Many investors, when deciding on a ventilation system and type of heat recovery unit, wonder how much it will cost to operate the system annually. This is not surprising, as a dream home is an investment for years to come, and it is important to make it wisely. We have a choice of gravitational ventilation, hybrid ventilation, and mechanical ventilation with heat recovery, or recuperation. The question is, how do we calculate this? This is not an easy task because each heat recovery unit operates under varying parameters of outside air.
The IX CHART program comes to the rescue, which, based on multi-year climatic data, calculates the energy consumption for each hour of the year and presents the results in the form of clear charts. In further considerations, we will ignore gravitational and hybrid ventilation and focus only on mechanical ventilation. A common mistake is to have a linear approach to the problem, i.e., if the operating cost of an 80% efficient heat recovery unit is 1000 PLN, then a 90% efficient heat recovery unit will cost 888 PLN, so I will not pay an additional 2000 PLN for a more expensive unit because the savings are only 112 PLN per year. But is this really the case?
To test this assumption, let's perform a simulation. Assumptions:
Heat recovery unit with a capacity of 300m3/h, operating throughout the year with a fixed work schedule, i.e., no work during the absence of household members, continuous work during the weekends.
Due to the fact that ventilation does not always operate at nominal capacity, we introduce a coincidence factor of 0.7, so the average ventilation capacity will be 300m3/h * 0.7 = 210m3/h
Electric heater. Price of electricity is 0.6 PLN/kWh (the most popular type of heater in recuperators)
Supply air temperature +20oC
Supply and exhaust fan with a total pressure of 400Pa (installation resistance + internal resistance)
Climate zone: Katowice
Variant 1 - Heat recovery efficiency of 80%. And what do we have? Below are the results - The operation of the recuperator will cost 901.5 PLN, of which the heater will consume energy for 625.4 PLN, and the fans will consume energy for 276.2 PLN.
Variant 2 - Heat recovery efficiency 90%
WOW - a 10% difference caused the energy consumption of the heater to halve! The energy consumption for the fans remained the same (assuming that the internal resistance of the more efficient heat exchanger of the recuperator remained the same).
So, ultimately, by increasing the recuperator's efficiency by 10%, we saved not 112 PLN but 312.7 PLN per year, which means that the simple payback period of the investment, i.e., paying an additional 2000 PLN for a more efficient recuperator, will be 6.4 years!
Therefore, it is really worth fighting for every percent of efficiency, as it translates into tangible savings!
Below, I will show a summary graph of how much the electric heater's operating cost (without taking into account the fans) depends on its efficiency, with a resolution of 5%.
PS: The use of a water heater powered by a gas boiler reduces energy bills for heating by approximately 2.5 times, due to the lower cost of 1 kWh (about 0.21 PLN/kWh), so it's worth considering ;)
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