Point of use Water Heaters

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jzadroga

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I am bidding a job that has 4 units each with the following:

256 sq ft.
2 small appliance circuits
1 laundry circuit

refrigerator...........1400 watts @ 120 Volts
Dish Washer...........744 watts @ 120 Volts
Microwave............1800 watts @ 120 Volts
Cook top..............7400 watts @ 240 Volts
Wash/Dry Combo...5000 watts @ 240 Volts
Water Heater......27840 watts @ 240 Volts
Electric Heat.........3000 watts @ 240 Volts

According to my calculations the service could be either 206 amps using the standard method for residential calculations or 77 amps using the optional method. This is a huge difference!

My question is this: The water heater can draw 116 amps (it is feed from 2 60 amp 240 volt circuits). If I were to size the service using the optional method I would by code need to use a 100 amp service. If I did the water heater could still draw more than the service could handle. what are the opinions of others on this situation. I feel that the only correct option is to use the standard calculation method but the competition might not. Would it be a violation to use the optional method?

Thoughts please. Thanks
 
Re: Point of use Water Heaters

I find it very difficult to understand how a 256 square foot apartment/condo could have a 28kw water heater. The typical water heater installed in a home up to 2000 square feet is usually around 2475 watts and even if it was quick recovery it would be somewhere around 4400 watts. I'm thinking if you indeed have a water heater rated for 28kw, it's central for all four units.
 
Re: Point of use Water Heaters

256 sq ft.
2 small appliance circuits
1 laundry circuit

refrigerator...........1400 watts @ 120 Volts
Dish Washer...........744 watts @ 120 Volts
Microwave............1800 watts @ 120 Volts
Cook top..............7400 watts @ 240 Volts
Wash/Dry Combo...5000 watts @ 240 Volts
Water Heater......27840 watts @ 240 Volts
Electric Heat.........3000 watts @ 240 Volts
Click to expand...
water heater are you sure that is full 27.8KW?? for each apt[unit ] ?? if for each one you will have serious problem with service system but really you should recheck the spec again to make sure you dont see the error there i am thinking this is a central point of tankless water heater if so how the heck the apt will deal with this hevey current useage like this ??

really for small apt like this you can get away with small tankless heater not the big monster type.

i think there are few smaller tankless type heaters on market too

merci, marc
 
Re: Point of use Water Heaters

I know it sounds crazy but there are tankless water heaters that have no storage capacity. They heat the water as it travels through the unit to the faucet. They have to rapidly heat the water as it travels quickly through the unit. As it has been stated the units are only 256 sq ft in size. Basically motel rooms. There is no room for a storage tank type of water heater. Each apt has its own unit. They want all the utilities separate. I have checked the wattage requirements and they are correct. You can see that there is a question of whether the optional calculations can be used with this type of install. It may be something that needs to be addressed in future codes.

Also I checked the water flow rate suggested by the manufacturer. It was about 3 gal per min for a bathroom, dish washer and washing machine. I think its reasonable that someone could do all three at once if they were on a cleaning mission.

[ January 21, 2006, 03:54 PM: Message edited by: jzadroga ]
 
Re: Point of use Water Heaters

Yeah, but look at the temperature rise for 3 GPM if it's similar to the Eemax unit that Bob posted. 65 deg @ 3GPM. Even if you've got 55 deg. ground water, that's 120 degree water. Plenty hot for a motel room, I'd say.

I dunno, maybe these are sized right if you want to be able to run the DW, shower and sink all at once. Not being of the plumbing persuasion and all.
 
Re: Point of use Water Heaters

In the 2002 code, it says to add the loads from 220.30(B) and (C). I would think the hot water heater would fall under 220.30(C)(3). So you would have to use 100% of the rating.

"100% of the nameplate ratings of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value. "

Here, I think "continuous" just means it will be on for a while. It is continuous as opposed to short term or thermostatically turned on and off. I don't think it is the same thing as a "continuous load" that is defined as runnning for 3 hours or more.

Steve
 
Re: Point of use Water Heaters

They are a serious problem to even a single family residense.They work but are not nearly worth the real cost.In this case i don't see any way out.Take it for what the numbers say.It will be costly

[ January 23, 2006, 08:35 PM: Message edited by: jimwalker ]
 
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