tankless water heater

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roy g

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have a question on load calculation for main service . recent residential project is adding two 24kw tankless heaters in place of standard 4500 watt tank. each tank will serve opposite ends of house. Using the examples given for load calculations, the water heater is included in the general load, with 1st 10000 watts at 100% and remainder of load (excluding electric heat & AC) at 40%. this obviously would not work if both heaters were being used at once. The house presently being served by 400 amp service , using load of 92KW. Will service have to be sized so both heaters can operate at once
 
Roy my guess, and only guess!! Would be as you stated.. if the two heaters can be operated at the same, then the service has to be sized accordily. My experience with tankless heaters has been more on a commercial installation, where normally I have to sub-panel to carry the new heater(s) load. Welcome to the forum!!:smile:
 
You have other items in your service calculation, so really these are going in at 40% each. It could easily work if you have enough other items, or the hot water use was short. But both heaters at full tilt are 100A each. So 200A of your 400A is gone when they both run at full tilt. There is usually enough load diversity that you won't pop main breaker, but it could happen if warming a cold house, cooking in the oven, drying clothes, and taking shows all at the same time. It would be prudent to leave yourself some headroom and not max out the 400A service.

I think the larger issue is if you have both of these on a 200A panel. The feeder calc may not have enough headroom for these. I'd try to put them on different panels if using a dual 200A panel service. But the electric heat is another big swinger to look out for and spread out if you can.
 
do these things turn on and draw 24kw every time someone turns on a faucet and draws hot water? Seems like that will happen often, laundry or dishwasher is going and someone turns on a faucet to wash their hands.....and poof. Wonder if it makes sense to put little instant water heaters on each sink, and leave the big units for laundry, dishwasher, tubs and showers. Just thinking out loud..
 
Two 24 KW tankless waterheater that something that will raise red flag from POCO side no question asked.

Did you stated that it is 400 amp service in that case it can be done by putting one on each 200 amp panel but you will have to get the POCO involed due the exsting transfomer that may end up getting larger one depending if it is underground lateral or Overhead set up.

Merci,Marc
 
roy g said:
have a question on load calculation for main service . recent residential project is adding two 24kw tankless heaters in place of standard 4500 watt tank. each tank will serve opposite ends of house. Using the examples given for load calculations, the water heater is included in the general load, with 1st 10000 watts at 100% and remainder of load (excluding electric heat & AC) at 40%. this obviously would not work if both heaters were being used at once. The house presently being served by 400 amp service , using load of 92KW. Will service have to be sized so both heaters can operate at once
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I have no doubt in my mind that if you are adding 48 kw to this house you will need a 600 amp service. 500 amps minimum.... If you have already calculated the load at 92KW for the existing house then take 40% of the 48kw for the heater. 48000*.4= 19200. Add 19200 to 92000 and you have 111200. Divide by 240= 463.33 amps.
 
A lot of the larger ones that I have seen are staged, depending on water flow. If it turned on full every time you cracked open a faucet, you would have a steam generator instead of a water heater.
 
I know this is a bit off topic, but I once replaced a 200 amp MB in a house with a 90 amp, since it was all I had. I forgot about it. I never heard from them in the 6 months it took for them to call me back. This is a 4000 sq ft home, with two subpanels. Funny that the house was fine on 90a, (but no 24KW monster water heaters).
 
Tankless Energy Savings

Tankless Energy Savings

I like all the ads for tankless water heaters! Save all the energy losses from the standing hot water heater! Have instant hot water on demand! They forget to mention that you need to replace the main panel, run 100A feeds to each one, pay the POCO to replace the service and transformer, etc. And then, live with the flicker every time one turns on!

They would be better off with a well insulated piping system and a hot water circulating pump.

Excuse me, I will now step off my soapbox!
 
Tankless gas is a good idea.
Tankless electric bad idea for just a retrofit.
New construction, maybe.
I had a HO call with a problem. The hack contractor he hired installed a 60 amp feed to each of two tankless heaters in a duplex without looking at the equipment.
The inspector looked at the heaters and saw a name plate that said minimum circuit, 100amps.
Needless to say it was a rope job and turned into a three digit change order.
Sad part is that after it was all said and done the HO called for some tech support and mentioned to the guy about the 100amp name plate.
The guy told the HO that he should have called, they would have sent him the 60amp nameplate. So who really knows whats going on inside of those tankless wonders.
My water heater has a label that says that it costs $414 per year to operate the unit. I'm happy with that.
May the good Lord help us all if we start getting charged with demand and have a 45kw tankless in the house.
 
Power Tech said:
They need gas.
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Maybe so but it will be the same headache have to upgrade the gas system as well due most resdentail meter they are only rated for about 150 KBTU max flowage so they have to bump up to 250 or 550 KBTU meter capitcy and the piping in the house will have to be bigger if stay on 7" WC { Natrual gaz} or 14" WC ( Propane ) or change the system to 2 PSIG rated system some of the newer homes allready ran on 2 PISG system now and seems handle the flowage pretty good.

Merci. Marc
 
I put in a propane gas unit 20 years ago after seeing it on TOH. I has nothing but problems with it just washing my truck. The unit would start up and sometimes not shut off when demand for hot water was not there. I found a puddle of aluminum on the basement floor from the unit melting and switched back to a "tank" the next day. Saving money is a good thing till you almost burn your house down doing it. :mad::mad::mad:
 
BLACK4TRUCK said:
I put in a propane gas unit 20 years ago after seeing it on TOH. I has nothing but problems with it just washing my truck. The unit would start up and sometimes not shut off when demand for hot water was not there. I found a puddle of aluminum on the basement floor from the unit melting and switched back to a "tank" the next day. Saving money is a good thing till you almost burn your house down doing it. :mad::mad::mad:
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Was yours a Munchkin? I've heard that same story several times related specifically to the Munchkin.
 
frenchelectrican said:
Maybe so but it will be the same headache have to upgrade the gas system as well due most resdentail meter they are only rated for about 150 KBTU max flowage so they have to bump up to 250 or 550 KBTU meter capitcy and the piping in the house will have to be bigger if stay on 7" WC { Natrual gaz} or 14" WC ( Propane ) or change the system to 2 PSIG rated system some of the newer homes allready ran on 2 PISG system now and seems handle the flowage pretty good.

Merci. Marc
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Im sure you might have to replace the regulator if you have NG. Propane, who knows, it burns much hotter (has more BTUs per CF) than NG.
 
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