Tankless water-heater theory

[JimD]

To save money, the effiency of heating for the tankless must be close to the tank type. The loss from the tank is not trivial but it does not equal a 10% difference in effiency (gut feel, I have not run the numbers).

The thing that amazed me about the piece on TOH was the heat input to the tankless they were using. It was nearly 200,000 BTUs/hour. I do not have nearly enough gas to run one of these. My furnace for the first floor (my biggest gas appliance) is less than 100,000 BTUs. I don't think my 50 gallon hot water heater plus my furnace is 100,000 BTUs.

I insisted on a gas hot water heater when we built our house because of the speed of recovery. We have 4 1/2 baths and have not run out of water - but they have also never all been used at the same time.

I also do not get the advantage of a tankless but one of my co-workers is getting one in his new house. They do take up less space. For two people (all he typically has home which is the same for me) with the house empty for along periods during the day maybe they make sense, if the effiency is high enough. On the other hand, it would be very hard to cost justify the whole house vacumn I spent over $1000 on. Not everything has to be hard dollar justified.

Jim

[gerti]

I did want a Takagi when our water heater died. But they were not available locally, nor did anyone seem to have experience with them. Venting would have had to be redone, and nobody could really tell me if with the cold water in winter here it would be able to heat it to the needed temperatures. So grudgingly I went with a conventional heater. I'll keep an eye on it though for next time...

[stewchi]

A lot of good info, thanks everyone and thanks again to Loring for coming through with the numbers. Which seemed about right, I loose more energy leaving my basement lights on than through my water heater insulation. Between the ATOH episode and my Jan Gas bill of $250 and Electric of $118 I have been thinking a lot about energy savings. Jan was are first month in the house and I think these bills have some connection fees in them but I still need to get the prices down. My water heater setup is unusual, I have 2, 50Gal tanks in series with each other. The first is electric and the second is gas. I am not sure why the builder set it up this way or what the advantages might be other than we have never run out of hot water in the 2 months we lived here. The house is only 3 years old so it should be pretty well insulated except last weekend I found 4 bays (cavities between unfinished ceiling joists) that went all the way to the outside under the deck (2’ off the ground). It was open for any rascal and lots of cold air to come through, I wondered why my basement was so cold and drafty. I sealed it up and insulated. That should help. I don’t have any good solutions for loose of heat and water from in the pipes, none of the pipes are insulated, I don’t think I can get insulation in the walls easily to insulate the pipes. It might help a little for the kitchen and first floor bathroom but the upstairs laundry and bathrooms will loose all the heat in the wall.
That wasn’t a problem when I lived in Tucson, my first house had the main water line coming up out of the carport into the attic then dropping down to the kitchen and baths. It got a lot of sun from out side and a lot of heat from the attic. In the summer you had to let the cold water run for a little while until it was cool enough to touch. I don’t think I ever used the hot water heater in the summer.

[scorrpio]

Here is a good read:
http://www.aceee.org/consumerguide/topwater.htm

Approximate system costs, including purchase and 13 years of use, according to their table:
Conventional electric: $5,680
Conventional gas: $2,707
Tankless electric: $5,982
Tankless gas: $2,730
Indirect heater hooked up to efficient boiler: $1,900.
Hmmm....

[jziegler]

Igor,

Do you know if the coil unit installed in boilers is considered an indirect, or what the difference is?

I have a few year old oil boiler with the coil and it works great. The burner is never on for more than about 15% of the time during a shower, even a long one. I really can beleive that it is the best system out there. Of course, mine is a bit odd with an electric water heater (turned off) after the coil as a holding tank, never quite understood that setup. But looking at the numbers, I think I should be glad for the setup I have.

-Jim

[scorrpio]

Editing - did not read post carefully.

My setup is identical to yours.
The problem with it is that once water itself is in the heater tank, it cannot be re-heated - you have to use some of it for boiler to pump some more in. So, if we don't use hot water for a day, it gets kinda lukewarm. This is a very old heater - about 20 years old - so maybe it has crap insulation.

The reason for this setup is simple: the boiler's heat output compared to a regular water heater is massive. It is like a sledgehammer compared to a woodcarver's mallet. It needs a sizable buffer for all that heat - hence the tank.

An indirect is a water heater that has a coil inside itself, and that coil is connected to the boiler's main output as a priority heating zone. When heater's thermostat calls for heat, boiler pumps 190 degree water through its coil, heating the water inside. The amount of heat the boiler can deliver is massive. Some consider the main plus for tankless to be its 'unlimited hot water' compared to tanks. Indeed, an electric 40 gal heater has about 40gal 1st hour rating. 40gal Gas heater can deliver about 50gal in 1st hour. To compare, a Weil-McLain Ultra Plus 40gal indirect heater, hooked up to a 105MBH Ultra boiler delivers 202gal of 50in-115out (or 154 gal of 50in-140out) hot water in 1st hour. The tank heat loss in these heaters is less that one degree F per hour, which is near negligible.

The low cost of indirect is due to one very important factor: pretty much all other water heating solutions are completely autonomous, while indirect requires hookup to an existing boiler. Indirect is also a lot simpler to install: no need for a fuel line, no electric hookup, no vent, no air source. Indirect is way easier to maintain and far less prone to breakage: no anode rods to replace, no elements to break, no valves, no pilot light. It is just an insulated tank with a big coil and a pair of thermostats.

[leehljp]

Japanese tankless

I have tanks here in the States and have lived in "Tank" houses in Japan. In one house, in 1990, our tank (gas) died. It was replaced with the tankless (gas) type. Our gas bill got cut in half. We had 3 kids at home. The delay in hot water was no more than the with the tank type.

One feature of the tankless type is the ability to adjust the temp easily from an indoor and close by control panel.

In two other homes in Japan, both had 3 tankless heaters on the wall outside the bathrooms and kitchen. Hot water was always within 10 seconds, as much as one wanted, as hot as anyone wanted.

When I retire, I would love to add electric tankless heaters to our house here in the States, but it is not really cost effecient unless a new home is designed for it - gas, electricity and or plumbing. This should be done in the beginning.

A single tankless source works OK if bathrooms, kitchen and laundry room are close. I still like the individual tankless units better.

My two yen.

[LCHIEN]

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.

Hey, I was rereading the thread, saw your note #9 I missed the first time.
I posted #15 on basically the same line, how much it takes to fight heat loss in a tanked heater.

My answer was

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.

I see we made the same assumptions (50 gal heater), I assumed Thermal resistance of typical batting and you go by the temperature loss from the mfr yet we arrived at remarkably the same number by entirely different ways with dollar costs on a par (me, $36/yr and you, $3.63 per month).
Wow, isn't engineering fun?

One error you make - 200,000BTU ~=58 KWH, not .7 KWH; but your KWH and my KWH are different anyway; you take efficiency adjusted before you calculate and my efficiency adjustment is in the $ rate of power. Going into the tank I assumed 100$ efficient KW->heat conversion.

[scorrpio]

Yes, I made an error converting BTU to KWh, but I used oil (140,000 BTU per gallon) to determine the cost.

Overall, I do agree that tankless is the best solution for independent systems, in particular in a new construction where you can set up proper locations for the units, alnog with either gas or sufficient gauge electric. Tankless units go down in price, their parts get more available, maintenance gets cheaper.

However, if the installation includes a high efficiency boiler, you'll be hard-pressed to beat an indirect heater.

[Stick]

Anyone have one on a farm water system? With REALLY hard water? Just curious about buildup of calcium in such a small tank. I know I clean probably 5 gallons of crap out of my 40 gallon tank every year. I like the idea of tankless, and could go either electric or propane. There's really only LOML left at home now, since I'm at work for 8 days at a time and 2 days driving every 18 days. There might be some substantial savings.

[mschrank]

Anyone have one on a farm water system? With REALLY hard water? Just curious about buildup of calcium in such a small tank.

Assuming your referring to a tankless here...

There is no tank to speak of...basically just pipes running back & forth. I don't think buildup would be much of a problem with these, since everytime you turn on the hot water, you are essentially flushing the system.

Nonetheless, on mine I installed some plumbing recommended by the manufacturer just for flushing the system (In case you want to get some kind of "solvent" in there...I think they recommend vinegar?).

Looks like this