Instant hot water must be defined. I see advertisements for instant hot water in ads for tankless water heaters, hot and warm water recirc and circulating systems, hot water demand systems, and point of use water heaters.
Truly instant hot water would be for instance if you have a traditional full time hot water circulating system with the proper plumbing layout. You would have hot water immediately upon opening an hot water fixture or faucet, that being within 2 or 3 seconds anyway.
Obviously for convenience this would be the best scenario. However there is a high price to pay for such a system. By continuously circulating hot water through your hot water plumbing you are wasting a huge amount of energy. Using energy costs money and contributes to greenhouse gas emissions.
There are also circulating systems that utilize the cold water piping as a return line for the hot water circulating system. These systems which include the Laing, Grundfos, RedyTemp, Watts, and others, are what I call “warm water circulating systems”. These are temperature controlled recirc pumps that typically turn on the pump when the water temperature drops to 85 degrees, and then shuts off when the temperature reaches 95 degrees.
Like the hot water circulating systems these systems keep warmer than normal water in the piping causing an increase in energy usage and corresponding green house gas emissions. However, the water isn’t really hot, it’s warm to cool. Your body temperature is 98 degrees so the water might not even feel warm, perhaps “cool”. Anyway, you still have to run water down the drain to get “hot” water and you are wasting energy too. I would have a hard time calling these systems “green”.
Another method is to use a point of use water heater where the heater is located so close to the fixture that there is virtually no wait for the hot water. This is a green type of plumbing system. You don’t waste water running it down the drain waiting for hot water to arrive, and you don’t circulate hot water and waste all that energy.
However, point of use water heaters are not very useful in most residential settings. For one thing you would probably need more than one water heater, and that can be expensive.
Not much else you can do to have instant hot water. And even point of use water heaters don’t always get you instant hot water. For instance, if your point of use water heater is a tankless model then you once again will not have instant hot water. A tankless water heater typically requires the water to flow all the way through the heat exchanger to reach full temperature. This means that you have to purge water out of both the hot water piping and the water heater’s heat exchanger before you get hot water.
A study done by the Australian government found that tankless water heaters typically take 10 to 20 seconds longer to get hot water to a fixture than a corresponding storage type water heater and therefore led to significant water wastage.
For a green plumbing system we’ve pretty much eliminated traditional hot water circulating systems, warm water circulating systems, point of use water heaters in most situations, and tankless water heaters which waste water.
Hot water demand systems are energy efficient, water conserving, time saving, and green by any standards. Manufacturers of hot water demand systems include Metlund, Taco Pump, Chilipepper Sales, and others.
A demand hot water system typically consists of a small pump mounted under the sink furthest from the water heater exactly the same as most warm water circ systems. When you want hot water you push a start button and the pump speeds the water from your water heater to the fixture, routing the cooled off hot water left in the pipe from the last use back to the water heater through the cold water piping.
When hot water reaches the pump it shuts off, preventing putting hot water into the cold water line. The pump has a higher flow rate than most fixtures making the waiting time shorter than a normal residential hot water piping system. Since no water gets run down the drain you save water. Since the pump only runs when you actually want hot water and only runs for a few seconds it uses very little electricity. Typically for a family of 4 the pump uses $1-$2 per year.
No extra water heating energy is needed because the pump doesn’t circulate the water, it just gets it to the fixture and stops.
How you define instant hot water and how you obtain instant hot water make the difference between a water and energy wasting system and a truly green system like hot water demand systems.
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