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Should we consider going tankless WH?

Posted by mdeleon (My Page) on
Sat, Dec 30, 06 at 12:54

My wife and I are moving into a new construction townhome in June. It's a 3 bed / 3bath. Should we consider swapping out the included tank with a tankless water heater? We take two showers a day, do about 4 loads of laundry a week. Any ideas / thoughts? I just want to save save save esp over the long run (and I guess not running out of hot water is a plus too!)

Mike


Follow-Up Postings:

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RE: Should we consider going tankless WH?

Hi,

I guess you are talking about gas heaters?

The energy factor for regular gas heaters is around 0.65, and the tankless are 0.85 or so. This would be an about 27% reduction in water heating cost for the tankless.

If you use the typical 15 gals per person per day, and your gas costs $1.50 per therm. Then the tankless would reduce your hot water bill per year from about $147 down to $112 -- a saving of $35 per year. This would go up as fuel prices increase.
The tankless heaters also have a much longer life, so you won't be faced with a replacement tank anytime soon.

You might also be able to get in on the federal tax credit -- it does apply to installing tankless heater -- not sure about new construction thou.

If you paid (say) and extra $350 for the tankless, then you would be getting a tax free, risk free, fuel price inflation protected 10% return on the $350. Try and get that from your investment broker :)
It would also free up a bit of space, since the tankless units are small.

If the tank model is an electric, then a gas tankless would also have the advantage of saving a LOT of greenhouse gas emissions each year.

Gary


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RE: Should we consider going tankless WH?

Well, in our new construction, I guess the price of the water heater is already built in to our home price. So we would have to actually buy the unit new at full price. I'm seeing bosch units for about $1000 but don't know what else is out there that is 'good'.

Mike


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RE: Should we consider going tankless WH?

Hi,
$1000 seems on the high side for the Bosch units.

Two other brands that have good reputations are Takagi and Ranai.

Gary


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RE: Should we consider going tankless WH?

A contractor installed a gas tankless water heater in my home about a year ago and I had nothing but trouble. I finally sent it back and opted for a conventional water heater instead. There were a variety of problems that I don't want to elaborate on here. The bottom line is that it was going to cost a couple of thousand dollars more than we expected to properly install the device, get sufficient gas pressure to it, and properly duct it. Then, for safety reasons, we still may not have been able to get the water hot enough at the remote faucets.

You have been warned.

John


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RE: Should we consider going tankless WH?

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The simple fact is that the VAST majority of consumption of ANY water heater is simply the incredible amount of energy required to actually HEAT the water. Tank losses, unless you've got an old bear; are a relatively small amount. With a well insulated electric tank or storage tank for indirect heating; the losses are way smaller.

"Instant" water heaters by definition; draw HUGE amounts of power ( be it gas or electric ) WHILE they are running. I think the gas ones require a 1" gas line . . the electrics a HUGE dedicated line. That can get expensive to properly install. Also, know that you MUST size the heater according to the MAX amount of water you'll be drawing at any time => lest you end up with an endless supply of warm water => such as filling a tub which HAS no flow restrictor . . while taking a shower / laundry / dishes etc.

At least some of these also have the odd quirk of a minimum flow rate before they will kick in => slow flow rates will NOT turn them on.

As far as longevity; it can vary. The passages through them where heating actually takes place, or the flow sensors in some kinds; can plug with minerals etc over time, depending upon you water and what it's got in it.

When you look at the initial cost, including the necessary power / gas connections; I can't see it paying off at all . . at least not for a very long time. For some unique situations I guess they can make sense . . . but overall you save little energy and will be a long time to recover any $avings in any reasonable amount of time . . .

Bob


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RE: Should we consider going tankless WH?

Hi Bob,

Bob: "The simple fact is that the VAST majority of consumption of ANY water heater is simply the incredible amount of energy required to actually HEAT the water. Tank losses, unless you've got an old bear; are a relatively small amount. With a well insulated electric tank or storage tank for indirect heating; the losses are way smaller."

I think this argument makes sense for electric water heaters where modern tank type models have energy factors of about 0.95.

But, for gas heaters. The energy factor for modern tank gas water heaters is about 0.65, and the energy factor for tankless is about 0.85.

Almost all of that difference is the standby losses of the tank type gas heater. Thats a 27% reduction in energy use -- seems worthwhile to me?

If you have a high mineral content in your water, it can leave deposits in a tankless heater. These are easily removed with a minearl disolver, and the tankless heater manuals tell you how to do this.

I have to say that the while its obvious from the two notes above that tankless heaters are not for all, there are many people who report having very good experiences with them.

--
Heating water with electricity from coal fired power plants (which is where the bulk of our electricity comes from) generates about 4 times as much greenhouse gas as heating water in a tankless gas heater.
7800 lbs per year of CO2 for an electric heater vs 1800 lbs of CO2 per year for a tankless gas heater.

(assuming: family of 4, 15 gal/person-day, 95% efic electric heater, 86% efficient gas heater, electricity from wester coal -- http://www.infinitepower.org/calc_carbon.htm )

Gary

Gary


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RE: Should we consider going tankless WH?

Wow, it actually sounds like a lot of trouble right at the beginning... Maybe I'll just sit on it for awhile longer while we get the rest of the house in order. Since it will be a brand new WH installed, it 'should' be okay then for now!!

Thanks for all the input!
Mike


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RE: Should we consider going tankless WH?

Hello SG,
How do you calculate the term Therm using gas?


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RE: Should we consider going tankless WH?

Hi Bob,

I did the estimate this way:

Heat for water heating for a family of 4 at 15 gal per day in BTU per year:

(15 gal/person-day)(4 people)(8.3 lb/gal)(120F-50F)(365 day/yr) = 12.7 Million BTU per year

Gas needed to supply this 12.7 MBTU with an 86% eficient tankless water heater:

(12.7 MBTU)/((100000 BTU/therm)(0.86 efic) = 148 therms/year

Electricity needed to supply this 12.7 MBTU with 95% eficient electric tank type water heater:

(12.7 MBTU)/((3412 BTU/KWH)(0.95 efic) = 3920 KWH/year

From the http://www.infinitepower.org/calculators.htm GHG calculator:

148 therms results in 1775 lb of CO2
3920 KWH from western coal results in 7839 lb of CO2

Just as a point of reference, a Prius driven 15K miles per year puts out about 5200 lbs of CO2.

Does the calc look OK to you??

Gary


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RE: Should we consider going tankless WH?

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Gary;

Take a well insulated electric tank. Heat the water up to x degrees, and shut it off. How long before it turns back on again ? . . . strictly a function of how well it holds it's heat => ie, insulation. With perfect insulation; it would be never. I haven't found anything like that yet; but if it existed the storage losses would be zero.

Take that same tank, and say after an hour it turns on for a few minutes to bring the water back up to temp => it cooled down from losses here and there. To heat it back up to temp; required FAR less energy to bring it up a few degrees than to initially heat that same water up from incoming temp.

I guess my point is; that when using electric or indirect; the storage losses are entirely a function of the overall insulation of the tank => insulation itself, other heat paths through the pipes, etc. So, with a VERY good tank; you only have to heat the water a few degrees once in a while. A "perfect" tank; and it would never turn on unless water was used.

I still believe that in a well-insulated non-gas ( with flue losses ) tank, that storage losses are a very small part of the total energy consumption . . . and this is one of my reasons for not being a fan of the ( very expensive to buy and install ) tankless heaters . . . .

Bob


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RE: Should we consider going tankless WH?

Hi Bob,

I think we are pretty much in agreement on this.

I agree that electric tanks today are well enough insulated that the standby losses are very small -- thats how they get energy factors of 0.95.
I don't see that there is any advantage to a tankless electric heater over a tank type electric heater. At the very very most they might save 5%, and its probably less than that.

My big problem with any kind of electric water heater is the several tons of green house gas emissions each year from the coal power plants that supply the electricity. I just could not do that.

With gas type water heaters where the flue goes up the middle of the tank, the standby losses are much larger. The energy factors of new gas water heating tanks are still only about 0.65 -- this has to be largely from standby losses.
This is where a gas tankless heater can save money and greenhouse gas emissions by eliminating standby losses.

Its odd to me that the manufacturers have not come up with a tank type gas water heater that does not have the high heat loss flue up the middle. At least I have not seen any.
It seems odd to me that its easy to buy a gas furnace that operates at 95% efficiency, but gas water heaters are only 65% efficient??

Gary


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RE: Should we consider going tankless WH?

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Gary;

Yup, that damn flue. You want it initmately connected WHEN running; but infinitely disconnected when NOT running . . . I've heard of flue dampers and such; but not sure how well they work or how widspread they are. Your energy factor for gas heaters sounds a bit low; seems to me the one I've got is rated .80. It was a Bradford-White / propane / power vented one.

Step aside from conventional heaters for a moment to condensing "boilers" . . . not really boilers as such, but actually GREAT water heaters . . . and run MORE efficiently down in the region where domestic hot water should be. Mine is into the 90's efficiency-wise, according to the flue readings when it was set up and installed. These things have apparently been in Europe for years ( we're one step behind, AGAIN ! ) and are now making inroads into the US. Quite a simple principle: run a small but HOT flame via LOTS of air flow; use a fingered and finned heat exchanger in a swirl pattern; that hold only a gallon or two of water in it. The rest of the water for the system / heater is in the heating system / piping etc. They are also modulating types; that is they essentially have a throttle => some basic electronics reads incoming and outgoing temps; and throttles up or down to meet the "demand". I'm quite impressed with it efficiency / operation-wise. The other amazing thing is the size and noise. Mine is about a 2' cube; and can heat a house twice the size of mine ( 1800 sq ft ) here in central New York state. It's a Munchkin 80M . . there are other brands out there too; this just happened to be the one I picked. They aren't cheap; but with energy where it is it's gonna pay back even faster every time stuff goes up.

Another nice feature is the ability to use this to do domestic hot water also, indirectly. When my present domestic water heater dies; that's what I'm gonna do. That's partly why I bought more capacity than I needed to just run the radiant heat.

Do a google on the Munchkin and check it out . . .

I've just passed 6,600 kWh's generated by my PV system . . so I'm doing my part for CO2 reduction . . . : < > any electric heaters of any kind are banned here . . . gotta break the wife's hair dryer still . . .

Bob


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RE: Should we consider going tankless WH?

Another factor to consider regarding a hot water tank is that during heating season (winter) any heat loss from the tank contributes to heating your home. Electric resistance heating, which is what a hot water tank is, is 100% efficient. The DHW tank is mainly a net negative, from an energy consumption point of view, during those times when were air-conditioning our homes.

I personally own and prefer the stainless steel variety of hot water tank, which in theory may never have to be replaced.

SR

Here is a link that might be useful: OSO Stainless Steel Hot Water Tanks


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RE: Should we consider going tankless WH?

I have looked at this from every angle and was considering
the Bosch Aquastar 125 however I cannot see that there would be much of a difference in energy. I have a Weil-Mclane gas boiler and a 30 gallon Super-Stor which is all stainless and heavily insulated. I am presently using 10 therms/month for hot water which I do not think the wall mounted unit could be much less.


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RE: Should we consider going tankless WH?

DH and I have an approx. 5 yr old 46 gal. electric water heater in the (uninsulated crawl space). The water heater is insulated inside, and with an extra "jacket" we added. We have also insulated the hot water pipes and about the first 10 inches of the cold pipe adjacent to the tank.

I was concerned about the heater being inefficient in the crawl space. Although we are in the South East, it can get down into the 40s below the house. However, in the summer it was up into the 80s (maybe 90s) with similar % humidity under the house.

I was considering a tankless, GAS water heater inside the house (in a space behind the bathroom). Reading what I have so far, it seems that this option might be less efficient than what we are using now? We would have the cost of the new heater plus the cost of installation to consider. Currently, we pay ~$0.09/kWh for electricity and ~$0.90/therm for gas (variable rate, excl. mo. charge).

We are about to update the HVAC system, which is currently under the house, and 28 years old. The A/C works, but is very inefficient and the gas heat is dead. I am now considering nixing the tankless water heater idea, but possibly considering a heat-pump or dehumidifier recovery system to boost our existing hot water tank.

What do you guys think?

We would also like to use the crawl space as more of a basement or storage area. It is 900 sq. ft. with a max. height of 7 ft. and a min height of 4 ft. We hope to level the floor as much as possible (it's stepped) and cover with concrete. It is surrounded by brick on three sides and the cinderblock garage wall on the other. It is hot and humid in the summer, but cold and dry down there in winter.


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RE: Should we consider going tankless WH?

SolarGary, Thanks so much for your reply.

I spent some time browsing the buliditsolar page, and feel I have more questions than answers!

I am very interested in making our home as energy efficient as possible, while reducing greenhouse gasses to help alleviate the "myth" of global warming! However, DH and I don't have a huge budget to work with, although we have several k set aside for a new HVAC system.

As I said, I was not happy with the position of the HVAC and hot water tank in an enclosed, but uninsulated crawl space, because it struck me immediately as being both inefficient and not ideal for the equipment to be exposed to extremes of temperature and humidity. I grew up in the UK, where hot water "tanks" and boilers are almost always in the house. In fact my mother's 10 yr old house has the gas boiler (like a tankless H20 heater with a pilot) in the garage, under the guest room, while the storage tank is in the "airing cupboard" off the upstairs hallway.

Anyway, we were already thinking of updating to a more efficient hot water heater and better HVAC system. I have been reading on the heating and cooling forum about heat exchanger technology to transfer waste heat from the AC refrigerant lines into a hot water tank, which also eases the load on the compressor, thus simulating an increase in SEER performance. Our current AC is a 28 yr old GE model, with unknowm SEER, but probably very low. We are def. looking for a more efficient system, since we are in GA.

How about if we did these things:

1) Level and concrete the crawl space
2) Insulate the walls of the crwal space
3) Install a new AC/furnace or heat pump handler in the garage closet adjacent to the crawl space (there's a hole in the wall for ducts already).
4) Drain and reposition the electric water heater and run a heat exchanger device between the AC refrigerant line and the water tank for FREE hot water while the AC is running.
5) Install a gas tankless hot water heater upstairs, adjacent to bathroom (above garage/crawl space cinder block wall) IF we go with a gas furnace also.
6) Install a grey water heat recovery device on the main drain (below bathrooms) in the crawlspace.

IF we did this, could we use the electric water heater as a "storage tank" for the water that is heat recovered by the grey system, before it is used by a tankless heater in winter? I'm sorry this is so complicated!
Basically, with my climate and energy rates, I am wondering if it might be worth keeping the electric tank to get FREE hot water from our summer AC use (which is pretty much every day from May-Sept in GA), but using a tankless water heater in the winter, since gas is a cheaper and greener fuel to use? I am hoping that adding a grey water heat recovery system could help us offset the loss of all that heat down the drain.

Of course, these all just ideas at the moment, and I have ZERO knowledge about how we might actually implement them. I would love others who are more knowledeable on the pratical aspects to give me advice. This is our first home and we are learning a lot as we go along. Although these ideas might be a little "odd" or "out there", I feel that energy efficiency is an investment in our home and future, as it seems that fuel costs will only go up, and maybe the average American is finally coming to terms with this!

BTW, I find it odd that most US hot water heaters are on constantly. Is this normal? Growing up, ours were always on a timer switch that you could set for any period during the day or night. I did see a timer for about $40 at Lowes, but it seemed a lot more complicated than what I am used to.

Thanks Again!


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RE: Should we consider going tankless WH?

bargainacious:

There are a lot of variables in what you are asking. My general advise would be to make a plan based on cost and projected savings. Do the cheap stuff first! It doesn't make much sense to spend a lot of money on a tankless heater that will save $60 a year when you could have spent less money to, say, insulate the attic and gotten more payback in terms of energy savings. A $20 insulation kit for the water heater has a great payback-- especially if you need to spend the big bucks somewhere else.

As part of the plan, I would look at furnace, water heater, A/C etc. and estimate a replacement date. If something needs to be replaced, I would look at a more efficient version. On the other hand, I would not replace a perfectly good gas water heater with a tankless if I needed a new furnace in a few years, for example.

Also, reducing energy demand is probably the most effective use of limited capital. For example, insulation will often have a greater payback than exotic energy saving appliances (with the possible exception of compact florescent bulbs)

If you need help making a plan, I would call someone to help you with an energy audit. This could range from something simple and cheap (you will get a lot of the standard suggestions) to something much more sophisticated where they would measure the insulation, check your appliances, take into account energy prices, climate etc.


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RE: Should we consider going tankless WH?

Hi bargainacious,

As you say, that is kind of complex.
Here are a few thoughts:

1) Level and concrete the crawl space
If you want the benefits of a conditioned crawl space, then you don't have to concrete the floor. You can use a heavy poly film over the dirt floor, and seal all the seams and edges with a caulk material. This prevents water vapor from the ground from getting up into the crawl space. You can then insulate the walls and rim joist, and close up the vents.
The concrete would just make it nicer if you want to actually use the space.


4) Drain and reposition the electric water heater and run a heat exchanger device between the AC refrigerant line and the water tank for FREE hot water while the AC is running.

I'm not very familiar with these, but it sounds like a good idea if the price is not too high.

5) Install a gas tankless hot water heater upstairs, adjacent to bathroom (above garage/crawl space cinder block wall) IF we go with a gas furnace also.

I guess that my preferences for using less water heating energy and generating less greenhouse gas would be: 1) a solar water heater, 2) the grey water heat recovery, 3) a tankless gas heater, 4) a regular gas water heater, 5) any kind of electric heater.
The electric heater being last because of the high GHG emissions from coal fired electric plants that supply most of our electricity.
I don't know enough about the recovery of heat from an AC or heat pump to have an opinion on it -- it might be fine.
I think you have to be a bit careful applying too many of these at the same time -- they all add cost, and the amount to be saved would diminish with each new device?

6) Install a grey water heat recovery device on the main drain (below bathrooms) in the crawlspace.

The most common of the grey water recovery devices (the GFX) needs a point where the drain pipe can take a vertical drop of about 4 ft. It sounds like you might have that. These (I believe) are fairly effective in recovering heat for situations where you are simultaneously using and draining hot water (like a shower), but don't recover heat in the case where you run hot water, and then later drain the water (like a tub bath).

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On the water heaters being on all the time.
There are some people who swear by water heater timers.
It seems to me that for a modern electric water heater with an energy factor of 0.95 there would be little to save (less than 5%).
It would seem to me that they might make more sense with gas water heaters, where the standby losses are much greater.
But, I'm not sure if the timers are made for gas heaters?

Gary


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