Sequential or Simultaneous

You have to be concerned with more than the relay capability. Most residential water heaters I've dealt with have 4500 watt elements and run them sequentially on a 30 amp, 240 volt dedicated circuit. If you run them simultaneously that would be 9000 watts. 9000 watts / 240 volts = 37.5 amps I suppose you could change out the elements to 3000 watts each, but I'm not sure you would gain a lot. The recovery time would probably be affected, since you wouldn't be putting as much heat into the top of the tank.

Bigg, your numbers match my situation exactly.

The relay is rated: FLA 30, LRA 150 and RES 40 at 240/207 volts.

My first thought was that the heaters were resistive loads so the RES 40 would be within specs, but I looked around the Web for the definition of those three ratings and could find nothing.

However, running the relay at 18.75A load in a sequential config would probably be prudent and make for a longer contact life.

What do you think?

bandership

Nothing is as economical as setting the thermostats to switch off at 60degC, and lagging the tank to its optimum thickness.

If the facility exists, power the lower element from low-tariff electricity or, better, income-based power such as solar or wind.

I just hooked my water heater into a pool pump timer. I can set two or three on-off cycles every 24 hours.

Works fine.

I just changed out my old propane fired water heater to an electric and then set it up so that both elements can come on at the same time. 40 amp breaker and 8 ga wire.

As far as only running the water heater when you need it I doubt you will save enough electricity by doing so to be worth the hassle unless you only use it once a week and know exactly when you are going to need hot water ahead of time.

Relating to the relay if it rated for 40 amps resistive and 150 locked rotor then yes it will work just fine at 38 amps.

Any time there is hot water in the tank there will be a loss of energy through the insulation. In the summertime this loss will be increased by the energy required by the AC to remove it from the residence. My object is to minimize these losses by heating water only when it is required for use.

Yes, a tankless point-of-use water heater would fit the bill but this would be an unjustifed expense.

A timer was considered, but as we do not live our lives on a rigid schedule it would turn on the water heater when it wasn't required thus creating heat that would be wasted.

What is "worth the hassle" is a subjective value. This project is "worth the hassle" to me or I wouldn't be doing it.

My push button latching relay solution has already been in operation for several months and has been satisfactory. This discussion developed when I replaced the lower heater and shortly thereafter discovered a defective upper thermostat. When wiring everything back up I discovered the sequential heating scheme and began investigating the possibility of switching to a simultaneous scheme, the advantage of which is quicker heating at a cost of heavier load (and possibly shorter life) on the latching relay. Since the goal is economy, at this point I am tending toward the sequential scheme to extend the life of the relay.

I am also considering adding an audio indicator to let us know when the hot water is ready for use (yet another interesting project), as the on-time will vary depending on if the water has been warmed by a previous use. If one starts using the water before the latching relay drops out (due to the water being completely heated) then more than just one tankful of hot water will be heated (recall we only need one tankful at a time), lowering the economy factor. It should beep when the relay (DPST NO 240vac coil) drops out and shuts off the heater. Any ideas on how to do that?

bandership

You should definitely make sure that you have properly functioning heat traps on the inlet and outlet to reduce heat loss from the thermosyphon effect - and insulate up to them.

Most electric water heaters have a high temperature limit switch. Which controls a relay that provides power to both heat element relays. This relay may not handle the current for simultaneous switching. A water heater with two 4500 watt heaters that was bought running elements sequential. This high temperature limit relay is usually only rated for 30 amps.

Switching the heater off if being used on a daily bases will not provide economical savings. Most are pretty well insulated. Over a 12 hr period they would loose some heat but not all so the water would cool some. It would not cool to room temperature though. So turning on after 12 hrs would just be adding heat back that it lost. All most the same amount energy that would have been used if it would have been left to keep the water hot.

I did say almost. The greater the temperature difference inside to out the more lost energy. Best way to be economical is to lower the difference and to provide additional insulation.

The are safety issue when heating with both elements simultanously.

The expansion rate (pressure build up) could cause the relief valve to open (water discharge) and the current draw may overload the wiring and/or circuit breaker or other overload protection such as fuses.

Yar

Where I live the electric company has a power line signal controlled energy saver option that could cut out at peak demand times. This option can save you a set amount each month. Other energy savers are...

1. So first the thing to do is to maximize the tank capacity by flushing it out at the bottom drain. Mud collects in the tank bottom over the course of several years.

2. Next, using 3 inch glass fiber insulation double wrap the tank and as much of the exit pipe as possible. Double because the stuff does compress quite a lot. Be sure to leave a "flap" for access to the thermostats. By the way this cut the temperature in the closet by about 15 degrees. Money saved.

3. Then considering the longer lasting storage temperature with the insulation, I decided to do some time control. Added a timer to only allow heating from about 5:am til 10:am on the upper element (usual shower hours). Testing this found that water would stay adequately warm for showers all 24 hours, unless laundry was done. So moved the lower element off the timer and to kick in anytime it got below about 110 degrees, and the upper also at 120 but on the timer.

I have a demand type unit bought at a flea market for $5. but decided it was not worth the bother to hook up.

We had a cottage with an electric water heater. It was wired sequential.The upper element was a higher wattage, and since hot water rises, it would provide a quick heat to a small amount of water. The bottom heater was a lower wattage and would slowly heat the rest of the water up, now providing enough hot water for a bath or doing the laundry. We had 5 kids - it was too slow. We replaced the lower element with the same wattage as the upper and we got faster recovery for a full tank.

Since the cottage was only used every few weeks, the heating was shut off until we returned. A faster recovery of the full tank was much appreciated.

For our main residence we have a commercial gas fired water heater - 75 gal US. We can take 3 simultaneous showers and get no complaints. Recovery is about 20 minutes.

Have you cleared this with your insurance carrier, in writing?

Seems to me thay'd not likely cover a modified electrical device.

Thank you everyone for your very helpful replies to my query.

Some points were well taken, such as the capacity of the thermal overload shut-off switch (no relay involved in my residential unit), just downstream of the electrical supply. Since the control unit is designed for sequential operation I should (for safety sake) assume that the thermal overload switch is designed to handle the 4500W load required for sequential operation, not the 9000W of power drawn in simultaneous operation. This factor by itself is enough for me to use the default sequential scheme.

So that's what I will do.

Some other points that were mentioned:

Several folks claimed that my efforts at economy were "not worth the trouble" because of a low payback despite the fact that there were no actual measurements cited to determine just how much energy savings we are dealing with. I thought of measuring energy usage by hooking up hour meters to the heating elements but it was "not worth the trouble".

Draining the heater did not remove the debris I found in the tank when I removed the lower (defective) heater element. There was no mud, but there was a large amount of whiteish granular material which I presume to be calcium. It was piled right up to the burned out heating element and might have restricted water flow around the element causing heat buildup inside the element so as to cause the failure. This material did not leave with the water draining. I was able to remove about 80% of it with 1/2" hose and a shop vacuum.

Switching to simultaneous operation will not necessarily heat the water up to a higher temperature than sequential operation so there will be no change in the degree of expansion in the tank inherent to either scheme.

Insurance company approval is a good point but sometimes can be unsafe. An excellent example was in the wiring of our condo's electrical outlets. In the early 1980's, when the condo was built, push-in wire connections to the duplex outlets were apparently approved because that is how the condo was wired. A couple of years ago the outlets in the master bedroom inexplicably went dead. Troubleshooting revealed that one neutral connection in one duplex outlet in the string had developed a bad connection in the push-in connection. The heat produced actually turned the white plastic insulation black near the outlet. It was our good luck that we did not have a heavy load, such as a space heater, on that circuit or a fire may have started. I opened all the outlets in the condo and changed all the connections to the screw terminals.

Again, thank you everyone for your input, it was most useful and enlightening.

bandership

Re your comment:- "Draining the heater did not remove the debris I found in the tank when I removed the lower (defective) heater element. There was no mud, but there was a large amount of whiteish granular material which I presume to be calcium. It was piled right up to the burned out heating element and might have restricted water flow around the element causing heat buildup inside the element so as to cause the failure. This material did not leave with the water draining. I was able to remove about 80% of it with 1/2" hose and a shop vacuum".

That debris is most likely the detritus from the sacrificial anode.

All glass lined tanks have one of these to protect the steel tank from corrosion.

The anode, which is normally a Magnesium alloy, but could be Aluminium if the tank was intended for use in a hard water environment, will erode more rapidly in areas closest to uninsulated fittings such as elements, pipe fittings, or holidays in the tank lining.

I would suggest that you check the anode for erosion and replace it if needed, but at the very least clean it. You can usually access it via a removable cover in the top of the outer skin, sometimes they can be a bit hard to replace if the ceiling above is a bit low.

Here is some info on saving energy on a water heater on this site.

http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

I did notice that temperature in excess of 140º F will cause some minerals to precipitate out of the water. Which could be the white sediment.

There is a page about using a timer. Don't think I could keep to a schedule that well.

Problem that I see with the information on that site is that the recommended temperatures for controlling Legionella are too low. Sure 130°F will kill it in 5 to 6 hours, which is OK for those germs in the tank, but the real problem with Legionella is that it is transmitted as an aerosol, and where better for that water to become an aerosol than in the shower head. Every time you turn your shower off, the water left behind in the piping and especially in the rose becomes an ideal breeding place for these beasties.

Legionella will thrive in water temperatures ranging from 65 to 125°F. Water at 140°F will kill it in 30 minutes and in just 2 minutes at 150°F. Of course, you need to balance the ideal killing temperatures against the dangers of scalding, and I am not suggesting that you set your thermostat to 150°, but for effective control you should not set it below 140°F, and you should regularly submit your shower head to high temperature and run it on hottest for a bit before getting in there.

Economy should take precedence somewhere behind personal safety, you're a long time dead.

Do you have a phobia to germs? Because they are all around us. Some good some bad. You just picked one. What of all the others? I'm sure we can find one that thrives in water above the maximum temperature of a domestic water heater.

If this bacteria is present in the water supply. Just increasing the temperature of the hot water heater is not going to resolve the problem. As you state the shower head would be a good means of dispersal. I do not think any one takes a shower with 160º F water which would kill it instantly. So it would also be in the cold water supply used to lower this temperature. Along with all the rest of the germs.

Also we have no idea of his water source. If from a municipal water supply it's being treated.

Hmm. Perhaps you should read up a bit more about Legionella and you may just change your previous opinions.

Have a look at this site, pay particular attention to the temperature ranges where it thrives, the very high amount of chlorination required to kill it, and the number of deaths in the US alone.

Legionella is a particularly nefarious bacteria, and the reason that I picked that one is because of its affinity to hot water systems.

I have and most of the reported major outbreaks of the disease are from other sources then the hot water systems.

I have been using a 60 gallon hot water tank to heat the house for the last 27 years.It is unpressurized and using dead water.

Both elements are working indepedently so I have 9000 watts of heat available.

As the thermal cutout and the thermostats are rated at 30 amps,I had to use two 30 amp breakers, one for each element.

Each pole of the thermal cutout is controling 1 element so it fit trips, both elements are shutoff.