The math of tankless water heaters

yeah, incoming water temp can be a problem with on demand heaters. basically they can only raise the temperature so much. so the colder your makeup water is to begin with, the cooler your final high water temperature will be.https://support.eccotemp.com/en/arti...-colder-months

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Originally Posted by singlesline

Lots of modern water heaters with smart features which usually sound stupid, but make sense for that kind of use case. Your guests don't have to do anything, you can just open the app and say "give hot water for the next week" as long as the place has a working internet connection. Probably also have some features that can let you know remotely if something seems wrong or is leaking which could be handy. Research I did recently suggests electric on demand is mostly a no go in places where the input water is pretty cold. Gas works, but still has tradeoffs. I'd love to reclaim the closet my water heater is in and be able to take longer showers (because the closet doesn't even fit a very big heater), but I don't have a gas line. I also have poorly planned pipes so it takes over a minute to get hot water out of the kitchen sink if I don't have the recirculation pump turned on...and my understanding is you can't use a recirc system with on demand heaters.

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Yeah, after I posted my question I started searching on Lowes/HD, and saw the smart heaters. In my situation, it does make sense. So will probably go that route. And yeah, my intake water is cold.

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Originally Posted by Danno

Yeah, after I posted my question I started searching on Lowes/HD, and saw the smart heaters. In my situation, it does make sense. So will probably go that route. And yeah, my intake water is cold.

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And in January it's probably really fuckin cold. :D

Sarah Connor would rip your throat out if you tried to install a smart anything in her mountain bunker.

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Originally Posted by Danno

those don't work with the current heater because we sometimes have guests stay there (we don't rent it) and we need something super user friendly; don't want to have someone crawling around the tank to plug it in. Current model doesn't have an easy temp adjustment but a newer one might.

Still like the idea of on-demand but it sounds like a bad idea.

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It's not difficult. Just install a double pole switch to turn it off.
When someone shows up, flip the switch on.
When they leave, flip it off.
I did this setup years ago at our condo.
Works great.


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Our incoming water is pretty damn cold in Truckee in the winter. Our heater has no trouble making scalding water. As far as recirc pumps you can use them with on demand heaters.

I dont feel like searching for it. Is there already a 'the meth of tankless water heaters' thread and did i create it?

I miss having endless hot water from my gas tankless, but like others I did the math on installing one to replace my gas tank water heater at my new place and ended up just putting in a new gas water heater. Some of the gas water heaters can almost keep up with producing hot water as fast as it is consumed in some cases.

You might look into rebates on heat pump H20 heaters from your electric supplier or state.

The electric tankless have a huge draw when turned on. Of the plumbers and plumbing suppliers I’ve talked to they rank them as follows:
One - gas tankless
Two - gas or electric tank
Five - electric tankless


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I imagine that ranking might be shifted if a person has solar panels and enough extra juice to run the heater? I don't have panels because not enough South facing roof, but curious of the maths in that regard.

An option for sinks far from the tankless heater is an electric point of service heater under the sink. I wish I had room under my kitchen sink for one.

If you reduce the copper run, the meths dont math.

Do the POS under the sink generally use two twenty?

No, those will use 120V, but deserve their own 20A circuit.

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Originally Posted by Rideski

I imagine that ranking might be shifted if a person has solar panels and enough extra juice to run the heater? I don't have panels because not enough South facing roof, but curious of the maths in that regard.

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Unless you're only showering or doing laundry in the middle of the day, solar doesn't change the equation without battery backup, and you'd need a huge battery to make it work.