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Parameters to size boiler room ventilation equipment

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katwalatapan

Electrical
Aug 9, 2011
153
Hello,

I have a query on the primary parameters required to size boiler room combustion air and ventilation air equipment. Would you please provide comments/feedback if I would require to consider any additional parameters than the following to size combustion air and ventilation air equipment for a boiler room:

1. The boiler room has Oil-fired furnace, 6.4 GPH, 924,000 BTU. The boiler has separate flue duct that exhausts to the building exterior.
2. The boiler room does not have direct access to the building exterior. No louvered boiler room door.
3. The boiler room dimensions are 9.5' (W) x 20' (L) x 8' (H).

Would I also need to consider factors such as:

1. Ventilation equipment to maintain either balanced or slightly positive pressure in the boiler room.
2. Have separate fan to bring combustion air from building exterior into the boiler room and separate exhaust air fan to maintain temperature in the boiler room.
3. Any consideration for combustion air fan be linked to the boiler, so that it opens the damper only if the boiler is in operation. Also thermostatically control the exhaust air fan.

I would appreciate your feedback and any special cases and formulas I should consider to size the combustion air and ventilation air system.

Thank you.
 
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Cannot be done with simple formula, that is quite responsible design work for HVAC engineer.

you have to take into account, boiler, burner and flue construction, than work with both generic formulas and manfucaturers recommendation.

heat rejection ventilation is not problem by itself, if heat rejecting fun is controlled by thermostat, it can only bring more than minimal fresh air requirement. design, however, has to ensure minimum fresh air quantities depending on boiler switching on, not on temperature, which can only be treated as addition.

negative pressure minimum is again related to combustion issues, not spacial issues. it is usually required when natural ventilation is planned, where you have to prove minimum flow in the most adverse conditions.


 
I don't have a copy of NFPA 31, however, I believe that this code will offer guideline on combustion and ventilation air requirements for boiler rooms. The size of boiler that you are posting is about that for a large residential home, therefore two separate air inlets may not be a requirement. I take issue when people don't post proper units and that refers to the 924,000 Btu which s/b Btu/Hr.
For total air intake(= combustion air + ventilation air) the rule of thumbs is Bhp* 8CFM/Bhp + Bhp* 2CFM/Bhp, respectively. Bhp means boiler horsepower =41,900 Btu/hr of heat input to the boiler. From the info, you can calculate total CFM. You can then size air inlet on 250 fpm air velocity thru a clear opening.
 
Hire an engineer who does this for a living and don't bootstrap this.
 
chicopee
I thought 1 Bhp = 33475 BTUH, not 41,900 BTUH
Looks like you are adding losses (assuming 80% boiler efficiency)
From a pure conversion point, tables from cleaver brooks list 33475 BTUH/Bhp
Have I been missing something?
 
Thank you all for the inputs. This does help me get on the correct track.
 
Yes, the value that I gave has an efficiency factor of 80%. The 1Bhp=33,475 btu/hr is a value given for the generation steam from water at 212dF.
 
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