Hacking an on-demand water heater for proper Fabric Rinsing

!!CAUTION!!    !!CAUTION!!     !!CAUTION!!     !!CAUTION!!   !!CAUTION!!    !!CAUTION!!

Increasing your water heater temperature can be very dangerous. Please make sure that every person in your household is aware that you have increased your hot water temperature and does not use the water. Always return the water to a safe temperature as soon as you are finished. Always use appropriate gloves and tools.

When I rinse my hand-dyed fabric, I need to soak the fabric in hot water. The minimum temperature is 140˚, but I prefer something closer to 170˚. This helps to remove the excess dye faster and more completely. Recently I installed an “on-demand” (sometimes called an “instant hot”) water heater in my studio: a Tagaki T-K1S. I bought this specific model because it’s rated for a max output temperature of 182˚F. That’s HOT! It worked great.

But after I replaced my studio’s old, clogged iron pipes with new copper pipes, the water pressure was better but my hot water got cooler. What happened?

On-demand water heaters work very differently than traditional “tank storage” water heaters. If you are considering getting one of these, you need to understand how they work else you won’t be able to get the hottest water possible to rinse hand-dyed fabric properly.

After some experimentation, I figured out how to get super-hot water again. Here’s how to do it:

Tagaki water heaterFirst, read your water heater’s manual. Every model and manufacturer is different, but once you get the general idea you’ll know how to approach your particular circumstance. The instructions below are specific to my Tagaki T-K1S. Your mileage may vary.

On-demand hot water heaters take water from your cold water supply and heat it directly in an intense boiler. Because they don’t try to store and heat water in a large storage tank, on-demand water heaters are supposedly more energy efficient than “tank” water heaters. Here’s two important distinctions: Because they have to heat the water in “real time”, they are designed to raise the inlet water temperature for a given flow rate.

– If you water supply is too cold, it won’t be able to heat water to your desired output temperature.
– If water flows through the heater too quickly, it won’t be able to heat it to your desired output temperature. Water flow is determined by either the amount of “demand” from your faucet or the amount of “supply” coming from your water company/well.

Once you understand these two limitations, you can start to control how hot your water can get. It’s very different than a “tank” water heater where you just crank up the temperature and wait for the tank to get up to temperature.

First experiment:
– Takagi set to “140˚F”
– Inlet water supply: 60˚F.
– Water supply into water heater fully open

Result: Water temperature at the faucet: 138˚F

Second experiment:
– Takagi set to “182˚F” (by adjusting DIP switches inside the heater…be very careful!)
– Inlet water supply: 60˚F.
– Water supply into water heater fully open

WarningDip Switches

Result: Water temperature at the faucet: 142˚F

It should have been 182˚F, but it wasn’t. I can’t change the inlet water temperature, so I need to limit the water flow. I tried reducing the water flow at the faucet, but it didn’t work: as I turned down the faucet, the water heater would turn off before the water got hot enough.

Things to check:
– Newer faucets have an “anti-scald” valves. These are designed to limit the output temperature to ~120-140˚F to avoid burns. If your faucet has one of these, find another faucet to use. I checked, and my faucet is rated for output a max of 176˚F, so that wasn’t the problem.
– Water supply may be too cold. My water supply was 60˚F. According to my water heater’s manual, it should be able to raise the temperature by  120˚F for a 2.6 gallons per minute flow.

Second experiment:
– Takagi set to “182˚F.”
– Inlet water supply: 60˚F.
– Water supply into water heater almost completely closed

Result: Success! Water temperature at the faucet: 172˚F.

Water SupplyBy closing down the water supply for the water heater, it could take more time to heat the water to a higher temperature. The more I closed down the water supply to the water heater, the hotter it made the water. But the slower it came out. I didn’t need more than 170˚F for my project, so I didn’t bother testing beyond that. When I closed it down too much, the water heater shut off.

Conclusion: If you have a tankless water heater and can’t get your water hot enough after turning up its thermostat, try turning down the inlet water supply! And be safe. Return the water heater to a safe level as soon as you are finished.

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