How do you pick between a tankless hot water heater and a traditional tank one?
Our fuel options are electric or propane. For the electric option, we can get a Marathon water heater from our coop for a very low price (around $220 for a 85 gallon or $275 for a 105 gallon).
I've read that Marathon are decent heaters, and the website specs say that for an 85 gallon the energy factor is .92. There are three different element sizes : 3, 3.8 or 4.5 KW with a first hour rating of 84, 87, 91 respectively.
I think an 85 gallon one would be enough for our household (2 adults, 2 kids for now).

But my DH is wondering if a tankless heater would be better in terms of efficiency and "endless" hot water. Though I'm not sure we can compete with the price of the tank one. But I don't know much about operating costs.

How do you compare between the two?

Thanks.

the Marathon is the best water heater made. that is what i would get. tankless is not really more effecient. the only savings is from standby heat loss and that is minimal in a
Marathon water heater.

It kinda depends on your fuel/electricity prices, and how you heat the house, etc.

If you're heating with a hydronic boiler, an indirect-fired tank running off the boiler will be the cheapest most efficient way of getting (nearly) endless hot water.

Tankless electrics require an upfront investment in the size of electrical service (they're ultra-uber power hogs when running).

Propane fire tankless can be expensive up front, a bit higher maintenance and has other quirks (google "cold-water sandwich".) If you have a spa to fill and fill it regularly, this might be the right option though.

HOW much are you willing to pay to go from 92% efficiency to 97% efficiency (electric), or from 60% to 80% (propane tank to tankless).

For the money, if you are a showering (rather than a tub-bathing) family of four you're better off spending the money on a drainwater heat recovery heat exchanger. Do the math (or read it, when it's been done for ya. :-) ) http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf A secondary but substantial benefit to drainwater heat recovery is larger "apparent capacity" in shower (but not tub fillin' ) applications, since more than half the heat is recovered & recycled while you're showering: you use less than half the amount of hot water during extended simultaneous drain/hot-water flow. Since showers account for typically 40% of water use in American homes, the net effect when used with a 0.92EF hot water heater is an "apparent" efficiency of greater than 1.0 (more than 100% apparent-efficiency) in high shower use homes. And it means you can live with a smaller tank and still get 4 sweaty people showered up in succession- it roughly doubles your first-hour rating on any tank, for shower use.

If you go fossil-fired (tank or tankless), it's worthwhile to go for a sealed-combustion/direct vent unit from both a health and actual-as-used-efficiency point of view. With combustion air drawn & ducted directly, there's no chance of backdrafting issues, and it doesn't suck conditioned-heated air out of the house driving outside air infiltration. It adds up- it's an untested-unseen 3-5% cost of operation in heating dominated climates that doesn't appear in an EF test.

If you bite on the Marathon, the smaller one should do unless you're planning a family expansion, or have a large soaking-tub to fill. The typical rule of thumb is 12-15gallons of storage per-person in the household. You could likely get by with a 50gallon unit if you don't have oversized tubs to fill. (With drainwater heat recovery a 30 could do it for a mostly-showering family.)

The standby losses of any tank can be dramatically reduced by adding at least R6 of insulation (fat fiberglass with inward-directed foil facers) to all of the near-tank plumbing, and and R2 (the cheap foam stuff) to the distribution plumbing. The first 3-5' from the tank is critical. If currently uninsulated, get up and feel it some AM before anyone has run the hot water- it's a significant radiator-convector 24/365.

A more detailed (and wholly conservative, based on not-very-efficient appliances & usage) lives here:

http://www.plumbingshowrooms.com/media/documents/Water%20Heater%20Sizing.pdf

First-hour rating is a huge factor, and quite different between electric/fossil tanks of equal storage. That 85 gallon Marathon might not be overkill, depending on how you & your guests actually use water.  (I tend to assume people are buying EnergyStar washers/dishwashers, etc and 2.5gpm shower heads, etc, where perhaps I shouldn't, or I should at least state that assumption when recommending.)

A few other things to consider is life span of the water heater. The marathon has a 45 year warranty, I don't think any tankless water heater can come close to that. Power outages are anotehr concern. If you have them on a semi regular basis, not having hot water (or having to go to greater lengths to get it) can be a concern. With a tank water heater you at least have 75-85 lbs or whatever your size is. Also how hard is your water? The water in my area is half rock..lol...so a tankless would need to be cleaned a lot more often. The marathon has a drain plug on the very bottom of the tank so you can get all the sediment out when you drain it periodically; which you should. Personally I would go with the marathon. That price is great and it could literally be the last water heater you would even have to buy. I'd rather just buy one and be done for good.

I just have to chime in and express my excitement for the Marathon. That is an INCREDIBLE price for an excellent water heater.

Dana,
If I am reading your posts correctly, your suggestion is that the Marathon HWH and a drainwater heat recovery heat exchanger would be a better investment that a tankless unit. Correct? Do you have suggestions as to the best drainwater heat recovery exchanger brands, as I am unfamiliar with these products?

In many cases, yes.

The key is whether your family tends to shower vs. tub-bathe.  With the heat exchanger in place you'll use roughly half of the energy to shower that you would otherwise have used at the same temperature and showering time.  If showers account for 40% of your total hot water use (the US average), it's tacking another (40% x 1/2=) 20% in operating efficiency to the system.  For a 0.92EF tank + heat exchanger becomes effectively a 1.10EF tank.   If showers are MORE than 40% of your hot water use (common, in 4-5 person households with EnergyStar washers/dishwashers) it does even better.  A tankless is never going to to better than 0.99EF, so your operating costs/power use will be lower if you go with the tank+ heat-exchanger instead of a tankless.

The only way to do better (short of solar) would be heat pump HW heater, but since it draws it's heat from the room, it's not always the best choice in heating dominated climates, since it adds to your heating system's load (in which case it would be more efficient to use the heating system directly to heat the hot water.)

National Resources Canada has heat exchanger models tested under standardized conditions to be able to make intelligent price/performance choices (with differing levels of subsidy based on performance, in Canada.)  I expect it'll be updated from time to time. The current list lives here:

http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/questions-answers.cfm#q45

You'll note that all else being equal, longer versions or fatter versions work better.  (A 4 footer for a 4" drain works about as well as a 5 footer with a 3" drain.)  It'll be more expensive than the Marathon, but it'll last as long, and is generally QUITE cost effective when the water is heated with electricity, in most US markets. See:

http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf

Thanks vehn. I don't know much about them, but what I've been reading seems impressive.

Excellent discussion Dana01, as always!
I forgot to mention that our electric rate is 0.12/KWH and that includes delivery and taxes. Our propane rates right now are $1.34 per gallon on the contract we're locked in for this season. I am a huge shower fan, so we're building a walk-in shower for me. The DH is getting a whirpool tub in the new house. Right now we only have a shower in the old house, so I can't say once we have a tub I won't be using it more than I should. :)

We are doing hydronic heat, but to qualify for the special rate for electrical HVAC (0.045/KWH in winter/0.065/KWH in summer delivered) we have to meter the electric HVAC separate, and domestic hot water can not be part of our HVAC system.
Right now we have a 50 gallon tank in the old house, and it seems to handle our needs. However, we don't own a dishwasher, only one shower, no tub, and our kids are still toddlers. This heater will be for the new house, where we hope the kids will grow up here, and we may still have more kids. Plus, my parents live outside of the country and when they come visit, they can stay up to two months at a time, and I thought an 85 gallon tank would not be unreasonable for those high demand months.

Looking at that sizing chart, if I'm doing it right, a 105 gallon tank is not unreasonable, but I'm not really sure we need that big.

Rykertest - that marathon is guaranteed as long as we own the home. If we sell, then it is only a 15 year warranty from date of installation. We have a wood fireplace for back up heat in case of loss of electricity, and cold water is not really a concern for a few days (we can do like in the old days and heat a kettle of water to take a sponge bath if required). Our water is like yours - hard and also full of rust, since we're on a well. But the new house is going to the rural water line, which is still not the best. I'd like a whole house purification system, but at a minimum we'll have a water softener. I hadn't considered that for the tankless option.

I was looking online for the prices of the marathon heater, and holy cow! are they expensive. I didn't know what kind of a deal we could get through the coop. And looking at tankless option prices (with install), I don't think we can beat the price of the marathon.

Dana - It won't be in the budget to initially install a DWHE, but it's on the list of things to get in the future (and we'd like to plan the plumbing to accommodate a future install). I have seen on other threads where you have posted that the best scenario is to pipe the DWHE to supply both the cold supply for the water heater and the shower simultaneously. Our house plan includes three full baths, and the location of the showers are roughly about 34', 28', 64' of pipe from the location of the main house drain (if they were piped separate). The water heater will be about 10' max from drain location. We're doing plumbing ourselves, so labor cost is not really an issue (is more of a time and headache issue), but would it be reasonable to hook up all three showers to the DWHE, or just go with the most used bath?
Do you have to pipe the water from the DWHE all the way to the actual shower cold supply (so essentially you're running two pipes to each shower? Do you run one "main" central line from the DWHE to the farthest shower and branch off that along the way for the other two? Or do you just pipe from the DWHE to closest spot of the main cold water line that would go to the showers? Or do you just hook up the DWHE to the main cold water supply that goes to all of the house?

Thanks for all the great comments!

The layout of the plumbing is often the feasibility determinant with DWHR.  But putting it in series with cold feed to the entire house makes it work automatically for all showers.  The only down side to doing that is that large draws of cold water while someone is taking a shower will be rather tepid to lukewarm.  There's about a gallon of water in every 40' of distribution plumbing, so for remote taps the cold water will be room-temp (not heat exchanger output temp) until you've drawn that distribution down.  This would not be a disaster for most households.

That's the way it is in our house, and cold water use during shower operation has not been much of an issue.  I haven't instrumented it, but I'd be surprised if the output of the heat exchanger was much over 80-85F (about the peak summertime supply temp from the street in parts of FL.)  But it's well above the basement's room temp (~65F) though, the temp to which most of the distribution lines would otherwise stagnate to.  (For reference, the model we installed runs about ~53% steady-state, according to the NRCAN listing.)

At 12 cents/kwh the economics are even more favorable than in that MN analysis (that used 7.5cents/kwh as the price basis):

http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf

It may not be in the budget, but the (after-tax) return on it will likely beat any interest you'd pay on it (or earn elsewhere, even before those earnings were taxed.)  Assuming 25 minutes of shower activity per day it's likely to pay for itself in 3-5 years.  (This analysis also shows that bigger is always better: The increased cost of a bigger unit is more than offset by the increased efficiency.  If you have the space for a 4" x 6-footer, go for it!  It's a much work to install a 3"x 3 footer as it is to install a 4" x 6-footer, but the payback will be quicker on the bigger unit. )

Since you're getting very favorable heating/cooling electrical rates, it probably DOES pay (eventually) to go with a tank-top heat pump type hot water heater, but the model gets a bit fuzzy around the particulars.  Maintenance on a heat pump is going to be a lot more than for the Marathon (or a  heat exchanger, for that matter), but in winter, assuming a COP of ~2 (which is typical), you'd be using half the amount of 12cent electricity, but adding a similar amount to the 4.5cent/kwh electricity use, making it the equivalent of running the Marathon at ~8cents/kwh.  In summer when your rates go up it would reduce your cooling load a bit.  Exactly when the rate shift occurs relative to the weather moves the financial model around a bit, but it's something you might consider (if and only if) there is a supplier near you who offers support for the product. Even with one of these, if you look at the MN analysis, drainwater heat recovery is still economic.

There's not much downside to going with the bigger tank if you have the room for it.  Mind you, the 105 gallon version is fully a half-ton of water + the tank for a dead-load. DO account for that if it's going to be supported by timber framing. (Sounds like it'll be resting comfortably on a concrete slab though, eh?) If you're filling a big spa you'll need every bit of it.

Yes, the marathon would sit on a slab, so the weight should not be a problem. I called the coop yesterday, and the guy made it sound like the cost to operate the additional 20 gallons on the 105 is not worth it.
Both the 85 and 105 gallon are 4500 watt for each element. Is there a way to compare operating costs for the two given a certain scenario of water usage?

Also, i asked the coop if they had any sort of incentive to use a DWHE, and right now they don't have one, but they are planning on rolling a new rebate starting next year. Right now they are proposing a $300 rebate on an installed DWHE, but until after the first, he couldn't confirm what the actual amount would be. So I'll wait to the 1st of the year to check the rebate. That would definitely change things budget-wise!

On plumbing the DWHE to the whole house cold water line, the HE would basically only operate when using the shower, since that would be the main simultanous use of hot water down the drain and cold water being drawn in (besides washing hands and dishes on the sink). In cases like the washer or dishwasher running, only the water sitting at the coil of the HE would get any heat off the drain water, since typically those appliances call for hot water, and don't drain it until end of cycle. That would still be a bit of "hot" water going into the water heater if there is call for hot water soon after the appliances drain.
But plumbing the thing to only supply showers and the water heater is definitely more complicated. The incoming cold water supply is right next to the water heater location, so that would be the easiest. Now I have to go measure to see what is the biggest pipe I can use. We have a 4" septic line, so it's a matter of getting the biggest length I can fit in the space.
The other thing I was pondering last night is that our septic tank is higher than our lowest level, so our basement plumbing drains into a sewage pit. I'm not familiar with how those operate, but I think if they eject the drain water as soon as it arrives, then that should help to include the basement shower into the DWHE output. If it sits in the pit until it reaches a certain level before it is pumped out, then most of the heat may be lost by then.

Good food for thought! Thanks.

The EF test numbers is reasonably apples-to-apples between two electric tank heaters. The test procedure & volume drawn from each are the same, so the EF basically measures the relative standby losses. The operating cost of a 0.91EF hot water heater (the 105 gallon Marathon) compared to a 0.92EF hot water heater (the 85 gallon version) will be pretty much 0.92/0.91 that of the 0.92EF heater (about 1.1% more.). That doesn't sound like much of a deal-breaker to me. Raising/lowering the storage temp 5 degrees would have as large an effect on efficiency.

The EF test is based on 62gallons/day usage. If you use more, your actual efficiency will be higher (but so will your bills), if you use less the efficiency will drop, (but so will your bills), since the standby loss is the same independent of actual usage. If you never use hot water, the efficiency drops to zero- since literally ALL of the energy is dissipated in standby. Assuming a storage temp of 125F in a 65F room, (60 degree delta), lowering the temp to 120F (making it a 55 degree delta), lowers the standby loss by 9%. If your original standby loss was 9% (for an EF of 0.91), cutting that loss by 9% brings the standby down to ~8% (for an EF of 0.92).

You're correct that batch draws don't get much benefit from the heat exchanger, and that it's only simultaneous flows that ever amount to much. But that may be improved slightly by insulating the heat exchanger if the heat exchanger is in a very cold space. Heat lost in the drain sump may still returned as space-heat to the adjacent room, provided the pit is insulated from the ground. The MN financial analyses have stated presumptions on number of showers/day (and their length & flow), and count only showering time as having any benefit at all. The more showering time your household sees, the higher the return on investment. It's not rocket science, but it's useful to put some realistic estimates to it before plunking the money down, since it IS a hunk o' change.

With the heat exchanger feeding both the input to the HW tank and the cold feed, you end up drawing ~20% less hot water from the tank, since you're mixing it with significantly warmer (but not hot) "cold" water, and since at the same time the water entering the tank is already pre-heated to its final temperature- the recovery time is shorter. When the shower has stopped or when batch drawas drain, draws from either hot or cold taps will purge the pre-heated water from the heat exchanger, and since it's only a few gallons on even the largest version, about 1 toilet flush & hand washing pretty much does it. With most EnergyStar dishwasher/washers you're only talking a few gallons in the first place, so they may in fact recover a reasonable fraction of the heat of HW used. (If using a warm-wash/warm-rinse in the laundry, a few gallons of the warm-rinse will have had a pre-heating benefit from the recent dumping of the warm-wash, etc.) But the volumes are small compared to a 10 minute shower at 2.5 gallons/minute. Top loading water-pig washers will have both a much lower fractional heat exchanger benefit, and a much larger loss overall.

Great question!  Consider the initial investment of the tankless vs. the Marathon.  You will most likely be looking at a savings of around $800 - $900 by choosing the tank type Marathon.  Operational cost can be compared if you know your Kw/hr rates from your utility company and the average cost for a gallon of LP. 

Often times, in my area (MN), under normal household demands lower operational costs are realized with the off-peak electric system.

I have installed dozens of the Marathon units and would highly recommend them to anyone.  There are also some very nice tankless units available.  Be cautious when selecting the proper size tankless heater.  There is no such thing as endless hot water.

Boontucky, one thing to be aware of is, as far as I have seen, the DWHE are vertical units, not horizontal. Depending on the elevation of your sewer line from the house relative to the first floor you may need to drop the shower drain water into your basement sewer sump. This will also exclude your basement shower from using a DWHE. You'll also have to size the sump to accommodate the shower drains.

I'll do some looking to see if there are any horizontal units available because I want one myself. I only have a crawl space and about 12" drop from the shower drain to the sewer outlet. If there aren't any commercially available horizontal units then I plan to say "to h***" with the code restrictions and build one myself. It most likely won't be as efficient as a commercial unit, but it'll be better than nothing. If I do go that route, and you can't fit a vertical unit in your house, then let me know. I'll explain how I built it. Of course, you won't get the rebate from the coop for a homemade unit, I'm sure.

What's this then?





We don't have any 3rd party test data on it, and I'm a bit skeptical about it's clog-proofnets, but  it's horizontal, eh? ;-)

See:  http://www.ecodrain.ca/

Contact info:  http://www.ecodrain.ca/en/where-buy

If you have the space, the vertical versions have more of a track record.

Hey thanks, Dana. I didn't know that one existed! I wish they had some pictures of how it's constructed. I see  comments about double wall and flattened tubes, increased turbulence, and teflon coating. The increased turbulence and teflon coating makes me think the drain water side isn't a smooth wall tube. That might cause problems with crud build up inside.

I'm thinking of building a simple tube-in-tube exchanger, about 10 ft long. Inside tube of 2" copper, outer tube of PVC, with copper wire spiral wrapped around the copper tube to swirl the potable water between the tubes. Now, before you, or anyone else, says yeah but, it's got to be double wall, tain't the case! Both the IRC and UPC state that if the transfer medium is non-toxic, and at a lower pressure than the potable water, then a single wall exchanger is OK. When's the last time you took a shower using toxic shampoo and soap? Even pee is non-toxic. The only thing my homemade exchanger would lack is a bona fide manufacturer's label limiting its use to non-toxic waste water and a UL label.

I wonder about its mounting position as they show in the diagram. Could be an air trap on the PW side. I think the PW inlets should be on the upper side to make sure there's no air trapped in there.

Dana, do you think a DWHR unit would be worth it if there is only one upstairs bathroom with shower? The other bathrooms are downstairs on a concrete slab foundation (no basements here in so. Louisiana).

It depends on a number of factors: How much does it cost you for hot water, how often (minutes/day or year) is the shower most used, etc. 

At 6cents/kwh fueling a 0.90EF tank showering 5 minutes/day at 2.5gpm it isn't going pay back as quickly as it would at 22cents/kwh & 25 minutes/day shower use.

But if it keeps your spouse from taking a cold shower right after YOU stepped out of a long relaxing shower, it pays for itself on the spot.   Increased apparent capacity can be a real reason for using DWHR, independently of cost savings.  (That apparent boost is only for showers though, not tub fills.)

In much of the gulf coast states slab-on-grade construction is the norm (high water tables, etc.) making it difficult to implement DWHR. But in much of that same area there's better payback on (pump & glycol free, plumbed in-series, often subsidized) simple batch hot water pre-heating.  At that latitude & climate it doesn't take much of a collector or batch storage to get a ~50% solar fraction out of it if you're reasonably conservative with hot water use (which beats the ~20-25%average return on DWHR. )

There's no "one size fits all" solution to boosting efficiency, but DWHR makes sense in a lot more places than it ever gets installed. (The guy I had install mine had never seen or heard of it before, and he's no kid.)  Batch solar would have to be freeze protected in most of the US, but the milder frosts of the gulf region makes it easier/cheaper to implement there, as opposed to MN or MI.

How about this: get a Marathon electric water heater, then mate it with an AirTap (by AirGenerate in Houston) heat pump water heater. Efficiency factor is claimed to be about 2.5.

I'd like to get the GE heat pump water heater, but it is not yet available in Texas. I'm torn between getting a small, inexpensive placeholder electric heater which I'll donate to Habitat for Humanity when the GE becomes available, versus going the Marathon plus Air Tap route.