The calculator below will get you started on figuring your payback period. You'll need to measure water flow rates. There are two ways to do this.
In both cases, you are measuring the total water flow. This includes unheated cold water, but we correct for this by using the mixed, warm water temperature to compute the energy required. If you are able to measure only the hot water consumption (you installed a meter on your heater?), then use the hot water temperature.
This calculator also assumes that your tankless heater has an electronic ignition. If it use a standing pilot, enter "0" for your tanked pilot light usage (because you won't get any savings there).
How to compute hot water delays:
It is important to locate your tankless heaters as close as possible to the faucets. The extra time it takes the tankless unit to start heating water will be noticeable and perhaps annoying. The large amount of water stored in the pipes also causes a larger "cold water sandwich."
If you are replacing a tanked water heater with a tankless and leaving the piping the same, then you'll notice an increased delay for hot water to reach your faucet. In a tanked system, the water leaving the heater is already hot, so you only have to suffer the delay caused by the piping. In a tankless system, however, the water in the heater is cold, so the heater has to turn on and get itself up to temperature before the water flowing out is hot or even warm. I measured this extra delay to be 23 seconds on the Rinnai tankless heaters. So, where it used to take about 40 seconds for hot water to reach our master bathroom shower, it now takes about a minute with the tankless heater.
The delay can be reduced by having your plumber downsize most of the pipe lengths between the heater and the faucets. For example, a combination of 1.25- and 0.75-inch diameter pipes could be changed to 0.75- and 0.5-inch, respectively. If you run the numbers (computing the volume of the water in the pipes), you'll see that this cuts by half the time it takes for the water to reach the destination.
Your local building codes might specify minimum pipe diameters for branch pipes and main lines, and that might limit your use of this trick. However, you could run the feed pipe for, say your furthest shower, all the way back to your heater and thus its diameter could be smaller. (That is, use separate feed pipes for each distant faucet.)
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The Cold Water Sandwich
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Details
Measure the inside diameter and the length of each pipe between your water heater and the faucet. For each length of pipe, square its radius, multiply by Pi (3.14159), and then multiply by the length. The result is cubic-inches (or cubic-feet). Add those volumes together to get the total volume of the standing water in your pipes. Then, repeat the computations, but use the smaller diameters. Multiply your current delay time by the ratio of the two volumes to compute your new delay. To this, add the time it takes the heater to get up to temperature.
Water travel times (in seconds) per 10-feet of pipe
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Flow rate | Pipe inside diameter (inches)
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0.5 | 0.75 | 1.0 | 1.25 | 1.5
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1 gal/min | 6.1 | 13.8 | 24.5 | 38.2 | 55.1
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2 gal/min | 3.1 | 6.9 | 12.2 | 76.5 | 27.5
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For example, it will take about 7 seconds for water to flow through a 1-foot-long, 0.75-inch diameter pipe.
Example
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| | | Original | | Modified
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| Pipe length | I.D. | Volume | | I.D. | Volume
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Direction | feet | inches | (in.) | (cu. in.) | | (in.) | (cu. in.)
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up | 1 | 12 | 0.75 | 5.30 | | 0.75 | 5.30
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up | 3 | 36 | 1.25 | 44.18 | | 0.75 | 15.90
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west | 4 | 48 | 1.25 | 58.90 | | 0.75 | 21.21
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west | 7 | 84 | 0.75 | 37.11 | | 0.5 | 16.49
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north | 26 | 312 | 0.75 | 137.84 | | 0.5 | 61.26
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up | 4 | 48 | 0.5 | 9.42 | | 0.5 | 9.42
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| | TOTALS | | 292.76 cu. in. | | | 129.59 cu. in.
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| | | | | | | 56% reduction
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Internal heater delay (measured): | | 23 seconds | | | 23 seconds
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Delay from heater to faucet: | | 43 seconds (measured) | | | 19 seconds (computed)
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Total time to get hot water: | | 66 seconds (measured) | | | 42 seconds (computed) 24 sec improvement
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To measure the delays due to the heater internals and the water in the pipes after you have installed your tankless heater, go to the water heater, grab hold of the hot water pipe leaving the heater, and have somebody turn on the faucet. Write down the times when you feel the pipe getting hot and when your helper shouts out that the faucet is running hot. From this you can compute both delays. You cannot do much about the internal delay, but you can change the pipe diameters (local building codes permitting) to reduce the pipe delay.
A side effect of reducing the pipe diameter will be lower pressure at the faucet, especially if you are using two faucets at the same time that share part or all of the same pipe. So, consider this solution in light of your own installation and usage patterns.
Tankless Water Heater Summary:
Pros:
- Higher efficiency; lower long-term costs.
- No continuous pilot light (no wasted gas).
- Only runs when water is flowing (quieter).
Cons:
- Longer time to get hot water.
- Small electric heaters restrict the flow rate to relatively low values (compared to gas units).
- Lower maximum flow rate before water is only warm (e.g., there's a multiple faucet limit).
- The "cold water sandwich" (see above).
- Lowering the flow rate might cause the heater to turn off, but you won't know that happened until the water turns cold.
- If your electrical power fails, you won't get any hot water.
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Post Installation Improvements
It was frustrating to be turning down a faucet and discover 20 seconds later that the heater
had turned off because the flow rate fell below the minimum.
So, I installed sensors on the water heaters that operate indicators
near the faucets showing if the heater is on or off.
This allows the flow to be set as low as possible but still keep the water hot.
These improvements involve custom-built electronic sensors and indicators.
Sensors: The problem is finding a point to detect whether the heater is on or off.
- Electric heaters: I connected a Radio-Shack "Power-Flash" interface module to the heater element.
This should only be attempted if you are
familiar with electrical circuits.
- Gas heaters: These heaters are very complicated. There's no convenient point for an electrical connection that
will detect the burner state. So, I mounted an infrared detector
in the little window that allows you to see into the burner.
I built a Darlington amplifier for the detector and wired that to a Radio-Shack "Power-Flash" interface module.
Indicators: X-10 lamp modules plugged into power outlets near the faucets.
I modified a lamp module to work with a small bulb, but a small nite-lite would work fine.
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| IR sensor and amplifier
| Interface module (1 of 2)
| Indicator 1
| Indicator 2
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Actual Results, Years Later (November, 2015)
I've been tracking our gas bills every month, starting about
five years before installing the water heaters and for another
five years (and counting) afterwards. Here's a chart showing the summer months:
Notice the lines appear to bunch into two groups during June, July, and August
(when our only gas usage is by the water heaters).
As if by magic, the top group are the five pre-tankless years and the bottom
group are the years since. Obviously, there's some savings there.
Here are ballpark estimates based on the graph:
- July usage before tankless: 20 CCF (hundred cubic feet)
- July usage after tankless: 10 CCF
- Average savings per month: 20-10=10 CCF
- Savings per year: 120 CCF
- Our current energy rate is about $0.85 per CCF.
- That's about $100 per year (for both heaters, combined).
We've had no problems with the water heaters -- none.
They've been flushed twice, and are due for another.
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Links to More Information
Tankless Water Heater - What You Need to Know
(About.com)
Tankless 101
A tutorial for the homeowner and the professional (profitableplumbing.com)
Homeowner FAQs and Considerations
More homeowner FAQs and advice (totalvegasrealestate.com)
https://www.mge.com/saving-energy/business/bea/article_detail.htm?nid=1776
(Madison Gas & Electric)
Some articles at the following links assume tankless heaters have a standing pilot
(e.g., "But gas-fired tankless heaters are only available with standing pilot lights").
However, modern tankless, such as the Rannai units described above, have an electronic ignition.
http://energy.gov/public-services/homes/water-heating
(U.S. Dept. of Energy)
http://hes.lbl.gov/hes/makingithappen/no_regrets/waterheatertankless.html
(Home Energy Saver, US DOE)
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Disclaimer: Your results might vary. IANAP. jmat .. at .. mac .. dot .. com
Updated: November 1, 2015; July 1, 2019
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