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Inline heater for water changes?

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Is it possible to use a high powered inline heater like 500 watts maybe to heat the tap water to temp on demand for water changes? Or has anyone tried one of those mini tankless water heaters that go under the sink that you can get for like 80$?
I simply do not have access to enough hot water to do all of the water changes even if I spread them out. And now adding a 150 gallon, 100 gallon, and another 60 breeder for large breeding projects… I need to figure out hot water on demand without breaking the bank

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My biggest single water change is like 50 gallons at a time but that’s just one tank… we are talking hundreds of gallons at a time potentially

Actually run out of water and have to boil water to do the smaller tanks after running out the hot water heater to do the bigger ones

I have over 20 tanks, 7 of them are 60+ gallons, there is a 150, 100, two 75, three 60s

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Electric tankless water heaters use 18 killowatts of electricity to make hot water on demand.  This is at about 3.5 gallons per minute and a raise from around 55 degree inlet to around 120 outlet temp.


now granted you are not looking to put 120 degree water in to your aquariums, but rather 75 or therabouts which is around 1/3 the temp rise…you would need a little over 6 degrees killowatts.  . And throttling flow down to 1.5 gpm you would be looking at least 3 killowatts…


so no, neither option you mentioned will come close to meeting demand on its own.


if using an electric water tank heater now, you could add a thermostatic mixing valve to it and raise tank temp to 140 degrees.  The mixing valve will add cold water at the valve to keep outlet water at 120 degrees which  make the tank virtually 30% bigger.  

alternatively you could install an additional electric hot water tank for more capacity.

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A killowatt hour of electricity assuming 100% efficiency is 3,500 btus. 

A 500 watt inline heater puts out about 1,700 btus if the water circulates though it for an hour..

3 gallons of water (3 gallons a minute flow) weighs 24 pounds.  Raising it from 50 degrees to 75 degrees is 25 degrees.


it takes 1 btu to raise 1 pound of water 1 degree farenheit.


600 btus needed to raise 3 gallons of water 25 degrees.

it would take that 500 watt heater about 20 minutes at 100% efficiency to raise three gallons of water 25 degrees.

Edited by Pepere
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