Tankless water-heater theory

[stewchi]

Last weekends ATOH (Ask This Old House) featured a tankless water heater and made many blanket statements about it being more efficient. I am not convinced and here is why:
1st the main assumption that heat loss occurs in tank systems and therefore on demand will not have this problem. Ok this certainly true but to what degree, how good is modern tank insulation are we talking about loosing 20% through heat loss or .001%. I think the number is pretty small since my tank stays pretty cool.
2nd I am speculating that flash heating the water as it is flowing by at 3 GPM requires a lot of heat and is less efficient (a lot of heat will be wasted through that huge vent) as heating stationary water in a tank, you need less heat and you have more time for the heat to be absorbed so less heat is sent out the exhaust.
3rd Again I am speculating but I guess that heat loss occurring long runs of hot water pipes will have a bigger impact by far the anything else tankless or tank.

The only way I see tankless to be more efficient is if you had one under every sink, which would eliminate long runs for washing hands and face, were you waste more hot water in the pipe than you actually use.

Has anyone ever tried putting there electric water heater on a timer to only kick in during off peak usage to save money???

What do you guys think, is tankless really more efficient, I think it will take some studies from somebody independent to convince me.

[greencat]

I looked hard at this last year. I think the big advantage is endless hot water as long as you don't exceed the flow rate. I know these are used in Europe but I think it's because of the size. I don't think any of these items can be justified on cost savings. They do make direct vent models so the efficiency should be pretty good. The same with high efficiency furnaces. If it is a new install then it makes sense but to rip out and expect a payoff is not realistic.

I just reduced the flow in the shower heads and it solved most of our hot water problems.

[maxparot]

Part of the expected savings is on your water bill. And yes the idea is to have the heater closer to the point of use. In my house this would require 2 heaters. Since the kitchen is on 1 side of the house and both bathrooms are on the other side. The savings comes from not having to wait for your pipes to be filled with hot water and become heat soaked so the temperature is stable in the line. All that water run down the drain is an environmental waste that costs you for the water. This water waste could be overcome with a hot water recirculating system but then you would be using electricity to run the pump and the hot water heater would be turning on more often thereby wasting more of that side of the equation. Also a lot depends on your utility costs for me it would be a electric heaters cost of operation vs water costs. Since I live in Arizona water is relatively expensive but electric costs are relatively reasonable. This swings the equation due to the water savings of an instant heater.

[Howard]

I curse my electric water heater regularly. I also looked at getting the tankless variety but unfortunately the gas service is on located on the other side of the house and it would cost big bucks to run a feeder line over to my water heater closet in the corner of the garage. The efficiency of an electric water heater is atrocious. Whereas a gas model can recover anywhere from 40-50 gph, the electric ones can only do less than half that. It runs on 220, and when it's on, the meter is flying. We do have a recirculating pump set on a timer to turn on a few hours in the morning and evening, otherwise, without the pump, it would take over ten minutes to get hot water to our bath on the other side of the house. With the pump left on all the time, we have instant hot water but an insane electric bill. Having an endless supply of hot water has a big attraction in my house, unfortunately, that dream will have to wait until we move to a house with a gas water heater already installed.

[LCHIEN]

From Consumer Reports:

Measure before you buy. Last year’s tougher federal energy standards require about 10 percent higher efficiency for gas water heaters and about 5 percent for electric models. But the insulation addded to meet those standards has made some heaters up to 4 inches fatter, a potential problem for closets and other tight spots.
Consider gas. Based on national average fuel costs, gas heaters cost roughly half as much to run as electric models and can pay for their higher up-front cost in as little as a year. Factor in the cost of running a gas line to your home if you don’t have one. Also consider adding insulation to hot-water pipes and the cold water pipe exiting the water heater.
You may have heard about tankless water heaters, which save energy by heating only the water you draw. Those savings can add up to some $50 per year compared with conventional heaters. But even at that rate, it will take more than 25 years for an average household to recoup the extra $1,300 or so those units cost to buy, install, and maintain.

your savings are twofold.
1. saving the wasted water as you circulate through the pipes waiting for the water at the faucet to heat up.
2. Saving the heat lost from constantly keeping a large vessel of water warm.

I think that #2 is probably the biggest waster of energy. Insulation is far from perfect and results in a small but steady loss of heat energy (proportional to the difference in temperature between the surounding and the water temperature) through the tanks walls. The loss is there Even though it may not feel warm to the touch - because a good insulator will have a steady drop in temperature across the insulation, the outside shold be at room temperature unless it were doing a totaly terrible job.
I think you are underrating that, because if you work outside the home, and sleep 8 hours, the heater will be keeping water hot for 23 hours just for the hour or so you do use it. I would not be surprised most of the heating energy were used to replace lost heat rather than heat what you use.

[billwmeyer]

I am well pleased with my tankless water heater. I disagree with a couple points though. It takes a little longer to get hot water to a tap because the water is cold until the tap is opened. It takes a little bit for the water to heat as it flows through the heater. With a tank style, as soon as you open the tap, hot water starts flowing out of the heater immediately, so the only water wasted is what is in the pipes. Since there is an unlimited amount of hot water, showers tend to take a little longer, so more water is wasted.

I feel my gas usage is down, but water heating is a small portion of my gas bill. I also like the small space that the heater needs.

My only real problem is that my gas line is undersized. In the coldest months of winter, my water does not get quite as hot as I would like it to. I plan on getting that fixed this spring.

Bill

[k2qo]

Efficiency Study at Bradford-White Website

Hi Gang,

I am a PE and do energy study stuff on the side. If you want the nitty-gritty of just how energy efficient these heater can be, go to BW's site and find the study (or Reader's Digest version to follow.) Very interesting stuff. Seems that best instants have an EF=0.85+/- and the tank types are at EF=0.65 for the very best. Now, take a family of 4, try out both units with average hard water and run 3 weeks. The real efficiency of the tank type is more like 0.73 and the tankless is down at 0.78. (numbers from memory here) Bottom line? In this scenario, gas would have to quadruple to get a 7 year payback. But change the number of occupants and usage criterea and you get different numbers.

Personally, I like the superinsulated indirect tanks hooked to a high efficiency boiler. One exhaust, sealed combustion, quiet, very efficient. And these tanks can last a lifetime. Just ask Rich Trethewy of This Old House.

Best regards,
Mark Adams
Clarence, NY

[just4funsies]

Two things: 1) everything will depend on the efficiency of the heat exchanger. If most of the heat goes into the water, you have the opportunity to save money. If the heat goes out the exhaust, then it's too wasteful. 2) You may also wish to consider solar. The equipment costs up front, but the sun is free.

[scorrpio]

That is the hookup I am currently considering as my heating system replacement: A Weil-McLain Ultra boiler paired with an indirect-fired Plus heater. The insulation rating on these heaters supposedly causes them to lose about 1 degree farenheit per 2 hours.

My physics is kinda rusty, so correct me if I got a hole in my calculations someplace.

1 calorie = amount of energy needed to raise temperature of 1 gram of water by 1 degree Kelvin.

1 farenheit is 0.56 Kelvin. A 50-gallon tank has approx. 189,200 grams of water. Thus, loss of 1 farenheit means approx 105,000 calories, or 417 BTU. (252 calories per BTU). In a 30-day month, at 1 farenheit in 2 hours, means 417x360 = 150,120 BTU loss. Supposing 75% efficiency, that's a loss of 200,000 BTU worth of fuel: 2 therms of gas, or 1.43 gallons of #2 oil or ~0.7 kWh. Can't recall gas or electric cost at the moment, but oil being $2.54/gal here, we are talking a whopping $3.63 a month loss from the tank. If heater works off a high-efficient boiler (like a 93% Ultra), that loss is even less.

[thestinker]

I have a gas tank-less WH, and I love it. I will never own anything else. For me, I only had a 30 gal WH, so it didn't take long to run out of hot water, and the size of my closet limited me going any larger with my tank. I got mine at HD, and paied about 2x what a tank heater costs, but the expected life on it is about 25 years. I will go through almost 2 tanked heaters in that time, and they do save $$$ on their operation. Now, as for the electric ones, run away. I know 2 people who have them, and both have major problems using them. But, I would go gas and not think a thing about it, especialy if you run out of hot water often. Also, they make small under the counter 110 heaters that will heat up the "cold water plug" in the hot water line. When they sence flow of cold water, they turn on, and once the water gets hot, they shut off.

[Warren]

With the quality of my well water and the average life span of water heaters (3 to 4 years), when the most recent one dies I'm seriously looking at a tankless as a replacement. The new tax break has really made the tankless attractive. My plumber, with whom I'm on a first name basis with, reluctantly agrees with my assessment. I say reluctlantly because he has come to rely on me to cover a substantial amount of the annual cost of gas for his yacht, ATV, and airplane.

[mschrank]

My .02

I have a Takagi T-K1S tankless water heater (natural gas)...been using it for about 6 months now.

Stewchi's real question is in regard to energy efficiency....I can't speak to the energy savings yet. I need to wait until July when I won't be using the gas furnace to get a real comparison.

That said, I can't restrain myself from going off topic and extolling the virtues of tankless...

Overall, I love it and will never go back to a tank type.

The biggest advantage to me is the increased space in my utility room. While the gain of 16 square feet sounds minimal, it has allowed be to rearrange things and make a "scrapbooking center" for the LOML.

Second big advantage is the endless hot water. As my kids grow, I expect this advantage to be even more important.

Third, while the initial cost was high, the warranty and expected life span are nearly twice that of a tank heater.

Fourth, a substantial (about $350, IIRC) tax incentive from the state for installing a "high efficiency appliance"

Plus, it's much quieter than the tank heater during the heating cycle.

You can save a lot by doing most of the install yourself (hanging the unit, installing the venting and rough plumbing). Just have a plumber come in to make the final connections. That way, your warranty is still valid.

I agree with Bill that I probably do waste a bit more water down the drain as it takes a bit longer for the hot water to get to the tap.

Also, as "thestinker" (what's your real name? ) points out, the electric models don't have such a great track record.

[bigsteel15]

I would have to agree with all of you on this...LOL
You had to expect many different answers here.
Having been in both the domestic and industrial side of the heating business and selling every variation of water boiler imaginable I have the following:
If you have a boiler for some form of hot water heating I would suggest a heat exchanger on a loop off that for on-demand hot water, as well as a pumped loop for the entire system to maintain hot water in the pipes. Newer houses usually have this whereas older homes won't. Or use a small (40 Gallon) storage tank with a coil in it.
If you have in-floor heat, you will need to have a seperate exchanger for that loop because you need to run the boiler too hot now to heat the dmoestic water.
Also have this system on a priority where any demand for hot water takes priority over building heat. If the boiler is big enough it will do both at the coldest times of year.
Personally I would stay away from "instant heaters" for maintenance reasons as well as inflated efficiencies. Where will the replacement parts come from? Most of these units are made in Europe and parts can be expensive.

My dream system would have an Quantum condensing boiler from Dunkirk (95% AFUE) for the radiant in-floor, forced air (having both is awesome) and an indirect storage tank. 1 PVC vent and a drain for the whole system.
Kind of pricey though. Last time I looked, the Quantums were around $4500.00. Their website says that you can get tax incentives with these units.

[LCHIEN]

heat loss from a Tank WH

Just to put some numbers to the continuous heat loss from a tank (which you do not have with a tankless):

I assumed a 50 gallon tank which might be 4' tall x 1.44' dia. with a surface area of 21.4 sq ft.
I also assumed an ambient air temp of 72F and a water tank temp setting of 130F.
I assumed insulation of the tank at R-11 which is approx the fiberglass batting found in 2x4 stud walls to insulate a house, nearly 4" thick.
And I assumed $0.13 per KW hour.

Based upon the heat loss calculation and thermal resistance of R11
I calculated a heat flow of 112 BTU which works out to a continuous loss of about 32 watts.

So if you have a heater as above, it will cost you about $36 a year just to keep the water hot replacing heat lost through the insulation.

[bigsteel15]

Loring,
You make a good point. The other option is of course no tank and just a small heat exchanger for the hot water. Also remember that any older unit with a standing pilot typically burns about 500-1000 BTU/HR