Electric insta hot water heater Calc

Status
Not open for further replies.
crtemp

crtemp

Senior Member
Location
Wa state
I have never come across this and was wondering if I need to calculate this at full load in my whole house calculation. I draws 150 amps (50 amps x3). It is an existing home and only has a 200 amp service and has a electric range, electric furnace (60 amp, 30 amp), dryer, dishwasher, disposal and is 2400 sq ft. I can’t imagine this service would acomidate this new water heater. I can do the service calculation I’m just unsure how to apply the water heater load.
Thanks
 
crtemp said:
I have never come across this and was wondering if I need to calculate this at full load in my whole house calculation. I draws 150 amps (50 amps x3). It is an existing home and only has a 200 amp service and has a electric range, electric furnace (60 amp, 30 amp), dryer, dishwasher, disposal and is 2400 sq ft. I can’t imagine this service would acomidate this new water heater. I can do the service calculation I’m just unsure how to apply the water heater load.
Thanks
Click to expand...

Yes, you would calculate it at full load.

Of course, it you use the optional method, 220.82, it would fall under the demand factor in 220.82(B).
 
I'm in the process of wiring a residential tankless water heater that uses four 40-a breakers, so I'm adding a second 200a M/L panel (no neutral) complete with second exterior 200a main and 400/320a meter.

Sounds like you need to do the same thing (other than exterior main, depending).


Speaking of which, anyone know the smallest 200a M/L Eaton (Cutler-Hammer) type BR panel available?
 
"But it works" :)
I'm amazed when I run across installs where on has been added to an existing 200 am panel
carrying the entire house. Seemingly the cycle enough the main doesn't see a sustained load that will cause it to trip.
I always figure the panel is living on borrowed time
 
In the case of the job I mentioned, the main breaker trips unless they shut off one or two circuits.
 
augie47 said:
"But it works" :)
I'm amazed when I run across installs where on has been added to an existing 200 am panel
carrying the entire house. Seemingly the cycle enough the main doesn't see a sustained load that will cause it to trip.
I always figure the panel is living on borrowed time
Click to expand...
The one I’ve seen installed by a DIY on a 150 Service worked quite well and didn’t appear to cause excessive VD. I didn’t know it was already installed when he was asking me questions and of course I said “don’t do it, it won’t work”.:ashamed1:
 
crtemp said:
I have never come across this and was wondering if I need to calculate this at full load in my whole house calculation. I draws 150 amps (50 amps x3). It is an existing home and only has a 200 amp service and has a electric range, electric furnace (60 amp, 30 amp), dryer, dishwasher, disposal and is 2400 sq ft. I can’t imagine this service would acomidate this new water heater. I can do the service calculation I’m just unsure how to apply the water heater load.
Thanks
Click to expand...
Is it three phase?
 
augie47 said:
"But it works" :)
I'm amazed when I run across installs where on has been added to an existing 200 am panel
carrying the entire house. Seemingly the cycle enough the main doesn't see a sustained load that will cause it to trip.
I always figure the panel is living on borrowed time
Click to expand...
All depends on power usage habits of the people that occupy the place. Single person living there - your chances of never tripping the 200 amp main increases. Family of 6 probably will trip quite frequently.
 
Besoeker said:
Similar here. I just thought 150A seems a lot for a water heater.
Click to expand...
Not for a whole house tankless heater with a high flow rate.
BTW, to me insta-hot denotes a brand of small tank (~ 1 gallon), near boiling, water heater installed at the sink for purposes of making tea or coffee, not a full flow tankless unit.
 
GoldDigger said:
Not for a whole house tankless heater with a high flow rate.
BTW, to me insta-hot denotes a brand of small tank (~ 1 gallon), near boiling, water heater installed at the sink for purposes of making tea or coffee, not a full flow tankless unit.
Click to expand...
We had a 9kW unit. It would supply hot water for the shower indefinitely.
We currently have an immersion heater rated at 3kW. It supplies the whole house. That's what prompted me to ask the question.
We have a 3kW electric kettle for tea or coffee.
 
One thing I think would help lessen the need for so much kW is to have a storage tank that is either unheated or at least only heated to near room temperature. Then you wouldn't need to raise the temp so high for the on demand type heater, lessening the needed kW.
 
kwired said:
One thing I think would help lessen the need for so much kW is to have a storage tank that is either unheated or at least only heated to near room temperature. Then you wouldn't need to raise the temp so high for the on demand type heater, lessening the needed kW.
Click to expand...
Good idea.
 
Besoeker said:
We had a 9kW unit. It would supply hot water for the shower indefinitely.
We currently have an immersion heater rated at 3kW. It supplies the whole house. That's what prompted me to ask the question.
We have a 3kW electric kettle for tea or coffee.
Click to expand...

A 3000 watt tea kettle?:jawdrop:Well, I suppose, since your mains are 220 over there. You won't find any consumer appliances (except A/Cs) here that top 1800W.

As for 9kW supplying a shower indefinitely, don't forget that it depends on the incoming water temperature and the temperature rise. I could see that happening if the incoming was 70 and you enjoy a 100 deg shower.

Originally Posted by kwired One thing I think would help lessen the need for so much kW is to have a storage tank that is either unheated or at least only heated to near room temperature. Then you wouldn't need to raise the temp so high for the on demand type heater, lessening the needed kW.
Click to expand...

But that would defeat the purpose of those type of heaters- no storage standby losses. That's why they call them tankless. If you were going to go that route just use a regular water heater.

I never liked electric tankless water heaters unless small "insta-hots" because at some point their power usage and the means to provide it becomes impractical. Many home owners don't realize that and think you only have to wire them up like an electric water heater. Better to use a NG or propane fired one. Haven't done a cost comparison between upgrading the service and installing NG or propane, but I'll bet NG or propane come out ahead, especially if you have NG on the premises already.

-Hal
 
Last edited:
hbiss said:
I never liked electric tankless water heaters unless small "insta-hots" because at some point their power usage and the means to provide it becomes impractical. Better to use a NG or propane fired one.

-Hal
Click to expand...
I concur. Our propane tankless is a luxury for when everybody needs to get ready in sequence.
One word of warning for those who plan to preheat (solar?), especially if using low flow shower heads:
There is a minimum product of flow rate and temperature rise that the burner can throttle back to (not necessarily a problem with electric) and too low a flow can lead to the burner shutting off, sending a slug of cold water down the pipe.
 
You will need a 320 / 400-amp service if you wish to run that instant water heater with the current loads.

With the loads you have, what will happen when the dryer the furnace are already on and that water heater comes on? Maybe the main holds 210 amps that time, but next time someone turns on the stove or oven on top of all that, they will be sitting in the dark.

If they do not want or can't afford to pay for a service upgrade it may be possible to hook up only 2 elements of the heater to have a hundred amp draw instead of 150

It seems to me that a 36 KW heater is massive overkill for such a small home.

PS: if they ever wish to have a generator they're going to need a monster size one to run that water heater alone. Looking at least 50 KW
 
JFletcher said:
It seems to me that a 36 KW heater is massive overkill for such a small home.
Click to expand...

Say you want to bring 45F water up to 120F, and have 2 fixtures running at once for 3 gallons per minute. I get 31KW.

I'd like to see a hybrid water heater, one that runs gas when the flow is high enough and uses electric below. I like _long_ showers. To not be wasteful I like to turn the water flow down quite a ways during the middle soak. My first experience doing this with a gas tankless was...enervating...

-Jon
 
hbiss said:
But that would defeat the purpose of those type of heaters- no storage standby losses. That's why they call them tankless. If you were going to go that route just use a regular water heater.
Click to expand...
If the storage is about room temp or even up to 85-90 deg F there is no loss or at least not much significant of a loss, and you don't need as many kw of heat out of your instantaneous water heater to reach desired temp compared to if you had to heat it from 50-55 deg F.
 
Status
Not open for further replies.
Top