Energy Efficiency Question: Water Heaters

The water heater for Chateau Steelypips is significantly older than the usual useful life for such devices, and it’s really started to show. I’m getting pretty sick of lukewarm showers, so we probably need to replace it.

As a good squishy liberal type, I of course want to replace it with something more efficient, and there are claims out there that the best way to go for efficiency is with the “tankless” models (a gas one, in our case, so we can still have hot water when/if winter weather takes out our electricity). Of course, there’s a lot of contradictory information out there. The usually reliable Consumer Reports is down on them, for financial reasons– the projected energy savings isn’t enough to offset the cost– but I’m less concerned about that. I’m willing to pay a little more for higher efficiency on general environmental grounds. That Consumer Reports piece also never quite gets around to giving useful figures on the energy usage,

So, I’m throwing this out to the environmental crowd here: Is the energy reduction from replacing our old tank model with a tankless one significant enough to be worth the bother (the installation would be significantly more complicated), or should we just try to find the most efficient tanked model. Please note that we are in a cold and rainy part of the Northeast and do not have significant southern exposure for our house, so ultra-green things like passive solar heating are not possible, so don’t waste everybody’s time talking them up.

33 thoughts on “Energy Efficiency Question: Water Heaters

  1. I called up the local outfit about a tankless, and mentioned that the well water around here is hard as a rock – at which point he said that tankless would not be such a great idea.

  2. We have one–a Baxi 380, a combo water heater/furnace. It’s gas, but it will NOT work if the electricity goes out. They’re very high-tech, with computerized controls. It does seem to be pretty efficient, though. We’re thinking about some sort of solar/battery backup for it.

    We went for it for a few reasons: the space in our basement made installation of a conventional unit more difficult, and it gets wet down there, so the old unit was prone to water damage. The new unit is mounted high up off the floor. This was a big plus for us. The price premium for the combo unit was not that bad over the cost of a new furnace, and tax credits and state rebates available at the time basically paid for the installation. (So you should check what’s currently available)

    It takes a few seconds more for hot water to arrive at the tap, but once it goes, you can’t run out. Rather than capacity, you have to plan for flow rates: what’s the maximum you need to deliver at once.

    Remember that without a tank, there’s no corrosion, so the units tend to last longer. The real enemy of the tankless unit is hard water, which can clog the heat exchanger over time (an expensive repair). So check your water supply.

    There are also high-efficiency tank units, but they also cost more than conventional (but they can qualify for some tax breaks).

  3. A friend who has a tankless heater was told that they typically last 9 years or so. From an environmental (and cost) perspective, that seems like an unnecessary number of replacements over the life of the house.

    Out here in the Northwest, they actually encourage people to use electricity for water heating because our energy sources are basically all renewable (hydro, wind, and solar). That’s probably not relevant in the Northeast.

  4. Is your current WH flushed free of sediment? It shouldn’t go lukewarm unless it is undersized for your use or something is wrong with the system. If it used to work OK maybe there is something fixable.

    My old WH started leaking, and when I pulled it out to replace it, it had maybe 5-10 gallons of disgustingly gooey sludge in the bottom half.

    For improved efficiency, you could add layers of insulation & reduce the losses. There are a couple “smart” water heaters out there that change the heating policies, but providing a certain number of gallons at a certain temperature is a pretty simple and tightly constrained problem. I’m not sure there’s much opportunity to improve the efficiency very much.

  5. I agree with Dave X. You have to get a big increase in efficiency over a long time to make up for the energy cost of a newly-built heater. In addition to Dave X’s suggestion of draining out sludge, make sure you check that the sacrificial anode is in good shape if you want to slow sludge build up as well as avoid premature failure of the tank.

  6. 1.) You’re a physicist. That means you have to buy the most advanced technology possible, independent of cost or energy considerations. You can take the physicist out of the lab, but your can’t take the lab out of the physicist.

    2.) More seriously, if the extra cost of the unit exceeds the projected energy savings, you have to ask why the unit is more expensive. “More expensive to produce” is often strongly correlated with “Takes more energy to produce.” Sorry I have no numbers for you, just pure speculation about facts that might be very difficult to dig up, but maybe a good chunk of the apparently-green energy reduction disappears if you include the hidden manufacturing/production energy. (In other words, don’t get me started on ethanol subsidies…)

  7. I’m with Dave on this one. Unless your old water heater is rusted out and full of scale and sediment you can’t get rid of, maintain and insulate it for now. Pull out the sacrificial anode and replace it (to forestall any more corrosion), and flush out as much sediment as possible. A wet/dry shop vac can help.
    Supplement the blanket insulation around the tank with some pieces of fiberglass insulation on the top of the tank and around the fittings. Put a second layer of insulation around the pipes as well.

    If you are looking at a new install – How long are the hot water pipes? If you have long runs (more than 30ft?), it might be worth downsizing the pipes from 3/4″ to 1/2″ or getting that tankless heater.

  8. I don’t know any details, but there is such a beast as a heat pump water heater. Rather than using resistance heating -or simple combustion, a heat pump model pumps heat into your tank, getting a pretty big boost in efficiency (i,.e.) it is a heat engine running in reverse. I think these babies are also quite a bit more expensive. But, as Emory said above, your a physicist, which means you gotta go for the geeky solution!

  9. “I don’t know any details, but there is such a beast as a heat pump water heater.”

    heat pump water heaters are only good in warm climates where the heat pump runs a lot in cooling mode.

  10. electric tank water heaters are 100% efficient.
    They are 100% efficient in converting electricity to heat. Unfortunately the process generating the electricity isn’t anywhere nearly that efficient.
    It really all depends on your water hardness. Our water here is so hard, we had 100 lbs of sediment in our water heater when it was replaced. If you have a water heater failing due to sediments (and the lukewarm showers sound like it) your expensive tankless model won’t last.

  11. I have seen combination fuel cell/hot water heaters advertised. These devices are fuel cells that generate electricity from natural gas, and use the waste heat to heat water. It was not really clear to me whether these are intended as hot water heaters that happen to produce electricity, or electric generators that happen to pre-heat water. The difference would matter:-/ But it is something to look into.

  12. electric tank water heaters are 100% efficient.

    They are almost certainly 100% efficient in turning electricity into heat. What they are almost certainly not 100% efficient at is getting all of that heat into the water.

  13. By “gas”, do you mean a propane tank or something coming from a utility line that runs under your street? If the former, and you want to keep your heat/hot water going during a power outage, you can get a generator that will run on your propane supply. Most furnace systems these days require some electricity to run, even if the heat itself isn’t derived from electricity.

    I agree with those who advocate getting a tank and wrapping it in insulation, especially if you aren’t replacing the furnace at the same time. Also consider wrapping pipes in insulation, to further reduce heat losses between hot water tank and shower. But first, check whether you can keep your existing tank after flushing out sediment; however, if you see any rust on the tank, better to replace it now.

  14. My parents and brother have both replaced their hot water heaters with tankless and been pleased. However, they both have had to run electricity to the hot water heater (which previously was just gas) because the tankless heaters required electricity, too (in addition to the gas).

    I’d double-check on the tankless working if the power is out.

  15. I assume you have natural gas, which should be even more efficient than the propane I have up here in rural Saratoga County. I’ve always used propane just for cooking and hot water, and since I put in a tankless “on demand” hot water heater 3 years ago, my propane bills are consistently close to 50% less than they were before.

  16. “They are almost certainly 100% efficient in turning electricity into heat. What they are almost certainly not 100% efficient at is getting all of that heat into the water.”

    you might want to think about this statement a while. maybe even use google.

  17. “They are 100% efficient in converting electricity to heat. Unfortunately the process generating the electricity isn’t anywhere nearly that efficient.”

    this is true but he did not ask that question.

  18. My experience with tankless water heaters is that they can have problems with the minimum hot water flow required to get them to turn on, which results in wasting water. Other than that, it’s obviously going to vary quite a bit with your water use patterns, and actually with your housing as well.

    The problem with gas tankless water heaters is that they don’t fit under the sink, so they retain two of the major heat losses of conventional water heaters — the heat lost to warming up the pipes going from the water heater to the faucet, and the heat lost due to hot water being trapped in the pipe after you shut off the faucet (plus any heat loss from the pipes while in use, but with decent insulation that should be unimportant). An electric under-sink model doesn’t have those losses, but runs on electricity rather than gas. For a faucet where you use small amounts of hot water intermittently, the under-sink model is probably more efficient even given that it’s using electricity, for something like your shower the larger gas heater should perform better.

  19. tankless heaters are not more efficient. the only advantage they have is savings from storage losses. with modern water heater insulation those are minimal if measurable.

  20. To answer a point that should’ve been in the original post: the gas service is natural gas from the local utility. We have a standby generator that runs off the same natural gas line, which provides several circuits’ worth of electricity to the house, enough to keep the fridge, lights, heat (gas furnace, but it needs electricity for the fans), and hot water (the trivial amount of electricity needed to control the gas water heater). We’re not switching to an electric water heater, because we wouldn’t be able to run that off the generator we have.

  21. 20 comments and no actual answers to the question.

    freethinker’s answers boggle my mind for its sheer utter nonsense. Water heaters use an “energy factor” to describe the ratio of useful energy put into water to the amount of energy inputted. A good water heater has an EF of 2/3. “Efficiency” is a poor metric to use as it is rather ambiguous. It is especially meaningless when you talk heat pumps since you put in fairly little energy to run the actual pump but you can get a significant transfer of heat thus having an “efficiency” of well over 100%, which is nonsense, but they have an EF less than one (unless your house doesn’t obey the laws of thermodynamics).

    Frankly, I don’t see this is a problem that is solvable in general. To solve it in general you have to make a lot of assumptions — precisely what CR did with their protocol they described — or you make a meaningless conclusion. What specific water heaters are you looking at? What specific usage assumptions are you making? What is the ambient temperature will they operate in? How much will installation cost for you (this is clearly different for replacing a conventional, replacing a tankless or building a new home)? Do you have copper pipes? Are they insulated?

    This is one of those problems in which the specifics matter as uncertainty in them means the answers overlap and there is no one True Answer.

  22. It is possible to use your furnace as the heating source if you currently have hot water baseboard. It’s called indirect heating and basically you have hot water from the furnace circulate through a coil in a storage tank. has a few more details.

  23. My first comment is about the sediment. You really should drain the water heater regularly, because you now have deposits insulating the water from your heat source. You should definitely take it as a warning to replace the heater before it fails due to corrosion. Not pretty.

    Tankless heating is good if the “run” from the heater to its use is really long, but the biggest gain is if you waste a large amount of cooled water to get a small amount of hot water. A small electric unit on a bathroom or kitchen sink (heating “hot” water if it isn’t hot), and that is NOT on your generator, might be the way to go. The big gas water heater is likely to be more energy efficient than lots of smaller gas ones for your major uses.

    Also, in your climate, your “wasted” heat from the hot water pipes is likely heating the house, so it isn’t all wasted.

    BTW, thermodynamics is perfectly happy about using a modest amount of energy to “pump” energy into the house. There is no reason the EF defined @22 can’t be bigger than 1 when the coefficient of performance is more than 3, as is the case for central heat/air systems in moderate climates.

  24. CCPhysicist, I don’t think you understand what I was getting at (probably because of the way I worded it). Sure, the COP can be > 1 but to call it “efficiency” is nonsense. To me, any use of efficiency over 100% implies perpetual motion-like. If you said a heat pump had a 350% efficiency then, to me, that means you can run the heat pump back and forth and get infinitely free energy out of it: nonsense. This is why EF and COP (and others?) were invented.

  25. This is not a question that can be answered without a lot of information about the installation circumstances.

    Keeping it fairly simple:

    Things that need to be included in the analysis are the cost of the fuel, the quantity of hot water used, and the quality of the water being heated (determines if pretreatment is needed. a softener may raise the cost well beyond reasonable, even if fuel cost and usage are favourable), among other things.

    Generally, high efficiency demand units make sense for very high usage and for minimal usage cases. High usage, because the fuel savings makes it worthwhile over a conventional unit (as well as the power are usually much higher for a demand unit: in the US, a 40 gal tank unit might fire at 30KBTU/hr, and a demand unit might fire at 100KBTU/hr or more, so no running out of hot water when all eight kids shower in the morning, while both washing machines are running). Minimal usage (like for a hunting cabin) since the water is heated only as needed, eliminating storage losses.

    Between the extremes, it is a tough question. Fueled by tank gas with good water, 4 kids and an in-law in the house, high efficiency demand unit makes more sense than for a childless couple in a home with hard well water and utility gas at the curb.

    As a side not: overinsulating the heater tank won’t help much. As long as the factory insulation is dry and in place, most heaters manufactured in the last 20 to 30 years have flue loss and combustion air loss as the major factor. These are losses you can’t prevent (unless you want to destroy the flue, reduce burner efficiency, asphyxiate or die from CO poisoning, etc) and the heater is probably designed for a near minimum on them. Yes, much of the storage loss is through the flue from convection through the heat exchanger. In a cold climate, this isn’t usually too a big deal, as much of the storage loss is recovered by reducing the cost of heating the home.

  26. We’re using tankless down the Jersey shore, which works fine because it’s a summer house (only in name, built like a tank and ruthlessly well insulated) that is occupied only occasionally, and the pipe runs are insulated, etc. Rinnai’s for the hot water loop, and a Boderus for the baseboard heat.

    The Samsung Quietside that was the old instant-hot for the baseboard heat gacked 10,000 gallons through the house three years ago, causing ~200,000 in damage. So Samsung sucks.

    But if you’re in central NY, and on gas heat, with hard well water, I’d stick in a really good electric/gas tank system. Like, use the home heating gas boiler as the primary loop, with an electric backup element in the tank for when electric is cheaper than gas, and it is not winter.

    But I’d stay away from glass-lined tanks – Vaughn is the last north american manufacturer of stone lined water tanks – – My parents, in Sullivan County, have been running a stone-lined insulated hot water tank (120 gallons!) electric/oil boiler system for thirty-three years, since they replaced the converted coal furnace that was put into the house from the old hotel when the house was built.

  27. It’s important to not that a physicist is respecting that you cannot really tell the answer to this question with some basic physics – depends on usage patterns etc.

    You should look into power-pipe type heat exchangers. They can give you easy 50% reduction in energy consumption and don’t cost anywhere near what a heat pump hot water heater costs.

    Interestingly, if you play your cards right you can save even more heat using heat exchangers but it’s rather impractical for a normal house…

    Also, can we talk about the absurdity of capturing solar energy with photovoltaic panels on your roof in a cold climate, when you could save way more CO2 for way less money by installing flat panel heat collectors? (on a kwh basis, electrical energy at the plug and heat energy from a furnace are about the same in terms of co2 emissions!)

  28. Northeast and do not have significant southern exposure for our house, so ultra-green things like passive solar heating are not possible, so don’t waste everybody’s time talking them up.

    I’m disappointed, based on actual experience looking at detailed weather data and looking at simulations as to energy collection, it is in fact altogether possible to do hot water heating under such conditions, although yeah it probably does not make much economic sense. But don’t say it is not possible, because it certainly is.

    Actually it does not make much sense in any case, as the amount of heat used is relatively small.

    Also, have you considered capturing the waste heat from the hot water to heat your house at least? Just a 55 gallon drum that the water sits in on a first in first out basis ought to do it.

    Lastly, to answer your question, condensing natural gas instant heaters are available that have a whopping >95% efficiency of combustion to hot water heating, so IMO they are probably inevitably going to be more efficient given the lack of standby heat loss though I have not checked.

    There are also heat-powered heat pumps such as the double and triple ammonia adsorption types but I don’t know if they are for sale anywhere.

  29. Over here in the UK, you’ve only been able to buy condensing gas boilers since 2005, and about 80% of homes are heated by gas boilers.

    We’ve got a relatively new combi condensing boiler which was installed by the previous owner, and its fine. The only problem was a need to repressurize it, which I should have let a professional do (I forgot to turn off the water, flooded the kitchen below and fried the boilers PCB – my plumber and wife were both very nice to me, considering!), but other than that, no problems. Apparently, they are now as reliable as the older boilers, so ignore stuff that says otherwise.

    The advantage of a condensing boiler is increased efficency, and the combi has the plus of constant hot water. The downside of this is that it takes a couple of seconds for the hot water to come on, and realistically you can’t run a hot bath and sink at the same time, but the increased space you get from having no tank (and this can include the cold tank in the loft) is a real plus.

    One possible downside is that in very low tempretures, the condense pipe (or even sometimes the boiler) can freeze up. A wide condense pipe is often use to overcome this, and to insulate the pipe, or to heat the pipe via a wire. Since the UK seldom gets weather cold enough to get this problem (about minus 10 c), its only occasionally an issue. In Germany (who often have worse winters than us), they often leave the boiler on (low) all night, and avoid icing up. Some boilers will apparently do this automatically.

    More cost-efficient? Certainly the general figure seems to be about 25% more efficient compared with an older boiler, and of course you will loose heat from a water tank, no matter how well its lagged. You will use electricity for the pilot light, controls, etc, but this barely noticable. Obviously, the more efficent the design( A+), the greater the margin, but also possible the cost of the unit.

    The best thing to do first (which has probably been done already) is to retro fit the house with thermostatic valves on the radiators, lag pipes, tanks and the roof (this last one is money for old rope – I’m paying less than last year thanks to doing that).

    Once you’ve done that, its spreadsheet time. According to the Energysavers website, you could get a rebate from your energy supplier/government, and if you get a good deal anyway, that might swing it. Looking at the differnce in initial cost, a couple of websites reckon that the difference can be recovered in 2-5 years.

    The other thing with gas boilers is the upward trend of gas prices over the long term. If the price of gas goes up 30% 2 years from now (and its gone up by a fair amount over the last year), the extra 25% effiency gain is going to effect your discount rate over the lifetime of the unit. Of course you might have to sping for a way to soften your water, but you’d probably have to do that which ever boiler you’d choose.

    You could try renovating the old boiler (desludge, lag, etc), but unless its a very small amount your spending, take the plunge and go for a new one.

    I’d probably go for the combi boiler. Its more efficent, just as reliable, and saves space. You’ll get a rebate, and will save money through lower energy costs in the long term. You only heat the water you need, and its always on tap. I’ve been happy with ours (so far), and this is in a ‘cold and rainy’ country (I think we invented it!).

  30. Most cases of poor water heating does not require a new unit.

    Annoyingly cool water can mean the thermostat/s are bad, maladjusted, or not making good contact with the tank. Compare actual output temperature to stat setting. Inspect thermostat mounting. Adjust/replace as needed.

    It can also mean you are making hot water but it isn’t getting there. Open the drain valve and check temperature. If the water in the tank is hot but it isn’t at the shower … Check the insulation on, the lines. Replace as needed.

    There is even a chance the anti-scald thermostat in the shower mixing valve is defective or maladjusted. Check temperature of water going into mixing valve and temperature going out with cold turned off at mix valve.

    A limed low-flow shower head can mean that the showers are colder. Time water flow and/or remove head and inspect.

    If none of that makes sense check the tank.

    Electrical water heater:
    Turn off the power.
    Turn off the water.
    Drain the tank. Easy and fast if you tap into the cold side or T&P line with compressed air to blow the water out the hot side. Opening the drain and hot on the sink or opening the T&P works but takes a long time.
    Once drained pop the element/s and anode/s.
    Shine a light in and examine the tank for rust, sediment, scale.
    If the glass liner is intact replacing the elements and anode/s can get you another seven to ten years.

    If you need elements replace with low-energy density models. They don’t save electricity but they last longer, lime less, and make less noise. Low density models are almost always easier to buy online. Same with anodes.

    Be sure to refill the tank before reconnecting the power. Helps to vent the air. The T&P valve will work for this, and its right there. Electrical elements energized before being immersed melt.

    For gas units there are analogous structures.
    Dirty or maladjusted burners don’t put out much heat. A maladjusted thermostat can be set too low.

    Tight houses can cause problems. Inadequate makeup air can cause the burner to backfire.

    I saw one where the heater had fire shooting out around the base. And it had been shooting out for months. Only when shower got unacceptably tepid did the HO call. In that case makeup air was blocked by a squirrel nest in the vent.

    In most cases if the tank is still sound it is far easier to replace defective parts. Most anyone handy with tools can handle the job. Gas piping can require extra care, perhaps an inspection (a friendly gas mechanic or knowledgeable plumber might check your work and advise you) , but it isn’t really hard to get right.

  31. If you get a tank water heater, put its electrical circuit on a timer. No sense heating water from (say) 10pm-7am, or 9am-4pm unless you
    need it. Timers are cheap. And lower its thermostat. Saves energy, the eqpt lasts longer, and prevents scalds–esp for seniors/kids. Don’t worry about the temp for the dishwasher (if present), they all have built-in preheaters these days. And it really pays to re-locate it as close as possible to main use point. Check the anode, if it’s too tight you won’t be able to replace it later. That one got me.

  32. Timers are okay but with modern electric water heaters they seldom repay their cost in a reasonable time. They made sense in the mid-70s when insulation on the tank was minimal and you were paying through the nose to keep thirty gallons of water hot. Pretty much all tanks are pretty well insulated so storage losses are low. IMO you’re better off investing the time and money you would spend on a timer and install better insulation on the tank and lines.

    Turning off the power can save money if you are going away for a few days. Most cost effective way is to simply flip off the circuit breaker in the panel. The exception there is where pipes might freeze with the water heater off. Except in the great frozen north, where power loss can kit your plumbing, most places can get along without power to the WH if the pipes are well insulated.

    I casually said to “pop the element/s and anode/s”. I use a electric impact wrench and impact rated sockets. The typical 1-1/2″ element and 1-1/16″ anode can be a bear to unscrew with professional-grade hand tools. Near impossible with most DIY-grade tools. Corrosion and heat cycling can pretty much weld them in place. A good electric impact wrench makes it almost fun, wear ear and eye protection, and use impact rated sockets because lesser ones tend to break catastrophically. Even easier if you grind the bevel off the sockets so they seat better on the narrow hex heads.

    For years I used a 3/4″ breaker bar, a ‘cheater’ pipe, and a second pipe for a hold-back. Sometimes a manual, hammer-driven impact driver and ten pound sledge. Helps to break lose the pieces after turning off the water but before draining the tank. 30 gallons of water weighs roughly 240 pounds that helps hold the tank down as you cuss and muscle out the plugs in the time-honored ritual of medieval brutality. Consider that you’re doing this feat of strength in a tight spot where everything has been placed in exactly in the worse possible spot and you know why I kiss my impact wrench every time I use it on this sort of job.

    Many tool rental places have the impact wrench and sockets for about $30 a day. Spend an evening sweating and cussing a stubborn anode out and $30 will sound cheap.

    You can make a second removal of anode or element easier if you use a electrically conductive threat compound. Avoid Teflon tape or other potentially non-conductive thread treatments on anodes. If your water supply has magnesium loving bacteria use an aluminum anode to avoid the rotten egg smell and black smut. Noted that neither elements nor anodes benefit from being horsed tight. They need only be snug enough for the gasket or threads to mate up and seal.

    And yes, I know some of this is talking past the original question. But the internet, barring the mythical HEMP, is eternal and someone, somewhere might be saved some time, money, effort knowing some of this when it pops up on a Google search.

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