The performance of tankless coil boilers – residential hydronic heaters in which a finned copper coil has been inserted into the boiler water volume for domestic hot water production – and of a residential boiler with an external heat exchanger for hot water has been evaluated in a detailed laboratory study. While heating boilers with separate domestic hot water (DHW) “indirect” tanks are also available and have higher annual efficiency, tankless coil boilers remain a very important part of the hydronic heating market with advantages including lower cost and smaller space requirements.
In this project, the output capacity of common tankless coils installed in boilers was evaluated following a standard protocol. In these tests, the measured capacity was found to be lower than the nominal capacity. The efficiency of these systems was measured over a 24-hour simulated use test and found to range from 30.8 to 40.8% under summer, DHW load only, conditions. The low efficiency under this low load condition derives from the need to maintain a high boiler temperature constantly to deliver DHW when needed.
Several approaches to improve the DHW production efficiency of systems of this type were evaluated. One of these involved new design approaches with an external, plate-type heat exchanger in a pumped loop. This system was found to have better thermal coupling between the boiler water and the domestic water, enabling better heat production at lower boiler water temperature. The system also included a control which enabled the boiler to fall to room temperature in the case of no domestic hot water draw. This system was shown to have an efficiency for domestic hot water production as high as 67% but there is a time delay in DHW delivery after a long idle period. This type of control approach was also evaluated with conventional tankless coil boilers.
Citation: 2020 Virtual Conference Papers
Product Details
- Published:
- 2020
- Number of Pages:
- 9
- Units of Measure:
- Dual
- File Size:
- 1 file , 1.7 MB
- Product Code(s):
- D-VC-20-C058
