I’ve talked to many plumbers about this and what I’ve learned is that if you don’t have gas, it won’t be as good (on average). New construction with dedicated power is a different story, but retrofits are often limited to whatever the old water heater was using, which by definition isn’t enough. Running new power is likely to be a headache.
Now that you’ve mentioned it, I’ve never considered tankless water heaters being gas powered also as an option. My house is all electric so I only have that variable to deal with.
I’m an electrician. I haven’t been impressed by most of the tankless systems I’ve hooked up. I would definitely go with gas unless you are willing to run some serious power. Most hot water tanks pull about 3000-4500W or 12.5-18.75A at 240V. A good tankless system is upwards of 18000W or 75A at 240V and some can be around 36000W which works out to 150A. This will easily be the single largest electrical load in your house, unless you also have an electric furnace or something. Depending the size of your electrical service and what else you have in your house you may need to upgrade the electrical service going to your house as well. Most Residential Electrical services are usually 100-200A in my area.
Edit: my bad, I was wrong. My unit is actually not using a regular plug, and pulls up to 21kW.
I mean I know things are different here in Europe but 18kW for a water heater? Why? I have one that plugs into a regular outlet, so no more than 3.5kW, and I have no issues whatsoever. Sure, water isn’t boiling hot out of the tap, but I can take showers no problem and usually need to mix a little cold water to make it comfortable. Why would you need any hotter water? Or is there some other reason why you need so much more power in an american system?
The 3.5 kW heater will have a tank reservoir that will have time to heat up the water when it’s not being used. Tankless means it has to be heated instantly.
So if a shower uses, say, 9 litres per minute, i.e. 0.15 litres per second, heating that water from 10°C (typical cold water temperature) to 40°C (comfortable shower temperature) is:
4 • 0.15 • 30 = 18 kW
Anything less heats less water per second, or to a lower temperature.
Like this 5.1 l/min unit at 9.5 kW will be able to heat that flowrate ~28°C above its inlet temperature.
For the shower you’ll want the 18 kW, otherwise you’d have to shower with a tiny stream of water. What you have is meant for washing your hands (tiny one under the sink).
Yeah, a tankless pulls down power like an EV, and since conversion to heat is 100% efficient (loss in the pipes on the way to your shower notwithstanding) you actually need that power to heat up the water at the speed a tankless sends it through. That’s why the gas ones work better, hydrocarbons store a shitload of energy.
I’ve talked to many plumbers about this and what I’ve learned is that if you don’t have gas, it won’t be as good (on average). New construction with dedicated power is a different story, but retrofits are often limited to whatever the old water heater was using, which by definition isn’t enough. Running new power is likely to be a headache.
Now that you’ve mentioned it, I’ve never considered tankless water heaters being gas powered also as an option. My house is all electric so I only have that variable to deal with.
I’m an electrician. I haven’t been impressed by most of the tankless systems I’ve hooked up. I would definitely go with gas unless you are willing to run some serious power. Most hot water tanks pull about 3000-4500W or 12.5-18.75A at 240V. A good tankless system is upwards of 18000W or 75A at 240V and some can be around 36000W which works out to 150A. This will easily be the single largest electrical load in your house, unless you also have an electric furnace or something. Depending the size of your electrical service and what else you have in your house you may need to upgrade the electrical service going to your house as well. Most Residential Electrical services are usually 100-200A in my area.
That is a lot of power… Surely with that much power you could afford a heat pump instead
Wowza! That’s a lot of power.
Edit: my bad, I was wrong. My unit is actually not using a regular plug, and pulls up to 21kW.
I mean I know things are different here in Europe but 18kW for a water heater? Why? I have one that plugs into a regular outlet, so no more than 3.5kW, and I have no issues whatsoever. Sure, water isn’t boiling hot out of the tap, but I can take showers no problem and usually need to mix a little cold water to make it comfortable. Why would you need any hotter water? Or is there some other reason why you need so much more power in an american system?The 3.5 kW heater will have a tank reservoir that will have time to heat up the water when it’s not being used. Tankless means it has to be heated instantly.
It takes ~ 4 kJ to heat 1 kg of water for each 1°C. If you want to do that in 1 second, you need 4 kW of heating power.
So if a shower uses, say, 9 litres per minute, i.e. 0.15 litres per second, heating that water from 10°C (typical cold water temperature) to 40°C (comfortable shower temperature) is:
Anything less heats less water per second, or to a lower temperature.
Like this 5.1 l/min unit at 9.5 kW will be able to heat that flowrate ~28°C above its inlet temperature.
Oops, my bad, I really thought it used a regular 240v plug. Turns out its hooked into my kitchen’s high voltage (400v) circuit and uses 21kW.
Thanks for explaining why physically my assumption could not be possible.
For the shower you’ll want the 18 kW, otherwise you’d have to shower with a tiny stream of water. What you have is meant for washing your hands (tiny one under the sink).
Good lord, well that puts that into perspective, thanks.
Yeah, a tankless pulls down power like an EV, and since conversion to heat is 100% efficient (loss in the pipes on the way to your shower notwithstanding) you actually need that power to heat up the water at the speed a tankless sends it through. That’s why the gas ones work better, hydrocarbons store a shitload of energy.
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