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I am considering purchasing a inline water heater and having it installed (Eco 24). Before I do though I want to know for sure that it is possible to install with my current breaker box (the water heater is electric). It needs 3x40 dp breakers, and 100amp service.

I have 240v 100amp service and my breaker box is currently full(see below). There are two breakers that can be removed due to un-use. but the rest are needed.

  1. Is it possible to use tandem breakers to gain enough space on the box to get the breakers I need?

  2. Can my box/service support this addition?

annotated circuit box

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I think you're going to need an electric service upgrade if you want to use this water heater.

Your 100 amp service is already somewhat loaded, as well as the box being physically full.

If you call for hot water while the dryer or range is in use (as obvious heavy loads on your panel), you may well trip the main, if you do not upgrade to 200 amp service before installing this water heater.

A different water heater might make more sense, unless you have other reasons to upgrade your electric service; one of the heat-pump water-heaters (or a heat-pump-add-on to an existing water heater tank), perhaps.

Edit, Add: Unless this is a house that stands vacant for long periods of time, there's surprisingly little "standby loss" (what you save by using an instant tankless electric resistance heater .vs. a tank-type electric resistance heater that's insulated to modern standards) - and if it does stand vacant for long periods, you can switch the water heater off when you leave. Changing to a heat pump water heater (HPWH) can, by contrast, save quite a bit and get you "free dehumidificaton" in the summertime. If you have gas available, it's more of a toss-up - gas and HPWH can be similar in cost to run, and a gas water heater will usually cost considerably less to buy. Most of the cost of running a normal (modern, well-insulated) tank-type water heater is, in fact, the cost of heating the water that you actually use. Most households will see little savings from using an instant-tankless waterheater, in practice.

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  • Well, shoot. I guess I will have to figure out something else. The energy savings is worth the $150 difference between this and the tank style it will replace. Upgrading my service for a $1500 difference, I don't think it is worth it.
    – user1463809
    Mar 31, 2014 at 15:20
  • Great thank you. I purchased a natural gas tank water heater.
    – user1463809
    Apr 1, 2014 at 21:26
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I have to agree with Ecnerwal here. Electric Tankless WHs just weren't designed with older service panels in mind. You'll probably be eligible for a tax credit and/or manufacturer rebate for going tankless, though. And you might be able to get a better deal from an electrician if you upgrade both your heater and electric service at the same time.

I just upgraded my service from 100 to 200 amps for $1800. You might be able to do better if you shop around.
Good luck.

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