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Condensing Boilers: Are they cost effective?
This document summarizes research on the cost effectiveness of condensing boilers. Field research was conducted on existing condensing boilers, which found average efficiencies of 90%. Retrocommissioning actions like lowering supply temperatures and adjusting reset curves improved efficiencies by 1-3%. On average, condensing boilers provided 13% annual savings over standard boilers. The average price difference of $2,300 between condensing and standard boilers means condensing boilers have a simple 25-year payback and are cost effective over the lifetime of the unit, with a path to a 10-year payback.
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Condensing Boilers: Are they cost effective?
- 1. Condensing Boilers—Are theyCost Effective? Rebecca Olson, Director of Residential Programs Ben Schoenbauer, Senior Research Engineer 2018 ACEEE Summer Study on Energy Efficiency in Buildings August, 2018 Center for Energy and Environment 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.
- 2. Pg. 2 Need forCondensing Boiler Research • Lack of modulating condensing boilers in residential market • Evidence that HVAC contractors and utilities have inconsistent confidence in products and savings • Potential need for quality installation protocol for utility savings and cost benefit confidence
- 3. Pg. 3 Condensing BoilerOperation • Has 2nd condensing heat exchanger • If return water temp is low, more heat is exchanged from the combustion gases into the boiler water • The low temp. combustion gases condense—increasing efficiency • Supply temperature, flow rates, and radiator type/size dictate return water temp
- 4. Pg. 4 Return WaterTemp. and Efficiency
- 5. Pg. 5 Research ProjectStructure • Field and market research • Existing condensing boiler monitoring • Draft retrocommissioning activities • Monitor savings after retrocommissioning • Development of Quality Installation Protocol • Work with contractors to install condensing boilers in homes using QI protocol
- 6. Pg. 6 Market Research •Interview HVAC contractors about installation • 6 companies interviewed, including a supplier • Some hesitation on cost vs. performance • Interview homeowners about performance • Comfort is very high in all sites • Four participants stated they’ve analyzed their bills and have savings between 10% and 40% • Analyze installation costs of condensing boilers vs. non- condensing boilers • 73 bids/invoices for 32 homes
- 7. Pg. 7 Field ResearchPhase I Set-up • Characterization of Typical MN households • 6 sites chosen with existing modulating condensing boilers installed within the last 7 years • All homes have cast iron radiators • Some have other convector types, (i.e. baseboard, in-floor, low mass) • Monitoring package: gas usage, supply/return water temp., flow rates, and condensation rate
- 8. Pg. 8 Phase I:As-found Efficiencies • Average space heating efficiency was 90% • Why were these boilers working so well? • Outdoor reset • Temperature control • Emitters capacity
- 9. Pg. 9 Outdoor Reset ColdestDay WarmestDay MinimumSetpoint Maximum Setpoint
- 10. Pg. 10 Controls &Supply Water Temp ~ 1 hour into the event
- 11. Large Capacity LowTemperature Emitters
- 12. Pg. 12 Phase I:Daily Measured Performance (As-is)
- 13. Pg. 13 Retrocommissioning Actions •Lowered Supply 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
- 14. Pg. 14 Phase ISite Example (exist_07) • Boiler for space heating only • 6 cast iron radiators • 2 low mass radiators 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
- 15. Pg. 15 Site Example(exist_07) Improvement
- 16. Pg. 16 Phase IIInstallation Procedure • 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
- 17. Pg. 17 Comparison ofAnnual Space Heating Efficiency • On average condensing boilers saved 13% annual space heating costs over 80% AFUE boilers
- 18. Pg. 18 Boiler PerformanceConclusions • Condensing equipment meets expected savings • Even boilers with non-aggressive reset curves are efficient • 1-3% savings potential based on intensive set-up procedure • Customer satisfaction very high • Emitter capacity in homes that have all or some cast iron radiators is more than enough to adjust re-set curve confidently
- 19. Pg. 19 Cost Effectiveness •Average savings over non-condensing equipment • 13% -- with an average annual cost savings of around $100 • Price difference required for 25 year simple payback is $2,500 • Average price difference was around $2,300 • However, price difference was highly variable • $3,700-$13,000 non-condensing, $5,700-$17,000 condensing • On average, cost effective if based on lifetime of the unit • Likely a path to get to 10 year payback based on our market research
Editor's Notes
- #2 Hi everyone. Thanks for attending today. I’m going to talk about the results from our field assessment of residential condensing boilers. As alluded to, the final report [for this project] will be out shortly and available on the DER and CEE websites. So, if you’re looking for more follow-up information afterwards, that will be available.
- #3 The low volume of condensing boiler replacements in the market and anecdotal evidence that contractors were talking customers out of condensing boiler replacement, prompted us to dig deeper. We learned that there was a widespread conception that condensing boiler efficiency wasn’t as good as listed and, therefore, not worth the upgrade cost. We also spoke with some utilities that had lower confidence in the energy efficiency of condensing units, particularly on installation procedures, so we wanted to explore a potential quality installation protocol as a way to increase performance of the systems.
- #4 Condensing boilers have a second heat exchanger in order to extract as much heat out of the combustion gases as possible, before sending waste heat up the chimney. In order to get the most heat out of the gases, the return water that it is being transferred to needs to be as low as possible to maximize that transfer. The factors dictating return water temp. are how high the supply water temp is to start with, how much radiating capacity the emitters have, and how fast the water moves through the system.
- #5 This chart shows the increase in boiler efficiency in relation to return water temperature. As you can see, efficiency slowly but steadily increases as the return water temperature goes down from 240˚ to about 130˚. At 130˚ return water temperature, you see an exponential jump in efficiency, and that has to do with the condensation of the combustion gases. This points to the 130˚ mark as quite an important threshold, and ideally we get return water temperatures as low as possible to maximize efficiency.
- #6 Now that I’ve laid out the background information, I’ll talk about our research project structure. Because we were interested in learning both about the performance of these systems, as well as the market penetration challenges, we did both market and field research. As part of the field research we monitored previously installed condensing boilers (up to 5 years old) as found. This was our non QI installation baseline. We then used what we learned from the “as found” performance and tips from other research conclusions to retrocommission these systems. We monitored performance within a range of climate conditions to see what kind of savings were possible. This helped us refine our QI protocol recommendations. Next, we worked with contractors and a different set of homeowners to replace non-condensing boilers with condensing to practice using our QI protocol. This allowed us to test out the ease of use and performance of that type of intervention. Now we are in the dissemination phase to let people know what we’ve found.
- #7 We interviewed 6 HVAC companies, one of which was an equipment supplier. Generally all HVAC contractors interviewed had a pretty low volume of boiler replacements as part of their business, and even fewer were condensing boilers. Again, I mentioned that we had some anecdotal evidence that contractors had discomfort over the price in relation to performance, and the interviews confirmed that evidence. One interesting thing is that there seemed to be pretty inconsistent responses to questions about the breakdown of labor and materials for condensing boiler replacements. During the homeowner interviews we wanted to get a sense of their motivation for replacement with condensing, as well as their satisfaction with this equipment. The responses were overwhelmingly and consistently positive. Comfort and energy saving satisfaction was high, and everyone said they would recommend this equipment to others. In our informal discussions with utilities about rebate development around condensing boilers, we received some indication that there was concern over condensing performance and savings claims. In addition, the cost/benefit calculations were likely not great because of high installation costs.
- #8 We knew we would have a fairly small sample size for this project, so it was important that we get a representative sample of MN Homes with boilers. To get homes across a wide range of heating loads, we used aggregate consumption data from existing CEE programs to determine typical heating loads for homes with boiler heating systems. We also analyzed the typical percentage of households with a condensing boiler integrated with domestic hot water. And, in order to address some of the findings from prior research about outdoor reset control, we wanted some with and some without re-set controls installed. There was also some evidence that emitter capacity could play a big role in the system efficiency based on the ability for the emitters to meet the load of the house, as well as scrub enough heat off to lower the return water temperature, so we looked for homes with a variety of emitter types and capacity to load ratios. Once we set up these parameters, we solicited participants and selected sites to meet our criteria. 6 sites were chosen for this phase. We accepted participants with boilers up to 5 years old. We didn’t want to study a system toward the end of it’s life because there would likely be too many unknown variables, but we did want to look at systems that weren’t brand new to see if there was any degradation of performance, since that was another concern we were hearing from contractors. We monitored gas usage, supply and return water temp, flow rates and condensation rate in order to make calculations on efficiency. Ben will discuss that detailed monitoring package in a little bit.
- #9 Phase 1 started with a detailed analysis of the already installed condensing boilers as we found them -- that is, without any CEE adjustments. In general, these systems performed really well. They had annual space heating efficiencies between 87% and 95%, with an average annual space heating efficiency of 90%. This is very good performance; a bit lower than the rated AFUE, but it is pretty common for a field measured efficiency to be slightly lower than a laboratory-rated setting, as the conditions of the test differ. Once we saw the high installed efficiencies, we started to look into why the boilers were preforming well. We found three key areas: outdoor temperature reset, general boiler temperature control measures, and the emitter types and capacities in our housing stock.
- #10 This first driver of high-efficiency performance was the outdoor reset. This control measure consists of an outdoor temp. sensor (upper left) and a control logic that changes the boiler supply temperature in order to match the boiler delivered capacity with the load of the home. The image in the upper right shows the control logic. The reset curve reduces the boiler supply water temperature as the outdoor temperature increases and the houses’ heating load decreases. The image shows the 4 parameters that can be controlled by the installer. First are the outdoor air temperatures that the coldest and warmest heating day temperatures can be set. For MN this is typically around -10 F to 60 F. Then the supply temperature can be set corresponding to these OATs. The factory default tends to be 180 on the coldest day (0 degrees) and ~100 or 120 on the warmest. We will go into more detail on how to set these parameters later, but even the factory defaults are a significant improvement over a fixed supply temperature. Finally in the lower left we can see an installed outdoor sensor. It is important to install the sensor in a location that can accurately read the typical outdoor temp. Follow manufacturer guidance, but in general you don’t want it always in direct sunlight or actually measuring the exhaust temperature.
- #11 This plot shows how the boiler controls actually help to optimize efficiency on their own. Each line on this chart shows a signal heating cycle at one site. We can see that the boiler temperature slowly ramps up over time. For the 30 Oat heating event (blue) it takes a little over half an hour to reach the OAT reset curve set point (130F in this case), and at 0 OAT it takes about an hour and a half to reach set point. If the load is met before the boiler reaches its target temperature the water temperatures in the system for the full event will be even lower than the reset curve setting, leading to even higher efficiencies. We can see on this chart a large number of heating events end around that 35/45 minute mark, where the temperatures drop sharply as the boiler shuts off. Those events were even more efficient than what the reset curve predicts.
- #12 The last key piece to the existing systems working really well was the emitters, with two major factors here: Most of the homes had some portion of the system (typically the majority of the area) heated with old cast iron radiators. Due to improvements in the home, boiler efficiency increase, and conservative sizing of past installs, these radiators usually have much higher capacity than needed to maintain the house; that meant there was room to reduce the supply water set point and run these high-capacity radiators at lower water temperatures. For new or replacement emitters there is a preference to move toward low temperature type emitters; these are radiant panels designed to work at lower temps (140 to 120) and radiator floors (~100 to 90). These two factors allow the other two factors (outdoor reset and temperature ramping) to meet the houses load at lower supply temps and lower flow rates, which increases efficiency.
- #14 Despite these high “as found” annual efficiencies we still wanted to try to optimize and improve performance. The optimization process was pretty straightforward, but took a little while to conduct. First I’ll go over the options and then walk through a couple case studies/examples. 3 types of adjustments were considered: Lowered Supply 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
- #15 The first example is an outdoor reset, supply temperature optimization. This is “as found” site 7. We found the “as found” efficiency was 89% for space heating. So, not a big potential for improvement, but we optimized anyways. Here are the basic specifications for this site: -We had a modulating boiler with a pretty good turn down, 99kbtu/hr. to 28 -We had a design heating load of 38500 btu/hr - We also looked at the emitter capacity and noted that at 180 supply temp., it had a capacity of 65 kbtu/hr, but at a lower 140 supply, it still had 35kbtu/hr capacity
- #16 Appling this optimization did improve the efficacy. We just didn’t have a lot of room for improvement. This plot shows the supply water temperature for every heating cycle in the “as found” mode, in black, and the optimized mode in red. We can also see the outdoor reset curve setting as solid lines. There are a few things to talk about here. We can see that we drastically reduced the supply water temps during optimization, because we calculated that the emitters had the capacity to meet the heating design load with a much lower temperature. The plot also shows that at we eliminated a lot of the highest temperature events in the “as found” cases. But due to the ramping controls we talked about earlier, we didn’t get as much efficiency increase as we could have. This plot sort of shows that density of cycles. Most events are clustered in the 120/130 supply range where only a small temperature reduction was seen.
- #17 The installation procedure was developed based on the phase II finding. The final procedure was pretty simple, since the “as found” systems were operating so well. First, ensure the boiler is sized proposedly using a manual J calc. When selecting a boiler, turndown rate is important. We want to allow that ramping behavior, so a range of modulation is necessary. Second, we want a good outdoor reset and we want it installed well, including locating the outdoor sensor in a place representative of the OAT. Finally, we want to get the reset curve set up well. While the limited savings of the optimization test DO NOT necessitate a full characterization of the emitters and site specific calcs., it is useful to follow the manufacturers recommendations and get as close as we can with reasonable effort. We can do this be selecting the reset curve based on the type of emitters present. Most manufacturers provide these adjustments, but a generalized version is shown here if they do not.
- #18 Here is a plot with the full project heating efficiencies. The big takeaway is that all phases and modes produced good efficiencies with an average annual performance of 90% efficiency, which resulted in about 13% savings over a baseline boiler. The baseline systems was the estimated performance of an 82% AFUE boiler. This was calibrated/verified against the utility bills prior to Phase II new installation. We only see 3 optimization cases here because exist 3 and 5 were only optimized, or DHW and exits_6 was so efficient (95%) that optimization was deemed unimportant.
- #19 So, in conclusion, our results showed that condensing boilers are meeting expected savings over non-condensing equipment. This is absent a QI protocol, however a few percentage points of efficiency could be gained if really dialing in the supply temperature based on the emitter capacity. This procedure is likely not very cost effective. We didn’t see any signs of pre-mature degradation at 5 years, even when little to no maintenance had been performed. Customer satisfaction is very high. The typical MN household tends to have plenty of emitter capacity to reliably reduce supply water temperature and still maintain comfort.
- #20 In order to understand the cost-effectiveness question, we need to look at the savings between non-condensing and condensing which Ben explained is around 13% or $100 annually on average. So, if we are looking at the simple payback corresponding to the typical lifetime association with a boiler of 25 years, we would need the price difference to be around $2,500. Based on our analysis, this is about where it is. However, most people want to recoup this investment faster, so looking at what it would take to get that ROI to be 10 years, the industry would need to shrink that to about $1,000. We did some investigation of equipment only costs between non-condensing and condensing and found that averaged right around $1,000.