Hot Water Heaters-What am I Missing?!?
Happy Thanksgiving!
I have been doing my usual OCD researching and math on natural gas HWHs and am would like someone to please check my logic. The results that I came up with were pretty surprising to me, as long as I didn’t jack something up…
Method: Go online to two box stores and compare specs and energy star labels for models I am considering that consists of old school atmospheric vent, fan assisted atmospheric vent, power vent, and finally a tankless.
I also looked at my most recent gas bill and divided total Therms usage and divided by total cost to find I pay $2.21/Therm including fees and taxes.
Old school atmospheric: 40 gal, 36kbtu, 54% efficiency, 1st hr= 64gal/hr. Purchase price = $519, annual operating cost (191 therms/yr) = $422
Warranty = 6 year
Total cost of ownership = $3,051 for 6 years ($508/yr)
Fan assist atmospheric: 29 gallon, 60kbtu, 71% efficiency, 1st hr = 93 gal/hr. Purchase price = $9oo, annual operating (238 therms/yr) = $526
Warranty = 12 year
Total cost of ownership = $7212 for 12 years ($601/yr)
Power vent (pvc): 40 gallon, 40kbtu, 69% efficiency, 1st hr = 86 gal/hr. Purchase price = $1100, annual operating (245 therms/yr) = $541.
Warranty = 6 year
Total cost of ownership = $4346 for 6 years ($725/yr)
Tankless: 199kbtu (11gpm)
94% efficiency. Purchase price = $1450, annual operating (180 therms/yr) = $398
Warranty = 12 year heat exchanger, 5 year parts
Total cost of ownership = $6226 for 12 years ($518/yr)
Based on the warranties (I’m sure each can last longer or shorter than the warranty period), I then figured total cost of ownership for a 12 year period since that was the longest warranty of all the heaters. This included initial purchase price and total gas usage costs. A 6 year warranty tank I added the cost of buying a second tank. I found total ownership cost for 12 years to be the following:
Old school atmospheric = $6,102
Fan assist atmospheric = $7,212
Power vent = $8,692
Tankless = $6,226
I often read that tankless are the end all, be all of water heating. But comparing this modestly priced tankless (compared to Rinnai) against the old school atmospheric, I am really surprised. It’s basically a wash.
There are a bunch more components and things that can go wrong on a tankless compared to atmospheric vent. Plus no power is required for atmospheric.
Have I overlooked anything in my calculations, or have we been “oversold” on the idea of tankless heaters?
Some claims state that tankless can last longer than 20 years if maintained, so do you only start seeing a true savings after you would have to buy your third atmospheric while still on your original tankless heater?
Replies
I did not see where you figure in the cost of heating the air you burn with the water heater.
The biggest variable is the cost of natural gas, which has until recently been very cheap.
( I over-estimated this cost when I did similar calculations for heating 15 years ago.)
You also may want to figure in your actual use pattern. the storage units all keep hot water hot all the time, (loosing heat) vs a tankless unit.
Just as a HVAC calculation would benefit from a manual J calculation for the structure, and include heating/cooling degree days, You may want to start with some idea of your actual hot water use patterns.
Try to include the reality of running hot water until it gets to the faucets.
Are any of these options condensing? They should come in well over 90% efficient.
I don’t think I understand what you mean by heating the air that I burn with the hot water heater, can you elaborate?
The tankless is a condensing unit, all others are not.
For each option, are you burning gas with air from outside the home, or with conditioned air from inside the home?
See if you can find a condensing tank water heater to throw into the mix.
and perhaps a non-condensing instant water heater.
As it is, you are comparing apples to bananas.
I only know my tankless heater, but do any of them use indoor air? Aren’t they all using only outdoor air?
Oh ok, I understand. It would draw inside air most likely
I don't pretend to know a lot, and I don't have time to redo your calculations, but I do have a question and a comment. Do the efficiency ratings account for stand-by loss? An instantaneous heater has none whereas a tank heater does. So when the tank heater says 54% efficiency is that simply how many BTUs it takes to heat the water and how many go out the vent? It's known as recovery efficiency. Stand-by loss is how much heat is lost by the heated water just sitting in the tank (often for hours) so you have to heat the water all over again to get hot water, a big loss of efficiency. I don't know how much all this affects overall efficiency, but if your "54% efficient" doesn't account for stand-by loss the difference is probably substantial. One comment, if over 12 years the total cost of a tankless is only $124 why wouldn't you want that? The convenience of instant hot water that can keep going until you run out of gas is more than worth the $10/year (if that number is correct). Here's another source for calculating costs (maybe you've seen or used it). https://www.energy.gov/energysaver/estimating-costs-and-efficiency-storage-demand-and-heat-pump-water-heaters
The efficiency, from my understanding, is something like if you spent $1 on gas only $0.54 would be used to heat the water while $0.46 would be wasted out the exhaust.
My assumption is that all of the DOE energy usage labels that I used to gather the information would include standby time where you are keeping a tank warm.I think that is where the efficiency factor comes in to play.
Regarding your comment about cost of $124/year, I’m not sure where you are seeing that number. Total cost of ownership spread out over 12 years is $6226. Basically not much savings compared to buying a tank unit twice.
Unlimited hot water would definitely be a benefit along with smaller footprint compared to a tank.
Drawbacks would be complicated circuits and components (much more fail points compared to a tank unit), required maintenance, and maybe not something that is financially prudent if you don’t plan on staying in your house long term.
I guess the way that these high end youtube builders are really pushing these tankless units (I can think of one specifically that rhymes with Cat Misinger), I was really thinking the tankless would be a no brainer.
I guess that is why I posted the question.
Old school atmospheric = $6,102
Fan assist atmospheric = $7,212
Power vent = $8,692
Tankless = $6,226
The difference between the tankless and the Old School option over 12 years is $124. I can’t speak to the maintenance cost, but you also have to factor in the labor cost of replacing the 6 year tank. Of course, it assumes the tankless last 12 years and the tank version lasts 6, only based on their warranty lengths.