Hot water heater argument 1

[ballpeen]

Hello all. I am hoping someone here can answer a question that would seem a no-brainer to me but, not being an electrical expert, I thought I'd pose it here. What it is, the facility where I work is on a electricity usage reduction program and are looking for anyway to cut back. One thing that was brought up was the use of hot water heaters. As it turns out we have quite a few in and around the plant.
The question is, would it be advantageous to replace the 4500 watt elements in most of our heaters with 1500 watt elements? My boss claims that this would save a lot of power. I really don't see it. It would seem to me that it takes a certain amount of power to get a certain amount of water up to a certain temperature. The only thing a lower wattage element would do, in my opinion, is to lengthen the time it takes. Is he correct or am I? Thanks, Ballpeen.

 

[davidbeach]

Yep, lower wattage element means more time. It takes a certain amount of energy to raise the temperature of a specified amount of water a specified temperature. Energy is power times time, so 1/3 the power means 3 times the time. The big risk of changing the elements is that you might not be able to keep up with the water usage.

 

[jraef]

Depends on the useage, but you are closer to being correct. If the hot water is used a little bit but all day long where the elements are on all day to replace hot water used, then he may have a point worth considering. Part of the reason is that a lot of hot water is wasted replacing cold water in the pipe, that's why you have to wait for the hot water to "get there" if the heater is far away. If the hot water is being trickled through semi-regularly, the losses would be slightly less (assuming the pipes are insulated). If the hot water is used in big slugs and replaced only a few times per day, you would be right because the amount of waste would be minimal compared to total useage, and the energy to heat it is essentially the same.

A better idea is to do what people elsewhere in the world have been doing for years, replace the big heaters with small instant heaters at the point of use. They don't store the hot water, they heat it only when you use it. Cuts down the losses to absolute minimum, and hot water is right there when you want it.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[VEBill]

Most facilities would be more interested in reducing their PEAK load (called: load shedding).

Therefore, what you would want to do is put in a control system so that all non-critical loads (and hot water heaters are typically ideal candidates) can be temporarily turned off when the essential loads start to climb. Since the tanks are full of hot water, if the power turn-off is not excessively long, then nobody will even notice. In the worst case, you might need to replace 40 gallon tanks with larger 60 or 80 gallon tanks to ride over the load shedding gap.

Instant 'On Demand' water heaters have some advantages, but they would be the exact opposite of what most facilities would be looking for if they're trying to reduce the peak load. Most tankless 'On Demand' water heaters need 7 or 8 kilowatts. That's a higher peak load than a normal tank.

Or switch to natural gas or oil fired water heaters (yuck).

Saving power overall (not concerned about when) means putting in water heaters with better insulation, closer to the tap (less running the tap waiting for hot water), insulate the pipes, slab of insulation under the tank, ask people to use less, etc. This is where 'on demand' heaters can help.

 

[itsmoked]

You should always look for ways to reclaim process heat into heating or preheating the water heater water. If you are running any kind of cooling systems,(cooling towers, radiators, fans), check out reclamation.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[waross]

Right on itsmoked.
The reclaimed heat doesn't have to do the whole job. Any preheating of the water before it goes into the existing hot water tanks is benificial.
A heat exchanger on an air conditioning system through which the refrigerant flows on its way to the condensor and through which the water flows on its way to the existing hot water tanks would be a win win situation. Lower head pressure and so more efficiency for the air conditioner, and preheated water for the hot water tank.
In times of light demand, the cost of energy for the smaller elements will be about the same as the larger heaters. Half as much heat for twice as long. The real saving is when the demand is high and people must use luke warm water because the 1500 watt heaters can't keep up to the demand.
Another energy saving technique that doesn't work very well is to just set the thermostats lower. The heat loss through the insulation of the tank will be slightly less but not enough to worry about. The users will just open the hot water tap wider as they mix the hot and cold water to achieve the desired temperature. The hot water will be at a lower temperature so the users will use more of it to compensate. Same energy bill in the end.
yours

 

[davidbeach]

waross, your points are much more valid if in fact the hot water is being used by people, particularly people who won't whine about a lack of hot water (or people who's whining will be ignored by those who make the decisions), but your points don't work well if the hot water is used by a process that has specific temperature needs. The OP doesn't state the use of the hot water, so I don't know. Hot water for hand washing, and the plant has a demand charge there might be some cost savings, but the pay back will be loooooong; hot water for most any other use, or no demand charges, the change may cause more problems than it solves, and the payback might even be negative.

 

[waross]

Hi davidbeach
I'm sorry if I didn't express myself well. I agreed with your original post and I thought that my post was complimentary to yours. I think I agreed with your latest post.

"The hot water will be at a lower temperature so the users {Proces or hand washing} will use more of it to compensate. Same energy bill in the end."
respectfully

 

[davidbeach]

waross, sorry, my attempted point in my 23:36 post was that process uses may draw the hot water faster than the lower wattage element can keep up with and the only way to maintain the necessary temperature is with the higher wattage element. Using more won't help if the temperature isn't high enough.

If the lower wattage element can keep up, than hand washing or process won't matter, and it is just a matter of the lower wattage element being on longer. But if the lower wattage element, even on continuously, can not heat a sufficient volume of water to a sufficient temperature, then the attempted replacement will fail. That's all, otherwise I think we agree.

 

[VEBill]

As mentioned, On-demand heaters are typically 7 or 8 kW. That sort of indicates that is the sort of power required to keep up with most normal demands.

In other words, it's obvious that 1.5kw wouldn't.

 

[Comcokid]

If the hot water draw from a particular hot water heater is intermittent, then the on-demand (or flash water heater) is the way to go. They are used extensively in Europe to save energy. In many situations it makes no sense to keep large tank of water hot all the time.

However if the water is in continuous use, then going to a on-demand or even changing elements won't save energy.

 

[stevenal]

Is this a 24/7 plant? If not, timers may help. Heating elements cycling on and off during periods of non-use waste the energy that is lost through the tank insulation. Warm water losses less energy than hot water does through a given insulation level. Set the timers so the water is hot prior to use. Demand charges and time of use may complicate matters though. If all the water heaters now switch on simultaneously during high load hours, the savings might be negative.

 

[VEBill]

"...makes no sense to keep large tank of water hot all the time..."

The standby losses can be reduced. In my own house the hot water tank has some sort of high-tech foam insulation (maybe R14). It's wrapped with an after-market insulated tank blanket. The tanks sits on a 2-inch slab of foam (R10). The cold line has an anti-siphon trap to block any convection circulation into the cold supply. All the pipes are insulated. I'm fairly confident that my hot water heating bill is primarily consumption driven.

OP really needs to double check about overall consumption versus peak load. The approaches can be opposite depending on the goal.

 

[Comcokid]

Energy savings is more than just the standby losses. The on-demand water heaters can be located very close to the point of use. Not only do you save running water until it's hot, but you save that hot water left in the pipe afterwards that just cools down. And, if you are in a warm climate, you save the AC cooling resulting from standby losses and the wasted hot water heat.

 

[ballpeen]

Wow, I didn't expect so much input. I guess our facility's water heaters would be considered to be occasional usage only. Practically all the heaters are for hand washing only. So lower wattage elements may make sense and as it turns out they're not all that expensive. I agree that we should replace our heaters with point of use type, but that will only be done as the older ones start leaking. We are currently monitoring our water heaters one by one with a wattage meter to see just what is actually being used on a day to day basis. Many of these are Mon.- Fri. and could be off over the weekends.
Right now I'm being told that "everything" is on the table as to electrical cutbacks. Even possibly moving our primary shifts to after peak hours instead of the traditional 9 to 5 environment. The reason being is that out electrical expenses have jumped to $1,000,000 over last years cost, or so we are told by our upper management. I guess some negotiating has been going on and we were able to shave off a considerable amount but, we still need to save more.... It never ends.
As far as heat reclamation, there is quite a bit of that going on already as our company is attempting to become as "green" as it can be. We try to save all the btu's we can.
Maybe the best thing to do is wrap the heaters with insulation (fairly cheap), install lower wattage elements ( also fairly cheap) and call it a day, where the type of timers we would need cost around $90 ( from Grainger's), it would take a while to recoup the $$$$$.
Does anyone have any magic "bullets" that would save a manufacturing company with mainly motor loads some $$$'s on electricity? And yes, we are cutting our lighting as well. My boss says that what we are doing will only make about a 1% difference in our overall bill. But you can't get the people in the Ivory Tower to understand it. Ballpeen.

 

[IRstuff]

You could do what one of my general managers did: put timers on the copiers (what a dork).

While it saved money on the utilities, it cost 10 times more in labor than the savings in utility bills.

TTFN

 

[itsmoked]

ballpeen; When your kind of question comes up the FIRST thing that must be looked at is your "rate structure". Because the biggest punishments dished out by suppliers can be due to power factor(yours being possibly)bad, and peak load penalties, a penalty charged because the power company has to have available enough capacity for your highest load time. Until these two questions can be answered, a whole lot of our suggestions would be flat wrong or ineffective.

That said a company with many millions in electrical charges should really have someone brought in to actually assess the zillion possibilities that are probably available in a place that large.

What kind of business are you? Textile? Food?

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[stevenal]

Please reread your initial doubts and Davidbeaches response. The lower wattage element will use the same energy and cost you in material and labor. Big deal if it's cheap, there is no payback, ever.
Now that I know the use, I have another suggestion. Hand washing requires warm, not hot water. You may be able to lower the thermostats. The possible downside might be insufficient warm water since more will be drawn from the tank and less from the cold water supply. As I stated before, more energy moves across the insulation from the hotter water. Somehow I suspect heating water for washing hands is an insignificant part a 1M increase.

 

[waross]

There are always exceptions, but many people who have used On-Demand water heaters are not that fond of them. I have several types in a lot of different places. I have used both gas fired and electrical. Your primary control of the temperature is the flow rate. It's really exciting in a cold climate to try for a hot shower. When the incoming water is very cold, the shower is just warm. You keep trying to cut back on the flow to get the water warmer. Then the flow switch cuts-out on low flow. Instant ice water. Now you have to turn the flow wide open to get it to start again, while trying to get out of the spray. One repair bill can wipe out a couple of years savings. I have seen and worked with the 7 to 8 Kw On Demand units. They were commercial units used to pre heat water feeding an institutional dishwasher. The dishwasher heats its own water, but that takes time. The wash timing cycle doesn't start until the water has reached a certain temperature (Set by the local health board).
When the demand for clean dishes exceeds the machines capacity, some suppliers increase the hourly through put by putting an on demand preheater ahead of the dishwasher to speed up the heating part of the cycle. The last one that I saw had a dedicated service and revenue meter.
yours

 

[waross]

Back to your primary problem.
I would suggest that you counter the smaller element suggestion with a better insulation suggestion. Better insulation for both the tanks and the hot water lines.

Aside from the water tanks, the first thing to check is your power factor, but I would guess that with a power bill like yours, any power factor penalty would have been noticed long ago and corrective action taken.
From the size of your bill you should be considering your own generators for peak shaving and/or co-generation. The economics depend on the relative demand charges, the cost of fuel, and the utility's willingness to buy peak power and pay a good price for it.
Do you have natural gas on site?
With a generator you will probably be able to heat all the water that you will ever need.
Do you use a lot of air conditioning or refrigeration? Using hot gas to preheat your water ahead of the hot water tanks has the potential for double digit percentage savings in water heating costs. Even solar water heaters will save a lot of water heating costs. Again use them as preheaters and let the electric heaters supply the final heating and control the temperature. That keeps the new system transparent to the users and no-one will be inconvenienced.
A good question to keep things sane is "How much a year does one of these units (Hot water tank, copier, whatever) cost to run, how much can we save and what will it cost to change it?"
Or if you prefer, "What's the cost benefit analysis?"
yours