Designing a Highly Efficient Combination Space/Water Heating System

Transcription

1 Designing a Highly Efficient Combination Space/Water Heating System Presented By: M. Thomas Contributors: ACS Hayden, E Entchev, D Mackenzie, of CanmetENERGY, NRCan P Edwards & H Tikiryan - Consultants CMX-CIPHEX 2012

2 Acknowledgements This work was enabled in part by funding from the OERD s Program of Energy Research & Development. We would like to acknowledge our NRCan Colleagues at the Office of Energy Efficiency (OEE) for their support of this project work.

3 Definition: Combined space and water heating system ( combo ) a product or a group of individual components that form an integrated system that is designed to provide space heating and water heating. Note: We discourage the use of Combos based on noncondensing storage tank water heaters + fan coils.

4 Combination System Layout - Non Segregated

5 Combination System Layout -Segregated

6 Why Build a Combination System? Integrating the space & water heating functions can lead to synergistic efficiency benefits. New, more-energy-efficient housing has made the overall space and water heating loads comparable As space heating loads drop the water heating load becomes more significant The combination of the two loads makes the technology suitable for low energy or near net-zero housing. Venting & gas/fuel connections are simplified resulting in a compact, potentially lower cost, installation.

7 Good Combination System Design Use a Condensing (90% efficiency plus) appliance as the heat source. Ensure return water temperature is low enough for condensing (i.e. have enough heat exchange area). Control Fan Speed - Too cool circulating air feels uncomfortable. Avoid excessive pipe / casing heat losses (heat doesn t go where you want it) Use R8 where possible. Use energy efficient ECM fan motors & water pumps. Avoid High cyclic losses due to poor control strategy. A segregated system is preferable to a non-segregated system (water quality issues). Overall system sizing and control strategy is important (Right- Sizing)

8 Pitfalls in Combination System Design Beware of Buffer Tanks, that can use a lot of energy to keep them hot Built-in (as shown) or External. The same goes for the use of a continuous domestic hot water re-circulation loop in a house. A Simple Dip Switch Change will turn off the feature

9 Pitfalls in Combination System Design -Example In testing a high-efficiency, off-the shelf, combination system based on a condensing gas-fired tankless water heater (Rated EF= 0.98) and a connection box that linked the water heater to the ECM fan coil. - The Water Heating was efficient (EF = 0.94), but space heating was either ineffective (too low a heat output) or inefficient (<70%). -The cause was insufficient heat exchanger size and thus return water temperatures were well above 60 ºC (flue gas dewpoint) so the system did not condense. ** By adding additional heat exchange area, we significantly improved the performance, getting a space heating Efficiency of between 86% and 91% while keeping the water heating EF at 0.94.

10 Pitfalls in Combination System Design -Example Original flat plate heat exchanger (1 of 2) Additional flat plate heat exchanger Note, it s well Insulated

11 Pitfalls in Combination System Design -Example The CSA P.9 Thermal Performance Factor: Output = 45,000 Btu/h, TPF = 0.82 Composite Space Heating Efficiency = 0.80 Water Heating Performance Factor = 0.94 The space heating efficiency at lower heat outputs is highly influenced by the electrical energy consumption, i.e. The Steady State 40% heat output may be 90% but when the electrical power is considered this can drop down to 80% with a Fixed Fan Speed.

12 Pitfalls in Combination System Design -Example Flue Loss Efficiency Fuel including Electrical Energy Efficiency Efficiency (%) y = x R 2 = y = x R 2 = Fraction of Maximum Heat Output (%)

13 Combination System Performance To Conclude (1) When using a Condensing Appliance heat source Ensure that it will condense across the input range. (2) Insulate the Pipework and Casing well. (3) The Control Strategy is important. (4) Consider the electric power consumption as well as the fuel efficiency (5) The Combination System TPF may not be as high as a Furnace AFUE, but the performance may be equivalent or Better.

14 Questions