Fuel-fired large space heater – how much supply air is needed, and other questions.

How is it possible to have a large space heater without an exhaust pipe?

It is made possible by using pure high-quality fuel that ensures perfect combustion. Of the exhaust, more than 99% is carbon dioxide and water vapour that are parts of the air that we breathe. There is practically no soot or other fine particles, not even carbon monoxide. Exhaust from clean-burning high-quality fuel contains very low nitrogen oxide levels that do not constitute a health risk. To learn more, read this article.

Water vapour is in no way hazardous to health, and the moisture in the exhaust gas of Airrex infrared heaters does not harm the structures of the heated space.

Oil heater and carbon dioxide load

There are two stages in warming up enclosed spaces, such as halls and warehouses: initial heating and continuous heating.

During the initial heating, the fuel heater blows or radiates heat at maximum power, producing the maximum amount of exhaust gases. Once the space is heated, continuous heating only requires a fraction of the heater’s heating capacity. In this stage, Airrex infrared heaters typically use only a small amount of fuel per hour.

Due to the two heating stages, the exhaust load in the heated space also varies greatly. Therefore, the ventilation performance required to ensure an appropriate amount of supply air and a level of carbon dioxide suitable for people to breathe varies.

Load on full heating capacity

Airrex infrared heaters are offered in three capacity classes. The heating capacity of the smallest Airrex AH-200 is 13 kWh, the heating capacity of the medium Airrex AH-300 is 15 kWh, and the capacity of the largest AH-800 is up to 22 kWh. The capacity is achieved by burning approximately 1.0–2.5 litres of fuel per hour.

Burning 1.5 litres of fuel or diesel oil produces approximately four kilograms, or 4,000 grams, of carbon dioxide. The burning process requires approximately 22 cubic metres of air. Correspondingly, to burn 2.5 litres requires approximately 36 m3 of air and produces approximately 6,600 grams of carbon dioxide as a combustion product.

The carbon dioxide produced by the heating makes working in the heated space more stressing. But when does it become a health risk?

Carbon dioxide is not dangerous as such, but large concentrations are considered harmful

Carbon dioxide is one of the normal gases in the air and, as such, not dangerous at all. On the contrary, the human body needs carbon dioxide to work. If the level of carbon dioxide in the air increases, we feel like we are “running out of breath” and the body reacts by automatically increasing the rate of breathing. A high level of carbon dioxide in the air is reported to cause headache, fatigue and a feeling of stuffy air.

The amount of carbon dioxide in the air is expressed in ppm, or parts per million. The amount of carbon dioxide in the open air is approximately 380 ppm.

According to the Decree of the Ministry of Social Affairs and Health on concentrations known to be harmful (HTP) and the Ministry of the Environment procedure on carbon dioxide levels in indoor air, the maximum allowed concentration of carbon dioxide during normal weather conditions and while the room is occupied is 1,200 ppm. The acceptable exposure limit at workplaces during an 8-hour workday is 5,000 ppm. This translates into carbon dioxide concentrations of half a per cent (0.5%) mixed in the air.

Something about the level of stress caused by carbon dioxide on the human body can be deduced from the fact that submarine crews work and live in an atmosphere that contains approximately one per cent (10,000 ppm) of carbon dioxide. Even a concentration of 2%, or 20,000 ppm, has not been found to cause adverse health effects during short-term exposure.

How much does an auxiliary oil heater increase the carbon dioxide concentration in a hall space?

The specific weight of air varies according to the temperature and humidity of the air, the average used in these calculations being 1.225 kg/cubic metre (at sea level). Of this figure, the amount of carbon dioxide is 0.038% = 0.0004655 kg, or approximately 0.47 grams.

The 5,000 ppm carbon dioxide exposure limit allowed at workplaces translates into approximately 6.125 grams of carbon dioxide per cubic metre of air. Considering the amount of carbon dioxide already in the air (0.47 grams/m3), the amount of extra carbon dioxide allowed to be mixed in the air is 5.655 grams.

If we now presume that there is no ventilation whatsoever in a heated space with Airrex AH-300 infrared heater blasting away at full capacity, how big must the space be in order to avoid any health hazard due to the exhaust?

The four kilos, or 4,000 grams, of carbon dioxide produced per hour requires a space of approximately 710 m3to dilute below the 5,000 ppm concentration level. The area of a hall space that is four (4) metres high should, then, be approximately 180 square metres. Using the more powerful Airrex AH-800 for an hour at full capacity would require a hall space of approximately 1,150 cubic metres to keep the increase in the carbon dioxide level below the recommended 5,000 ppm limit.

Continuous heating requires only one-third or significantly less fuel compared to full capacity heating. This means that carbon dioxide emissions are only one-third or less by comparison. Subsequently, the volume of the heated space only needs to be 230–380 m3 to keep the carbon dioxide level below the recommended workplace exposure limit. The floor area of a 2.70-metre high hall space with a volume of 230–380 m3 is 85–140 square metres.

The significance of ventilation

The above examples are theoretical in nature because there are not that many heated spaces that are completely sealed. On the other hand, a combination of a particularly low ventilation setting and use of a fuel-fired heater will result in continuously increasing indoor air carbon dioxide levels, something that should be considered. If the heated space is ventilated in a normal manner in compliance with the regulations, meaning a complete air change per every two hours, the carbon dioxide level in the heated space cannot rise to a harmful level under any circumstance.

In principle, sufficient ventilation is achieved by air supply matching that used by the heater. In case of the AH-300 infrared heater, this means that, in practice, the required ventilation is 15–20 m3 per hour during initial heating and approximately 10 m3 during continuous heating. In case of the more powerful Airrex AH-800, the ventilation requirement during initial heating is approximately 30 m3, while approximately 10 m3  suffices during continuous heating, same as with the AH-300.

Theory and practice

The high efficiency with which the fuel-fired Airrex infrared heaters utilise and burn fuel means that normal ventilation arrangements are enough to ensure recommended working conditions. To circulate ten cubic metres of air, you only have to operate an overhead door once.

As discussed above, the amount of carbon dioxide in the heated space increases. It should also be remembered that carbon dioxide has been used to extinguish fires for a long time now. High enough levels of carbon dioxide extinguish fire by replacing oxygen. This also applies to the heater’s oil burner if the heated space is sealed and/or insufficiently ventilated. If there is no exhaust pipe from a heater with an oil burner to the outside, the level of carbon dioxide in the heated space continues to rise, and the level of oxygen drops. Finally, the burner may go out due to lack of oxygen, but its performance will suffer even before that. It starts to produce smoke, creating soot that builds up in the combustion chamber and nozzles.

Even if an overly high level of carbon dioxide in the air does not constitute a direct health hazard, symptoms like headache and fatigue should be considered as a reason to check for sufficient ventilation. The effect on the heating costs is not significant enough to justify compromising workplace comfort.