furnace or boiler vented into the chimney with a water heater. In the past with two pilots operating (each pilot around 1 cubic foot of gas per hour) at 1000 BTU’s each the wet time was reduced. In some cases the chimney was adequately heated with those two pilots to give you a good jump start on draft when either of the appliances came on. This was especially true on an inside chimney. When we now remove the furnace or boiler and side wall vent the new appliance the water heater may not be able to power the vent by itself, and in many cases the chimney is now too large. The solution many times is to run “B” Vent for the water heater up through the chimney which now becomes a “chase” by code. Once it is no longer a chimney no other appliances can be directly vented into it. Often times we are alerted to this problem with pilot outage problems on the water heater.

 

 

 

 

Tim McElwain

What about the vent connector?

The vent connector can also be a source of excessive heat loss if it is too long, affect­ing performance if the venting system is already a borderline operation. When we have mid-efficiency furnaces and boilers without draft hoods, the performance of the vent connector takes on even greater importance. In all cases, a vent connector should be kept as short as possible. Since the chimney or Type "B" gas vent is often positioned first, or already in position and can’t be moved, this means paying close attention to the positioning of the furnace or boiler by the installation crew. In addition too many 90° elbows or tees can also affect its operation. Changing a vent connector from single wall to “B” vent double wall vent can often make the difference in its ability to draft sufficiently.

 

A final point on the effect of vent system heat loss: While it is generally understood that a vent system can be too small to handle a cer­tain volume of vent gases, the fact that it can also be too large is often overlooked. The min/max tables in NFPA 54/ANSI Z223.1 National Fuel Gas Code were developed for this very reason. Low vol­ume vent gases from small appliances, such as the water heater, can cool too rapidly, with the result that "spillage" may oc­cur at the draft hood. Condensation can also develop.





In all cases from a safety point of view a combustion test should be conducted on all gas appliances. As part of that testing proper draft  -.01, -.02, -.03 is a must venting (draft) means ventilation (air for combustion) for burning. Keep in mind the rule say 50 BTU's per cubic foot of space in the Combustion Zone. A quick example is a 40' X  25' ranch house with a full basement with an 8 foot ceiling (or close to it) is 8,000 cubic feet of space. Divide 8,000 by the 50 BTU's per cubic foot and you get 160 CFH times 1050 BTU (heat value in BTU's per cubic foot) that gives 168,000 the maximum allowable BTU in that full open basement. Anything in excess of that as far as total BTU's of equipment and that full open basement becomes a confined space. It would then have to bring in air from outside or from rooms communicating with outside.



Now cut that basement in half with a remodel to have a finished basement family room, then a workshop, laundry room, sewing room etc. The next thing you know the boiler/furnace - water heater is in an 6' X 6' room with a solid core door. NO AIR FOR COMBUSTION! CARBON MONOXIDE IS ON THE WAY!

 

 

 

Tim McElwain

How do you feel

about removing the draft hood on designed gas equipment or blocking it off and going with a Barometric?



Have you ever heard of such a thing?



Does it change the design of the equipment?



Does it add anything to the overall efficiency of the equipment?



Is it true that a draft hood actually disconnects the appliance from the chimney?



What is "CURTAIN EFFECT"?

Henry

Draft hoods

In our B149 code, we must follow the manufacturers certified instructions. If there is no mention of replacing a draft hood with a barometric, then one cannot do this. The manufacturer has certified his apliance do work with a draft hood or barometric under all sorts of conditions. Any change by any installer that is not in the certified instructions is illegal and probably dangerous.

I beleive that NFPA 54 uses the same reasoning as we do. What is not specificaly prohibited, is allowed unless the manufacturer states that what is permited. Therefore, what the manufacturer does not state is not allowed.

So yes, it changes the design and operating perimeters of the apliance! Unless it is permited in the certified instruction manual of the apliance manufacturer.

We modified our code in 2007 to let replacing an apliance whitout having to line a chimney. If one replaces an apliance with one of the same capacity and type, verifies that the chimney is in good condition, then one would not need to line it. Unfortunately, most AHJ still refer to the paragraph that says it needs to be lined. So A hot water tank replacement on a 14 story building is no longer a $6 to $8K job, it becomes $100K due to the need to install a liner. Hopefully we will have a proper amendement in the next code in a few years. It is very difficult to convince the AHJ in commitee meetings!

neophytehomeowner

what about venting oil boilers ?

This is an interesting topic.  Do the chimney dynamics work any different for oil boilers than for gas boilers as described above?  I have a big old house with a humongous oil boiler running radiators, big hydrocoil and large HW indirect. It vents into a very old 3 story unlined masonry chimney that used to vent a big fireplace. Do I have trouble brewing? thanks

Tim McElwain

Pretty much the

same factors apply just that oil vents tend to be at higher temperatures therefore they have less difficulty creating draft. All oil burners are also power burners so they need less excess air which also helps for better venting and less issues with drafting. There is also the advantage of being able to take an over-fire draft. All oil systems pretty much use a barometric so the ability to adjust versus the fixed draft hood for gas.



Oil is not my typical topic so I defer to those who know oil system better than I do. NFPA 211 and NFPA 31 would apply I know to oil installations