One Water Heater Or Two

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toolguy504

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The house is new construction approximately 2250 sq. ft. There is no gas so the water heaters have to be electric.It has two bathrooms. Three adults will be living in it. One bath has a walk in shower and whirlpool tub (55 gal) with heater and one with regular shower and tub combo. Both baths are on the same side of the house. The kitchen and laundry are on the other side of the house approximately 42 ft. away. The original plan was to put one 40 gal water heater above the two bathrooms and one 40 gal heater above the kitchen near the dishwasher. Since the washing machine uses very little hot water now, I was wondering if the two heater system is overkill. I was thinking maybe going with just a 50 gal just over the bathrooms. Any opinions on this. Thanks. Earl
 

hj

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heaer

Sewer... I didn't think they made an ELECTRIC model of that heater. One 50 and one 40 would give the fastest hot water with the least waste. They are also MUCH, MUCH cheaper than an 80 gallon. I question the " heaters above the bathrooms" statement. Heaters in the attic are the most expensive to install AND replace, and there is no way you would want the expense of putting an 80 gallon heater up there.
 

toolguy504

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Master Plumber Mark. I was just discussing the idea of putting an 80 gallon tank in instead of two smaller tanks. I think I am going with that plan.
SewerRatz. Thanks for the info.
hj. Thanks for the advise but I can get an 80 gallon tank with a Life Time Warranty for $599. That is $175 cheaper then if I buy a 40 gallon and a 50 gallon with a 12 year warranty. As for as getting it in the attic it really is no problem because I have a regular set of stairs going to the attic with a landing in the middle. It really has easy access to where the water heater will go. The roof is a 10/12 pitch and there is plenty of head room.
I do thank all of you for the replies. Earl
 
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The 80 gallon route makes a lot of sense with an electric and the 55 gal tub mentioned. It will take a long time for an electric water heater to catch up, so having first hour capacity is the key. Two water heaters would seem an unnecessary complication when what is really needed is storage capacity. The redeeming feature of electric water heaters is that they are more fully insulated than gas, so standby losses are minimal. Wrapping a blanket around the heater and insulating lines should minimize this as well.

The OP is right about the clothes and dishwasher not needing that much water (assuming front loading washer and Energy star dishwasher.) Compared to older top loading clothes and dishwashers these are minimal users. Baths/showers will be the primary users in terms of volume.

p.s. make sure you have plenty of support in the attic for the extra 1,000 pounds of water and gear.
 

Gary Swart

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I suggest you check the structural strength of you attic ceiling. 80 gallons of water weight about 650 lbs and that doesn't count the heater.
 

Dana

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I suggest you check the structural strength of you attic ceiling. 80 gallons of water weight about 650 lbs and that doesn't count the heater.

NO KIDDING! You're looking at nearly an 800lb dead-weight- not sure if that would pass structural loading code in the average attic. A 40-gallon tank will be a bit more than half that, a 50 gallon (which is the minimum you'll need for the 55 gallon tub or you'll be sorry) will run about 500lbs. Electric 50 gallon heaters typically deliver ~60 first-hour gallons- it'll be enough as long as the loads on that tank are minimized while filling the tub, and don't expect someone to take a shower 20 minutes after filling the tub.

The 2- heater option will be far more efficient, but you don't need more than ~5-8 gallons for the laundry/kitchen. Doing an under-sink mini-tank in the kitchen will have lower standby losses, and significantly lower distribution losses than a 40 gallon tank in the laundry, (and orders of magnitude lower distribution loss from a tank 42 feet away.) Recovery times on mini-tanks is very short- even if you run it out it'll recover in 10-20 minutes (shorter for a tiny one, longer for a bigger one, since the wattage is usually about the same.) But unless you're running a continuous warm rinse in the sink for 30 minutes while running a load of laundry AND a load of dishes a 5-6 gallon unit would be adequate. You may even do just fine with a 3 gallon unit 99% of the time.

Standby loss is roughly proportional to surface area- a mini-tank has less than a third of the surface area of a 40 gallon tank, and most of the energy in low-volume intermittent use locations like that will be standby + distribution loss. In this case bigger isn't better. Placing the tank closest to the point of use with the highest FREQUENCY use (probably the sink) will minimize the amount of heat that gets abandoned in the plumbing, and minimize the wait for hot water where it matters the most- the washer & dishwasher doesn't care if the first slug o' hot is on the cool side, but your hands do. Under the sink is usually best from both an efficiency and human-factor POV.
 
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Surface area and losses won't scale 1:1. That's because for fixed proportions surface area increases more slowly than volume. With some fairly standard width water heaters it scales a little better, except the head area is unchanged in that case, and head losses are substantial. There isn't a lot to be gained in reducing the size of a tank until you get to mini sizes.

Another problem with scaling the tank losses is that the connected piping/relief is a big source of heat loss as well, and is relatively fixed quantity (rather than a fixed multiplier) independent of the tank size.

Electric water heating already has rather complete insulation, so standby loss from the tank itself is a smaller factor than with conventional gas storage which necessarily has some uninsulated parts and air flow past them.

A well insulated mini-tank at the kitchen end might make sense to reduce the piping/tubing run from say ~50 total feet to about 12, and therefore reduce the dead volume. However, the tub still sets the minimum size for the main water heater. I've not yet determined the fill capacity of my tub, but it takes a little more than my 50 gallon nat. gas heater can supply at 120 F thermostat with a 40,000 Btu/hr burner. Fortunately, with gas it catches up rapidly enough that I can stop the fill when the tap temp begins to fall, then add the remainder a few minutes later during the bath.
 

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Surface area and losses won't scale 1:1. That's because for fixed proportions surface area increases more slowly than volume. With some fairly standard width water heaters it scales a little better, except the head area is unchanged in that case, and head losses are substantial. There isn't a lot to be gained in reducing the size of a tank until you get to mini sizes.

Another problem with scaling the tank losses is that the connected piping/relief is a big source of heat loss as well, and is relatively fixed quantity (rather than a fixed multiplier) independent of the tank size.

Electric water heating already has rather complete insulation, so standby loss from the tank itself is a smaller factor than with conventional gas storage which necessarily has some uninsulated parts and air flow past them.

A well insulated mini-tank at the kitchen end might make sense to reduce the piping/tubing run from say ~50 total feet to about 12, and therefore reduce the dead volume. However, the tub still sets the minimum size for the main water heater. I've not yet determined the fill capacity of my tub, but it takes a little more than my 50 gallon nat. gas heater can supply at 120 F thermostat with a 40,000 Btu/hr burner. Fortunately, with gas it catches up rapidly enough that I can stop the fill when the tap temp begins to fall, then add the remainder a few minutes later during the bath.


Surface area and losses DO scale roughly 1-1 (for the same R-value) just not 1-1 with VOLUME (as you rightly point out.) A 5 gallon mini is 1/8 the volume of a 40 gallon tank but ~1/3 the surface area (as I pointed out.) Standby losses are more like half that of a good 40 gallon tank, not 1/3 due to the near-piping issues, but the distribution losses of the tank-to-faucet plubming are an even bigger factor. Insulating the near-tank plumbing is more important for standby than some might believe (as you point out), but reducing the raw distance between the high-frequency taps outweighs it. Something like the foam-insulated Bosch Ariston mini tank series fit pretty nicely under most kitchen sinks, and are nearly spherical, with a minimum surface area/volume ratio, making it not quite as lossy as the mini-cylinders. And the piping length...

To get fully 12 feet of plumbing between an under-sink tank and the faucet would take some real creativity- even making it longer than 3 feet is tough, eh? ;-) A 12 footer would be a more typical run to a 40 gallon tank in the adjacent room. Something between 18-30" is typical for under-sink installations.

Boosting the thermostat to 140F you can comfortably fill a 55 gallon tub with a 50 gallon electric tank, but probably not with it set at 120F. There's ~8.3 KBTU more stored energy in a 50 gallon tank at 140F than 120F. I agree it's marginal, and with 40 gallon electric tanks, no way- 50 is the absolute minimum. With the slow recovery of an electric they won't be able to get an extra 5kbtu in just a few minutes the way you can with your gas fired tank- it'll likely need higher temp storage from the get-go, but it should still do it (if barely- don't expect anyone to be taking a shower right after filling the tub.)
 
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I'm not assuming it will be under the sink, but above as the OP mentioned. And if it is under the sink, it is still likely that the run to the clothes washer will exceed 12 feet. Honestly, I don't think a mini electric could keep up with our needs during dishwashing, especially since it is not uncommon for us to be running laundry and/or the dishwasher while doing a sinkful of some items (typically about twice/week.) It's not a large volume over a week, but the usage is high enough in short bursts. I doubt the volume I would need would fit comfortably under the sink, and even if it did I consider storage space in the kitchen to be a premium so that factors in.

Running a 50 gallon tank at 140 F will cost more energy than running a larger gallon tank at 120 F because your storage losses in the 50 will increase by roughly 40% due to temperature alone (e.g. 140 - 75 F ambient driving force, vs. 120 -75 F ambient average driving force for heat transfer.) Add to that a shortened life for the tank and probably a shorter life on elements, plus the scald issue.

I'm well aware that electric won't keep up with gas in the same tank size, which is why I've pointed that out several times in this thread.
 
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Surface area and losses DO scale roughly 1-1 (for the same R-value) just not 1-1 with VOLUME (as you rightly point out.)barely-

Which begs the question, why did you feel the need to repeat it as if I was wrong. I explained the why of it in the next sentence.
 

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12 feet to the washer isn't nearly the hit that 12 feet to the sink would be, since it's used many fewer times in a day. Volumes of high-efficiency washer draws are amazingly small these days, as are those for modern dishwashers. With the fast recovery times of mini-tanks it they'd rarely be run dry by simultaneous laundry/dishwasher loads- the time between wash & rinse cycles is usually more than sufficient unless you're using flat-out hot for both. But if you're convinced you'll need more than 5-6 gallons, you know more about your appliances & habits than me. (I've seen this work in other people's houses.) And since you're saving the under-sink space for storage, so be it. A 40 gallon tank can be as cheap up front as a mini-tank, and reducing the call for water from 42' down to 12' away is a huge improvement.

140F storage is lossier, but insulation is cheap (cheaper than beefing up the structure to handle the weight an 80 gallon tank.) With a tempering valve on the output set 120F (or even 115F) you avoid the scald issue- the amount of retrievable stored heat is the same, and the distribution loss lower. The difference in tank life from running hotter is miniscule- the elements don't care what the storage temp is, only the number of heating cycles they run through, which is far more a function of the volume of your HW use than the standby loss. (If you add an R13 wrap to an R20 tank you'll have mitigated the difference in standby loss anyway- the duty-cycle of the elements will be about the same.) Liming/scaling deposits go up slightly with temp, that's about it. (140F storage is safer from a legionalla point of view too, but that's a separate issue.)

I pointed out the mis-statement "Surface area and losses won't scale 1:1" because your explanation for had to do with volume, which is not a standby loss factor. Standby losses are INDEED roughly 1:1 with surface area, quite independently of the fact that losses aren't linear with volume, which is where your explanation seemed to go. The smaller nonlinear aspects of near-tank plumbing only become apparent when you get to the very small sizes, but that's not reason enough to avoid going very small, especially when one considers how much additional can be saved on distribution losses by by point-of-most-frequent-use location. The standby may only be cut in half, instead of 2/3, but you can cut the even larger distribution losses by more than that simple as a function of plumbing length. You run the sink a dozen times/day, but probably not the washer or dishwasher. Many of those sink draws are very short too, abandoning as much or more heat in the plumbing as the pint or three you use washing hands as the distribution path grows ever longer. The washer is going to draw at least a couple of gallons at a time, making the fractional loss lower. Point-of-use hot water really is better on several counts if it fits and can serve the load...

...and if it doesn't, oh well... (it was just a suggestion, eh?)
 
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12 feet to the washer isn't nearly the hit that 12 feet to the sink would be, since it's used many fewer times in a day.
Depends on how you use it. It is wise to compensate for long runs by changing habits, that does impact convenience though. We don't run the hot water much at the kitchen sink unless we are piggybacking uses--as in running the dishwasher, etc. I don't do short draws of hot water much there unless it is already hot. So quick food rinses are done in cold rather than defaulting to hot, unless it will be needed shortly, in which case the rinse can be done with cold "hot water" until the hot arrives.

Volumes of high-efficiency washer draws are amazingly small these days, as are those for modern dishwashers. With the fast recovery times of mini-tanks it they'd rarely be run dry by simultaneous laundry/dishwasher loads- the time between wash & rinse cycles is usually more than sufficient unless you're using flat-out hot for both.
They are small compared to the old gear, but still more than the mini-tank can provide in any 2 of 3 scenario I mentioned. They do the mix "flat out" from what I've seen in my new appliances. The dishwasher only takes hot and fills quickly. (Complicating things it does some heating of the water if the temp is not sufficient.) The front load washer on warm seems to be about 50/50. The fill takes a few minutes as it has an interesting protocol of adding a little at a time and turning the drum.

140F storage is lossier, but insulation is cheap (cheaper than beefing up the structure to handle the weight an 80 gallon tank.)
I would insulate anyway, so there is no net gain available from that. I don't see why the structure can't support the tank, just that it must be designed with that load in mind, which is why I mentioned designing for the extra ~1,000 pounds.

The difference in tank life from running hotter is miniscule- the elements don't care what the storage temp is, only the number of heating cycles they run through, which is far more a function of the volume of your HW use than the standby loss.
Doubtful, the retrograde solubility of the carbonate minerals suggests the opposite. The elements will have to run longer to do as you suggest (closer temp approach as well as increased external losses = longer runs), and in water that will produce more scaling at the element (and lines) due to the temp being called for. Corrosion of the tank will occur more rapidly at higher temperature, that's something that has been a given in the heat exchange equipment I've worked with. If there is fouling and corrosion related to it or other aspects of water chemistry, temperature almost always works against you in attacking the vessel. The temperature range you are talking about is above what I would normally have targeted in design if I could avoid it--particularly for fouling service. About 50 C (122 F) was a breakpoint at which bad things began to happen at an increasing rate. 20 F delta is typically huge when comparing corrosion rates or solubility.

(If you add an R13 wrap to an R20 tank you'll have mitigated the difference in standby loss anyway- the duty-cycle of the elements will be about the same.) Liming/scaling deposits go up slightly with temp, that's about it.
It's a logical fallacy to suggest doing the wrap on one tank and not the other. As indicated above, the higher temperature has some major holes and not just surface insulation.

I pointed out the mis-statement "Surface area and losses won't scale 1:1" because your explanation for had to do with volume, which is not a standby loss factor. Standby losses are INDEED roughly 1:1 with surface area, quite independently of the fact that losses aren't linear with volume, which is where your explanation seemed to go.
What I said was accurate, not a mis-statement because I qualified it. I intentionally avoided going into a detailed dissertation and explained why in the 2nd sentence. Cherry picking it as you did is BS. As it is, for the same throughput, a tank that has 2/3 more volume tends to take an efficiency loss of maybe 1%...on a 92% efficiency factor that works out to about 1/8th more standby losses. Sometimes it is 0% different. As near as I can tell looking at energy guides, the water output and energy usage is the same for these comparisons.

Volume is indeed what we are discussing and as you say, it is not of itself a standby loss factor. However, unlike you state, standby losses do not appear to be anywhere near 1:1 with surface area. Nice try, but you missed by a mile.

The smaller nonlinear aspects of near-tank plumbing only become apparent when you get to the very small sizes
Judging by efficiency factors they become apparent well before that. There is a reason that the efficiency factors don't trail off rapidly for a given insulation thickness when the tank size goes from 30 gal to 50 gal, or 40 to 80 gallons (looking at a State High Efficiency electric table for the latter...same 0.93 EF for both, with R20 insulation on both, roughly 40% more area on the 80 gallon.)

My point is that "going small" with the tank in-and-of itself does little for efficiency. Reducing the length of runs has benefits, that we agree on. One has to compensate for the lag time in long runs...or waste considerable energy. However, one has to weigh the expense and upkeep for the second system against this waste. If I was going to sell the second water heater in the kitchen end of the home, it would be for convenience. And I would still insulate the tank and lines.
 

hj

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tank

IF it is a "lifetime" tank, would that mean it is a "Marathon" model? If so, it is a fiberglass tank, which means a single person can "carry" it up the stairs. But do not equate a "lifetime guaranty" with "lasting a lifetime", because it just means when the tank goes bad, they will give you a new one, but you will either have to remove and install it yourself, or pay to have it done. The advantage of TWO heaters is that you get twice the recovery rate, since EACH one will heat water at the same rate as the single 80 gallon one will.
 

Master Plumber Mark

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what is the point???

what is the point here anyway????
how can you debate something like this
ad-nauseum for this long...??

either you get 2 40s or get 0ne 80...

or you could get 2 50s....

or how about lets debate the co-efficinetecy
of a 50 and a 40


wheres my gun, I got to end this misery...
 

SewerRatz

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you are a genious Mr Ratz....

the only thing you got to figure now

what is the co-efficient ratio .
...would these 4 25 gallon heaters
be 220 volt or ony 110??
and which would be cheaper over the long haul

and in series or parallell??

parallel of course 220 is the way to go, and have a 80 gallon storage tank with an aquastat and recirc pump just for them heavy use days.
 
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