[Greenbuilding] Commercial Water Heaters

Paul Eldridge paul.eldridge at ns.sympatico.ca
Mon Jul 16 19:56:29 CDT 2012


My firm just wrapped up a lighting retrofit at a local municipal 
building where, as best as I can tell, there are four high capacity 
electric water heaters.  The two shown in this first picture 
(http://i362.photobucket.com/albums/oo69/HereinHalifax/Img_1547.jpg) 
each draw 36.0 kW and the remaining two (one of which is shown here: 
http://i362.photobucket.com/albums/oo69/HereinHalifax/Img_1550.jpg) are 
rated at 18.0 kW. Thus, the combined load of these four tanks is 108.0 kW.

These tanks are used almost exclusively for hand washing purposes, and 
so we will be disconnecting all three top elements in the two 36.0 kW 
tanks, as well as two of the three bottom elements, effectively 
de-rating each tank to 6.0 kW.  As you can see in the first photograph, 
the two tanks feed a common supply, and 12.0 kW combined with almost 
1,000 litres of storage capacity is more than sufficient to meet all of 
their requirements.  The two remaining tanks likewise serve mostly hand 
washing needs and will be de-rated to 6.0 kW as well; with that, the 
total connected load falls to 24.0 kW, for a net savings of 84.0 kW.

In terms of cost savings, an 84.0 kW reduction in coincident demand will 
reduce our client's demand charges by $9,350.21 a year, i.e., 84.0 kW x 
$9.276 per kW, per month x 12 months/year.  It will also shift some 
16,800 kWh of energy each month to Nova Scotia Power's lower cost second 
tier, for an additional savings of $5,842.37 a year, i.e., 84.0 kW x 200 
kWh/month, per kW x ($0.09904 - $0.07006 per kWh) x 12 months/year.  
Taken together, this represents a savings of over $15,000.00 a year, at 
current rates, achievable with no discernible loss in water heater 
performance.

By simply reducing the power draw of these tanks, we will save our 
client more money each year than by upgrading the facility's entire 
lighting system, and will have done so at effectively zero cost (their 
lighting retrofit will reduce coincident demand by an estimated 37.7 kW, 
and de-rating their water heaters will more than triple that).  Five to 
ten minutes is all that's required to remove the jumper wires that 
connect the terminal block to each corresponding heating element.

We've de-rated dozens of similarly oversized water heaters over the 
years and in many cases implemented timer controls to lock-out their 
operation during normal business hours, thereby reducing the customer's 
peak demand even further; thankfully, there have been no complaints of 
hot water run-out to date, and so the results have proven more than 
satisfactory.  This particular building initially served as a police 
station, and so these water heaters originally feed a bank of showers; 
the building was subsequently converted to general offices and, 
consequently, their DHW usage is vastly lower.

The other thing we like to do is run the circulator pumps on 
multi-program timers so that they operate only as required.  For 
example, we may run a circulator pump a couple hours at the start of 
each weekday morning, shut it off, then turn it back on for an hour or 
so mid-day and perhaps another hour later in the day.  In most cases, we 
can limit the operation of a pump to twenty or twenty-five hours a week, 
as opposed to one hundred and sixty-eight; after all, there's really no 
need to push hot water through an extensive network of pipes at 02h00 or 
03h00 in the morning (with all the inherent losses), when the last 
person had left the building eight hours prior.

BTW, you can view one of the rooms that we had upgraded at: 
http://i362.photobucket.com/albums/oo69/HereinHalifax/Img_1546.jpg. 
Here, we replaced 3-lamp F34T12 prismatic troffers that consumed 
130-watts each with 2-lamp 28-watt 850 series T8 troffers that draw just 
42-watts, for a two-thirds reduction in demand.  Amazingly, light levels 
in this area increased two and a half to three fold (originally, 14 to 
17 foot candles, now 43 to 46 foot candles).  Far more light and much 
better light quality, with fewer watts to boot.

Cheers,
Paul




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