demand water heater

[JDB3]

Electric house with the only gas being cook-top, & now the owners are talking about electric demand water heater. For the load calculation, is it kept @ 100% or is d-rating allowed?

 

[GoldDigger]

For some combinations of temperature settings and flow rate, it will draw its full power for as long as the water is flowing. Do you think you can derate that based on diversity?
For a range top, not all burners will be used at full heat at the same time, and not for a long time.
Can you derate that based on diversity?

If you can calculate the maximum water flow in that site and heat rise and determine that full power will never be used, could you derate then?



Tapatalk...

 

[suemarkp]

Usually no. The larger units typically have multiple branch circuit feeds (typically 2, perhaps even 3). Those will cycle on as required, so the branch circuit can not be derated. But if you're doing a service load calc, each branch circuit could possible get a 75% factor a long as there are 4 or more fastened-in-place appliances. Or use the optional calculation and the whole thing will just go in at 40% anyway.

In most places, gas is significantly cheaper than purse resistance heat. I wold probably be cheaper to use a gas unit unless running the pipe to the heater area is especially difficult or venting is difficult.

 

[mivey]

If properly sized for a full family home, I doubt it would run 100% for three hours. If the family is big enough, an under-sized unit might.

If I had to choose an instant water heater for a house with a full family, I would go with a gas unit because the electric ones tend to be under-sized and might require a service upgrade if properly sized. For a small out-building that sees very infrequent use (like a shop with a utility sink) I would select an electric instant because of the reduced demand and maintenance.

But given a more complete choice for a full family home, and if I were that concerned about efficiency, my choice would be a heat pump water heater as it is much more efficient than gas and instant types.

 

[jumper]

If I had to choose an instant water heater for a house with a full family, I would go with a gas unit because the electric ones tend to be under-sized and might require a service upgrade if properly sized.

Also, some members have posted that many POCOs require that they be contacted before any on demand water heaters are installed so they can see if their transformer is sized for the additional load.

 

[kwired]

... I doubt it would run 100% for three hours...

I don't think continuous duty or non continuous duty is that much of an issue here, but being able to apply demand factors goes beyond just continuous duty.

When the average household is using hot water, there is often also other coincidental demand from other areas such as laundry, the kitchen or the bathroom(s) which puts more emphasis on whether we are able to use demand factors or not.

 

[mwm1752]

IMO no difference than a Cooktop as for usage so the load calc would be at nameplate

 

[kwired]

IMO no difference than a Cooktop as for usage so the load calc would be at nameplate

For branch circuit calculations - yes, for service and feeder calculations allowable demand factors are not going to be the same. The cooktop has a specific section for applying demand factors to that type of equipment, the water heater falls more into a "other loads" classification. You may or may not be able to apply a demand factor to the water heating equipment but if you can, it will not be the same method as used for cooking equipment.

 

[jcassity]

i wont get into a debate of what others have said,, I own a Stieble eltron Tempra24.

You best darn well size that thing to its full name plate rating.
here is why...

you can not control the input water temperature as the seasons change.
you cant be expected to insulate all free air exposed plumbing feeding the house if that situation exists.
what you can do is look at the water line, examine if it stands risk of being exposed to a delta in enviromental temps

the lower the input water temperature, the higher current it will draw

the higher temperature the user selects, the higher the current draw

the water heaters have an impeller flow meter that triggers the thermal couple heater in the mini tanks before sending out to the faucets.

in a city supply water system, the water presure is fairly linear and the tankless system is easier to control.
in a well situation, the home owner has to "calibrate" each individual cut off valve on the hot side to a point where they are able to run two water supplies at once without the tankless system going into "abort" or into a "throttle back" mode. Wells generally range in 40 to 60psi supply which effects the input gallons per min due to the pressure tank ect. make sure thier presure tank is properly filled with the correct amount of air also, this part if wrong will make it look like your installation is bad... target 15lbs of air in the presure tank.

the good brands will do the math on flow and compare the input temperature, then they will make an efficiency determination that says,,,

"ok,, i give up,, i cant heat up this many gallons per minute, im just gonna give them "warm" watter".
The above statement is what separates a very good tankless system / efficiency from the ones i suggest you avoid that are available at lowes ect.

dont go cheap on these units,, and by all means do not derate, follow the manuf suggest single, double or tripple two pole breakers. My unit which is the size of a VCR was about 650$
I wouldnt feel bashful at all in telling the customer this one small change will save them 15 to 20 percent on the electrical power consumption. This one item / change represents a very quick return on investment ~ROI

 

[GoldDigger]

the good brands will do the math on flow and compare the input temperature, then they will make an efficiency determination that says,,,

"ok,, i give up,, i cant heat up this many gallons per minute, im just gonna give them "warm" watter".

Others, like mine, instead operate a flow restricter so that you can draw as much water as it can heat to the set temperature and no more than that.

The effect is that when you turn on the second use the flow at the first use point MAY decrease.




Tapatalk...

 

[suemarkp]

But given a more complete choice for a full family home, and if I were that concerned about efficiency, my choice would be a heat pump water heater as it is much more efficient than gas and instant types.

Not to derail this thread, but I've always wondered where the cold side goes on these. Is it just dumping cold air into the room with the water tank? If so, doesn't that mean you need to add heat to the room (which could be at a similar efficiency or even worse electric resistance heat)?

 

[GoldDigger]

Not to derail this thread, but I've always wondered where the cold side goes on these. Is it just dumping cold air into the room with the water tank? If so, doesn't that mean you need to add heat to the room (which could be at a similar efficiency or even worse electric resistance heat)?

Exactly right.
During the summer you hope that the room with the cold side needs cooling.
In the winter you want the cold side to be in a basement that picks up heat from the earth or waste heat from furnace, etc.

It the heat source is natural gas or another heat pump, you may still be no worse off than with a conventional heater.

They are at their best where outside air will work as a heat source all year.

Tapatalk...

 

[mivey]

Not to derail this thread, but I've always wondered where the cold side goes on these. Is it just dumping cold air into the room with the water tank? If so, doesn't that mean you need to add heat to the room (which could be at a similar efficiency or even worse electric resistance heat)?

That's correct. Placement is key. Some equipment can act as a heat source for a room and reduce the heating demand and increase net efficiency.

In this case, you could see the water heater as an A/C supplement. If you have a location that stays hot and needs cooling, you gain efficiency by locating the water heater there. If you can't take advantage of that, then a better place is usually the garage or unheated space like a utility room.

 

[kwired]

They are at their best where outside air will work as a heat source all year.

May work well in most of CA, but some places get too cold for outside air to be useful year round. Maybe I should restate that, as there is always some heat in the air, but once it gets below a certain temp the heat pump is no longer efficient enough or possibly can not keep up with demand anymore.

Any storage type water heater no matter how well insulated will also contribute heat to the space it is in from the stored hot water. In the heating months this simply reduces heating load if the water heater is in the heated space, but during the cooling season it does contribute to cooling load. One of my HVAC buddies says a water heater contributes an average need of 1/4 ton of cooling, and he would rather see a single 80 or 100 gallon water heater for cooling load purposes than to see two 50 gallon water heaters. Most plumbers though will install two 50 gallon units because they sell in high enough volume that two of those usually cost less than a single larger unit does.

 

[greenspark1]

100%

100%

I believe NEC requires instantaneous hot water heaters at 100% nameplate for feeder/service calcs. It is a major pain if you are designing a commercial building using solely instantaneous. Maybe your local AHJ allows some diversity. Seems like there should be some allowed, odds of say 10 water heaters firing at the same time for an extended period is quite low.

 

[kwired]

It seems to me the best install may be to use a storage type heater to preheat the water to around 100?F, then use an instantaneous heater to gain the remainder of needed heat on demand. That would not lose much heat from the storage tank and would not require as large of a unit to raise the temperature on demand either.

 

[GoldDigger]

It seems to me the best install may be to use a storage type heater to preheat the water to around 100?F, then use an instantaneous heater to gain the remainder of needed heat on demand. That would not lose much heat from the storage tank and would not require as large of a unit to raise the temperature on demand either.

As I may have mentioned before on other threads (or was it another forum?), some if not all on-demand heaters will have temperature regulation problems when working with low flow rates and/or small temperature deltas. Using preheated water can reduce the operating delta easily by a factor of three:

​

(120-100)/(120-60)=1/3
A storage type heater can work very well with an on-off thermostatic control because of the high thermal mass of water in the tank. But an instant, on-demand, heater must proportion the heat output to exactly fit the amount of water going through per unit time, with no room for clumsy on-off cycles.

An electric heater with PWM controls is most likely to able to handle this, but not all do this. The older units once popular in UK, for example, would have at most two heat settings and you regulated the temperature by controlling the flow rate. Just fine for a tub, but not very safe for a shower.
And a gas source on-demand heater can be even more limited in its low power output range because you cannot safely modulate the gas flame below a minimum level.

Before going down this route, carefully check out the specs of the unit you are thinking of using.

 

[kwired]

As I may have mentioned before on other threads (or was it another forum?), some if not all on-demand heaters will have temperature regulation problems when working with low flow rates and/or small temperature deltas. Using preheated water can reduce the operating delta easily by a factor of three120-100)/(120-60)=1/3
A storage type heater can work very well with an on-off thermostatic control because of the high thermal mass of water in the tank. But an instant, on-demand, heater must proportion the heat output to exactly fit the amount of water going through per unit time, with no room for clumsy on-off cycles.

An electric heater with PWM controls is most likely to able to handle this, but not all do this. The older units once popular in UK, for example, would have at most two heat settings and you regulated the temperature by controlling the flow rate. Just fine for a tub, but not very safe for a shower.
And a gas source on-demand heater can be even more limited in its low power output range because you cannot safely modulate the gas flame below a minimum level.

Before going down this route, carefully check out the specs of the unit you are thinking of using.

Good points. I still think there may be some benefit of increasing incoming temperature in places where incoming water is otherwise pretty cold. but maybe the preheating should only be to 75 or 80 degrees at the most. Maybe just an unheated storage tank would gain enough heat on its own in many applications resulting in room temp being the max input to the instantaneous water heater.

 

[GoldDigger]

Good points. I still think there may be some benefit of increasing incoming temperature in places where incoming water is otherwise pretty cold. but maybe the preheating should only be to 75 or 80 degrees at the most. Maybe just an unheated storage tank would gain enough heat on its own in many applications resulting in room temp being the max input to the instantaneous water heater.

:thumbsup:

 

[jcassity]

I believe NEC requires instantaneous hot water heaters at 100% nameplate for feeder/service calcs. It is a major pain if you are designing a commercial building using solely instantaneous. Maybe your local AHJ allows some diversity. Seems like there should be some allowed, odds of say 10 water heaters firing at the same time for an extended period is quite low.

well you said commercial and odds in the same sentence ,

exhibit A ~ Hampton Inn