Volume 27, number 3, December 2013

The importance of the energy source for ventilating residences and multi-tenant buildings

Residences and multi-tenant buldings are characterized by their common areas, which require optimal ventilation and heating in order to ensure the comfort of the residents and maintain healthy air quality.

To accomplish this, some of the ambient air is exhausted and replaced by the same volume of fresh air. The fresh air thus has to be heated in order to maintain a stable ambient temperature. The stale air is exhausted by extraction fans, while the fresh air supply (also called make-up air) is introduced by air compensators.

Electricity is frequently used to operate the air compensators, sometimes unwisely. The reasons often given for such a choice are the precision of the temperature, as well as the costs of installation and operation.

The degree of precision of an electric coil may, in fact, prove to be more precise than natural gas exchangers for heating fresh air­­­– even more so when there is modulation of the air flow. But the problem is that air compensation units are not designed for modulating air flow or for space heating – they are designed to selectively heat the fresh air used to ventilate and replace the air exhausted from a building’s common areas, which would be better served by natural gas heating, as we will show later.

The fresh air supplied by the compensators represents only some of the ambient air, and when compensation units are well chosen, the degree of precision of natural gas appliances is sufficient to avoid overheating, to maintain the quality of the ambient air and optimal comfort.

As for the costs of installation and repair, building owners and managers have everything to gain by requesting studies that compare natural gas and electricity, particularly the operating costs of their ventilation appliances. Below is a comparison of the energy consumption of a multi-tenant building equipped with a fresh air compensator with electric heating coils versus natural gas exchangers.

Technical data and operating conditions of a multi-tenant building:

Air flow: 4,720 L/s or 10,000 ft³/min.
Temperature: 20°C or 68°F
Operation: 24/7 continuous
Efficiency: 100% electricity; 80% natural gas
with an exchanger
Electricity required: 622,330 kWh/year
Maximum power demand: 235 kW in January

Electricity Rate M – energy calculated only on the 2nd tier of Rate M

Simulation in continuous operation

Energy consumption – electric coil

Electricity required: 622,330 kWh/year
Maximum power demand: 235 kW in January
Cost of electrical energy: $20,282/year, or 3.36¢/kWh –
2nd tier only
Cost for power demand: $21,395/year, or $13.71/kW
Total cost: $41,677/year

Energy consumption – natural gas exchanger

Natural gas required: 71,697 m³/year
Cost of natural gas energy: $30,489/year –
Rate D1
Total cost: $30,489/year

Since the price of natural gas is currently very competitive, it would be tempting to think that the simulation favours natural gas because the appliance operates continuously.

However, this is not so. Here are the results of a simulation based on the operation of an air compensator limited to 12 hours, that is, from 6:00 am to 6:00 pm.

Simulation of a 12-hour operation

Energy consumption – electric coil

Electricity required: 308,743 kWh/year
Maximum power demand: 235 kW in January
Cost of electrical energy: $10,373/year
Cost for power demand: $21,395/year
Total cost: $31,768/year

Energy consumption – natural gas exchanger

Natural gas required: 36,669 m³/year
Cost of natural gas energy: $16,444/year
Total cost: $16,444/year

As can be seen, using natural gas is even more economical than using electricity.

Notes:

Simulation of the electricity bill: the 2nd tier of Hydro-Québec’s Rate M was used since this was the lowest and most advantageous rate for this type of building.

Simulation of the natural gas bill: Gaz Métro’s Rate D1 was used, assuming just one appliance, i.e., a fresh air compensator running on natural gas. This is the least advantageous situation possible for this type of building.

The simulations used rates in effect on October 15, 2013.

Finally, since the costs of the appliances studied (electricity or natural gas) are usually similar, the choice of appliance depends more on the installation costs. In this respect, Gaz Métro offers its customers financial assistance aimed at compensating up to 45% of the costs of purchasing and installing natural gas appliances.

For more information

Conclusion

In conclusion, the operation of a ventilation air compensator is by far more economical when it runs on natural gas. Owners and managers of residences or multi-tenant buildings who wish to reduce their energy costs should always analyze their choice of energy sources and evaluate the feasibility and affordability of natural gas for ventilating their existing buildings or those under construction.

Roger Gauvin Eng., CEM
Advisor, Energy Efficiency &Technologies
DATECH Group