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We are planning to build a family house, 2 adult, 2 child, 100m2, eastern EU.

We got stuck that what kind of water heating should we use.

Where we use hot water?:

  • shower
  • kitchen
  • sink in bathroom

Two solutions:

https://en.wikipedia.org/wiki/Tankless_water_heating

vs.

https://en.wikipedia.org/wiki/Storage_water_heater

The question: which one is cheaper?

  • As far as I see the tankless looks much cheaper to create/build up vs. storage/boiler. (Initial cost is lower.)
  • For operating expense: again, the tankless looks cheaper, since it doesn't need to always heat up a 200L sized boiler to ex.: 60C. Only heats up the water what is really needed at the given place.
  • Only minus points for the tankless: maybe bigger chance of getting an electric shock?
  • Evidently you are going to power the water heater by electricity not by natural gas, correct? – Jim Stewart Feb 16 at 12:15
  • Only electricity! – cirka547 Feb 16 at 13:23
  • What's the 99%ile outdoor temperature for your location? (i.e. the temperature that it's warmer than out 99% of the time) – ThreePhaseEel Feb 16 at 15:42
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Sometimes on-demand saves money, sometimes it doesn't.

The initial cost - the cost of the equipment plus the cost of the installation - can vary quite a bit. You have to have the capacity on your gas or electric service to add this large load. Don't forget to consider the life cycle of the equipment - storage tank water heaters don't last long.

The ongoing cost is harder to nail down. The on-demand is usually less efficient heating water, but the storage tank heater has to make up heat losses from the stored water all day and night. The more the water sits unused in the tank the more heat loss there is to make up. For example if there's just one or two people in the household and they only use hot water a short period in the morning and a few hours at night, the heater spends a lot of energy just keeping water ready around the clock. For a vacation home that's only used weekends, there's even more heat lost keeping hot water on standby.

You can look even harder at the cost of the nuisance heat. In some cases nuisance heat isn't really much of a loss - the nuisance heat the water heater makes takes a little load off the furnace - others, the nuisance heat isn't just wasted, you pay for it twice - it adds a little to the air conditioning.

If you can talk to some people that have on-demand water heaters it may be more helpful than trying to do a solid to-the-penny cost analysis.

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A central or whole house electric tankless water heater would require two or three very high power circuits (40 A each) and your electric panel must have the capacity to supply this. If your incoming water is very cold, then a tankless heater will be limited to a relatively low flow rate (a few L/min) if you want very hot water.

By contrast a 150-L electrically (resistance) heated tank requires only one circuit and it would be of lower power. A storage tank allows accumulation of hot water which is both a benefit and a liability.

The performance benefit of a hot water storage tank is high flow rate of very hot water . . . until the stored water is used and then you get no hot water until the tank "recovers". The main liability is that a tank is more prone to catastrophic failure in which 150 L of 60 deg C water would be dumped into the house. Better home design would have a tank heater placed where a catastrophic failure would not flood the house, but this would increase construction costs and would usually mean a longer wait for hot water.

Will the water pipes in this planned home go underground, i.e., through the earth? I ask this because that is the common practice for "slab" foundations where I live, but it is to be avoided in my opinion.

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With two kids I'd be leaning towards a tankless. Not a junky one like in the link provided but specifically a Navien. They are 98% efficient, very quiet and you will never run out of hot water. The cheapest option up front usually costs the most down the road. And electric water heater is very inexpensive to buy but you will be paying for it in electric consumption. A Navien tankless will be among the most expensive installations. With two kids in your family home it will pay for itself in savings. Kids waste a ton of hot water, especially from about 12 until they move out and pay their own utility bills.

  • I think two teenagers is a good reason NOT to buy an on-demand. The tankless can be nice if everything works out exactly right because you never run out of water. Which means kids will stay in the shower forever rather than until the tank is empty. It's more expensive to heat water with on-demand, the energy savings is in the heat loss during standby. – batsplatsterson Feb 16 at 13:21
  • If this house will have only one "bathroom", then demand from others to use it will limit the time any one person occupies it. Will the toilet be separate from the bathing area? Will the house have one toilet or two? What type of foundation will it have--full basement, crawl space only or slab on grade? – Jim Stewart Feb 16 at 13:40
  • Where in the house will this tankless water heater be placed? – Jim Stewart Feb 16 at 14:21

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