Adding a small tankless to supplement 40gal tank

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fyremaster

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I replaced our 40gal electric tank water heater a year ago. It doesn't seem to be enough water to make it thru a longer shower as the water gets cold.

I am thinking of supplementing the tank with a small electric tankless prior to the tank so the water is preheated before going into the tank.

Can anyone tell me if this is a good idea or if there is another option that would be more effective? I don't want to reduce the pressure so am concerned that too small a tankless would cause more harm than good.
 

Dana

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Raising the storage temperature on the tank will increase the showering capacity. The thing SHOULD have a tempering valve or thermostatic mixing valve to limit the temperature that's being delivered to the hot water distribution by mixing in some cold whenever the hot water heater's output is over the setpoint of the valve. If it doesn't have one you'll need to add one, but once you do you can crank the water heater as high as it will go without increasing the scald risk. If the tempering valve is already there, this solution only costs you the increased standby loss on the hot water heater due to the higher temp. If you have to add one, it's still pretty cheap compared to another hot water heater.

If it's primarily showering performance you're after, if you have at least 4' of vertical drain downstream of the shower you can cut in a drainwater heat recovery heat exchanger. With a 4" x 48" or 3" x 60" or larger it will at least double your showering capacity, but it will cut your water heating energy use, since it's recovering half or more of the heat that was going down the drain and putting it into the incoming water stream(s). The highest performance is achieved when the potable output of the heat exchanger feeds both the cold side of the shower, AND the cold feed of the hot water heater, but if you have to choose just one or the other, make it feed the water heater.

By delivering 70-80F water into both the hot water heater and cold side of the shower, you end up mixing a larger fraction of cold water at the shower, and the hot water heater only needs to raise the temperature 25-35F to hit showering temperatures, not 50-70F. With a typical gas hot water heater burner and a low flow shower head the showering times can be nearly infinite, but with a typical electric tank it'll still eventually drop to below body temp.

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They are on the crazy-expensive side when purchased through retail outlets (such as the big orange box store). Some manufacturers discount a bit when purchased direct. But EFI has a few suitable models in the $$550-600 range (including shipping) if you have an account with them. (You can open an account online with a credit card to get that pricing.)
 

BadgerBoilerMN

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With all due respect and unflagging devotion...

A 40 gallon electric is too small for anything, save had washing or sponge bathes.

I have a 105 gallon Rheem Marathon and find it just right, even with guests. In residential construction the only practical way to make DHW with electricity in most markets is to do it in the off-peak, if you have it and then at low amperage with lots of insulation, a la the Marathon.

OK, a heat pump if it is pre-qualified, and paid for, in an appropriate HVAC system.

Heat recovery is effective but the cost of making DHW is relatively low and the ROI bad. Moreover, if you are recovering while showering the will not go cold as fast but the, likely, 4500w element has essentially no meaningful recovery in the time it takes to shower.

If you are showering the volleyball team with tankless gas water heaters, I will buy into a recovery coil.
 

Tom Sawyer

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Unless the waste system was specifically designed for recovery, the chances of it being effective or ever recovering its cost is just about zero. Least expensive option, put another 40 gallon tank beside the one you have now. Pipe them parallel, not series.
 

Dana

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It's not about the net present value of energy cost savings- it's about increasing showering capacity.

Drainwater heat recovery been proven effective for significantly increasing showering capacity for electric tanks in retrofits in Canada, a country which SFAIK uses the same laws of physics as the rest of the known universe. :)

The amount of the showering time increase with electric tanks depends on the incoming water temp, the storage temp, the shower flow, and to a lesser extent the power rating of the heating elements.
 

BadgerBoilerMN

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Perhaps. But having seen the inside of more waste pipes than I care to recall, the heat transfer will also depend on how fouled the pipe gets in use.

For an appliance with no moving parts this one has a lot going on and the cost to have us install it will require a whole lot of recovery.

Again, in a commercial setting or for the uber-green or those seeking net-zero... ;)
 

Tom Sawyer

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Agreed and the so called figures are heavily weighted using perfect conditions. If the shower is on the 2nd floor and you have 50 or so feet of pipe between it and the recovery unit, it's delivery is next to nothing. They work well in commercial locations but residentially they are never going to pay themselves off.
 

Dana

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There's nobody here asking them "... to pay themselves off...", only to increase the showering capacity.

Aging tests on first-generation units done by the US DOE back in the 1980s indicate that heat transfer efficiency would drop to about 75% of it's day-1 efficiency after being in service somewhere between 30-40 years. That means a unit delivering 55% steady-state energy return at 2.5 gpm drops to about 40% return after a few decades, which is still enough.

The amount of heat lost to the drain pluming through the "extra" length of another story of height in a 7-8 minute shower is remarkably small, even with cast-iron drain. It's really barely more than the amount of heat necessary to raise the temperature of that additional 10' of drain from 70F (room temp) to 90-100F (drain water temp), which is a function of it's thermal mass. The fraction of total heat being radiated off the 90-100F pipe is miniscule.
 

Tom Sawyer

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Sorry, but it couldn't disagree more. I don't know where you're getting your numbers from but they are grossly inflated. If the unit isn't going to ever pay for itself than why install it? Granted another 40 gallon tank won't pay for itself either but it will damn sure cure his volume problem and I doubt like hell a recovery coil will even come close. Remember, we are putting about 90 degree water down the drain at 2 gpm. The numbers don't even come close. I've done the math on these things and ven installed them in a couple of extended care facilities but we're talking a whole lot more volume passing over the exchanger than a residence with a single shower running. In truth, his best option would have been a heat pump type water heater installed in the first place.
 

Dana

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If mine wasn't working pretty close to spec (after ~6 years of service) I'd be taking cold showers and be in divorce court by now! :)

I'll trust those Canuck engineers' math on this one.
 

BadgerBoilerMN

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I wouldn't challenge Dana to a Math contest hehehee

But, still skeptical on every front save the squeeze the tea bag for another cup mentality. I like the seemingly lifetime serviceable life.

And the tons of CO2 used to produce this energy/marriage saving device?
 
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I replaced our 40gal electric tank water heater a year ago. It doesn't seem to be enough water to make it thru a longer shower as the water gets cold.

I am thinking of supplementing the tank with a small electric tankless prior to the tank so the water is preheated before going into the tank.

Can anyone tell me if this is a good idea or if there is another option that would be more effective? I don't want to reduce the pressure so am concerned that too small a tankless would cause more harm than good.

What flow rate is your showerhead and how long of a shower do you desire? If you don't know on the flow rate a simple bucket test with known volume or using a bathroom scale and a stopwatch can give you the flow rate.

If you have multiple showerheads or an old showerhead with some crazy high flow rate then a 40 gal just won't cut it. In one old home we had a 5 or 7 gpm showerhead and my wife would run the 50 gallon tank out to lukewarm, leaving me with a cold shower.

I use some good 1.5-1.6 gpm showerheads (High Sierra's and the original Roadrunner's) that give good showers. With a 50 gallon tank we could run three showers simultaneously and not run out of hot water. This was with a set point of 125 F. I typically set the water heater at 125-130 in order to be able to fill a whirlpool tub without having to wait for the tank to recover (higher set point in winter, lower in summer). Never have seen any need for a tempering valve, but I've never had to run at temps that could result in quick scalds.
 
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I looked at the drainwater heat recovery systems in a previous home. I agree with Dana's perspective on them. There was one that would have been a good fit in that home, but I had reduced hot water use so much and nat. gas was so cheap there that the payback was overly long in that location. For someone that regularly uses the capacity of 40 gallon electric tank payback might work out depending on electric rates. Washington state is pretty cheap as I understand it, so that won't help.

I wasn't concerned about fouling because the limiting side coefficient was not that high from what I recall and would not have been affected much by some fouling. Being a "falling thin film" device and having had some industrial experience with similar gear I didn't see why it wouldn't work long term. Plus it looked to be cleanable in that large bore section if needed (which related to how I would have arranged the install.)

The primary consideration is whether or not it is a good fit. In our previous home 75+% of the showering was done in the upstairs which would benefit from the device which would have fit nicely on a wall in the utility space right where the main upper level drain pipe ran. The other major concern is how much would it cost to hire a good plumber who would be receptive to the project (rather than someone fundamentally negative on the whole venture, often because they don't want to work outside their comfort zone.) This latter part is the major variable.

So in the OP's application one could spend X dollars installing another tank or Y dollars adding drainwater heat recovery (if the fit is good). Add to X the annual additional operating costs while Y has none. Over 10 years I wouldn't be surprised if the drainwater heat recovery option ended up being cheaper.
 

Dana

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Again, it's not about the net present value of future energy use savings against the installed cost of the heat exchagner. Rather, it's about gaining the showering capacity without taking up more space or throwing away a perfectly good hot water tank for a bigger one. Compared to the installed cost of a tankless big enough to make up the difference (the original question) it's comparable (or cheaper if done as a DIY.)

There's also no "payback" on raising the storage temperature which was my original suggestion.

Nor is there "payback" on installing a second tank in parallel. Though the initial cost of a second tank may be lower than a drainwater heat exchanger, it takes more space, and you'll go through three or four of those before the heat exchanger would crap out.

Oh yeah- AND it would use more energy. From a lifecycle cost point of view compare the installed cost of 2-3 replacement tanks plus the difference in energy use cost and it's not hard to come up with discount rates and energy price inflation rates that would make it financially rational, even if purely on the energy savings alone it doesn't "pay".

If you're only planning to live there another year, find a low-cost gym membership and shower there! :)

If you plan to spend a couple decades there, do what's rational for the longer term. The "install it and forget it" aspect of the long lifecycle is itself appealing.

The embodied CO2 of the heat exchanger is roughly 2-6x that of it's copper weight. (source1, source 2 ) The shipping weight of a 3" x 60" or 4" x 48" is 25-35lbs, so pessimistically it's 35lbs x 6= 210 lbs of CO2. Burning natural gas is worth about 11.7lbs of CO2 emissions per therm, so the lifecycle embodied carbon emissions of the heat exchanger it's no worse than lighting off (210lbs/11.7=) 18 therms of natural gas.

If heating hot water with natural gas it'll save a typical showing family of four about 1.5-3x that amount of fuel every year. But if you're heating hot water with all hydro grid power in the Pacific Northwest it may have a net-lifecycle carbon footprint, but not a huge one. Assuming a 40 year lifecycle that's about 5lbs of carbon emissions per year, which is the same order of magnitude of climate damage of the average human's flatulence-methane emissions per year (about 2lbs of CO2-equivalent.)
 
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