Domestic hot water energy consumption represents the second largest energy end-use point in houses after space heating. Though the heat-on-demand, no standby energy loss features of tankless water heaters can be expected to reduce hot water-related energy consumption, little data is available about the actual energy savings of tankless water heaters compared to conventional hot water tanks.
Notwithstanding the energy-efficient features of tankless heaters, there are certain operational aspects of such systems that may affect their overall performance. For instance, when a hot water tap is opened, a clothes washer is activated or a dishwasher is started, tankless water heaters may allow water to flow through them for some short period of time before they start heating water. Once started, it may also take the heating unit some finite time to become fully operational.
This can result in a delay of hot water arriving at the tap, and may also result in a waste of water as the user waits for the hot water to arrive. Additionally, the capacity of tankless water heaters to provide "endless" hot water for showers may lead some users to consume more hot water than they otherwise would (or could) with storage-type water heaters.
To better understand the relative performance of storage and tankless water heaters, Canada Mortgage and Housing Corporation (CMHC) initiated a research project in collaboration with Enbridge Gas to answer the following questions:
1. Do gas-fired tankless water heaters result in less gas being used for water heating than their storage tank alternatives? If so, how much?
2. Is there a difference in the amount of water used by a family with a new tankless water heater, compared tothe family's old usage with the storage-type heaters? If so, how much?
3. Are there any other notable differences in how occupantsperceive the performance of tankless and storage-type water heaters?
The overall approach for the research involved the characterization of water heating energy use and hot water consumption in houses, before and after the conversion of the water heating system from storage-type water heaters to tankless water heaters. Thirty-one houses, in various cities across Ontario, were initially identified as candidate test houses, given the intentions of the homeowners to convert from storage-type to tankless heaters. Twenty-three houses continued through the full length of the project. The number of occupants in the houses ranged from one to five, with an average of 3.5 people per household. The pre-retrofit storage-type heaters ranged in age from two years to over 25 years with an average age of 10.2 years. Just over 70 per cent of the storage tanks were rentals.
Hot water use was measured with water meters installed on the inlets to the water heaters and natural gas meters were installed to measure the natural gas consumed by the water heaters. After a three-month monitoring period, the storage type heaters were removed and replaced with tankless water heaters and monitoring then continued for another three months. Hourly gas and hot water use data, pre- and post-installation of the tankless units, were then analyzed.
The gas consumption data were adjusted to remove the impact of typical changes in water heater energy consumption due to changes in the water temperature delivered from the water mains. This was accomplished by normalizing the gas consumption for each day to an August reference using monthly seasonality factors that have been developed by Enbridge.
The monthly seasonality factors were applied to each day's gas consumption, corresponding to the month in which each day fell. Water consumption data were not adjusted.
Surveys were conducted to gauge homeowner impressions of the performance of their new tankless water heaters; and to determine if there had been any changes in the households during the study period that would have impacted hot water consumption during the pre- and post-retrofit monitoring periods. Based on the results, two houses were removed from the study and two other houses had specific time periods removed from the analysis that corresponded to homeowner absences. Individual days that showed less than four litres of hot water usage during the pre- and post-installation periods were removed from the data set that was used for analysis.
Note that the methodology did not consider the impact that the hot water heaters may have had on space heating and cooling energy consumption. Therefore, overall household natural gas savings resulting from a conversion from storage-type to tankless water heaters may not be as large as the natural gas savings reported in this study.
The daily water and gas consumption (normalized to August) data for each site were tabulated and compared for the "before" and "after" installation periods. To investigate how the rated efficiency level of the new water heaters might influence the results, the data were separated into two categories, representing the non-condensing and condensing tankless water heaters. Non-condensing water heaters were installed in four homes, while condensing water heaters were installed in the remainder (19).
The results of the analysis of the energy and water consumption pre- and post-retrofit are shown in Figure 1. More detailed information for each house, including the average daily normalized gas consumption of the water heater at each site before and after the retrofits, is available in the original Research Highlight.* An analysis of the extent of the changes is also provided.
Overall, there was, on average, a 46-per cent reduction in natural gas used for water heating after the installation of the tankless water heaters. Even though the condensing tankless units are designed to be more energy efficient, on average, the non-condensing tankless heaters saw the greatest energy savings. However, given the small sample size and the differences in the storage-type water heaters installed in the homes, no conclusions can be drawn concerning the performance of the condensing and non-condensing tankless heaters.
One reason the homes with the non-condensing tankless heaters achieved greater natural gas savings is that the four houses that had these systems installed saw their usage of hot water (see the detailed table in the original Research Highlight*) decrease by an average of 13 per cent, compared to an average increase of five per cent for the houses with the condensing units. Considering all of the sites, there was an average increase in hot water use of two per cent after the installation of the tankless water heaters.
There was a large fluctuation in the pre- and post-installation hot water usage from house to house that was influenced by how tankless water heaters heat and deliver hot water and changes in occupant usage. The availability of an endless supply of hot water might explain the 51-per cent and 62-per cent increases in hot water use seen in two of the sites.
Fourteen of the 25 survey respondents expressed that the "endless" hot water was something they liked about the tankless system, while 10 respondents indicated that they enjoyed the energy savings. Fourteen of the 25 survey respondents expressed a dislike for the increase in the time it took to get hot water to the tap with the tankless heaters.
The average increase in the time delay to get hot water at the tap was reported in the occupant survey to be around 20 seconds. Three of the respondents (one owner, two renters) commented that they disliked the high cost of the units.
Implications for the Housing Industry
Based on the results of the study, replacing the existing natural gas-fired, storage-type water heaters with gas-fired tankless water heaters can result in significant natural gas savings for water heating. On average, 0.63 m3/day or 230 m3/year of natural gas was saved. At $0.30/m3, this translates to an average savings of $69 per year. It may also be concluded that, based on the study results, the installation of tankless water heaters may lead to an increase in hot water usage, though the extent of any such increase would be highly dependent on individual occupants.
The availability of tankless hot water heaters provides builders, renovators and consumers with a water heater option that can save energy and associated costs, reduce space needs, and provide "endless" hot water. Careful design and installation of tankless water heating systems may help overcome issues related to hot water delivery time.
Source: Canada Mortgage and Housing Corporation (CMHC). Monitoring Performance of Retrofitting from Tank to Tankless Water Heaters –Research Highlights – Technical Series 11-101, 2011. All rights reserved. Reproduced with the consent of CMHC. All other uses and reproductions of this material are expressly prohibited. *www.cmhc.ca/odpub/pdf/67548.pdf