tankless water-heaters are a scam

[RustyShackleford]

A tankless water heater may make sense if you have a need for unlimited hot water or if space is a huge concern. But I see them mostly marketed as a "green" alternative to traditional tank-style water heaters, the idea being that they don't waste energy in stand-by losses, that is, heat lost into the environment through the walls of that big tank of hot water.

So I decided to check for myself, what the standby loss would be from a typical (i.e. the one I'm probably going to buy) "energy saver" electric resistance water heater: the Bradford-White model M-2-40L6DS. This unit is 24" in diameter and 32" high. It is covered with 2" of insulation with an R-value of 16. The surface area of the water heater is about 23 sq-ft. If we assume the thermostat is set at 120 degrees, and it's sitting in a room at 70 degrees, the temperature differential is 50 degrees. Since R-value is sq-ft*degrees/(btu/hour), the heat loss through the water heater jacket is 23*50/16 = 72 btu/hour, which is 21 watts. That's right. The amount of energy you're saving by installing that expensive, trouble-prone, maintenance-requiring tankless unit is equivalent to one typical CFL light bulb.

Meanwhile, the energy lost in the hot water that's sitting in your pipes after you turn the faucet off, is estimated at 15-20% of your hot-water usage. So far greater energy savings can be achieved by minimizing the piping distance between faucets and hot-water heater. And if you're still concerned about the electricity usage, take the $1000-2000 you're saving by going with an inexpensive water heater, and put it towards a solar electric system.

 

[IFIXH20]

Rusty, sounds good in theory, but a average water heater temp setting is 130-135 degrees. The 70 degree will be hard to achieve in Garage,Attic, and Basements. Things to account for with water piping, Piping In Attic, Under Slab, Inside & outside walls. I am not saying water heater one is better than the other, just saying the numbers does not jive 100% with real world condition.

 

[stevemachine]

I agree ifix, I think the avg water heater should be set above 140 F to avoid the risk of legionnaires disease. It depends on what you want in a water heater. If it was my house I'd stick with a tank, however some people like tankless. There are advantages and disadvantages to both

 

[RustyShackleford]

Thanks for chiming in guys, I was trying to stimulate some discussion, not saying I'm right for sure ...

Seems like 140 is pretty hot, serious scalding risk, no ? But even if we set temp to 130 and crawlspace is 30 (which ain't gonna happen most places), my number is only doubled. As far as piping losses, those tend to be the same with tankless, no ?

 

[stevemachine]

140 is probably a little on the hot side.I looked again and you want it above 120 for sure, however I think most are around 140, if you like hot showers then 140 or so is about right, but that's true even if its set bare minimum 120, tanks aren't as inefficient as they claim. But the gas ones are around 75% efficient burning gas constantly cause its storing water, whereas the tankless burn 95% or so efficient, and only run on demand. But they need more maintenance. Just depends on what you want, and what you're willing to spend up front.

 

[RustyShackleford]

... if you like hot showers then 140 or so is about right ...

I think 140 is way too hot, but looks like 130 is safe:

http://www.legionellaprevention.org/Scalding/Scalding.htm

But I guess you need a tempering valve to really be hot enough to be dead safe on legionella but no risk of scalding. I've never actually seen a tempering valve in real life - but I'm not a plumber, just a DIY homeowner. My propane heater (that's lasted 25 years so far) is probably not set above 120 (because it will not scald at all) and I haven't died of legionella yet - just lucky I guess (or could it possibly be because I'm on a well ?)

... tanks aren't as inefficient as they claim. But the gas ones are around 75% efficient burning gas constantly cause its storing water

The reason gas ones are only 75% efficient is not because they're storing water. It's because they don't burn the gas efficiently and largely because they can't be that well insulated because of the exhaust flue. The condensing type are an exception to this, but they are very expensive and somewhat noisy I'm told.

, whereas the tankless burn 95% or so efficient, and only run on demand.

Actually, the best electric-resistance storage water heaters achieve 95% efficiency (the AO Smith Conservationist line), and efficiencies in the low 90s are commonplace now, with the ones with 2" insulation.

 

[stevemachine]

Yeah but there are some slight piping losses, but yeah if it's set to 140 you'll probably get 135 at the tap, I know on my sisters tankless its set at 135 and its pretty good for hot. You're right though a mixing valve is truly the only way to have perfect temperature at the tap. And for the gas yeah that's true, the way they heat the water isn't efficient cause of the flue. The power vent around here go for about 1500, they are about as noisy as a furnace. I think these would my be my preference, its like a compromise between the two. The power vented water heaters have an awesome recovery rate, they are like 70 or so gallons per hr recovery rate. But the electric I didn't know they were that efficient. However from my knowledge the recovery rate on an electric is half of a gas water heater. Depends on your needs whether you want electric, natural draft, power vent, or tankless. They all have ups and downs. Depends on the person living there.

 

[Beni]

Legionella is not killed until 135 degrees F. 120 degrees is still a breeding ground for the bacteria. The VA hospitals have a procedure for raising the temperature in the hot water piping to 140 degrees F. for an hour or so once a month to kill the bacteria in the piping.

Shower heads are a delightful place that Legionella likes to hang out in. As is those canister water filters attached to sinks. Great growth media!!

Most homes turn over the water in the tank enough that legionella does not grow well. It will grow after a week or so. So run the water after being on vacation for a week or two.

Good call on the OP that the temperature loss from the tank is not that much for electric tanks. Much more so for gas water heaters without a flue valve (a commercial option on some heaters).

The other issue is gravity circulation of the hot water to piping over head. Hot water is slightly lighter (less dense) then cold water and will rise in the hot water pipe. So if the piping is not insulated there will be a lot of heat loss there. A heat trap (S trap 18" ish high) on the hot water discharge will prevent that from happening.

Electric water heaters for residential is limited to about 4.5 KW from the power companies. One KW will head about 4.1 gallons per hour at 100 degree rise. So the typical residential unit will do 18 GPH @100 degree rise. The element is about 100% efficient in that all the power/heat goes into the water.

Gas is about 63% for the typical gas water heater. High eff gas can get that up to 96% but many have issues long term.

Just FYI

 

[stevemachine]

Well if you really want to get specific 75% efficiency is the minimum allowed by the standards in my area, it depends on jurisdiction I guess.

http://oee.nrcan.gc.ca/regulations/bulletins/5160

 

[stevemachine]

Here's a chart for legionnaires disease and the temperatures required to kill it.

 

[RustyShackleford]

Electric water heaters for residential is limited to about 4.5 KW from the power companies.

I'm not sure what you mean by this, but I don't think it's true.

If you look at the manual for a typical Bradford White unit:

http://www.bradfordwhite.com/images/shared/pdfs/manuals/238-44422-00K.pdf

... on page 13 it has a dozen different heater element options, on up to 6000 watts. I called 'em, and they said you can special order with any size you want, for minimal cost but a bit longer lead time. What say would the power company have on this ? They provide me with a certain number of amps (typically, and for me, 200) and I can use them how I choose. Of course, I'll want to make sure the wiring and breaker can handle the additonal current. I think AO Smith just gives you the choice of 4500 or 6000 watts.

 

[Beni]

Well yes you can get up to 6 KW elements in residential now a days. But the standard or default for the typical residential unit is 4.5 kw or it was back 20 years ago. Why 4.5 and not 4 or 5 kw. Well back when a house had a 60 amp service, it determined that about 20 amps was the most you could safely use for a water heater, when you also had a electric stove. 4.5 KW is about 18.7 amps. I actually remember reading this from the then local electric company's manual about 30 ish years ago.

Nowadays with 200 amp services its less an issue but the basic electric water heater is still 4.5 kw.

Sort of why the american railroad gauge is actually based on roman chariot wheel width. (true store BTW) It became a standard long ago and keeps being used.

On the 63% thing...
Water heater efficiency is much like boiler efficiency and how its measured. They measure efficiency by using the flue gas temperature and mass air flow. They subtract the flue gas energy from the input of the burner and that then becomes the efficiency of the water heater or boiler.

It does not actually take into account the radiant heat losses from the burner area or shell of the water heater. That 75% water heaters real efficiency is going to be a bit less when you factor in the combustion efficiency and radiant heat loss from the bottom burner area...

Oh it might be 66% or 68% for the typical heater, My memory does get a little faulty with age. The high efficient heaters with a submerged flame area are better.

 

[stevemachine]

Fair enough, I see what you are saying Beni.

 

[plumber_in_training12]

140 is probably a little on the hot side.I looked again and you want it above 120 for sure, however I think most are around 140, if you like hot showers then 140 or so is about right, but that's true even if its set bare minimum 120, tanks aren't as inefficient as they claim. But the gas ones are around 75% efficient burning gas constantly cause its storing water, whereas the tankless burn 95% or so efficient, and only run on demand. But they need more maintenance. Just depends on what you want, and what you're willing to spend up front.

id like to know what tankless hot water heater your lookin at thats 95% efficient. most are 85-90% efficient.

 

[stevemachine]

Navien and Rinnai

http://www.navienamerica.com/__DATA...0/[1]_2. Brochure_NPE-180A_210A_240A_1112.pdf

http://www.rinnai.us/tankless-water-heater/buyer-guide

Not to say all are 95% but if you're gonna sped the money may as well get an efficient one

 

[Christoph]

I completely agree with your take on tankless water heaters Rusty. People often think that they are going to save money with a tankless water heater, but they also don't take account for the fact that they generally cost more than an electric water heater, generally require more maintenance, and have a certain life span and need to be replaced every so many years. At the end of the life of the water heater, did you really save any money? Maybe, or maybe not. And not only do conventional water heaters not have very much standby heat loss, but that heat (energy lost by the water heater) goes towards heating your house (which is usually a good thing up here in Alaska where I live).