That is 415 CCF, or 425 therms for hot water.
As a comparison I used 1085 therms for space heating of my home last year inluding hot water (tankless with pilot) with a 50 year old gas furnace (in New England).
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What the *&^!! I read all 11 pages, and it ends like this?? Any updates? I would be very interested in any outcomes.
In all fairness, if you needed a water heater, the price of the standard tank and install should be deducted from the install price of a tankless before the payback is calculated. Then it works out to about 7 years which is accurate.
But remember the #1 reason for buying tankless today is not savings but endless hot water by people who are sick of cold showers, and # 2 is space required, followed by energy savings as 3rd.
Interviewing plumbers shows it's the older generation that has difficulty accepting the new technology, becuase of difficulty in learning and understanding it.
Get with it, tankless is here to stay and growing.
To compare, why did you get a flat screen TV? That 25" would work just as well to watch the evening news.
15 year payback is what tank manufactures spread around.
this thread has been dead and gone for a long time....
last night my 75 gallon heater finally started leaking and
today I had to replace it with a new 75 brad white..
it was a 2005 unit and only lasted 7 years.... not too good
anyway the GAS METER has been churning away since I started this
experiment way back on sept 16 2007
here is the final reading off that meter.....199122 therms units were used
to heat my water for basically 4 1/2 years...
... so now all I got to do is count the days and divide by the number of therms
time flies when you are busy
I am sorry that I let this little experiment go dead
the meter is now installed on the new FVIR ICON heater and I wonder
how many years that one will last......
You need to make sure you size your gas tankless correctly, otherwise you will not be able to run simultaneous applications, like 2 showers going at the same time. All gas tankless water heater manufacturers advertize flow rates in the 6 -8 gallon per minute range. Read the small print - these flow rates are based on a 35 degree rise, so in your case with inlet water temperatures of 45 degrees, anything smaller than a 180,000 BTU model will not work for you. If you are told 140,000 BTU is more than enough, then the salesman or installer is not doing you justice. Here is why... If oyu buy a gas tankless rated at 140,000 BTU, Energy Star Rated, the Energy Efficiency being that of 82% it means that the 140,000 max BTU is really that of 114,800. If you want to figure out what this 114,800 represents as it pertains to Temperature rise and flow rates, here is the formula. Convert the 114,800 to KW by dividing 114,800 / 3414 (1KW=3414 BTU) = 33KW. Now that you know the KW you can figure out the temperature rise of the unit. Take 33kw and convert to watts (1kw=1,000 watts) so 33KW = 33,000 watts, then divide by 147 (this is the wattage needed to raise 1 galon of water by 10 degrees in an instantaneous basis) ( its actually 1470, but divide by 147 so you won't need to carry the decimal).
33kw x 1,000=33,000 watts / 147 = 224 (this is the tamperature rise at a 1 gallon per minute flow rate) Now take the 224 and divide by the actual flow rate you need, say 2 showers simultaneously. Keep in mind that all US made shower heads are rated at 2.5 GPM at 80 psi, with normal residential water pressures in the 55-60 psi they really only put out about 2 gpm. So 2 showers running at the same time with an inlet water temperature of 45 degrees as you mentioned will only come out at 101 degrees, (244 temp rise at 1 gpm / 4 gallons of flow = 56 temp rise + 45 inlet = 101. Get my point, the advertized 7 to 8 gallon per minute flow rates are not accurate. In your case you need at least a 65-70 degree rise so your outlet hot water is at least in the range of 110-115 degrees and for this you need to have at minimum a unit rated at 180,000 which is only going to supply a max BTU output of 147,600, enough to run 4.8-5 gallons per minute or 2 showers at the same time.
Probably not long
Slightly off topic, perhaps, but Mark -- did you investigate the old WH to see why it started leaking? I'm considering a powered anode to eliminate any electrolytic issues, but if these things just up and decide to start leaking for more basic reasons, it's probably a waste of money. But fabricating a tank to hold water at 100psi for 25 years or more doesn't seem like rocket science.