How to Calculate Propane Usage for Tankless Water Heater

lamb_abbey_orchardsDecember 11, 2009

I hope there are some techies following this forum who may be able to help me do some basic calculations regarding anticipated propane usage.

I'm building a new home this coming year and one of the indulgences I'm allowing myself is a luxury shower with 6 shower heads, each using 2.5 gpm. At full bore, it will therefore use 15 gpm of hot water. I know how incredibly wasteful this sounds, so I'm utilizing some technology to get my costs down on heating the water. It's also water coming from my own well and going back into my own land.

I'm trying to calculate the amount of propane I'll need to use on a daily basis in two different scenarios--summer and winter.

My basic set-up consists a pair of a 119 gallon indirect hot water heater (and holding tank) that is preheated by a boiler loop coming from the wood stove heating the house in the cold months. Additionally, I'm be using a GFX Drain Heat Recovery System which also preheats the water going into the on-demand water heaters by means of a copper coil wrapped around the shower drain. The preheated water will then be fed into a pair of on-demand water heaters.

In the summer months, my well water is around 56F and my goal is to be able to produce 15 gpm at 110F. The GFX coil on my drain will be able to preheat the well water from 56F to 80F, leaving a needed rise in the summer of 30 degrees to get to 110F.

In the winter months, my well water is around 45F. I'll have the benefit in the winter of preheating the water twice, once with the boiler loop between the wood stove and indirect hot water heater, and a second time with the GFX coil on the shower drain. The boiler loop should get the water from 45F to 65F, the GFX from 65F to 89F, leaving a needed rise in the winter of 21 degrees to get it to 110F.

I'm anticipating using the shower at full capacity (15 gpm) for a full hour every day. How do I calculate how much propane I'll need to use on a daily basis, both in the summer and in the winter?

By the way, the pair of on-demand water heaters I'm using are 94% efficient, if that needs to be factored in here. The also each have a maximum consumption of 199,900 btu/hr, for a total of 400,000 btu/hr for the pair.

I'm at a loss how to calculate the BTUs needed on a daily basis, and the subsequent amount of propane needed.

Anyone willing to help me calculate this? I'd greatly appreciate it!


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An hour of use at 15 gpm with a 60F temp rise and 90% efic would be:

(15 gal/min)(8.3 lb/gal)(60 min)(60F)(1 BTU/lb-F)
= 450,000 BTU/hr, or with 90% efic, an even 500,000 BTU/hr

Propane has 92,000 BTU per gallon, so this is 5.4 gallons of propane per hour of use. Not to mention 80 lbs of CO2 emissions a HOUR!

I'd also be surprised if the GFX performed up to its specs at these flow rates.

I have to say that your plan sounds totally insane to me -- not to mention totally irresponsible.


    Bookmark   December 13, 2009 at 10:12PM
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I'm building a new house next year where this 6-head shower will reside. It's the one big indulgence I'm allowing myself. The house itself will be super-insulated and airtight to the point that it'll require no more than 12,500 BTUs to keep the house at 70F when it's -20F outside. I'll basically be able to heat the house with a Zippo lighter, although I'll be heating the place with wood.

The shower that I'm allowing myself will be using well water, coming from and returning to my land, which will be preheated both by a loop from the woodburning boiler stove that heats the cottage as well as by a substantial GFX drain heat recovery system to get the Delta T somewhere between 20 and 30 degree (depending on the time of year.) I'm sorry you're skeptical about the performance specs of the GFX, but that's your issue, not mine. I'll also be using a pair of incredibly energy efficient condensing tankless water heaters to heat the pre-heated shower water to the final desired temperature.

I've done the math now and have calculated that to take two 30-minute showers per day, utilizing all six shower heads going full bore (450 gallons per shower!), each will require an average of 1 gallon of propane costing approximately $2.40 per shower. This is less than the smallest Starbuck's Latte you probably throw back on your way to work.

I'm hyper-aware of my carbon footprint. And I'm willing to bet that the footprint caused by my home will be less than the one caused by yours. I just happen to place a high priority on having a really, really great shower.

Bottomline: The calculations you provide are erroneous and based both on a lack of information and a number of false assumptions. More importantly though, your perception of what I'm doing is really myopic. You should be concerned about ones NET carbon footprint, not about any one individual indulgence.

Don't be so quick to jump to conclusions. You just end up looking foolish.


    Bookmark   December 13, 2009 at 11:37PM
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By the way, the CO2 emissions from this system amount to around 25.4 lbs/DAY, as opposed to 80 lbs/HOUR. The system is only running 1 hour/day, so to calculate it by that one hour per day is grossly misleading.

2 gal x 12.669 lbs of CO2 per gal of propane = 25.4 lbs


    Bookmark   December 14, 2009 at 1:55AM
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Gary wrote:

"You asked for a calculation, and I gave you a correct one with all the variables spelled out. . ."

No, indeed you did not. You outright ignored the temperature rises provided, made calculations using inaccurate information, and then proceeded to climb upon your moral high horse and screech like a hyena on the basis of your erroneous results:

"this is 5.4 gallons of propane per hour of use. Not to mention 80 lbs of CO2 emissions a HOUR! . . . your plan sounds totally insane to me -- not to mention totally irresponsible."

Why would I blow 2 gallons a day on propane for showers? Because of the sheer pleasure I get from them. It's basic Cost Benefit Analysis. I'll be using an amount of propane for the 2 killer showers I take in a given day that's equivalent in CO2 emissions to about 1.1 gallons of gasoline. Yet I work from a home office and drive very little, opting to produce the CO2 emissions I do from things that provide me with more satisfaction than simply driving every place I have to go. It's a trade-off. I could just as easily get indignant about the fact that you likely drive when you could ride a bicycle instead. But how you get around is a choice you get to make, not I.

Again, you need to focus on one's NET carbon footprint, and stop your bloviating and pompous moralizing about any one indulgence.


    Bookmark   December 14, 2009 at 8:09PM
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Did you ever bite off on this project? One thing I'd highly reccomend is a heat recovery system on the drain. Rather than attack you as a wasteful basturd, I'll make a useful comment... I'd love a shower like this! There are many many options here and they typicaly consist of a 2" copper drain tube that is about 3 feet long where the film effect of the drain water causes the inside of the copper tube to be "coated" by the drain water thus heating the tube. The tube is wrapped with copper tubes for the cold water to run though that would be going to the water heater. It picks up the waste heat on the way to the water heater. You can significantly reduce your need for huge capacity hot water heating by recovering the heat that is otherwise going down the drain. With this much volume, you might need a few stages.. This works best on a second story, or if you have a basement for the water to go down vertically for the film effect to work, IE just putting the drain pipe on it on it's side will not get the heat transfer you'd otherwise get with it vertically.

    Bookmark   August 30, 2012 at 3:33AM
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