# On demand tankless water heater



## georgepds (Dec 29, 2014)

In order to make a little more space for a full tub, I’m thinking of putting in an on demand tankless electric water heater. Right now I use a really small electric tank ( ~20 gallons) . It works OK except when trying to take a shower when the dish washer is running. There are only two of us.. can someone recommend a size or model? Anything special I should look out for?
I think it should be slightly more efficient than the 20 gallon tank (no need to keep the water heated when there is no demand). If not, please let me know
I’ve got plenty of electricity with ~4.5 kW of solar on the roof. I generate ~6 MWhr/year, and have not paid an electric bill in a long time
Regards
George


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## seige101 (Dec 29, 2014)

I have wired many and installed one of them. The largest i wired was a 25kw whole house unit. It will draw 100 amps of power at full throttle requiring 3 50 amp breakers  and associated wiring involved ( 3 50 amp #6 wire 240 volt circuits) It worked well with with the cold well water coming into it and could supply 2 sinks running full tilt at once. You would see the lights in the house dim slightly when it was running at full throttle.

I installed a 12 kw model at the local animal shelter, the only thing it supplies is a set tub for cleaning dishes and washing hands and a washing machine. It needed a dedicated 60 amp circuit for it. It will not keep up with just 1 faucet running, despite a low flow sink faucet being used. With the tap wide open you get warmish water flowing, if you have the tap open at say 1/2 way you will get hot water.

I would NOT install one if you have hard water or a high mineral content in your water as the plumbing will clog up over time.

With a larger model you WILL need a 200 amp service coming into your house and 6 open panel spaces.

Some manufacturers will put a calculator online to help you determine the size but i highly recommend consulting with a plumber who is experience installing these.


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## begreen (Dec 29, 2014)

I would simply skip showering when the dishwasher is going, or turn down the dishwasher temp if it has an eco-mode. As an alternative, some dishwashers have their own heating element.


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## claydogg84 (Dec 29, 2014)

I did some research on these units myself. I was in shock once I saw what kind of wiring/breakers they required, especially since it wouldn't be located anywhere near the panel. That alone made it a no go for me, but I also stumbled upon a website that had tests done to compare power usage between the ready hot and the tank system. Their results showed virtually no power savings with the ready hot system. This was several months ago, and I can't remember the source of this information, but I'm sure there is plenty of information on Google. Have you considered a bigger tank?


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## valuman (Dec 31, 2014)

Electric resistance heating is a super inefficient use of electricity and the higher you crank up the heat output, the more that becomes apparent. For demand water heating I think gas is the way to go, but I'd seriously consider begreen's suggestion of simply altering the timing of your dish washing and let the PV continue to produce the kWh needed to heat the water in your 20 gallon tank.


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## semipro (Dec 31, 2014)

Given that you're on PV I'd stick with the lower instantaneous loads of a tank heater. 
There are other issues with tankless units with low flow usage which may be a problem also.
Since space seems to be the main issue could you relocate the existing unit, possibly elevating it. 20 gal. of water is about 160 lb plus the weight of the empty tank (? lb) so maybe 250 lb.  You could put that much weight on an strong shelf.


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## sloeffle (Dec 31, 2014)

When we put our addition on I looked into the tankless electric hot water heaters. It would of cost me $300 worth of wiring just to wire it up. I also would have overloaded my panel.

Have you looked into a HPHW ? They only pull around 600 - 800 watts when running and you can get them as a large as 80 gallons. If it is in a basement they will also dehumidify.


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## maple1 (Dec 31, 2014)

valuman said:


> Electric resistance heating is a super inefficient use of electricity


 
You won't get a much more efficient way to heat hot water though - with a well insulated tank. And piping.

Our 80 gallon electric water heater costs about $25/mo (family of 5) @ $0.18/kwh. Compared to the 3/4 gallon of oil per day our old way used, should have got rid of the oil long ago.


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## AK13 (Dec 31, 2014)

OP, I don't understand your situation. Do you want to go on-demand to get rid of the tank so that you can get a full bathtub installed? That is what I got from your question. If that is the case then its true that an on demand heater will take way less space. But I don't think that I'd go that route. 

The issue with electric tankless was already explained above....huge service sizes. Take a look at the following chart (by the way, I recommend this brand if you go tankless. Hubbell makes a great product.) 

I'd figure maybe 50 F entering water temperature. Then figure 70 F rise for 120 F shower water. I don't know what your DW draws, but even if you use a low flow shower head (1.5 gpm) you are probably looking at at least an 18 kw unit. And you'd probably want to go to 21 kw. So you are looking at somewhere around 80-100 amps. 

http://www.hubbellheaters.com/model/hxtx/#formulas


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## AK13 (Dec 31, 2014)

valuman said:


> Electric resistance heating is a super inefficient use of electricity and the higher you crank up the heat output, the more that becomes apparent. For demand water heating I think gas is the way to go, but I'd seriously consider begreen's suggestion of simply altering the timing of your dish washing and let the PV continue to produce the kWh needed to heat the water in your 20 gallon tank.



What type of efficiency are you talking about? Cost efficiency?  Natural gas is surely a cheaper way to heat water, but then again NG is the cheapest energy available right now. Propane....well then it depends on what you are paying. Oil, well depends on what year it is!

As far as combustion efficiency, electric HWH's are almost 100% efficient both as on-demand and tank type heaters. Losses from modern well insulated tanks are very low. But I agree with you that a tank type heater makes more sense than tankless.


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## valuman (Jan 7, 2015)

maple1 said:


> You won't get a much more efficient way to heat hot water though - with a well insulated tank. And piping.
> 
> Our 80 gallon electric water heater costs about $25/mo (family of 5) @ $0.18/kwh. Compared to the 3/4 gallon of oil per day our old way used, should have got rid of the oil long ago.


Hmmm, not too sure about this. Why don't we use electric boilers for space heating?


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## AK13 (Jan 9, 2015)

valuman said:


> Hmmm, not too sure about this. Why don't we use electric boilers for space heating?



I actually do use an electric boiler in an apartment that I own. Its about 98% efficient at turning electricity into heat. So you can't argue with the efficiency of heating with electric. But you can certainly argue with the cost. With oil and propane down and electric up this winter, the electricity is far from the cheapest heat that I could be using there. Last winter it was extremely close to oil and probably better depending on the situation (age and location of oil boiler or furnace).


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## valuman (Jan 9, 2015)

AK13 said:


> I actually do use an electric boiler in an apartment that I own. Its about 98% efficient at turning electricity into heat. So you can't argue with the efficiency of heating with electric. But you can certainly argue with the cost. With oil and propane down and electric up this winter, the electricity is far from the cheapest heat that I could be using there. Last winter it was extremely close to oil and probably better depending on the situation (age and location of oil boiler or furnace).


Interesting. Does your boiler work on resistance heating, or heat pump technology?


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## AK13 (Jan 9, 2015)

Electric resistance. Heat pump would be way better, but also way more expensive of course. 

The unit that I use for a boiler is commonly used for on-demand DHW as well. I think its a 14 kw model, but I forget. I might have gone with 11 kw. Electric baseboard would have been more sensible, but I already had the hydronic baseboard in place so it was cheaper to do it this way.


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## maple1 (Jan 9, 2015)

I also have an electric boiler. Used for backup heat only.

Virtually all of the electricity used in heating hot water via resistance heating goes into the water. Hard to get much more efficient than that - except for heat pump technology. But I will agree that it is likely not the least costly. If I needed our electric boiler for more than backup, or for more regular use, I likely would not have it & would likely still be using oil. Which although cheaper, wastes a greater % of it's heating potential - up the chimney.

Unless maybe I could do time of day metering with it - which our utility hasn't quite got the complete hang of yet. Maybe some day...


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## valuman (Jan 9, 2015)

Interesting, again. I've long been told that electricity is a very inefficient way to heat, but from what you all are writing here, it's the cost that makes it inefficient, not the physics.


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## dougstove (Jan 9, 2015)

Many years ago, in Paris, my wife and I had an apartment with a gas-fired, on-demand hot water heater.
When we turned on the shower, the water would be stone cold, and we would scream.
If you turned the flow rate up high enough, it would trigger a big gas flame, and there would be a WUMPHHHH of concussion propagated through the apartment.
Then, after a ~1 min, boiling hot water would gush from the shower.
So we would turn down the flow, the flame would go out, and after ~1 min, the water went icy again.
Rinse, repeat....
I assume modern units are better, there were certainly no romantic joint showers in our little Parisian love nest...


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## DBoon (Jan 10, 2015)

dougstove said:


> When we turned on the shower, the water would be stone cold, and we would scream.
> If you turned the flow rate up high enough, it would trigger a big gas flame, and there would be a WUMPHHHH of concussion propagated through the apartment.
> Then, after a ~1 min, boiling hot water would gush from the shower.
> So we would turn down the flow, the flame would go out, and after ~1 min, the water went icy again.



This has also been my experience with gas-fired on demand water heaters, in Germany (travel for business) and in Hong Kong (friends lived there for a while, and I would visit while on business).  I can't believe anyone puts up with that type of operation....


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## valuman (Jan 11, 2015)

DBoon said:


> This has also been my experience with gas-fired on demand water heaters, in Germany (travel for business) and in Hong Kong (friends lived there for a while, and I would visit while on business).  I can't believe anyone puts up with that type of operation....


Hmm, the one we have at camp produces predictable and steady streams of hot water. I wonder if those erratic producing units that you guys are dealing with in other countries are older designs. I know some countries embraced on demand hot water systems long before we moved toward them here so it would make sense that there would be some older designs still in use there.


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## Seasoned Oak (Jan 11, 2015)

Iv been looking at these as well but am leaning more towards the heat pump water heater if you need any kind of volume.
Point of use seems better suited to faucets and sinks than showers.


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## valuman (Jan 12, 2015)

Seasoned Oak said:


> Iv been looking at these as well but am leaning more towards the heat pump water heater if you need any kind of volume.
> Point of use seems better suited to faucets and sinks than showers.


I've looked at the HP on demand water heaters a bit and am not convinced they're always a good option for northern climates, because basement temps are lowered by running one. How that impacts winter heating and potential for pipe freeze ups is a big question in my mind.


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## maple1 (Jan 12, 2015)

HP water heaters aren't on-demand heaters. They rely on some storage capacity as they can't heat it as fast as it is being used.

I don't have one because I think I would be looking at a pretty long payback considering a savings of in the area of $15/mo for here - but I don't think cooling of the basement is a great factor overall from what I have read on here from people that use them. I think it could certainly be offset by the dehumidification factor in the summer. In those really cold winter cold snaps, you could always just run it in resistance mode in which case it would be the same as an ordinary resistance water heater.


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## AK13 (Jan 12, 2015)

valuman said:


> I've looked at the HP on demand water heaters a bit and am not convinced they're always a good option for northern climates, because basement temps are lowered by running one. How that impacts winter heating and potential for pipe freeze ups is a big question in my mind.



I'm running one of these. I'd say they have zero impact on basement freezeups because they won't run that low. They'll shut off around 40-45 F I think so they can't cool the basement any lower than that. I'm on my first winter running one. It must be pulling heat out of the basement (obviously), but I can't tell any difference at all. My basement is insulated, but unheated except for the fact that the oil furnace and ductwork is down there (providing lots of heat when it runs every morning to warm the house before I get the fires going). 

Its probably just stealing a bit more heat from the first floor and i'm not noticing it. My basement has been running at about 47-50 F pretty consistently. I think the bigger issue with the HPWH is that it isn't very efficient at that temperature. I bet its only running at a 1.5 to 1.75 COP. Still better than straight electric though.


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## Highbeam (Jan 12, 2015)

valuman said:


> Interesting, again. I've long been told that electricity is a very inefficient way to heat, but from what you all are writing here, it's the cost that makes it inefficient, not the physics.


 
It could be terminology. Efficiency and cost are often interchanged by folks who don't know what they're talking about.

Electric resistance is 100% efficient at delivering energy to the water. There are no losses through a flue, pump, blower, etc. The standby losses of keeping the tank hot are what makes up the less than 1.0 energy factor of an electric tank water heater. In some locations, such as mine, electricity is relatively cheap. I pay under 10 cents per kwh. Which makes each btu cost less than the same btu from the much less efficient propane. If NG was available then it would likely beat the others. It too has relatively low efficiency.

Also consider equipment costs. That gas boiler cost several thousand dollars when an electric boiler is under 1000 and with no burners or moving parts is less likely to fail.


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## DBoon (Jan 13, 2015)

I have a Geyser in a pretty big basement (800+ square feet) in a cold climate, and after the first winter, I found that the basement did cool off a lot more with the HPWH running.  Come December, I usually run the top electric element overnight on a timer (I have a day/night electric rate) and run the HPWH overnight to circulate the water so that the top element warms the whole tank, and I keep the HPWH on until about Noon so that we don't "run out" of hot water. 

This approach works pretty well.  The HPWH doesn't run all day (and work too hard) in the winter, and it doesn't cost me much of a premium with the day/night electric rate.  In April, I'll go back to straight HPWH when I want the dehumidification as well (one of my primary reasons for installing the HPWH) and it is warmer to run at a higher COP.


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## jebatty (Jan 14, 2015)

Won't solve the OP's concern, but due to a long plumbing run, I installed a 5 gal electric resistance hwh under the kitchen sink just to provide hot water to the kitchen sink and the dishwasher, although can't use both at the same time as that will quickly deplete the tank. Also can't just run the hot water for long either, and that's good because water waste has been reduced considerably.


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