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October 22, 2013 3:48 PM   Subscribe

I would like to use my house's water heater to fill a small outdoor soaking tub. Can I do this while the unit is running?

There is a drain at the bottom of the water heater and I would like to connect a hose to it and fill my tub. I would assume that as the water level in the heater dropped a float switch would trigger the heater to be refilled and that eventually the water draining from the bottom of the tank would begin to run cold.
Am I going to damage my water heater doing this? Is it possible to "overheat" the water heater by draining water out of the bottom of the unit while it is running? Is this a stupid idea?
posted by Mr.Me to Home & Garden (4 answers total)
This is functionally the same as filling your bathtub with hot water. There is no float controlling the level; line pressure causes cold water to flow into the water heater as hot water flows out. Just don't close the supply valve.
posted by coldhotel at 3:56 PM on October 22, 2013

Response assumes normal tank type water heater, not a tankless insta-hot unit.

I've filled my waterbed like this many times.

Water heater is designed to stay completely full, no float switch. Water going out causes water to come in.

The damage you are worried about is typically caused by someone shutting the water supply valve off and draining the tank with the heater operating. That will expose the hot heating elements or surfaces to air and damage the heater. Leave that valve open, and it's no harder on the water heater than taking a shower till the hot water runs out.

You are correct the water will start to come out unheated once you've used the supply of hot water in the tank, it will need time to heat the new, cold, water up.
posted by rudd135 at 4:08 PM on October 22, 2013 [1 favorite]

Best answer: Generally, there is a part of the water heater called the dip tube which delivers cold water from the supply line to the *bottom* of the tank. If you just ran water out with the supply on, you would quickly end up with luke warm water at best, as the cold water is delivered to the bottom of the tank where the gas or electric heating element and drain are located. If the volume of the water heater is the same/less than the volume of the tub you are filling, I think following the normal directions for flushing your hot water heater (turning off the supply and heating element while draining from the bottom tap) would give you the largest volume of hot water. Two seconds to google "flush hot water heater" and you're good to go. It would be a bit more work, but would maximize the amount of hot water available (and prevent harm to the heating element) if that is what you are looking for.
posted by k8oglyph at 4:32 PM on October 22, 2013

If you haven't flushed your hot water heater in a while, you might not want to put the initial water into a tub you are going to soak in. Put it into a bucket and see if you like the looks of it first.
posted by yohko at 12:10 AM on October 23, 2013 [1 favorite]

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