Electric water heater questions

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richfloe
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Electric water heater questions

Post by richfloe » Sun Dec 10, 2006 11:19 am

Tired of being ripped off by my natural gas supplier, I have been thinking of replacing my gas-fired water with with an electric one. My cost for gas is high enough that electric will be cheaper! (Yes, I did the math)

My current water (gas) is 40 gallon capacity with 34,000 btu input. It barely keeps up with my wife and daughter who like long, hot showers but they are getting by. With the slow recovery time of an electric heater I figured life would quickly become unpleasant for me when they ran out of hot water. I don't have room for a much larger heater.

I have connected a couple of electric water heaters in years past and they both said "Upper element 4500 watts, Lower element 4500 watts, Total 4500 watts" which doesn't seem to make much sense, obviously they only run one element at a time. Why or how it makes the determination of which element to run I don't know but curiosity is getting to me. On two of the water heaters I had installed, I wired a 30a 240v circuit which obviously would not be enough for both elements so I know that it must only be running one at a time, one of them I had even measured the current draw, it was 19a just as you would have expected for 4500w at 240v.

Now for the real question... Aside from needing a 50a supply and doing some rewiring, is there any reason I can't have it turn on Both of the heating elements at the same time to heat the water? To rewire the unit to do this is a simple matter for me, what is need to know is if there is some reason having both elements on is a problem.

I have looked at some commercial higher-powered units but didn't find anything that I thought was a good option for me. Using a common (and reasonably priced with replacement parts easily available) comsumer model is the direction I would like to go rather than some special order unit.

It seems to me that if I can run both elements at the same time, the electric heater will equal (if not surpass) the heating performance of my current gas-fired water heater. The women will remain happy.

Comments and suggestions welcome.

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Post by bigkim100 » Sun Dec 10, 2006 11:41 am

Well, now you can look foreward to being ripped off by your electricity supplier.
:???: Kim
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Chris Smith
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Post by Chris Smith » Sun Dec 10, 2006 12:55 pm

Have you ever heard of boom, followed by a melt down and a fire?

ITS Stupid, the heater has limits [pressure relief valve from the excess heat] and now you want to play with them?

Ever seen a HW heater hit the moon?

Quite impressive if the pressure valve sticks, waste all the excess heat and hot water if it works. And its illegal.

If heat is the issue, run an instant hot water heater into the units cold water supply, it uses up a little more power when its on but it will place a large warm supply of water into the heater, then the heater can keep up with not having to deal with warming up all the cold water supply from scratch. [And some are adjustable to set for warm, not hot]

You can even switch it for shower usages like night or morning timers, or not.

And they are small.

The smaller, possibly the better, as you are not trying to make hot water, just trying to supplement the incoming cold.

bodgy
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Post by bodgy » Sun Dec 10, 2006 1:18 pm

Do you have electric showers in the US? This would answer your problem.

Instant hot water for the shower - just need a fused electrical connection + mains pressure cold water.

Unfortunately they're illegal in Australia (can't see why). You can even get fancy electronic ones that can have electronic controlled heat output and 'personal' settings.


Colin
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markp
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Post by markp » Sun Dec 10, 2006 2:03 pm

The water heater "usually" uses the bottom element because that part of the tank will get cold first. When a lot of water is drawn, the top of the tank will get cold and the top element will be used. ( Note: If the top element fails, you will run out of hot water and NOT switch back to the lower element )

If you can get at the shower head water supply, an "on-demand" heater mounted in the wall would be a good bet. It would be supplied by the hot water tank and not be used until the tank ran cold.

Not using both tank heating elements is strickly a safety issue. ( That same issue exists to a lesser degree with only one element. )

P.S. It is a good idea to check the resistance of the lower heating element occasionally. If it fails, you would not normally notice because the heater starts using the upper element.

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Lenp
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Post by Lenp » Sun Dec 10, 2006 2:32 pm

markp, you're right on target!

By way of a little more info, the HWH draws the water from the top of the tank. It also uses a long pipe (dip tube) inside the heater to provide the cold water to the bottom of the tank.

Because of stratification, the top water will be hotter than the bottom. So, when the top reaches the setpoint, the thermostat switches power off of the top and onto the bottom thermostat and element to 'finish' the tank heating and reduce the stratification.

If the dip tube breaks off, and they can, then the cold water is dumped in with the hot water at the top of the tank and the heater is unable to satisfy the demand.

Safety issues about connecting the elements in parallel are mostly regarding the current capabilities of the controls and wiring. Any HWH can over pressure if it is set way too high or the controls fail. That's the reason for the T&P (Temperature/Pressure) safety valve.

Efficiency may also falter with parallel elements since there will still be stratification in the tank!

Quick recovery heaters either gas or electric, have no reserve capacity. They use enormous amounts of energy, short term, to heat the water on-the-run to provide a constant supply. They are available both as point of use as well as central units. I have had no experience with the central units, but we used point of use units school shops that were distant from the central boilers to provide hot water for handwashing. Seemed to work well, but I'm not sure about maintenance issues.

Clear as mud, huh?

Len

richfloe
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Post by richfloe » Sun Dec 10, 2006 3:32 pm

Thanks for the quick responses.

I did the math, it will be cheaper to use the electricity. My gas supplier recently raised the "customer charge" to $13.50/mo, I pay this just to be a customer! The public service commission is going to force them to reduce it but then allow them to raise it over the next couple years. This charge is over half of the what the cost of the gas is! Yes, 25 years ago it was cheaper to heat with gas, while that may still be the case in some places, it sure isn't around here. My cost per KWHR varies from about 3.5 to 7.5 cents (depending on usage).

I had looked into the point-of-use heaters but the only ones that would provide continuous heated water were gas-fired with btu inputs of 100,000 and up. To do this with electricity would have required 100a at 240v, obviously not a real practical solution in a residential environment. The only electric point-of-use units I find are for washing your hands or making instant coffee, they have very small tanks and wont last for more than a few minutes at a time though one hand-washing unit would flow continuously and at least warm the water to a comfortable level, nowhere near enough for a shower.

Chris, I wasnt going to be bypassing the thermostats or anything silly like that, I just wanted to increase the power input. Just what about that would be illegal?

I see that stratification is perhaps going to be my biggest issue, I would have to see if it is possible to put a couple of sensors into the tank to monitor the temp and switch to only the lower element when the water is hotter at the top (but still needs heating). It would seem to me that when there is water flow and the discharge temp is lower than desired that it would ok to run both elements.

Thanks again for the responses.
Rich

bodgy
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Post by bodgy » Sun Dec 10, 2006 4:11 pm

This was the type of thing I'm thinking of - electric shower

Constant and instant (about 3 seconds) hot water.


By the way my water meter project could be altered to being a shower timer - connect to an electric valve and the water shuts off after a predetermined time.


Colin
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Chris Smith
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Post by Chris Smith » Sun Dec 10, 2006 4:13 pm

Bodgy, in West Australia that’s all we had was the instant type of heater.

It was the 240 type [standard wall power] for the whole house including the shower.


richfloe

http://www.plumbingsupply.com/eemaxso.html

http://www.spec-net.com.au/company/zipindustries.htm

http://www.rewci.com/whhohotwaci.html


In our local electrical outlet shop we have several 120 and 240 volt instant heaters that can even run off of an extension cord, [small draw] ... all the way up to the larger types and 240 v that are hard wired.

Because your not trying to shower or use them as a total hot water source, the small ones will preheat your water heater intake just fine.

And not all use that much power especially when specifically switched for time of shower only.

That plus the fact that only when the water is used, the instant heater turns on to the other heater inlet while shower time switching can restrict all other usages.

This will keep ahead of the drain and load with out breaking the bank.

Over heating the common HWH will simply blow the extra water temperature out through the excess pressure valve which is a total waste and dangerous if it sticks..

Yes and even the small instant heaters may need a little more wall power to the general area.

The whole idea of the instant heater is to raise the general cold water temperature into the main hot water heater from cold to luke warm.

This is half way to a shower.

Raising it 20 degrees from the taps cold temperature means that the hot water heater doesn’t have to start from scratch.

You can also use a by pass pipe parallel to the heater to mix in cold and hot water at the same time into the main heater so that the average water temperature is hotter than the mains.

Valves can be used to adjust the inward water to be raised to any given temperature, making the main HWH receive a warmer temperature and not just cold water, thus being able to keep up its out put in the hot region almost indefinitely.

A hot water heater must take complete cold water and raise its temperature to the showers out put heat and then maintain this hot water flow indefinitely?.

With WARM water entering the heater, its job is vastly reduced [especially during showers] and it can keep up the out put of hot water indefinitely during showers.

Showers are only at 100+ degrees [109 is real hot] so from the pipes inlet temperature to the hot water out put temperature is not that hard to maintain this way

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dacflyer
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Post by dacflyer » Sun Dec 10, 2006 8:51 pm

they do make electric tankless water heaters also..they never run out of hot water.. friend here got one for his house.. it stores no hot water..but makes it on demand..
the ones in europe are the same way...but almost all of them are gas powered.
but they do make them here in electric versions.. the one my neighbor has,, uses 2 heater elements.

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haklesup
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Post by haklesup » Mon Dec 11, 2006 4:50 am

Modifying any HWH is ill advised because if in the unlikely event it does go bang, your insurance company could deny your calim and god forbid if anyone were injured you could be held criminally neglegent. An unacceptable risk IMO

Second, the math could change next year and NG or propane could become more cost effective. In my city a permit costing $141 is required for replacement HWH plus the $250 or so cost of the heater. Your savings on electric had better be darn good to payoff that in the lifetime of the heater (warranty plus 1 year). You'de have to save about $50 per year to break even on a 7 year (warranty) model using my numbers. Did your math inclule a term for ROI? Teflon tape the anode when new so you can replace it (easily) in 4 years and the numbers get better cause you can now expect 10-12 years out of the tank with proper PM.

A little trick that works great for me is to run the hot water for 2 minutes then wait 15 minutes before showering or filling the tub. This works best if your hot water pipes are insulated the whole way.

Running the hot water until it just reaches the tap (the 2 minute part) does two things. It fills your hot delivery pipe with hot water thus increasing the system capacity by that amount and preheating the pipe so that fresh hot water is not cooled by the pipe. Also it introduces cold water into the tank causing it to fire up and heat the full volume (15 min or so) to max temp thus requiring a leaner mix of hot to cold when you shower. (I set my tank at about 80% of max, hotter than normal but not all the way up)

Without this trick I can't fill my big tub without running cold (which takes over 2 hours to recover) but with the trick, I can completely fill the tub and have a bit in reserve if I want to top it off. Without depleating all the hot water, the tank reheats in just over 1 hour. I have a 40gal tank.

I just repiped the house and the trick works even better now because the volume of the hot pipe is over 2x what it was with those rusty 40 year old galvinized steel pipes and the copper holds less heat than the thick walled and rusty steel thus robbing me of less heat when I first run the tap. (not to mention I upgraded the insulation on the pipes at the same time).

Next I'll add an on demand (button activated, not timer or passive) HW recirculator so I can prime the pipe without wasting water either. Just a simple pump between the hot and cold risers at the furthest tap, no seperate return line to waste energy or expensive copper.

For icing on the cake, an indicator (like an LED in the bathroom) to tell me when the HWH is firing so I can know the exact moment when it is finished and the water within is at its maximum temp. I haven't worked out how to do this on a gas WH yet though but that shouldn't be too hard. Would be easy with electric, just a coil of wire around the hot wire (like a clamp on ammeter) and a comparitor etc etc.

Nothing is free, even preheating the cold inlet with waste heat from waste grey water (presumably from the shower drain) has a significant cost in plumbing supplies to make a heat exchanger and redirect the drain flow.

A passive solar inlet preheater is the most practical solution I know of to reduce HW costs. Every Calorie/Joule/Watt/Therm (take your pick) you can put into the water before it gets in the tank is one you don't need to pay to the gas man. Winter is naturally worse case because inlet temps are at their lowest (challenging marginally powered tankless systems) and any passive solar system is robbed by radiational heat loss not to mention a short day. A passive system can be made at home for a cost less than that of a HWH even with a plumbing permit. If you have a greenhouse window or sun portch put it indoors.

A storage tank ahead of the WH would help extend the solar benefit into the night. In such a system, the solar heated water is seperate from the inlet cold water. water heated from the collector is recirculated into the storage tank (via gravity convection or a pump) and the inlet cold water is passed through a heat exchanger inside the storage tank. The water in the solar loop can contain antifreeze or brine to increase the heat content and prevent freezing on really cold days. If you just passed the inlet cold through the solar collector, you risk freezing on cold nights when you're not home to run the water.

IMO, if you really want to save gas, this solar option is actually practical and has a reasonable ROI depending on the collector size and efficiency, unfortunately copper pipe is at an all time price high right now (1/2" type L going for over $1.80/foot recently). With such a low pressure split system, you would not need to use copper, plastic tubing, PVC or black ABS will work as well at a fraction of the price. The inlet pipe must be capable of full house pressure and cannot leak inward to be contaminated by the water in the heat excahanger. Satisfying that last bit is the hard part in complying with plumbing code.

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Lenp
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Post by Lenp » Mon Dec 11, 2006 5:36 am

Another thought..

In large buildings a domestic hot water recirculating loop is used. A small pump moves hot water from whatever heats it, circulates it around the building and returns it for reheating. The loop is tapped to provide hot water to the fixture using as short a path as possible. A time control sometimes turns off the pump when the building is closed. The benefits are reduced time to wait for the hot water thus reducing waste water and reducing storage tank stratification. With efficient pipe insulation I guess there's still some energy waste but maybe it's tolerable

Len

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Post by k7elp60 » Mon Dec 11, 2006 5:37 am

Another solution that I don't think has been addressed is restrict the flow of water to the shower head. I have done this and I never run out of hot water with a gas water heater. Even in a travel trailer the water heater is small(something like 6 gallon). Once the water is hot and with the proper shower head taking a shower in the trailer can be about 15 minutes.

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haklesup
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Post by haklesup » Mon Dec 11, 2006 6:59 am

In large buildings the closed loop hot water recirculator is a practical necessity because there can be dozens of gallons of water standing betwween a tap and the tank. I do believe it is also required in hospitals for sanitary reasons but in a residential setting, such a system can be a huge waste of energy.

With intermittant or continuous use, heat just bleeds from every uninsulated spot and even insulated pipe looses energy, just less. Heat is also lost as it conducts up to the metal fixtures and into the room. As in my post, I highly recommended an on demand system without a dedicated hot return to the tank. You simply don't want hot water in the pipe when you're bnot using it. Recirculated water is injected back into the cold supply and only when desired. The only drawback is the first glass of cold water may be tepid or warm. A good system shuts down as soon as the hot water reaches the pump so as not to fill the cold line with hot. This works out because people rarely need hot and cold water at the exact same time

Low flow heads and taps are the first line of attack (and thus are required by plumbing code and other laws on all new fixtures). Conservation of all water and reduced use of hot will certainly reduce energy usage. Unfortunately lots of people like a gusher when they shower and I see lots of NG posts asking how to remove flow restrictors and rarely if ever see ones asking where to find lower flow fixtures. This also does not help with a bathtub, just makes it take longer to fill.

And since wives and daughters often don't pay the gas bill, getting them to sign off on lower flow (than required by code) is an uphill battle. On the other hand, if you just go on and change out the shower heads, what they gonna do, go natural, I doubt it.

If you also heat with gas, your money is probably better spent adding insulation to the attic or crawlspace rather than switching to electric WH. Insulated curtains or window inserts also work wonders but make gloomy time of the year even more so. With so many ways to conserve energy, I have to stop myself digressing into that topic.

And about that math you did, how? Did you convert watts to therms and compare? what unit did you convert to make the two comparable? Did you do it with a particular heater in mind (so you could know its efficiency) or did you assume the efficiency was the same as gas. I suppose the easiest way would be to compare the energy star rating stickers in the store on current models then scaled that for your actual cost of electricity or gas.

In CA, electricity (and NG too for that matter) is priced in tiers, the more you use, the more expensive each kWh gets. Right now it ranges from $0.12 to $0.34 per kWh on my bill (which reaches a bit into the 300% above baseline usage range). Did you factor this in or are you a lucky one with flat rate or time of day rates.

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Chris Smith
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Post by Chris Smith » Mon Dec 11, 2006 7:56 am

It barely keeps up with my wife and daughter who like long, hot showers but they are getting by.


Assuming this is your main goal, preheat the intake to let the heater keep up with the girls. Its a One time usage that will cost any way.

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