I have a bathroom in my shop building that I use infrequently. I do not want to maintain hot water 24/7 there. I want to use a Bosch 2.5 gallon instant hot water device connected to a cold water line. I would like the outlet to become activated when it detects motion in the bathroom and stay on for 5 minutes or so, or until there is no motion after that time.

The main reason I want to do this is that if I have a switch or plug-in/unplug the heater when needed that I will forget to turn it off or unplug it when I am through.

I see most motion detectors can only control low amperage devices. This water heater is 1,400 watts. Is there such a thing as a motion-controlled outlet that can handle this load?

If not, I suppose I could rig a relay to be activated by the motion sensor to energize the outlet, but I would like to make it more simple.

Another option would be to use a rotary timer like on a hot tub. Are those direct connect to an outlet or do they need a relay involved?

This is the heater I am looking at: https://www.amazon.com/Bosch-Tronic-2-5-Gallon-Electric-Mini-Tank/dp/B0148O658Y/ref=pd_ybh_a_5?_encoding=UTF8&psc=1&refRID=SPZ8JFCMYAS3QM1A36WB

It is 110v and has an adjustable thermostat.

  • Does your point of user heater run off 120V or 240V? Commented Mar 24, 2018 at 14:50
  • I would anticipate the relay path to be just as simple, and far less expensive. Chances are the device you are looking for has a relay or triac set in it. If it is an on demand system, letting it have power is the right thing to do. They dont cost much when idle.
    – noybman
    Commented Mar 24, 2018 at 15:14
  • Does an instant heater use power when the waters not running?
    – Tyson
    Commented Mar 24, 2018 at 15:20

2 Answers 2


You misunderstand these products. Yes, "tankless heaters" are quite popular. And this isn't one.

  • Tanked water heaters have a reservoir (tank). The tank takes 10-30 minutes to heat up. Once you use the tankful, you are out of hot water for awhile.
  • Tankless water heaters have no tank, water streams through them and they flash heat it. Water is endless. But this takes a LOT of electricity (in the moment) and so you need fat, dedicated wires to the heater.

The tankless is a net win, because though it surges huge current during instants of hot-water use, the rest of the time it uses none at all, and that is a net win since it never heats water no-one is using.

You are talking about shutting off the heater when you're not using it to save energy, and only turning it on when you need it. Let's think about that.

  • Tanked water heaters use energy all the time. Once they initially heat the water, it cools (slowly due to the insulation) but nonetheless the heater must re-fire periodically 24x7 while on standby. This is a fair amount of "vampire power".
  • Tankless water heaters don't heat water when not being used, so they don't use any appreciable amount of energy when on standby.

So you see, the very idea of putting one on a motion sensor is pointless.

  • For a tanked water heater, it won't even start to warm up by the time the user wants to use the hot water. The user will need to do jumping jacks in front of the motion sensor for 10-20 minutes to get hot water.
  • For a tankless water heater, it will make no difference whatsoever, because the tankless isn't using energy while in standby. It's a waste of a perfectly good motion sensor.

A fair solution is to fit a 1500-1800 watt tankless in that location, with a valve to limit flow so you get respectable heat. But it won't be much flow.

An excellent solution is to run a new #12 or #10 homerun and install a 4000-6000W tankless.

  • Did not call it "tankless". It has a 2.4 gal tank. Commented Mar 24, 2018 at 22:31
  • You called it instant, hence my confusion. Instant usually denotes tankless,since instant isn't really relevant to a tanked heater. Commented Mar 24, 2018 at 22:42

1400W isn't a tankless (too low wattage to give enough heat for an instant-heat).

In your case: rarely used (sink only) a point of use 3kW is the right choice. It doesn't require any motion sensor because it starts heating only when hot water is flowing and stops as soon as water is turned off and can be plugged to a 30A circuit.

I suggest you, if you don't want to change your water heater, to add a timer instead.
Timer that kicks in the boiler once (or twice) a day for 2 hrs that is enough time to heat water with this timings 04:00->06:00 and 11:00->13:00. Because at least one hour is needed to warm up all the water (10L if it's a small size) and, once heated, water will stay hot for some hours.

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