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#55492 08/31/05 09:00 PM
Joined: Apr 2005
Posts: 27
Rich R Offline OP
I have a customer requesting a price to hook up a tankless water heater. I know a service upgrade would be involved so I told him to get me the specs on the unit before I submitted a bid.

He called back and said the unit is 28.5 KW ! At first I thought he was mistaken on the load but after doing some searching I found that they actually draw that much for a whole house unit.

My question is, I don't see where the code actually adresses this type of unit, so what is the normal way to figure this into your service feeder calc ?

I see if I use the optional method I can put it in as the water heater load but would hardly make sense since it is almost 3 times larger than the central heat of the next step in 220.82 (c)

Any ideas or suggestions from people who have installed these would really help

#55493 09/01/05 03:18 PM
Joined: Jul 2002
Posts: 717
Rich, my first suggestion is to make it clear in your proposal that you are not responsible for lights dimming whenever somebody turns on a hot water faucet. Second suggestion means less revenue for you, but a good possible alternative. They also make gas powered tankless water heaters. They still sometimes need a 120 volt outlet installed for controls, so you can still make some dough on it, depending on the make and model.

#55494 09/01/05 07:05 PM
Joined: Feb 2005
Posts: 693
28.5 KW???! Holy moley, that's (28,500/240) 118.75 amps! Add in the required 125% overhead, and that means a (118.75*1.25=148.4375) 150-amp circuit.

I'd want a separate service for a single load like that.

"Are you sure about those five minutes?!" - My Cousin Vinny

Larry Fine
Fine Electric Co.
#55495 09/01/05 08:26 PM
Joined: Apr 2005
Posts: 27
Rich R Offline OP
Thanks for the reply's, I'm thinking of leaving the existing 150 amp main panel alone and then upgrade the existing meter to 300 amp and then feed a separate 125 amp main panel just for the heater.

I doubt he will want it done after he sees the cost, I just don't understand why they would make these things so big unless they think every house just has a spare 125 amps to play with.

Here is a link to a similar unit that is 22kw

#55496 09/01/05 09:10 PM
Joined: Apr 2002
Posts: 914
We've seen a few of these and I really don't understand why someone would want one. I know they only run when hot water is needed, but they draw so much more than a standard water heater when in use, I doubt there is an energy savings. One of our customers put in one of these with a recirc pump, so it runs constantly. The same customer had us install a steam generator that draws 126 amp constantly when in use. He was surprised when I told him he had to have a service upgrade.

#55497 09/01/05 09:21 PM
Joined: Dec 2003
Posts: 886
One of our customers put in one of these with a recirc pump, so it runs constantly.

Betcha it doesn't last very long. I believe they have a duty cycle or at least are designed for intermittent duty. Also, a recirc pump pretty much goes against what the heater was designed for, that being on-demand water heating. If it's going to maintain hot water there are other much less expensive alternatives such as a conventional water heater.


#55498 09/01/05 09:36 PM
Joined: Jan 2005
Posts: 109
we installed one of these tho not quite as large. Step one was a service upgrade. The unit we installed came with a panel with the unit. Fed 125A to the panel and it had 3 50A two poles. The controller put power where it was needed in the unit. It had 6 heating elements and demand dictated how much power/how many elements were used. Interesting set up. Rod

#55499 09/02/05 02:28 PM
Joined: Dec 2001
Posts: 2,498
Oh boy... something like that in the US... here in Europe those beasts are wired across the three phases and still require a 3x35A 400V feed (that's a standard service for a single family building here...)
They only have one single advantage... they provide instant hot water where no gas is available. That's the only reason they were invented.

Tankless gas water heaters have been de facto standard here for ages. Apartments built around 1900 and later (the big majority of Vienna's houses) didn't have hot water originally, or if they did it was already tankless gas water heaters. They were invented in 1885 I think. Over the years functionality to add hot water central heating to small wall-mount combi units was added (caught on around the 1970ies). Nowadays you can see such a tankless water heater/central heating combi boiler in almost every renovated apartment and smaller single family home (those combi units serve a floor area of roughly 2000 square feet for heating).

#55500 09/02/05 02:30 PM
Joined: Dec 2001
Posts: 2,498
It's not a good joke, but I have to add it... in Austria electric tankless heaters are basically unknown, whereas in Germany they seem to be quite popular. And each time somebody shows up at a German board asking: "What line do I ned for a 28kW tankless?" I say: "3/4" iron pipe!"

#55501 09/03/05 09:07 AM
Joined: Aug 2001
Posts: 7,520
"Great melting service cables, Batman!" [Linked Image]

I thought the 9.5kW units we have over here pulled a lot of current. They've become more common in recent years, often fitted to supply hot water in a location remote from the main tank, for example:

Triton 9.5kW Water heater

One thing you need to remember with these units is that the output temperature is inversely related to the flow rate. In the U.K. one above, for example, to get water out at 140 degrees you'd be looking at only about 3/4 gal. per minute flow rate based on our average water supply temperature.

According to the specs on that 22kW American unit linked to above, you could get only just over 1-1/2 gals. per minute for 140 degrees out with water going in at 50 deg.

[This message has been edited by pauluk (edited 09-03-2005).]

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