Tankless water heaters are terrible....

[Pkmmte]

I'd like to install a tankless electric water heater for my property but none of the options I've found are outdoors. They all recommend being installed indoors. Does anyone know any that are allowed outdoor or the reason why? I'm not interested in gas nor traditional tanks.

Closest reason I found were due to freezing temperatures but I don't remember ever getting those here in Southern California.

My existing gas tank water heater is already outside near my electrical panel (but not too close) so I'd like to use the same location. I'm not sure where to put it if I were to choose an indoor location. It'd look weird in my living room/kitchen, our closets are too small, our garage is becoming an ADU, and I'd like to avoid bathrooms/bedrooms.

I've been eye'ing this Rheem unit for a while:
https://smile.amazon.com/dp/B01MS9DVEE/ref=cm_sw_r_other_apa_i_oW-8Eb4FR5288

 

[BrettS]

The tankless heater that I use and love is made by EcoSmart, like this one...

https://www.amazon.com/EcoSmart-ECO...=1&keywords=ecosmart+27&qid=1593046789&sr=8-2

They make similar units with more or less capacity.

It can’t be installed outside and exposed to weather, but it does include the following note in the manual:

This product is designed to be installed indoors only. You may install your unit in an outdoor location so long as it is mounted in a suitable enclosure that protects it from rain, splashed water, direct sunlight, debris, and insects.
DO NOT install this product in a location where it may be subjected to freezing temperatures. If the water inside your tankless water heater freezes, it can cause severe and permanent damage that is not covered under your warranty.

So it sounds like you might be able to build some sort of an enclosure or something for it.

 

[Merrill]

I have always heard that electric tankless water heaters were not that efficient.

 

[BrettS]

I have always heard that electric tankless water heaters were not that efficient.

They are less efficient than a heat pump water heater, but they can be more efficient than an electric tank water heater because there are no heat losses when the water heater is sitting idle. In my case when I switched from a traditional electric tank water heater to a tankless I saved about 33% on the energy I was spending for heating water. (I had an energy monitor on my water heater circuit, so I was able to tell how much the old and new heaters used).

 

[arnolddeleon]

I have always heard that electric tankless water heaters were not that efficient.

That's because the best you can do is get a kWh worth heat out from a kWh of energy in (that's assuming 100% efficiency). If you are comparing a tankless resistance heater versus with one with a tank then the tankless will likely win, no standby loses. On the hand a heat pump typically beats resistance heaters (Heat pump - Wikipedia). Now a tankless might make sense over a heat pump one for a location is infrequently used. A tankless water heater would be consuming approximately zero while a heat pump would have to overcome the standby loses. I haven't done the math to see where this cross over is.

 

[Merrill]

I have a regular 60 gallon electric hot water heater that is getting old and would love to replace it with a tankless, would like to hear some thoughts on this.

 

[brkaus]

It’s reasonably easy to add additional insulation to an electric water heater vs a gas tank heater. The more insulation, the closer an electric tank heater approaches a tankless.

But yes, there is an effeciency benefit to an electric tankless as the tank will always radiate some heat. No where near as much is lost in the vent stack of a propane tank heater, but there are still losses.

The real problem is many houses don’t have the electric capacity to run a tankless electric heater. One linked above draws 112a / 240v and requires 3 40a circuits.

A heat pump electric water heater will put more heat into the water, so that would be my preferred choice for electric.

like everything, lots of choices and trade offs.

 

[SageBrush]

That's because the best you can do is get a kWh worth heat out from a kWh of energy in (that's assuming 100% efficiency). If you are comparing a tankless resistance heater versus with one with a tank then the tankless will likely win, no standby loses. On the hand a heat pump typically beats resistance heaters (Heat pump - Wikipedia). Now a tankless might make sense over a heat pump one for a location is infrequently used. A tankless water heater would be consuming approximately zero while a heat pump would have to overcome the standby loses. I haven't done the math to see where this cross over is.

I think most of the losses are not from the tank, but in the piping.

 

[nwdiver]

I have a regular 60 gallon electric hot water heater that is getting old and would love to replace it with a tankless, would like to hear some thoughts on this.

..... there's a reason this thread has the title it has... tankless electric water heaters ARE terrible.

Two of the biggest challenges for the grid moving forward are large loads and non-dicretionary loads that can't easily be shifted with demand response. Tankless electric are the WORST in both areas while a heat pump water heater is pretty much the best in both areas.

Rheem just refreshed their line of HPWH. Their COP is now ~3.75 - 4 so you get up to 4w of heat for every unit of electricity. The best tankless can do is 1.

The thing I really hate about tankless electric is that they're pitched as being more efficient when they're really not. Best case is they might use ~10% less than a traditional electric tank while a HPWH uses ~70% less.

I've been eye'ing this Rheem unit for a while:
https://smile.amazon.com/dp/B01MS9DVEE/ref=cm_sw_r_other_apa_i_oW-8Eb4FR5288

The '24' in RTEX-24 is 24kW. Do you understand how much power that is? The peak demand my entire house had this month was 5kW. Better hope you're never exposed to demand fees... those usually run ~$10/kW so your hot showers could cost $240 per month excluding the cost of energy. Can't repeat it enough.... Tankless water heaters are terrible. It's also responsible for tripping one of the solar inverters I installed. Instantaneous ~24kW load creates a pretty good voltage dip inverters don't like. Tankless water heaters are terrible

 

[wws]

_Electric_ tankless hot water heaters are terrible. We have two natgas Rheem tankless, mounted outdoors. After 12 years of use they continue be great.

 

[nwdiver]

_Electric_ tankless hot water heaters are terrible. We have two natgas Rheem tankless, mounted outdoors. After 12 years of use they continue be great.

.... well..... fossil fuels are it's own kind of terrible... but I will grudgingly agree that electric tankless is generally speaking worse than gas tankless.

 

[BrettS]

The '24' in RTEX-24 is 24kW. Do you understand how much power that is? The peak demand my entire house had this month was 5kW. Better hope you're never exposed to demand fees... those usually run ~$10/kW so your hot showers could cost $240 per month excluding the cost of energy. Can't repeat it enough.... Tankless water heaters are terrible. It's also responsible for tripping one of the solar inverters I installed. Instantaneous ~24kW load creates a pretty good voltage dip inverters don't like. Tankless water heaters are terrible

I think we hashed this out a year or two ago earlier in this thread, but just because the tankless water heater can draw 24kW doesn’t mean that it draws it whenever it is in use. The power draw will vary based on the water flow.

For example, when someone showers my tankless heater will draw about 6-7kW for the duration of the shower. If I turn on the hot water at the kitchen sink it will draw about 3kW.

Obviously 6-7kW is still a pretty big demand and it’s certainly possible that it could draw the full 24kW if, for example, three people are showering at once and the washing machine is running. But it’s not like there’s an instant 24kW draw as soon as any hot water is used.

 

[nwdiver]

I think we hashed this out a year or two ago earlier in this thread, but just because the tankless water heater can draw 24kW doesn’t mean that it draws it whenever it is in use. The power draw will vary based on the water flow.

For example, when someone showers my tankless heater will draw about 6-7kW for the duration of the shower. If I turn on the hot water at the kitchen sink it will draw about 3kW.

Obviously 6-7kW is still a pretty big demand and it’s certainly possible that it could draw the full 24kW if, for example, three people are showering at once and the washing machine is running. But it’s not like there’s an instant 24kW draw as soon as any hot water is used.

Sure... energy is conserved; So the power draw is going to vary depending on incoming water temperature and flow rate.

The formula I found is 147w for 1gpm per F. The outlet from my HPWH is 112F and in incoming water averages ~50F so (147w)(62F)(2GPM) = 18.2kW

But even best case it's still terrible. My HPWH draws <750w and I can pull that 750w at 3am off-peak while maintaining the freedom of taking a shower at 5pm in the middle of peak demand w/o adding to peak demand.

There's so many perks to a HPWH over tankless; Here's one more. I was able to take my house off-grid from March to May. My grid-tied PV system stayed on-grid and just exported ~100% of production. I've seen a lot of posts from people trying to determine how many powerwalls they'd need to run Air Conditioning... try doing that with a tankless water heater I was able to set my HPWH to 'reheat' while the sun was up so I wasn't using the batteries to heat water.

 

[SageBrush]

The formula I found is 147w for 1gpm per F. The outlet from my HPWH is 112F and in incoming water averages ~50F so (147w)(62F)(2GPM) = 18.2kW

Give me metric, or give me death

One gallon = 3800 ml
60 minutes = one hour
one calorie = 4.12 joules
one calorie increases 1 ml of water 1 C
62 F = 34.44 C. (9F = 5C)

Now, some arithmetic:
One hour of 2 gpm = 3800 * 2 * 60 ml = 456,000 ml
456,000 ml raised 34.44 C = 456,000*34.44 calories = 456,000*34.44 * 4.12 joules
456,000*34.44 * 4.12 joules = 18 kWh. (Google)

18 kW it is, although the beauty experts on the web say 112F is too hot; they recommend ~ 105F. I thought you would want to know.

 

[SageBrush]

My grid-tied PV system stayed on-grid and just exported ~100% of production.

Similar to an EV, programmable HPWH really shines when matched with home PV. They are a trifecta

 

[nwdiver]

18 kW it is, although the beauty experts on the web say 112F is too hot; they recommend ~ 105F. I thought you would want to know.

~105F is probably what makes it to my shower. I've found that 105F out of the water heater is generally too cold.

 

[SageBrush]

Give me metric, or give me death

Chris knows of my OCD nature; for everybody else I ask forgiveness for beating a dead horse:

The grotesque Imperial units in this discussion are most easily converted to metric using the second:

2 gpm = 2*63.1 ml per second (thanks, Google)
33.4 calories to raise an ml 33.4 degrees C
4.12 joules per calorie

Thus heating is 2*63.1*33.4*4.12 joules/second, or watts

 

[mongo]

Chris knows of my OCD nature; for everybody else I ask forgiveness for beating a dead horse:

The grotesque Imperial units in this discussion are most easily converted to metric using the second:

2 gpm = 2*63.1 ml per second (thanks, Google)
33.4 calories to raise an ml 33.4 degrees C
4.12 joules per calorie

Thus heating is 2*63.1*33.4*4.12 joules/second, or watts

Oh come on now, conflating ml with g???
Is your water at 4C (0.9998395 g/ml)?
At 49.3C (leaving the tank) it's 0.98870 g/ml.

 

[SageBrush]

Oh come on now, conflating ml with g???
Is your water at 4C (0.9998395 g/ml)?
At 49.3C (leaving the tank) it's 0.98870 g/ml.

I'm ashamed. I'll return my metric merit badge, today

 

[ohmman]

The grotesque Imperial units in this discussion are most easily converted to metric

I may have told this story here before, but when I graduated with a degree in engineering, we were required to take the Fundamentals of Engineering exam, which is a precursor to becoming a registered P.E. The exam was offered in Imperial or metric units, and it was an 6-hour timed exam. I was stunned, stunned, that over half of my class chose Imperial units. A timed exam! Conversions!

That was 25 years ago and I'm still shaking my head.