Load Calculations Tankless Electric Water Heaters

[deb4523]

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!

 

[ramsy]

Military installations require practice drills, training excercises, and other roll-call events that require everyone to be present and accounted for, at the same time.

Not a good idea to force 2/3 of tenants to be present for roll-call inspections without a shower.

 

[junkhound]

Our requirements for hot water are to provide 1200 GPH

at what temp rise??

Every millitary contract I've ever worked they want what they ask for, you get no favors for telling them different unless it will IMPROVE performance AND REDUCE cost at the same time.

 

[JFletcher]

I cannot think of a worse application of tankless water heaters than a ton of people that need to take hot showers at virtually the same time. 1200 gallons per hour is only 20 gallons a minute enough for maybe 30 showers with a 50-50 hot / cold mix. maybe that's all the barracks needs, I don't know it's size.

no you cannot put a demand Factor on this, you're going to need to size the service for the full calculated load. pretty much all other options are better than a tankless set up here... Gas powered water heaters if available, running a commercial type water heater at a high temp and using mixing valves to bring the hot water back down to a hundred and twenty-five degrees, half a dozen 80 gallon residential type water heaters... I take it natural gas is not available otherwise the on demands would be gas-fired

 

[Ingenieur]

how many showers?
you need to size demand at 100% based on water supply fixture units wsfu
this must be done for all fixtures

and make it a recirculating loop
agreed pos or instantaneous are not good for this application
you want industrial heaters with circ pump(s)

 

[deb4523]

Ok, Thanks guys.

 

[petersonra]

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!

650A at 208V is 230kw or just under 800,000 btu/hr. That will get 20 gpm 50 deg F water to about 130 Deg F. So you mix it 50/50 and it is about 40 gpm at 90 deg F. That might be enough for 8 guys to shower at the same time.

 

[Ingenieur]

a low flow shower head is 2 gpm
so 40 gpm would supply 20 heads
???

 

[petersonra]

a low flow shower head is 2 gpm
so 40 gpm would supply 20 heads
???

maybe in your world but no one leaves those stupid washers in place. they take them out so you can have a real shower.

 

[Ingenieur]

maybe in your world but no one leaves those stupid washers in place. they take them out so you can have a real shower.

federal facility
destruction of gov property
felony

 

[ramsy]

650A at 208V is 230kw or just under 800,000 btu/hr. That will get 20 gpm 50 deg F water to about 130 Deg F. So you mix it 50/50 and it is about 40 gpm at 90 deg F. That might be enough for 8 guys to shower at the same time.

Alaska in winter can drop below 32°F where pre-heated rooftop thermal arrays may freeze and burst overnight, and a 1200 GPM tankless system would require its own electrical sub-station with no energy to spare for lights.

If insulated hot-water tanks only need capacity for a few minutes @ 1200 GPM, then re-heating water can take 8-hours between drills, and require less power.

1200 GPM x 5 minutes =
1) 6000 Gallons of hot-water tanks
2) 3000 Gallons @ 50/50 Hot/Cold mix
3) 1500 Gallons with 50/50 + low flow shower head @ 2 gpm

 

[JFletcher]

Hey didn't even see that the original poster is in Alaska. That makes the idea of tankless water heaters doubly stupid, having to bring water that is near freezing up to 120 +. They could install 9 80 gallon residential type heaters that would have roughly 144 amps per leg draw versus 650 of a tankless system. But hey, military intelligence isn't an oxymoron for nothing.

Ramsy, original poster stated 1200 gallons per hour, not minute. And it's not stated how long that flow rate needs to be maintained. 720 gallons of hot water at 140 degrees mixed 60/ 40 with 40 degree cold water gives approximately 1200 gallons at 100 degrees

 

[Ingenieur]

Alaska in winter can drop below 32°F where pre-heated rooftop thermal arrays may freeze and burst overnight, and a 1200 GPM tankless system would require its own electrical sub-station with no energy to spare for lights.

If insulated hot-water tanks only need capacity for a few minutes @ 1200 GPM, then re-heating water can take 8-hours between drills, and require less power.

1200 GPM x 5 minutes =
1) 6000 Gallons of hot-water tanks
2) 3000 Gallons @ 50/50 Hot/Cold mix
3) 1500 Gallons with 50/50 + low flow shower head @ 2 gpm

GPH not GPM

 

[ramsy]

GPH not GPM

Ok, redo:

Assuming 5 minute showers: Substitute any # for 5.
1200 GPH = 1200/60 GPM = 20 * 5 Gallons Hot Water

1) 100 Gallons of hot-water tanks
2) 50 Gallons @ 50/50 Hot/Cold mix (Store 100G @ boiler room temp)
3) 25 Gallons with 50/50 + low flow shower head @ 2 gpm

 

[Ingenieur]

I use something like these for hotel, locker rooms, etc

2 heaters/boilers
1 storage tank
recirc loop

http://www.hubbellheaters.com/downloads/oms/model_v_om.pdf


assume 20 showers x 2 gpm hw x 10 minutes = 400 gal
2 x 60 kw, 80 deg rise 300 gph each
operate in sequence based on demand
400 gal insul stg tank
100/400 ~ 25% replentished in 10 min shower cycle
100% in 40 min

 

[ramsy]

I use something like these for hotel, locker rooms, etc

2 heaters/boilers
1 storage tank
recirc loop

http://www.hubbellheaters.com/downloads/oms/model_v_om.pdf

assume 20 showers x 2 gpm hw x 10 minutes = 400 gal
2 x 60 kw, 80 deg rise 300 gph each
operate in sequence based on demand
400 gal insul stg tank
100/400 ~ 25% replentished in 10 min shower cycle
100% in 40 min

Yes, but before bid is awarded to you, I also found "A.O. Smith" hot water calculator that assumes a 10min shower standard.

www.hotwatersizing.com/ApplicationData/MilitaryBarracks.aspx

Fill-in fields for Electric Fuel, 2-Heater Elements, 400-Gal room-temp. storage, 10min shower
Results: DRE-80 13.5 kw, 1st Hour Delivery:551 USGPH, 100 °F Rise
Two 80-Gal Units approach 1200 USGPH using 27kw/208/1.732 = 80A 3Ø Load

These typical 80-Gal tanks are relatively maintenance free with sacrificial anodes for hard water.
1) "Sacrificial Anodes" replace water-softener treatment of Hubbell & Tankless boilers without voiding Warranty.
2) Your Hubbell boilers also require Megger Testing to check & dry any damp elements (condensation) before start-up.

 

[Ingenieur]

Yes, but before bid is awarded to you, I also found "A.O. Smith" hot water calculator that assumes a 10min shower standard.

www.hotwatersizing.com/ApplicationData/MilitaryBarracks.aspx

Fill-in fields for Electric Fuel, 2-Heater Elements, 400-Gal room-temp. storage, 10min shower
Results: DRE-80 13.5 kw, 1st Hour Delivery:551 USGPH, 100 °F Rise
Two 80-Gal Units approach 1200 USGPH using 27kw/208/1.732 = 80A 3Ø Load

These typical 80-Gal tanks are relatively maintenance free with sacrificial anodes for hard water.
1) "Sacrificial Anodes" replace water-softener treatment of Hubbell & Tankless boilers without voiding Warranty.
2) Your Hubbell boilers also require Megger Testing to check & dry any damp elements (condensation) before start-up.

I would store hot water
need to know
number of showers, lavs, etc
number of people
use schedule

 

[deb4523]

During the winter our water temp is around 37 degrees F. We need to supply 1200gph water at 100-130 degrees F. The specs I listed in my original post were from a specific product rated for this service. Yes, it is industrial. And can supply that rate indefinitely. The heater requirements are to be able to heat and supply 1200 gallons per hour, every hour.

It is not my project, but I've been asked to look at this as an option because of the cost of putting in the equivalent oil boilers and the associated oil tanks and exhaust stacks. No, natural gas is not an option. It can be electric or oil..

There is a re-circulation loop already in place and we would utilize it. The product I am looking at has a recirc function, so that is also not an issue.

Personally, I think tankless heaters are a good idea here, if we could get the power, but I do agree with you guys about being prepared for the worst case scenario and not putting a demand factor on this.

We could get the power to the building, but I'm unsure it would be more cost effective than putting in the boilers.

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.

 

[JFletcher]

Tankless are a bad idea because the electrical requirements are 4 to 10 times higher than a tank system, and in the event of a power failure you have no hot water whatsoever. At least with storage tanks or large residential / commercial tank type heaters, if you lose power, you still have hot water for a while.

I worked Hotel maintenance when the hotel took contracts to station almost 500 Personnel from Fort Eustis. our occupancy would go from 3% to 100 + percent overnight. those 500 guys would take showers in the morning almost at the same time. the hotel was basically three separate hot water systems, heavy duty boilers with storage tanks and mixing valves to make the hot water 120 degrees again. one part of the hotel had its mixing valves removed, and the boilers there were turn to 125 degrees... The guys in that section of the hotel, had about 5 minutes of hot water before they were taking ice cold showers.

I just don't see building a completely separate 800 amp service, or going 1,200 amps plus just to accommodate on-demand water heaters, especially if there are is space for conventional water heaters. Why not use all of the Dead time when nobody is using water, like from Midnight to 0400 to gradually heat up the water, then have enough storage So that everyone can take hot showers at once? There is no way they need to maintain a 1200 gallon an hour flow rate indefinitely for the hot water.

 

[deb4523]

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-heat...electric/gold-series-dre-and-dve/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.