Tankless water heater s

[JDB3]

I had posted this in the "NEC" section. Just remembered about calculations . Please forgive me.
Tankless water heater s

I have just recently found out that, for a house I am bidding on, the owner to be wants tank-less water heaters & probably "Instant Hot" heaters @ kitchen sink & bath vanity. The heat will be 15Kw electric with 4 ton "Heat Pump" unit. The square footage of the house is around 2,500 sq. ft. .
Are the tank-less water heaters & instant hot heaters figured @ 100% or what d-rating is allowed?
This will be my first load calculation with this type of water heaters. So help is appreciated!

Thanks,

 

[GoldDigger]

The nature of a good tankless water heater is that a flow switch turns on the heating element and ideally the current through that heating element is proportionally controlled to produce the set temperature at whatever flow rate the faucet is demanding.
Some may also have a means of limiting the volume of water flowing if the heating element is unable to keep up with the demand.
Given that, you can say that:
1. This is not going to be a continuous load requiring a 125% factor.
2. Whether or not it pulls full rated current through the heating element is a function of the flow rate, the incoming water temperature and the set outlet water temperature.
Since you have no control over the conditions in 2, IMHO you have to size the wiring for 100% of nominal power.
The NEC does not make any provision for reducing the calculated load on the basis of load diversity. And you do not have any basis either, unless, for example, you can be assured that there is only one person living in the house and that person will never be running all of the hot water sources at once. Even then, once again, the NEC does not let you consider this unless there is a positive interlock.

Instant hot heaters, on the other hand, are just local tanked heaters with a very small tank. Suitable for making hot drinks but not washing dishes. Those are very unlikely to all be heating at once, but when they are on they always draw full power until the thermostat shuts them off. You can expect non-coincident loads here but I still see no code provision for reducing calculated demand. You could make an analogy to multiple tanked DHW units in a multifamily residence, but you probably[ could not convince an inspector to apply those factors.

POCO, OTOH, will simply ignore their existence.

 

[Little Bill]

Unless they are going with a gas tankless I don't think they will be happy with the results, especially form what they add to the electrical load. If it's for the whole house type you can expect at least 100A just for the tankless. Then there's the "cold spots" that can happen if more than one thing is using hot water at the same time. Being the OP is from Tx it probably won't matter much, but the temperature of the water coming in has a lot to do with how well/fast they heat.

@golddigger: they have a point of use heater that are tankless. Just a box mounted under the sink.

 

[GoldDigger]

@golddigger: they have a point of use heater that are tankless. Just a box mounted under the sink.

True, but Instahot (TM) is a specific brand/model of tanked heater (about 1 quart) which provides near boiling water instantly.
How much power do the OP's small tankless units draw.

 

[George Stolz]

Are the tankless water heaters & instant hot heaters figured @ 100% or what d-rating is allowed?

They are not a continuous load; they don't store water. 422.13.

You can apply a 75% demand factor to any group of four appliances. 220.53.

I don't know how to build a watch, and won't bother trying to tell you how; but there's the time, set your own.

 

[George Stolz]

True, but Instahot (TM) is a specific brand/model of tanked heater (about 1 quart) which provides near boiling water instantly.
How much power do the OP's small tankless units draw.

Whatever the nameplate states? Why make it complicated?

 

[JDB3]

The units will most likely be electric. They have yet to give me the number of units, size, amp draw, etc..
So 75% demand factor for 4 or more appliances, just what all would these apply to? Cooking units, water heaters, disposal, dishwashers, ?

 

[jumper]

The units will most likely be electric. They have yet to give me the number of units, size, amp draw, etc..
So 75% demand factor for 4 or more appliances, just what all would these apply to? Cooking units, water heaters, disposal, dishwashers, ?

Here is the code section that George S. cited earlier:

220.53 Appliance Load — Dwelling Unit(s). It shall be
permissible to apply a demand factor of 75 percent to the
nameplate rating load of four or more appliances fastened
in place, other than electric ranges, clothes dryers, space heating
equipment, or air-conditioning equipment, that are
served by the same feeder or service in a one-family, two family,
or multifamily dwelling.

 

[gadfly56]

The units will most likely be electric. They have yet to give me the number of units, size, amp draw, etc..
So 75% demand factor for 4 or more appliances, just what all would these apply to? Cooking units, water heaters, disposal, dishwashers, ?

I think you'd want to apply that just to the tank-less heaters, depending on the their nameplate. When they're on, they're 100% on. I've never seen a proportional controller for this sort of thing but I'll grant someone may have one.

I remember looking at tank-less heaters for an industrial setting, and we'd have had to boost our 3,000 amp service based on the water flow rates the process was calling for. The power called for was head-shakingly awesome.

 

[JDB3]

Thanks, one & all.