Load Calculations Tankless Electric Water Heaters

[JFletcher]

As far as I know, Legionnaires disease is an extremely rare condition resulting from Air Conditioning or Chiller water coil runoff. I have never heard of it resulting from a hot water tank or heater system, though not saying it is impossible.

I am most definitely suggesting a tank system, however if they are willing to pay for the service to run tankless water heaters, and live with the limitations that I already mentioned, mainly that if electricity goes out they will have no hot water at all, then go ahead and wire it up that way and put in the tankless units. Also, if they ever want a generator installed to run the barracks, they will need an industrial sized one just to have hot water.

[winnie]

Can you guys give me a reason why tankless electric water heaters would be such a bad idea? I'm just not seeing it and would rather seem like an idiot to you guys and ask this question here than to put it out to the military and have it not work.

As has been noted, the big problem with electric tankless water heaters is their _huge_ power demand. Many hot water users do not have continuous demand, so by using a storage (tank) type water heater your electrical demand is closer to the average power consumption.

Say that you need 1200 GPH for a half hour at a time, every 4 hours. If you have a 600 gallon tank, then you can spend 4 hours heating it up, rather than having to heat it up in 1/2 hour. You use the same number of kWh, but at 1/8 the kW. Smaller service, smaller wires, etc.

However, if the requirement is 1200 GPH of water on a continuous basis, 24x7, then the above benefit goes away. If you need 1200 GPH, every hour, then you need enough thermal power available to satisfy that requirement.

To meet that requirement you need about 233 kW of thermal power, which means 233 kW of electricity if you are using resistance heating.

You started this thread asking about demand factors, which is another way of comparing the average to the peak consumption. If the requirement is 1200 GPH, then your demand factor is 100%. If you were told that you need to supply 1200 GPH 25% of the time, then your demand factor would be 25%, but... With electricity the 'time constant' is very short; even if you _know_ that average consumption is only 400 GPH, if you design for this and by chance all 1200 GPH gets used, you will have breakers tripping in 10s of seconds to scant minutes.

This is were a tank comes in; by sizing your tank you can design the 'time constant' of your system. If your actual usage is only 400 GPH _on average_ but your peak consumption is 1200 GPH, then a tank large enough to run for the longest expected 1200 GPH period would let you cut down on the peak electrical power consumption.

As far as the choice between electric resistance heating or burning oil to provide the thermal energy, the question is 'where is the electricity coming from'?

If you are burning oil to run a generator to make the electricity, then keep in mind that the efficiency of that generator is somewhere around 40%, meaning to get your 233 kW of electrical power you are putting about 580 kW worth of fuel into the generator (in terms of heat output). So burning fuel directly to heat your water means you use quite a bit less of the stuff.

You can, in fact turn this equation on its head. Use a 200 kW diesel genset. Heat your water with the waste heat coming off the genset. Export the electricity to other users. Probably a hell of a lot more in the way of design hassle, and well off tangent of your original question!

-Jon

[Ingenieur]

not enough info
how many showers
how many users
in what time frame
???

you say you need continuous 1200 gph
so 20 gpm will do?
or will the 1200 be used in 20 min? 60 gpm
you need a recovery rate of 1200 gph?

the 2 heaters you selected will only generate 250 -300 gph
you would need
0.10 x 37 + 0.90 x 120 ~112 deg at 1200 gph
so you need 0.9 x 1200 ~ 1100 gal of 120 deg water
the 40 kw makes 140 so 1100/140 = 8 heaters

[deb4523]

the 2 heaters you selected will only generate 250 -300 gph
you would need
0.10 x 37 + 0.90 x 120 ~112 deg at 1200 gph
so you need 0.9 x 1200 ~ 1100 gal of 120 deg water
the 40 kw makes 140 so 1100/140 = 8 heaters

Sorry - I misunderstood the table. There are two numbers per size per temperature rise. I was assuming that there was one number for single phase and one for three phase, as there is in the following table. But now I see that it is likely one for gallons and one for liters. That definitely gets rid of any benefit of having the electric water heater tanks as far as power consumption goes.

you need a recovery rate of 1200 gph?

Yes.

Now that I've thought on this some more and have heard back from you guys, I think the real problem with a tankless system is that a recovery rate of 1200 gph is not the same as providing 20 gpm hot water. They may need more hot water at a given time, and then the heaters would reheat that amount within the hour.

[kwired]

So you are suggesting electric water heaters with tanks?

My understanding is that the requirements are different with tanks because of legionnaire's. This is second hand information, but I was told tanks require the water to be held at a higher temp (I think they said a min 140F) and then mixed to cool it back down for service so that legionnaire's disease doesn't grow. If we don't use a tank, we don't need the extra heating.

The requirement is for 1200 gph regardless of when the tenants actually use it. Any tank system would have to have the ability to reheat and supply that amount hourly. Not everything makes sense around here.

It looks like this product may work:
https://www.hotwater.com/water-heate...e/gold-xi-dve/
If I used two of the 40.5 kW tanks it would have a 225A full load current and satisfy the requirements.

I know nothing about legionaire's, but if you have a circulating pump aren't you still maintaining a ready to use temp in the lines anyway? Plus storage tanks are used all over the place and you seldom hear of health issues from the water that comes from them, and if you do it often is water quality issue of the incoming water anyway.

In Alaska I you won't have much "wasted heat" issues as long as the tanks are in a space that needs heating anyway, any heat they do lose is just less heat the HVAC system needs to produce.

[Russs57]

What is being used for heating for the building? If there is a boiler that brings more options to the table.

If you are on a municipal water supply chlorine levels should negate legionnaires concerns.

[petersonra]

What is being used for heating for the building? If there is a boiler that brings more options to the table.

If you are on a municipal water supply chlorine levels should negate legionnaires concerns.

while it might also help with some pathogens, chlorine is mostly used to control algae growth in potable water systems.

[kwired]

while it might also help with some pathogens, chlorine is mostly used to control algae growth in potable water systems.

Kind of what I get out of what searching I have done is that chlorine is effective, but keeping a sufficient chlorine level isn't necessarily easy, especially on warmer water applications.

But at same time there is millions of storage tank water heaters and this doesn't seem to all that big of a problem. I can see the issue being in the water source. Ground water is probably low risk. Drawing water from surface sources I can see being a higher risk.

[Ingenieur]

while it might also help with some pathogens, chlorine is mostly used to control algae growth in potable water systems.

chlorine is the primary disinfectant
mainly for ecoli/fecal coliform and prevent the spread of waterborne diseases such as cholera, dysentery, and typhoid.

[sameguy]

Hi,

We are working on a building remodel for barracks. Our requirements for hot water are to provide 1200 GPH, because it is assumed all residents will be showering at the same time. In reality, the residents are on rotating shifts and the most that would be showing at a time are 30%.

The question is this:
If we have tankless electric water heaters to provide the full demanded amount of hot water but there is never a scenario when they would all be running at the same time, can I apply a demand factor to the load for load calculations? If so, what numbers would I use?

I am looking at approximately 650A Full Load Amp Draw at 208V, 3 Ph.

Thanks!

If it's a spec. of the bid then that's what you have. Design/build you can explore more ideas.
How is heat for the mess, buildings, quarters, etc, generated? Scavenging waste heat.
How many showers are there total 8,12,32 that would be the true water flow.
Good luck!