Induction Water Heaters: Are They Worth It?

[sophiecentaur]

How is that water heated? It seems like an ideal application for a heat pump.

I was thinking about the mutual effect of a lot of air sourced heat pump installations operating in close proximity. There could be a 'battle' between systems for available Joules in the surrounding air. Otoh, the microclimate within cities tends to produce measurably higher air temperatures compared with those in the surrounding countryside. Wildlife and plants often thrive (partly) because of this. That adds a further confusion factor.
Clearly the big money is in reducing heat loss in the first place but that has nothing to do with the stark facts when comparing an electrical heater, immersed in water and other small scale methods for making a cup of tea.
This thread could run and run, I think.

 

[artis]

I think you stated it good enough that these solutions might be worthwhile in keeping ambient temperatures within rooms but definitely out of scale for producing lots of energy in a small area in minutes or even hours.

 

[Baluncore]

Engineering involves considering every available possibility, then pruning the options tree only once the economics of each individual application is known.

Heating water is not a fully specified application. There are too many possible confounding variables to rule anything in or out, now or in the future.

 

[Nugatory]

I've never seen a heat pump water heater in a residential setting

Residential heat pump water heaters are available in the US - I’ve spent a lot of time in a house that has one. They are a particularly good fit for a summer house because they turn cold water and hot air into hot water and cold air.

 

[Averagesupernova]

Residential heat pump water heaters are available in the US -

Aren't they a requirement over a certain size? Has been my understanding.

 

[made_change]

Hi! To your Question, Both are electrical heaters with 100% efficiency, but they have varying use and they differ in the way they produce heat. Induction heater, in general, are not used to heat water. Instead, a conventional heating element is a more viable option. In conventional Heater, the current is made to flow through the resistance wire by applying a potential difference while in induction heater, the current is made to flow with the tune of external exciting electromagnetic wave. if you are interested you can go to https://resources.mrheater.in/2019/09/how-induction-heater-works.html for a brief understanding on how an induction heater works.

 

[essenmein]

Heatpump HW heaters do have limitations, specifically the maximum hot side temperature is the problem, from memory its difficult for them to exceed 50-60C output temp without multi stage pumps, basically good for low grade heat at best.

So they are perfect for areas that are warm already, but almost no better than a resistor if the cold side temp drops too much, good luck making them work if your outside air is -20C! As the delta T grows, COP -> 1. At COP 1 a heat pump is a very mechanically complicated resistor!

Induction water/fluid heaters do exist, but generally I've seen them used in applications where heat density is a problem, eg burning the material you want to heat. So for example in milk processing, you want to heat all the fluid to a temp (say pasteurization), but not have a energy density so high that the fluid in contact with the heat source exceeds some limit or burns. Restive elements are also sold with different "densities", if you've ever home brewed properly this would be familiar!

http://waterheatertimer.org/watt-density.html

 

[hutchphd]

Anyone who has used a Metcal soldering iron understands how spectacularly induction heating can deliver lots of power without exceeding a fixed (Curie) temperature.
The other capability of an inductive heater is lack of penetration of the pressure vessel. Most of the appliances rely on coupling to ferromagnetism so if the vessel is not ferromagnetic there is little surface heating.

 

[FJT2]

Makes sense, thanks

I understand that in theory an induction boiler can put the induction coil and the magnetic metal plates inside the tank just like the heating element in an electric boiler. The induction coil doesn't heat up, just the iron in the plates heats up. the plates don't touch the induction coil, so the space allows water to completely surround the plates. as a result they could be just as efficient as a resistive coil. The improved efficiency could come from use of a continuous flow heater that uses an Iron pipe to heat the water on demand, giving instantaneous heat, and eliminating losses due to storing heated water for a long period. Unfortunately the iron pipe may have a relatively short lifespan.

 

[sophiecentaur]

If you are talking in terms of the proportion of electrical energy supplied that provides water heating then you should really include the efficiency of RF equipment to provide the RF power. That makes the simple resistive heating solution the winner.

 

[Baluncore]

The improved efficiency could come from use of a continuous flow heater that uses an Iron pipe to heat the water on demand, giving instantaneous heat, and eliminating losses due to storing heated water for a long period. Unfortunately the iron pipe may have a relatively short lifespan.

Welcome to PF.
You may never beat an immersed resistive element, but you might come a close second.
Why not replace the iron with a thin walled bronze tube that acts as a shorted turn ?
What range of operating frequency would you consider. Maybe down to 50/60 Hz ?
Can you pre-heat water by cooling the primary induction coil and the frequency generator ?

 

[artis]

As @sophiecentaur and @Baluncore already pointed out RF heating is not as efficient as simple resistive heating due to the extra energy being drained in charging discharging the capacitance of the heating element and driving the overall reactance of the circuit.

I guess we could go further and say that even among resistive heating elements the absolute highest efficiency would be for one being fed by DC current as AC even at low frequencies still have some losses due to reactance of the of element and wires, but those are so tiny that I think it would be hard to even measure them.

 

[Baluncore]

... as AC even at low frequencies still have some losses due to reactance of the of element and wires, but those are so tiny that I think it would be hard to even measure them.

The reactance does not cost real watts, only imaginary VAR.

When you need to limit current, such as in or with a solenoid, it is better to use AC with inductance than DC with resistance. That is because the inductive reactance does not generate heat.

To heat something there need to be real watts developed in a resistive element.

 

[artis]

That is because the inductive reactance does not generate heat.

Not by itself but the extra current associated with it does in the form of heat generated within the wire. Isn't this the main reason utilities tend to care about controlling the grid power factor, apart from voltage stability ?

 

[sophiecentaur]

The reactance does not cost real watts, only imaginary VAR.

The wasted I²R power is dissipated elsewhere. "Zere ist no escape Breetish Tommy".
But my initial objection to induction heating was based on what you can buy for cooking etc.. But a beefy transformer could be very local to the heating coil so the low voltage parts of the circuit could involve less loss.
Total cost and scale should also be considered - as in the arguments for Electric Cars - (even down to the transformer and non-mass production of parts).

 

[ZeFrenchman]

I absolutely love this thread. It is one of the clearest demonstration of the complexity of "energy efficiency" discussions on one end, and of how something as trivial as heating water is in fact, a pretty deep and somewhat convoluted thing.
I am one of the few human beings alive having visited multiple electric water heater factories (these guys don't joke around their industrial secrets!) I have been amazed at how complex and counterintuitive heating water can be.
In strictly electric heating, there's no doubt that resistive wins, if for no other reason that there's little else to measure it against and for most of the reasons listed below. In no way does this mean it's the best or the most efficient, but merely that "it's the best we have for the money".
The conductive heat loss is not insignificant and (for most north-american models) hovers around 90W h. So efficiency should also relate to conservation. Or storage. Whatever energy buzzword you want to tack on there.

Efficiency also means having a durable, reliable equipment. And truth be told, the vast majority of EWH elements have grown to be absolute garbage over the past decade, leading to a record number of failures. These are generally not noticed in 2-elements tanks, or at least, not as much.

It's also helpful to know that US-made and Canadian-made EWH are not the same: they do not answer to the same efficiency standards and while they look similar there are key differences in their designs, not the least of which the number of elements and the temperature set point.

Finally, take other means "more efficient" means with a grain of salt: heat-pump EWH are incredibly efficient. They are also the worse possible idea in nothern climates... Geography counts a lot for "efficiency" and one needs to consider that the current resistive type might not be perfect, but it is the sum of all compromises and currently the one that serves us the best. Can we do better ? I sure hope we can :)

As for the OP's question, well, there are people (yours trully being one of 'em) working on induction-based water heating, not out of efficiency concerns, but out of public health concerns. Legionnaire's disease is far more rampant in EWH than most like to admit, and for areas where only electric is an option that are unfortunate to have that particular bacteria in abundance, there needs to be cheaper, more efficient solutions. That's part of what we're working on.

 

[berkeman]

As for the OP's question, well, there are people (yours trully being one of 'em) working on induction-based water heating, not out of efficiency concerns, but out of public health concerns. Legionnaire's disease is far more rampant in EWH than most like to admit, and for areas where only electric is an option that are unfortunate to have that particular bacteria in abundance, there needs to be cheaper, more efficient solutions. That's part of what we're working on.

Welcome to PF.

Can you say more about how inductive heating can help cut down on the bacteria that cause Legionnaire's disease? It seems like you are right that it can be a problem for resistive WH designs:

https://plumbingperspective.com/does-your-hot-water-heater-protect-you-from-legionnaires-disease

 

[ZeFrenchman]

Welcome to PF.

Can you say more about how inductive heating can help cut down on the bacteria that cause Legionnaire's disease? It seems like you are right that it can be a problem for resistive WH designs:

https://plumbingperspective.com/does-your-hot-water-heater-protect-you-from-legionnaires-disease

The prime issue with EWH and Legionnaire's is the configuration of the pressure vessel: with the resistive element a few inches from the bottom, there is a layer of cold water that simply never gets heated. And before someone screams equilibrium, the fact of the matter is that an EWH will typically never rest long enough (at least 3-4 days, in the best of cicumtances) to reach that state. Stratification is well known in WH but it is more problematic in electric, as that bottom layer will never get heated. The bottom is also, unfortunately, where the sludge of biofilm gets deposited, providing a superb environment for both areobic and anaerobic critters. Add to that a broken lower element, and you have yourself a petri dish of Legionella pneumophilae...

I'll add as a political statement that US-made EWH are somewhat worse in that fashion than Canadian-made EWH: in the US, an EWH is asked to "deliver hot water". So it makes sense to have 1 or 2 elements closer to the top of the tank, to provide faster recovery and more deliverability. This also means that the "cold zone" at the bottom is waaaay cold, a problem compounded by the fact that most state mandate 50°C as a storage setpoint. So, a perfect growth environment if contaminated.

Canadian-made EWH are tested to "keep water hot", meaning they want their bottom element closer to the bottom to promote uniformity, at the cost of a slower recovery time. As well, federal law mandates that water be stored no lower than 60°C.
Now, I'm cynical enough to accept that this is by accident, rather than design, but the problem exists nevertheless and the fact that EWH are now being promoted to the rank of energy storage devices in homes means we need to take a long look at that problem that went unnoticed or ignored for decades.

Gas-fired WH circumvent the issue by roasting the biofilm to a crisp with each cycle, so we are trying to achieve a similar effect with in induction-fired WH, where we can eliminate that cold bottom layer. The alternative of course is high-temperature WH, but that's a different beast and these make sens in different settings (think solar heating, energy storage, et al). Also, HTWH bring a load of new issues to the table (mixing valves, pressure vessel material, insulation, etc) and that's some distance out in left field from the original topic :)

 

[ZeFrenchman]

And on the topic of tankless WH: they certainly do help the bacteria growth issue, but unless they are gas fired, they should be considered an abomination to our not-so-modern North American power grid: it just can't take the strain of toasting your bread, making your coffee AND charging your car, let alone heating your shower at the same time... That dynamic alone warrants that we look into different EWH solutions, even at the cost of apparent efficiency, if it means displacing load on the power grid.

It's another direction to that topic, but it ties well into the conversation.

Full disclaimer: the reasoning above is only sound in my world, the frozen hell of Hoth. Not so much in warm-weather geography or where wind/sun/thermal might be available on the cheap.

 

[artis]

@ZeFrenchman or should I greet you by "Salut!"?

What you say is interesting. Where I live we use a lot of electric "boilers", almost every other home has one, as the only other option to heat water is from the heating furnace that heats the house to pass through circulation through an accumulation boiler, which is also another popular way, the third is to use natural gas.
Honestly I have never heard of a single outbreak in the past couple of decades , well I have heard about them but in fact not in association with the boilers, rather in places where water has been stagnant for a long time, like cases where owners go for a long trip or the flat/house isn't inhabited for some time.
Could it be that it is more prevalent for standing water?
I myself have a electric boiler which works exactly like you described, vertical pressure vessel, heating element resistive and placed at the bottom.

One other important thing is to clean them regularly. I clean myself either once a year or twice , lately the water quality has become better so once is enough.

I am not sure with this one so correct me but I would think the filtration of the city water supply before it is even pumped into pipelines for distribution to houses is also important, in Europe we in general tend to have strict controls over that to prevent microorganisms from building up in the pipes.
But as I said I have mostly only heard about the disease with respect to long standing water , wherever water is used regularly I have never seen a problem with it.

But it seems your logic is correct, a heating supply that heats the water and surrounding container evenly to high temp is better than one which only partially heats to high temp while leaving other parts to lower temps where bacteria can thrive, combine that with a local badly filtered supply coming from a lake or ground/river and you potentially have problems

 

[ZeFrenchman]

@ZeFrenchman or should I greet you by "Salut!"?

What you say is interesting. Where I live we use a lot of electric "boilers", almost every other home has one, as the only other option to heat water is from the heating furnace that heats the house to pass through circulation through an accumulation boiler, which is also another popular way, the third is to use natural gas.
Honestly I have never heard of a single outbreak in the past couple of decades , well I have heard about them but in fact not in association with the boilers, rather in places where water has been stagnant for a long time, like cases where owners go for a long trip or the flat/house isn't inhabited for some time.
Could it be that it is more prevalent for standing water?
I myself have a electric boiler which works exactly like you described, vertical pressure vessel, heating element resistive and placed at the bottom.

One other important thing is to clean them regularly. I clean myself either once a year or twice , lately the water quality has become better so once is enough.

I am not sure with this one so correct me but I would think the filtration of the city water supply before it is even pumped into pipelines for distribution to houses is also important, in Europe we in general tend to have strict controls over that to prevent microorganisms from building up in the pipes.
But as I said I have mostly only heard about the disease with respect to long standing water , wherever water is used regularly I have never seen a problem with it.

But it seems your logic is correct, a heating supply that heats the water and surrounding container evenly to high temp is better than one which only partially heats to high temp while leaving other parts to lower temps where bacteria can thrive, combine that with a local badly filtered supply coming from a lake or ground/river and you potentially have problems

Et salut à toi ;)

Yes, geography matters a great deal when it comes to the Legionella conversation, since areas see both various water heating technologies and exposure to the bacteria. But last we checked, not a single water utility (in North America anyway) filtered or otherwise treated water against Legionella pneumophila, a bacteria found in many water ways around the world. Frankly, I'm not certain they could even if they wanted to...

Standing water does increase biofilm, thus risk, but isn't a requirement to bacteria growth in water heaters, mostly because of the advantage provided by the mild water temperature. France has seen its fair share of challenges with this in the early 00's if memory serves.

The challenge in under-reporting and the associated issues in sampling (See Dr Janet Stout on that topic, great work) is exactly that: under reporting. Whenever an outbreak appears, it drives ressources and energy to finding the source, often times an HVAC cooling tower or some public water feature. But when people, usually a single person, shows up at the hospital with Legionaires and often enough dies from it, no one ever investiguates.

And unfortunately, once corrected for under reporting, the stats can be a lot grimmer: Dr.Michel Plante (Hydro Quebec, Canada) reported recently that he figured 7 people died yearly from Legionaires contracted from their EWH (which makes up the majority of WH in that area). Some argue that his studies take a few shortcuts, but the fact remains that somewhere between "no problem" and "we'll all die from a hot shower" lies a poorly documented and understudied problem.

 

[anorlunda]

no one ever investiguates

My best friend is retired from the US Public Health Service. He spent almost all his time in recent decades running to every outbreak to investigate.

 

[anorlunda]

My son is a licensed plumber. He says that the plumbing code in his state specified 120F (50C) as the maximum legal hot water temperature. Scalding burns are possible if hotter.

However, the legionnaires literature here says that hot water tank temperatures must not be lower than 60C (140F) to prevent growth of the bacteria.

Both can be satisfied with a mixer between tank and faucet that blends hot water with cold water to guarantee water no hotter than 120F at the faucet, regardless of tank temperature. But I've never seen a residence with such a mixer.

My son also said that using up your hot water, prevents it from being stagnant. Problems occur if hot water is stored weeks or months below 60C with no circulation. Hot tubs have that situation more often than hot water heaters.

 

[Baluncore]

All is lost without the technology to contain and heat water.
You know you are civilised, when you can make a hot cup of tea.

 

[Tom.G]

My best friend is retired from the US Public Health Service. He spent almost all his time in recent decades running to every outbreak to investigate.

And his general conclusions are?

 

[artis]

And his general conclusions are?

All is lost

 

[ZeFrenchman]

Well, that escalated quickly... ;)

 

[ZeFrenchman]

My best friend is retired from the US Public Health Service. He spent almost all his time in recent decades running to every outbreak to investigate.

To clarify: Legionnaires disease is a reportable disease in most countries with a solid public health system. This means that there generaly are investigations for reported cases, but they are generally centered around reports with more than one case for a general area as it often points to a community source (think of a HVAC cooling tower on top of a building blowing bacteria in the wind...). As literature points out however, investigating individual cases and the technical difficulty of sampling water heaters makes it nearly impossible to identify and link bacterial sources to an individual's disease.

Technological advances in DNA field analysis might help this in the future.

 

[ZeFrenchman]

My son is a licensed plumber. He says that the plumbing code in his state specified 120F (50C) as the maximum legal hot water temperature. Scalding burns are possible if hotter.

Funny enough, the consensus around the scalding temperature seems to be a correlated with litigation: a very cynical research collaborator once told me: "burn victims cost a fortune, dead people can't sue". A bit harsh, but it does seem to represent the industry's desire for lower water temperature vs preventing bacteria growth in systems. That particular aspect isn't much of an issue for residential, but it seems to be responsible for many outbreaks in institutional settings.

Both can be satisfied with a mixer between tank and faucet that blends hot water with cold water to guarantee water no hotter than 120F at the faucet, regardless of tank temperature. But I've never seen a residence with such a mixer.

True and it's certainly part of the solution. We have a number of consumer-grade high-temperature water heaters on trial here and they all have a built-in mixing valve. It does look promising.

My son also said that using up your hot water, prevents it from being stagnant. Problems occur if hot water is stored weeks or months below 60C with no circulation. Hot tubs have that situation more often than hot water heaters.

Also very, very true about hot tubs or whirlpools. They are certainly guilty of harboring nasty critters (I'm also told that it would explain in great part their disapearance from hotels and such). For the water heater, using it up might not do much unfortunately, since the pressure vessel doesn't "circulate" per se. Depending on make an model, some areas of the tank can remain fairly undisturbed, specially at the bottom, where evil lurks...

 

[russ_watters]

You know you are civilised, when you can make a hot cup of tea.

On a campfire, in a clay pot?