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Condensing Boiler Optimization
The document summarizes research on improving the efficiency of hydronic heating systems through retro-commissioning of condensing boilers. Researchers monitored the performance of 6 homes with existing condensing boilers, identified opportunities to optimize settings, and measured efficiency gains after adjusting the outdoor reset curve, supply water temperature, and domestic hot water settings. The research aims to develop best practice installation and operation guidelines to improve contractor confidence and consumer savings from condensing boiler upgrades.
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Condensing Boiler Optimization
- 1. Better Buildings: BetterBusiness Conference 3/3/2017 This project was supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through the Conservation Applied Research and Development (CARD) program. Research Update: Rebecca Olson Neighborhood Energy Connection Ben Schoenbauer Center for Energy and Environment
- 2. Introduction to HydronicHeating
- 3. Introduction to HydronicHeating • Non-condensing vs. condensing – Conventional boiler: condensation of combustion gases can rust out heat exchanger – Condensing boiler: condensation of combustion gases is optimum for efficiency • Difference in return temperature requirement – In order to get combustion gases to condense, the return water temperature needs to be below ~130° • Radiator types – Radiator types and size play a significant role in the ΔT between supply and return temps. • Issues with replacement from one to other – It’s important to optimize efficiency when replacing a conventional boiler with a condensing boiler based on the above factors
- 4. Hydronic Heating inMN • Approximately 30% of MN homes are heated by a boiler • Most of these are in older cities like St. Paul and Minneapolis • Some in northern locations where central A/C is in less demand
- 5. Condensing Boilers • Howit works: – 2nd condensing heat exchanger – Less waste heat up the chimney – If return water temp is low, more heat is exchanged from the combustion gases to the boiler water: increasing efficiency – Supply temperature, flow rates and radiator type/size dictate return water temp.
- 6. Need for CondensingBoiler Research • Lack of modulating condensing boilers in residential market • Evidence that HVAC contractors and utilities have inconsistent confidence in products • Prior research showing how important return temperature is on condensing boilers— commercial and hydronic air handler studies • Need for quality installation protocol for utility savings and cost benefit confidence
- 7. Contractor Hesitance andCost • Cost of condensing units is generally high and variable • $6,500--$15,000 installation cost range • Lack of confidence in operation at high efficiency • Issues with early models and maintenance callbacks • Confusion about supply set-temps and condensing rate optimization • Not as many model options in this market as condensing forced air systems
- 8. Prior Research andInformation • Conclusions: – Return water temp is a primary factor – Flow rates can influence return water temperature – Outdoor reset needs to be installed and set-up properly – Needed more info pertaining to MN housing stock, radiator types, and climate as well as more field implementation guidelines • Building America—Butcher/Arena • Commercial Boiler study—CEE Russ Landry • ASHRAE Handbook
- 9. Prior Research andInformation
- 10. Research Project Structure •Field and Market research • Existing condensing boiler monitoring • Draft retro-commissioning activities • Monitor savings after retro-commissioning • Development of Quality Installation Protocol for Utility rebates based on savings from retro- commissioning • Work with contractors to install condensing boilers in homes using QI protocol • Information dissemination through webinars, presentations and published reports
- 11. Research Project Timeline TaskName 1 Assessment 2 Existing boiler monitoring 2.2 Exisiting monitoring 2.3 recommissioning 2.4 Post monitoring 2.5 refinement of recommissioning checklist 3 New boiler installation 3.2 Installation 3.3 Monitoring 3.4 refinement of installation checklist 4 Final report Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
- 12. Market Research Structure •Interview HVAC contractors about installation • Procedures • Pricing • Barriers • Incidence • Interview homeowners about performance • Comfort • Maintenance • Issues • Interview Utilities about rebate development and rationale
- 13. Early Market ResearchResults • Interview HVAC contractors about installation • 6 companies interviewed so far including a supplier • Low volume of boiler replacements and even lower volume of condensing • Some hesitation on cost vs. performance • Costs seem to be inconsistent with equipment and labor details • Interview homeowners about performance of existing condensing Boiler • Comfort is very high in existing sites • Maintenance does not seem to be an issue with any of the sites • Most sites relied on contractor to choose model • All 6 residents said they would recommend condensing system to others • Interview Utilities about rebate development and rationale • Preliminary discussions indicate some utilities were worried about cost effectiveness. • They may be getting high installation cost estimates, and not have a lot of confidence in the efficiency
- 14. Field Research PhaseI • Characterization of Typical MN households • Based on aggregate consumption data from existing programs • Participant solicitation/selection • 6 sites chosen with existing modulating condensing boilers installed within the last 5 years • Sites have varied heating loads and construction characteristics • All homes have cast iron radiators • Some have other convector types, (i.e. baseboard, in-floor, low mass) • 3 sites have indirect water heaters • Monitoring • Gas usage • Supply and return water temperature • Flow rates • Condensation rate
- 15. Field Research PhaseI • 1st half of 2015/2016 heating season, monitored as installed • Made minor changes to optimize efficiency • Adjusted supply temp • Optimized turn-down ratio • Attempted to change DHW supply temp and flow rate • 2nd half of 2015/2016 heating season, monitored after adjustments • Measured savings from 1st half to 2nd half • Developed draft quality installation protocol
- 16. Site Selection Criteria •At least 1 home per typical heating load quartile (420 to 700, 700 to 830, 830 to 1275, and >=1275 therms/yr)—based on MN aggregated residential utility program data • At least 1 of each of the top 3 manufacturers—identified by utility rebate and local sales info • A variety of installers • MN program databases suggest between 30-36% of condensing boiler installs had indirect water heaters • National Grid study found 30-40% of outdoor reset were not installed or installed poorly • A variety of emitter types. Cast iron radiation, Low mass radiation, baseboards, and in-floor heating
- 17. Site Selection andRecruitment • 17 recruited homes had smaller loads than typical homes (Avg 720 therm/yr) – In selected 6 sites, larger usage homes were slightly under represented • Identified 6 different manufacturers in recruitment. – Top brands based on supplier and utility rebate data are represented in 6 selected sites – Triangle tube, Buderus, Bunham, Weil Mclain all included • 11 different installers in recruited homes – 5 different installers in selected sites
- 18.
- 19. Phase I PreRetro-commissioning Daily Measured Performance
- 20.
- 21. Phase I: As-foundperformance Site Mode Space Heating DHW Combined Space Heating DHW therms/yr therms/yr d215_ex_02 As-found 837 na na 88.4% na d215_ex_03 As-found 536 112 88.0% 90.5% 76.1% d215_ex_05 As-found 88.0% 91.6% 70.3% d215_ex_06 As-found 669 na na 95.1% na d215_ex_07 As-found 731 na na 89.0% na Annual Energy Use Annual Efficiency 459 94
- 22. Retro-Commissioning Actions • LoweredSupply Temperature – Determined reasonable level to still meet load, but lower return temp. to optimize efficiency • Adjusted overall Reset Curve – Maximum supply temperature output at -12° vs. default of 0° – This will lower the slope of the curve making more points along the curve in the condensing mode • Adjusted DHW Supply when possible – Based on lower efficiencies of indirect tanks as well as indications of unused capacity – This was either impossible, or didn’t actually have an effect on the return temp because of heat exchange capability
- 23. Phase I SiteExample (D215_ex_07) • Boiler for space heating only • 6 Cast iron radiators • 2 low mass radiators • 3 zones w/ 3 t-stats Min Max 28,500 99,000 at 140 Sup T at 180 Sup T 35,000 65,000 Capacity Estimates and Ratings (Btu/hr) Boiler Output Emmiters Design Heating Load (Bill Analysis) at -12 F OAT 38,500
- 24. Supply Temperature Optimization •Calculate or estimate the home heating load • Calculate or estimate the emitter capacity • Minimize the supply water temperature so that the house load can still be met
- 25. Phase I SiteExample (D215_ex_07) At 0 °F the house needs ~25,000 Btu/hr
- 26. Site Example(D215ex07)Emitter Capacity 130°Fwill deliver the needed 25,000 Btu/hr at 0°F OAT.
- 27.
- 28. After retro commissioningperformance Add figure
- 29. Site Example (D215_ex_03)- DHW • Boiler and indirect water heater • 6 Cast iron radiators • 41 Gal WH set at 130°F Min Max 22,745 75,200 at 140 Sup T at 180 Sup T 22,234 41,997 Design Heating Load (Bill Analysis) at -12 F OAT 28,925 Capacity Estimates and Ratings (Btu/hr) Boiler Output Emmiters
- 30. Site Example (D215ex 03)
- 31. DHW Optimization • Doesthe current DHW capacity meet load? • Minimize the supply water temperature – Many older boilers fix the DHW temperature • d215_ex_03 • Minimize the DHW loop flow rate – Manual pump adjustment – Or boiler settings
- 32. Site Example (D215ex 03)
- 33. After retro commissioningperformance Pump Speed DHW GPM Return Water T Capacity (Btu/hr) Avg Eff As-Found High 2.8 133 53,100 76.1% Opt_1 Med 2.5 131 49,300 77.3% Opt_2 Low 2 129 45,500 77.9%
- 34. Comparison Chart Site Mode Annual Combined Eff Annual SpaceEff Annual DHW thermal Eff d215_ex_01 As-found 82.6% 86.2% 62.6% d215_ex_01 Opt - OAT Curve d215_ex_02 As-found na 88.4% na d215_ex_02 Opt - OAT Curve na 90.2% na d215_ex_03 As-found 88.0% 90.5% 76.1% d215_ex_03 Opt- Clean 88.2% 90.5% 77.1% d215_ex_03 Opt - DHW 1 88.4% 90.5% 77.3% d215_ex_03 Opt - DHW 2 88.6% 90.5% 77.9% d215_ex_05 As-found 88.0% 91.6% 70.3% d215_ex_05 Opt- NA d215_ex_06 As-found na 95.1% na d215_ex_06 Opt - NA d215_ex_07 As-found na 89.0% na d215_ex_07 Opt - OAT Curve 90.4% Analysis On-going No Optimization to be Done No Optimization to be Done
- 35. Conclusions Optimization • Enablingrecommended OAT reseat curve will achieve majority of savings – Further optimization is possible, but not typically cost effective • Water heating has room for improving efficiency – However the system must be designed for low water temperatures (which appears to be uncommon)
- 36. Field Research, PhaseII • Selected 7 homes looking to replace non-condensing boiler with condensing boiler • Used similar solicitation and selection criteria to Phase I • Worked with 4 contractors to install new boiler in accordance with draft QI protocol in selected sites • Monitoring (same as existing sites) • Gas usage • Supply and return water temperature • Flow rates • Condensation rate • Measure efficiency and compare to Phase I to estimate potential savings associated with QI protocol
- 37. Phase II Set-upProcedure leading to QI guide • Boiler sized according to manual J – Ensure minimum firing rate is as low as possible • Ensure outdoor reset is installed and meets manufacturer specifications. – Ex. Not located in sunny location or in exhaust path • Set appropriate reset curve
- 38.
- 39. Comparison of AnnualSpace Heating Efficiency • On average condensing boilers saved 14% annual space heating costs over 80% AFUE boilers. • Indirect water heaters were equivalent to a 0.70 EF power vent WH
- 40. Preliminary Conclusions • Condensingequipment meet expected savings – Even boilers with non-aggressive reset curves are efficient because set temp is reached before supply water temp gets to high limit – 1-3% savings potential based on intensive set-up procedure—emitter capacity measurement – Not cost effective • DHW performance is more aligned with Power vented water heater than condensing – Not recommending unless cost competitive with power vent
- 41. Dissemination Plan • Holdseveral webinars and live presentations about project findings • Work with contractors involved in interviews as well as installations to adopt QI protocol and hone in on pricing • Work with WX agencies to determine assessment protocol for condensing boiler work scope • Work with gas utilities not currently offering rebates for mod cons to implement rebate with QI protocol required • Publish final research report and send out with DER newsletters etc.
- 42. Future plans • Analyzecost effectiveness • Finalize optimized reset curve recommendations • Communicate with utilities about Basic set-up protocol • Get information out based on Dissemination plan • Publish final research report and send out with DER newsletters etc.
- 43. THANKS This project wassupported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through the Conservation Applied Research and Development (CARD) program. Ben Schoenbauer Senior Research Engineer Center for Energy and Environment bschoenbauer@mncee.org 612-244-2413 Rebecca Olson Residential Energy Program Director Neighborhood Energy Connection rebeccao@thenec.org 651-789-5705
Editor's Notes
- #17 Clean this up with the next slide
- #18 Clean this up with the next slide
- #21 Comment on the fact that the heating efficiencies from these boilers are actually pretty good annual efficiencies in the 85-95% range. In our 6 sites were haven’t seen the horror case of a 96% afue boiler running at below 80% Note the efficiencies ~80% at the 100-120 return water temperature are DHW only efficiencies. Not sure what’s going on inside the boiler that is limiting the efficiency at similar return water temps in the space heating loop. More investigation is required. If you’d prefer to avoid this discussion point there is an efficiency vs OAT plot pasted underneath this image. You could use that instead
- #22 Comment on the fact that the heating efficiencies from these boilers are actually pretty good annual efficiencies in the 85-95% range. In our 6 sites we haven’t seen the horror case of a 96% afue boiler running at below 80%
- #25 Can boost those efficiencies by reducing the supply water temp How do we do that? We need: - House load (NEAT Manual J utility bills) - Emitter capacity (we are developing a form) - Set the supply temperature at the min temperature (reset curve) needed to meet the houses need with the emitters it has installed
- #27 This is supply water temp. We will need to provide a way to estimate this without monitoring
- #28 Shows the impact of outdoor reset curve on the supply water temperature of the boiler for individual events. Why do we only see minimal savings on next slide for the large impact here? 1. This plot doesn’t really show the number of events. Vast majority are between 20 and 50F where difference in supply between opt/as-found is about 5 to 10 deg 2. Heat transfer rate isn’t linear with supply T. Ie supply temp reduction does not directly equal return temp reduction
- #29 Daily efficiency before and after optimization. Increase is small, but present. Despite big changes to the supply temperature, boiler isn’t gaining much in actual daily efficiency
- #31 Note this is thermal efficiency for the water heater. DOES NOT include tank losses. This data estimated the split of the space heating and dhw efficiency for each day. So every monitored day has both a red and a blue point (except days with OAT>60 where there was no space heating). at temps near -10 the whole site efficiency will be very close to the red point. Space efficiency >>> water heating efficiency
- #33 Hot water capacity is about 53,000 Btu/hr Typical 40 gal gas water heaters have a capacity of 30,000-40,000 btu/hr, so this site also has more capacity than a typical water heater The average runtime is about 10 minutes. There are relatively few long draws. That means most of the events are just firing to reheat from a small tank temp drop, and aren’t responding to large hot water events. All runtimes are below 25 min. THE overall point is. THERE IS ROOM TO REDUCE WATER HEATING CAPACITY. Current the site is at about 75% dhw efficiency and 90% space heating efficiency -> annual combined efficiently around 88%. Boosting the DHW efficiency up to 85% would increase the annual efficiency 2-3%. These are small increases, but they show that potentially the problem isn’t as great as we considered and the there are steps to be taken to reach the desired efficiencies.
- #34 Models for daily efficiency at site:d215_ex_03 Differences in as-found, cleaned heat exchanger, opt_1, opt_2 -> all small and not statistically significant on their own, but combined analysis shows they are significant. However, as-found to opt 2 increased eff DHW eff 1.8% -> overall site eff 0.6% Take away here is that our ability to modify the DHW is largely dependent on the set up. DHW efficiency will only be improved if the indirect and DHW heat exchanger can operate at low temps Optimization focused on flow speed reduction via manual pump setting as-found=2.8 gpm, opt1=2.5gpm, opt2=2 gpm Only about a 2 deg reduction in return water temp
- #35 Optimization results: “Savings” at 3 sites. OAT reset yeilds savings, but when already running at 90% only 1-2% increases feasible DHW savings are difficult with in-effective double wall heat exchangers Site 5 wasn’t responding to our changes. Take Away: Meet the basic install requirements and optimzation will not be necessary!
- #39 Space heating efficiency to date for the newly installed condensing boilers
- #40 Comparing the new, existing as found, and existing optimized space heating efficiency Based on what it would be if 80% AFUE calculated.
- #41 General practice is pretty good, and potential for 1-3%
- #42 Combine this with next slide
- #43 The 14% space heating increase is equal to about $120/year in a home that has a 1000 therm/year heat load (~45,000 btu/hr design load)