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Properly Designing a Boiler Exhaust System Webinar FAQs

03/21/12

1. Any new or innovative applications coming to market that should be considered? 1”

Double Wall Systems : available in 1” Air Gap / 1” Mineral Insulation / Ceramic Insulation for UL103 Condensing Systems. Available in 1” Air Gap and 1” Mineral Insulation for UL1738 Condensing Systems.

2. How are liners installed inside existing Masonry Chimneys? The best way to find out

is to read the Installation Guide already provided on the web site by Cleaver-Brooks. (http://www.chemineelining.com/media/other/473807Ins_MCL.pdf)

3. Are flue gas economizers gaining any popularity? Flue gas heat recovery

economizers are definitely increasing in popularity with resulting payback typically within 12-24 months. Depending on the overall efficiency of the boiler/economizer/feedwater system, economizer installations can impact the selection of suitable exhaust system design and materials. For instance, in a condensing (or near-condensing) economizer, an existing steel stack will corrode quickly from condensation of the flue gases at the lower economizer stack temperature. A Category IV style exhaust solution may be recommended to ensure long life for this type of application.

4. How would SCR or other BACT equipment fall into design considerations? The pressure losses of these equipments, if any, must be considered in the stack calculation sizing in order to maintain the appropriate draft in the exhaust system.

5. What about renovations of existing stack systems that originally supported larger power

burner systems, but now have less load. Is there an accepted control method for condensate control? First, the breeching must be insulated in order to maintain high flue gas temperature into the horizontal exhaust. Where applicable, a liner must be designed and installed inside the existing stack in order to maintain a proper flue gas velocity in the vertical exhaust. Drainage device at the bottom of the vertical exhaust must be installed. With new condensing boilers, the existing breeching may have to be changed to be in accordance with the special gas venting application (Cat 2 and Cat 4).

6. I have a particular job using a common vent stack where the boiler room runs in

positive pressure when the wind blows at the fresh air vents and in negative pressure when the wind flow reverses. Can you suggest ideas to alleviate the effect on the stack? Sometimes, when wind blows in the opposite directions, the outer surface of the building becomes positive or negative. You can use mechanical air intake to avoid the pressure fluctuation or in some conditions, you may use a vertical air intake in order to minimize the wind effect.

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7. For a Cat IV condensing boiler, can the combustion air be provided with a combination of combustion vent pipes and room combustion vents, if the vent stack space is limited? No, the combustion air has to come from outside the boiler room in order to avoid the room to turn negative. If the space is limited, you may use a mechanical combustion air intake or multi vent pipes in order to keep the minimum air intake area as specified in the local standards.

8. In the configurator, I did not see an option in the drop down box for condensing boilers for the Aerco boiler? Also, the Aerco boiler requires a special starter section that is offered by Heat Fab and Metalbestos, however, I do not see it in our offering? The first phase of the Sales Configurator does not cover all brands at this time. Nevertheless you can configure a new boiler for any brand if you select “Other” under the manufacturer brand. Then you have to fill out the settings for your particular boiler, which also includes the category of the boiler.

9. Is cpvc good pipe material to use for condensing boilers exhaust, air intake and

condensate drain piping? It can be a good material if recommended by the boiler manufacturer. It has its limitations and you have to be careful about the maximum temperature allowance. The cpvc is only used in Canada and must be ULC-S636 approved.

10. Do retrofit economizers effect the draft in the chimney do to lower exhaust temperature.

Yes it is. The natural draft in a chimney is proportionally linked with the gas temperature and the ambient temperature. So moving the gas temperature down will automatically decrease the natural draft in the chimney. Also, the natural draft calculation for the stack will begin after the economizer at a neutral pressure. The pressure loss in the economizer must be covered by the positive pressure supplied at the burner.

11. If performing an energy audit what would be some key indicators of existing stack

design that may benefit from improved design? Is the potential energy savings easily quantifiable? any rules of thumb? Perform a combustion analysis on the boiler, including a measurement the stack draft and stack temperature at all firing conditions. For example, if there is a high negative draft (>0.25” negative) with high excess oxygen in flue gas, an efficiency improvement can be estimated with a simple calculation. A 2% reduction in excess O2% is approximately 0.5% improvement in combustion efficiency. A properly sized/designed stack and draft control can help bring a boiler back into it its optimum operational efficiency.

12. roof termination See local code for roof termination details (height, clearance…)

13. How are liners installed inside existing Masonry Chimneys? See answer #2

14. Are automated dampers more efficient than barometric dampers on a non combustible boiler. See answer #19

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15. At what height of a stub stack do draft controls such as sequencial dampers become cost effective? Each case has to be calculated individually in order to find if a sequential damper become cost effective. Multiple variables have to be considered, such as, the boiler maximum input, the annual gas consumption, draft calculation, the location of the stack…

16. Are variable speed exhaust fan systems on the stack effective to control draft on common stack and multible boiler configuration with widely variable loads? On a vertical standard exhaust and with a proper stack sizing, the use of a draft control system, control dampers and draft transmitters is very effective. The use of an exhaust fan should only be considered when space is limited, proper sizing of the stack is impossible and on horizontal exhaust.

17. Do you recommend the use of mechanical exhaust control devices such as "exhausto"

brand? Do these devices have an affect on the package approval of the boiler? See answer #16 for the use of exhaust fan. These devices will have an effect on the package boiler approval if the electrical sequence of the boiler is modified.

18. What are the percent of energy savings for economizers according to heating hot water

boiler categories? Potential energy savings depend on the existing boiler’s operating efficiency, water inlet and outlet temperatures, and whether you want condensing or non-condensing in your stack. For non-condensing applications, the maximum efficiency one can expect is ~86% firing natural gas. If the water inlet temperature is low enough to achieve condensing, efficiencies of 90+% can be realized.

19. Are automated dampers more efficient than barometric dampers on a non condensing

Boiler? Yes it is. Using automated dampers along with pressure draft controller allow you to set the exact negative pressure needed at the boiler outlet. However, a barometric damper become more efficient if you do not use a draft controller system along with the automated damper.

20. With every degree reduction in exhaust temp you get a 1% increase in efficiency. Rule-

of-thumb is a 40 deg F reduction in exhaust temperature results in a 1% increase efficiency.

21. Where on your website is the new boiler vent calculator mentioned during the seminar?

Not on the website and it will never be. Form to be provided along with the answers.

22. What are the most common uses for the heat recovered from an economizer?

Preheating boiler feed water, preheating make-up water for deaerator or feed system, cold hydronic system return water, and heating a “cold” plant process stream, such as a washing system.

23. In joining late, did I miss a point? Did you mention that it's possible to combine Cat IV condensing boiler vents through the vent calculator? It can be combined but not

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recommended by most Standards, Codes and manufacturers. They are more complex designs and they most likely need Control Systems to perform as they should do. Often they turn out to be more expensive than going 1 stack per boiler.

24. Does CB offer designed stacks with their boiler systems? Yes, Exhaust solution

offers UL listed single wall and double wall chimney systems along with freestanding stacks.

25. For dual fuel burners, fuel oil and natural gas, does the vent become type III or IV due

to pressurization? Since the appliance could not have dual certification, the vent will be in accordance with its category (III or IV). Just keep in mind that the special vent for category IV is approved for maximum continuous gas temperature of 550°F. Exhaust gas from oil burner appliance may be higher than that.

26. What are the percent of energy savings for economizers according to heating hot water

boiler categories? Brian – repeat of 18

27. Is it's possible to combine Cat IV condensing boiler vents through the vent calculator? See answer #23

28. Can Cat 3 and Cat 2 (condensing) boilers be combined on the same stack? What are

the considerations? It is not recommend to combine any positive pressure (Cat. 3) with a negative pressure (Cat. 2) exhaust. Especially when using barometric damper or draft hood connector. The venting system must prevent backflow of exhaust gas through idle boilers.

29. What are the percent of energy savings for economizers according to heating hot water

boiler categories? See answer #26