# on demand water heater question



## uarelovedbygod (Nov 1, 2003)

This week I re-filled our propane tank---$850.00!!! Yikes!! Propane here is almost $3 per gallon. I realize that the biggest user of propane in our house is the 40 gallon, 5 year old propane water heater. So I am now for the first time looking into switching to an on demand propane water heater.

Does anyone know how to calculate how much less propane these use than the older fashioned heaters? The rating on the side of the water heater says it uses approximately 288 gallons of propane a year. I don't know how the water heater guys predict this, since they don't know how often our family of 8 showers or does laundry....But I do know that we fill our propane tank outside twice a year, and our only gas appliances include the water heater, our kitchen stove and a small space heater.

Any experts on the subject out there??????


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## WisJim (Jan 14, 2004)

They have a comparison system for figuring out the "average" use of fuel by water heaters, just as they do for MPG for cars and electrical usage for refrigerators. If you know the label rating on your existing water heater, you could compare the gallons per year usage of propane to the demand type water heater--that is what those figures are good for. They aren't really saying how much LP you might use, just giving a figure to compare efficiencies of different appliances.


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## uarelovedbygod (Nov 1, 2003)

Thanks WisJim! Do you know where to find one of those comparison charts?

--Chris


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## 12vman (Feb 17, 2004)

Chris..
Do you have enough room to install another tank beside the existing one? I have an idea if your interested..

~Don


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## Gary in ohio (May 11, 2002)

keep in mind on demand or talk less water heaters use more water since cold water runs until its heated.


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## SolarGary (Sep 8, 2005)

uarelovedbygod said:


> This week I re-filled our propane tank---$850.00!!! Yikes!! Propane here is almost $3 per gallon. I realize that the biggest user of propane in our house is the 40 gallon, 5 year old propane water heater. So I am now for the first time looking into switching to an on demand propane water heater.
> 
> Does anyone know how to calculate how much less propane these use than the older fashioned heaters? The rating on the side of the water heater says it uses approximately 288 gallons of propane a year. I don't know how the water heater guys predict this, since they don't know how often our family of 8 showers or does laundry....But I do know that we fill our propane tank outside twice a year, and our only gas appliances include the water heater, our kitchen stove and a small space heater.
> 
> Any experts on the subject out there??????


Hi,
You can make a rough estimate of how much propane you should be using this way:

Gallons of Propane/year = (gallons per day of Hot water)*(8.3 lb/gal)*(365 days/year)*(Temperature rise)*(1 BTU/lb-F)/((92000 BTU/galpropane)(Efficiency Factor))

For a typical few years old gas HW tank Efficiency Factor is about 0.62

You could figure 20 gallons per day per person.
If your ground water is 60F, and you heat to 130F, temperature rise is 70F.

So, for 8 people -- this comes out to:

(160 gal/day)(8.3)(365)(70)/(92000)(0.62)) = 595 gal per year.

This is just an estimate -- you can plug in better numbers if you know them.
This probably comes out a lot higher than the 288 because you have more people using hot water.

The tankless heaters have efficiency factors around 0.85 ish -- this would drop the 595 gallons to around 430 gallons -- saving about $500 a year -- pretty good payback!

If you use hot water for clothes washing, think about using cold. If you use hot wash and hot rinse, this is about 8.2 KWH a load, or about a half gallon of propane per load for your 62% efficient heater. 
We have not used an drop of hot water for clothes washing for years -- I don't see any difference.

Low flow shower heads?

Or, think about making a solar batch water heater for a few hundered dollars?
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Batch
if you are in a good place for solar.

Gary in Ohio, I'm not understanding your comment on tankless heaters using more water? The water that comes out of the tankless is always hot, so how do you use more water? Maybe I'm missing something?

I just put in a Takagi Jr tankless heater about a month ago, and we like it a lot. Compact, quiet, and endless hot water.


Gary


---
you can look up the exact Efficiency Factor for your tank here:
http://www.gamanet.org/gama/inforesources.nsf/vContentEntries/Product+Directories?OpenDocument


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## Helena (May 10, 2002)

We looked into the tankless water heaters..infact I wanted one many years ago..but anyways..and really liked them...a bit pricey for us right now but when our tank goes I will surely look into one, maybe before that the way prices of propane are going up..up..up !!I was concerned with our hard water issue and heating the water but was told it only heats when running through the heater so no problem with that or they had something in it that would prevent hard water problems..(old age..can't remember what or where I hear things sometimes..).. If the price is right for you I would buy one..I was actually told by our company that the less hot water/propane you use the higher the cost..go figure...Let us know..me especially..if you get one and how you like it and the savings...Thanks !!


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## mdharris68 (Sep 28, 2006)

As I am getting used to my new to me, (2 year old) tankless water heater, I know what Gary in Ohio is talking about. i.e. cold running water first. My problem is that I have too much water line between the heater and the outlets. I am in the process of cutting out all unneccasary plumbing and rerouting my hot water supply lines in the most direct route to the outlets.

Like noted above, you can figure that a tankless water heater will produce the same amount of water for approximately 20 percent less when you compare with the same fuel. My heater has a .84 efficiency factor. 

There are some manufacturers out there that claim to have a 99 percent efficient unit. I can't recall any names right now, but if I find a link, I'll edit this post and add it in.

I guess the 99% heaters are generally electric, and that can be an expensive option if your electric rates are high. 

If I could recommend a brand, I would recommend Bosch. Mainly because I have dealt with thier technical support for parts ordering and those guys are great. I have had to work on a few of them over the years.


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## Ken Scharabok (May 11, 2002)

Dumb question: Can a tankless hot water heater be used as a 'boiler' for a baseboard hot water radiator system?


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## Tilly (Oct 16, 2007)

Just talked to a rep from First Day Cottages yesterday about radiant floor heating, he said that the company recommends the on demand water heaters for their radiant floor system. When we build in two years, we are going to use a tankless system.

Tilly


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## pruane (Nov 18, 2007)

I do not see the difference in tankless or tak heater for amount of water used?


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## Jim-mi (May 15, 2002)

Ken, Yes
but remember those things don't throttle back.
they are on or off.
and when *on* they are consuming lots of fuel.
you would need plenty of intelligent controls to run things.


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## Jim-mi (May 15, 2002)

uareloved......
Do you have a double insulation blanket around the tank..??
Have you considered ways to decrease your HW usage..??
If there is some shower takers using *more than needed* then you will still be sucking lots of fuel for those *over long* showers.

Think conservation first . . . . . . . . . .


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## SolarGary (Sep 8, 2005)

Jim-mi said:


> Ken, Yes
> but remember those things don't throttle back.
> they are on or off.
> and when *on* they are consuming lots of fuel.
> you would need plenty of intelligent controls to run things.


Hi Jim,
I think that most of the tankless heaters do modulate the burner.
Mine will go from 19K BTU/hr to 140K BTU/hr depending on the incoming water temperature and the flow rate. It does quite a nice job of regulating the temperature to what you set it to.
I actually got the optional remote, so I can set the temperature from the bathroom -- OK -- I'm a hopeless gadget freak 

Gary


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## SolarGary (Sep 8, 2005)

pruane said:


> I do not see the difference in tankless or tak heater for amount of water used?


Hi,
Not sure what you mean?
In the calculation, I assumed the same water usage for both tankless and tank type. As near as I can tell they should be the same?

Gary


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## SolarGary (Sep 8, 2005)

Ken Scharabok said:


> Dumb question: Can a tankless hot water heater be used as a 'boiler' for a baseboard hot water radiator system?


Hi Ken,
The Takagi website has a system diagram for a radiant heat system powered by one of their tankless heaters -- it looks pretty simple.

The only disadvantage I can see is that the efficiency of the best boilers is up around 95%, where the tankless heaters are more like 85%. 

Maybe Ross knows more on this?

Gary


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