Geothermal implications
Geothermal operating temperature limits in hydronic applications are typically around 46 C (115 F). At higher temperatures most geothermal systems will not operate well, or will operate with a very poor coefficient of performance (COP) (the amount of heat required divided by the amount of energy used to do produce that heat). However, the low-operating temperature and ‘free energy’ heat source of geothermals permit the design of an extremely efficient heating system.
Boiler implications
Modern energy efficient condensing boilers achieve four things:
- Operate at a very high annual fuel utilization efficiency (AFUE) value;
- Modulate supply water temperature based on outdoor air temperature;
- Modulate gas usage based on system demands; and
- Extract energy from exhaust gases by cooling them to the point where moisture contained in the gases condense and release additional energy (‘latent’ energy at an approximate rate of 1,000 Btu/L[lb] of water).
Even though a boiler can produce 82 C (180 F) water, this should not be set as the system’s operating temperature. High-efficiency boilers are meant to condense. Condensing is a process in which the exhaust vapours in the combustion process change from gas to liquid. To achieve condensation, the return water temperature must be 54.4 C (130 F) or less. A good rule of thumb to follow is the lower the exhaust temperature, the more efficiently the system is operating. Designing the system to operate at 60 C (140 F) or less will typically allow the boiler to operate in the high-efficiency range.
Conclusion
Careful system and heat exchanger design is critical when making use of the latest low-temperature and low-energy cost heat sources that are available. Proper selection of the heat exchanger component is essential to providing long-term customer satisfaction.
Steve Hamoen is the founder and acting president of Zonelife Inc., a contracting and design firm specializing in custom hydronic and geothermal systems for residential and commercial applications in Cambridge, Ont. Hamoen can be reached through his blog at www.geothermalinstallers.ca, or by e-mail at steve@zonelife.ca.
