Problem / conflict with gas appliance ventilation standard

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Summary:

A ventilation standard for unflued gas appliances appears to give unsafe ambient levels of carbon dioxide. I need someone to check my calculations.
Unflued gas appliances emit all of their combustion products into the room in which they are installed. In the UK, ambient carbon dioxide in the commercial workplace is limited to 2800 ppm.

The UK/European standard BS EN 13410 (and many appliance manufacturers) give the formula for mechanical extract ventilation for unflued radiant heaters as: Extract flow rate (cubic metres per hour) = 10 x Net heat input (kW)

Consider a 20 kW heater installed in a space of 900 cubic metres. The required extract ventilation will be 20 x 10 = 200 cubic metres per hour. This equates to 200 / 900 = 0.22 air changes per hour.

In terms of gas rate, the appliance burns approximately 2 cubic metres of gas per hour. For natural gas appliances, cooled carbon dioxide production (by volume) is approximately equal to gas rate.

At equilibrium, room carbon dioxide will be: Carbon dioxide production rate / (Air change rate x Room volume)

In this example we get: 2 / (0.22 x 900) = 10,000 ppm - far in excess of the 2800 ppm limit. In a smaller room the problem would be worse.

Are my calculations correct? And, if so, how can the ventilation formula given in BS EN 13410 be correct?
 

Answers and Replies

  • #2
Lnewqban
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It seems that manufacturers specify the minimum area of building to be heated according to the size of each unflued gas heater.
Oversizing this type of heater can produce more air pollutants, especially lethal carbon monoxide (CO) and nitrogen dioxide (NO2).

Calculating exhaust volume requires proper consideration of actual achievable make-up air, as well as avoidance of stagnant air pockets in the main airflow through the building.
 
  • #3
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Yes, the same standard gives a minimum effective room volume of 10 cubic metres per kW net heat input.

In my example, the room space meets and exceeds this minimum volume requirement. Yet, according to the equilibrium calculation, emissions will still exceed safe limits.

I can't understand it.
 
  • #4
256bits
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Summary:: A ventilation standard for unflued gas appliances appears to give unsafe ambient levels of carbon dioxide. I need someone to check my calculations.

The UK/European standard BS EN 13410 (and many appliance manufacturers) give the formula for mechanical extract ventilation for unflued radiant heaters as: Extract flow rate (cubic metres per hour) = 10 x Net heat input (kW)
Is that the amount of air needed to ensure a good flame perhaps, and NOT the required ventilation for CO2 safe levels?
 
  • #5
Lnewqban
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Your specific appliance uses ##2~m^3/h## of natural gas or methane.
Domestic appliances normally use low pressure: 6" to 7" WC (0.25 psig).
For a perfect combustion of methane to happen, 10 cubic feet of air should be mixed with each cubic feet of burned gas.
Lack of air tends to make a flame yellow and generating carbon monoxide.
Could you show us details of your calculations of the ppm?
 
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  • #6
bigfooted
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OK, I checked the norm to make sure I have it correctly:
http://home.aktor.qa/External Docum...h Standards/BS EN/BS EN 13410-2001 (2003).pdf

  • 4.1 installation room:The installation room shall have a volume of at least 10##m^3/kW## of installed nominal heat input of the radiant heaters
  • 4.2.2.5 [4.2.3.4] Ventilation by thermal [mechanical] evacuation is sufficient if 10 m³/h of exhaust air per kW of operating heat input are ventilated out of the installation room
  • 4.2.2.7 Calculation of the necessary exhaust air volume rate:
$$ V_{tot} = \sum Q \cdot L$$

where ##V_{tot}## is the total necessary exhaust air volume rate in ##m^3/hr##, ##\sum Q## the total operating heat input of all heaters in kW, L the specific air exhaust rate ##(\geq 10 m^3/hr)/kW##

Let's take the limit case of a 1kW heater in a 10 ##m^3## room. Your calculations for the consumption are correct: you need 0.1 ##m^3/hr## of gas and 1 ##m^3/hr## of air, and you produce approximately 0.1 ##m^3/hr## of ##CO_2##.
The equilibrium concentration is ##x_{eq}=\frac{Q_{CO2}}{Q_{out}} = \frac{0.1}{10}=0.01##, this is 10,000 ppm. A transient calculation shows that you will reach 99% of the equilibrium value in 5 hrs (for the 10 ##m^3## room). for the 900 ##m^3## room, it takes 2 weeks to reach 5,000 ppm.
The 8-hour exposure limit in the EH40/2005 workplace exposure limits however is 5,000 ppm (I could not find your 2800 ppm limit). So yes, there could be a potential risk here for small rooms and large exposure times.
 
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  • #7
256bits
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The 8-hour exposure limit in the EH40/2005 workplace exposure limits however is 5,000 ppm (I could not find your 2800 ppm limit). So yes, there could be a potential risk here for small rooms and large exposure times.
  • British Standards BS6896, BS6230 and BS5990 which cover various gas fired heaters state that CO2 concentrations should not exceed 2,800 ppm where people are working
BS5990 has the 2880ppm, but that is for >330kW and < 2MW
EH 40 limit is 5000ppm
BS 525 has another limit to 2500 <330kW

Which standard is actually applicable.
 
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  • #8
Baluncore
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... For natural gas appliances, cooled carbon dioxide production (by volume) is approximately equal to gas rate. ...
That would be true for Methane only, where 1(CH4) + 2(O2) => 1(CO2) + 2(H2O).

Natural gas is composed of Methane CH4, and Ethane C2H6, so each single molar volume of natural gas will produce between 1 and 2 molar volumes of CO2.

LPG is composed mainly of Propane C3H8, with some Butane C4H10, so each single molar volume of LPG will produce just over 3 molar volumes of CO2.
For Propane the combustion reaction is; 1(C3H8) + 5(O2) => 3(CO2) + 4(H2O)
 
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  • #9
russ_watters
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Unflued gas appliances emit all of their combustion products into the room in which they are installed. In the UK, ambient carbon dioxide in the commercial workplace is limited to 2800 ppm.
I assume that is a time-weighted average over an 8-hour workday? Not an instantaneous or short term limit?
[edit]
I didn't see the standard at first...

If the exhaust is above the heater, the concentration you are calculating is the concentration in the exhaust, not the concentration in the room. The idea would be to capture and exhaust the combustion products directly, without allowing them to mix uniformly into the room.
 
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  • #10
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I assume that is a time-weighted average over an 8-hour workday? Not an instantaneous or short term limit?
[edit]
I didn't see the standard at first...

If the exhaust is above the heater, the concentration you are calculating is the concentration in the exhaust, not the concentration in the room. The idea would be to capture and exhaust the combustion products directly, without allowing them to mix uniformly into the room.
Yes, I think this must be the answer.

I always consider worst-case situations. Here, this would be if the combustion products didn't go out through the exhaust vent (which definitely should be above the heater as specified in the standard) but mixed uniformly into the room.

I can imagine this happening if the exhaust vents are too far from the heater.

This must be why the standard specifies a maximum horizontal distance from heater to exhaust vent (6 times the exhaust height for wall vents). I can now see why this is of crucial importance.

Thanks for all of the informative answers. They have been very helpful indeed.
 
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  • #11
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In the gas industry, we normally simplify by assuming natural gas = methane. In any case, if heat input (power) is used for ventilation calculations then the increase in CO2 production is balanced by an increase in heat produced, so ventilation per kilowatt is considered the same for methane, natural gas and even LPG.
 
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