Thread: OT: Tankless hot water vs 3ph Demand charges?

Hi all,

As some of you know, I live in a rather large building (an old elementary school).

My industrial grade shop is contained in what used to be the cafeteria of the school, yet it gets used (mostly) as a home shop.

I have a 1200a 230v 3ph service (only) and I get charged for "Demand".

As near as I can tell, this means something like... they take the largest current drawn in a 15min period during the month, then calculate some amount to "reserve" this capacity for me.

The way I think this works is that if I turn on my 20hp 380v lathe, it costs me between $125 and $250/month even if I only turn it on for 10 minutes. If I don't turn on the lathe for a month (which happens), I don't get charged that, or I get charged whatever the "demand" maximum was for smaller items.

Many months this place operates just like a household, so the 3ph demand consists of things like heaters, and in the summer, not even those (=low demand charge).

OK, so on to the question....

I've been considering going "tankless" for 1 or 2 of my hot water heaters. These are in locations that prohibit a gas installation.

The reason of course is that I would like to save money in parts of the building I'm not using (ie not continuously heating water), and have an endless supply.

I think some of the larger commercial tankless units suitable for my application are 30A X 3 @230v.

Sooooo....

If I'm not otherwise creating a large "demand" by turning on the big equipment, will the "demand" charge for the tankless heater eat up, or surpass any savings that I might see by owning one?

Think 30A X 3ph @ 230v X 30min (shower)

Interesting eh?

Thoughts?
Sean

Hello Sean, sounds like what is most enerfy effiecent and what is more economical are at odds with each other. Running an electric tank water heater would be cheaper because, as you said, you are paying the demand "reserve". If using the lathe or other machinery sets the demand higher than the tankless water heater would, then it should not be a problem.

if using the lathe or other machinery sets the demand higher than the tankless water heater would, then it should not be a problem.

Yeah, that's kind of my dilemma. I'm wondering if even though I'm supposed to be saving energy/money with the tankless, that I don't end up with a $100 demand charge each month because of it (thereby well killing any savings I would see). Even scarier would be if I happened to turn on the big lathe when someone was taking a shower (15 min of lathe+tankless = $400 demand charge kind of thing).

If that's the case, I'm going to make a stink with the power company.

Sean

One way to look at it.

The tankless must heat every gallon of water from more or less room temp up to say 120F at the prevailing flow rate. A conventional tanked hot water heater cannot do that as we all know once you deplete the "thermal capacitor" it puts out lukewarm water at best.

Such a heat transfer rate on the tankless requires a high input power rate, and since you can't change the voltage, the current drawn must increase.

So..if you are "tanked" right now the only thing that can happen is the potential for increased demand.

Another basic thermodynamic way to look at this is high power (not equals) high efficiency.

Electricity used for heat of any kind is the highest priced form of energy.
Maybe if you run a Nat. Gas generator to make the electric, and you have no demand charge on the gas, and the waste heat to heat water, besides the electricity. IOW a CHP (combined heat and power) system.
Maybe solar for the water heaters would be worth investigating?

I think demand charges are OK as a theory, but they always seem excessive....

FWIW....I have a tankless GAS HWH and it's working out great.....
Can't even stand the idea of a 30 gallon tank of "X" dollars in energy sitting there cooling off most of the time.
Matt is correct on the high "energy" input......this thing is something like 7 gal/minute capacity and 200K BTU's.
3/4 gas line required. BUT.....it gets used only a few times a day.

dk

You might look into a soft starter for the lathe. Ought to cut that demand down considerably. Either that or install a VFD and set the acceleration to keep the instantaneous current draw down. Once you do that then look at your power bill and figure out how much that water heater is costing you.

DK, you make a good point...

Besides simply heating the water up to temp...the tanked has to keep up with the rate of heat loss, while the tankless has to deal with inefficient combustion while it is running. Tanked heaters run at pretty decent combustion efficiency IMHO.

However, I can't say which of the above is greater "loss" or rather "greater expense".

One thing that I have read is that the tankless likely has a shorter lifespan...and so those costs might exceed the energy savings (integrated over time) altogether.

You mignt want to check oil fired hot water heaters, they are not cheap but they will put out the hot water. A 55 gal drum of oil will probably last you a year. You could shut it off until you need the hot water. Rinnai or Toyo made a small one that was direct vent out a side wall at one time and they are on demand hot water heaters. One thing to remember, they work better on K-1, (kerosene) not K-2 (fuel oil)

fwiw: I installed a tankless water heater in a laundry room I built in a first floor garage area as wife has problems with the basement stairs. I located a Co. on the internet and spoke to a lady to whom I told of my requirements to feed an automatic washer and she was unsure of my requirements and after checking with someone else came back and gave me the mod. # of a unit to order which required 220V. 40 amp.I did not at the time realize this was a distributor of the heater and after plumbing it up and wiring it, I was totally useless as to GPM. I called the number on the directions and found out it was the factory and they told me I needed a double element unit that required 80 amps to satisfy a washer. Of course, I am all amped out in the garage area as it is sharing a full house/ garage 200 amp service.The only way to get any warm water is to put the mixer knob on HOT and then the flow is very restricted as on the warm cycle the cold water overrules the hot flow and never gets even warm.It is the type that you adjust the water flow to the heater unit to control the temp.It is ok for washing your hands at the sink where you can tolerate the lower water volume.My point is if you are using more than one unit as in 2 showers, the electric cost savings are gone.

Sean,
How do they monitor this, is there a special meter on the building?

Have you looked into propane? My house is propane & it is expensive but I think it is cheaper then electricity.
I have a tankless water heater it works great. It has a 20 year warranty .

Jackmo

What makes installing gas heaters impossible? If there is an outside wall, many of them are direct-vent these days. Of course, in an old school building there are going to be a lot of rooms with no outside walls, I suppose.
Andy

Besides simply heating the water up to temp...the tanked has to keep up with the rate of heat loss, while the tankless has to deal with inefficient combustion while it is running. Tanked heaters run at pretty decent combustion efficiency IMHO.

Matt,
I disagree on two counts.....
I haven't heard of short life span issues on the Tankless HW heaters, so don't know about that issue...
But....
The tankless has a very efficient copper/stainless heat exchanger that exposes very little water to a large area of flame in the burner. Also, it is a modulating burner, and it throttles in response to water flow. During installation and testing, I watched the flame thru the view window as I varied the water flow on the outlet. The flame height and intensity varied as I worked the water valve. It was so responsive.... as though the water valve was the gas valve. This unit doesn't put out 200k BTU's unless the flow demands it. A 3/8 inch stream means there is a low fire in the heater....1/2 GPM minimum flow rate.

Gas tank type water heaters:
A tea pot in the basement.
I find it appalling in these times that tank water heaters are still built with one lousy central flue. Straight! Not a spiral path, not even corrugated to increase area or decrease laminar flow! (When the heater is burning, try sticking your fingers
up under the bonnet at the top of the tank.....see how hot the flue gas still is that's going up the chimney).

I have studied Steam boilers over the years. They use many flues for obvious reasons.
When will some manufacturer offer a multi flue water heater that has inherently increased efficiency by design?
One central flue does little to expose the maximum amount of water surface to the flame. I suppose there would be condensation issues to deal with, but that's why we have copper and stainless metals. (And condensing furnaces that deal with that issue by design...it can be done)
I don't think there is anything remotely efficient about a tank type water heater, and all the while the burner is off, the flue pulls cold air thru the unit to enhance the rate of cool-down :rolleyes:

Do you actually need a 1200a 230v 3ph service? Thats a shade under half a megawatt capability. Could you re-negotiate your supply with the utility and possibly drop off the demand billing?

Originally Posted by dkmc

Matt,
When will some manufacturer offer a multi flue unit with vastly increased efficiency?
One central flue does little to expose the maximum amount of water surface to the flame. I suppose there would be condensation issues to deal with, but that's why we have copper and stainless metals. (And condensing furnaces that deal with that issue by design...it can be done)
I don't think there is anything remotely efficient about a tank type water heater, and all the while the burner is off, the flue pulls cold air thru the unit to enhance the rate of cool-down :rolleyes:

Do you mean to say that condensing gas boilers aren't the norm yet in the States? I'm truly appalled

How do they monitor this, is there a special meter on the building?

The meter at work (480v 3φ) looks like the one above, with a digital display that has 'peak demand memory'. Older meters had a resettable mechanical hand that provided a similar function, but did not have quite the accuracy.

I think a generator is what you want.

all the best.markj

Sean,
To make a valid decision on this you really need to know the details of the demand charges from your utility.

The time at demand for resetting the meter can vary depending on the particular utility. Some utilities have what they call a small commercial 3 phase service which allows you to avoid demand charges as long as your demand stays under some specified kW. Ours is set up like this, and as long as we don't exceed 35kW twice, or 50kW once, within a specified time period (6 months IIRC) then we have no demand charge. If we do exceed the limit, the demand charge is $XX per kW per month, and remains in effect for a year.

Our service is 800A 208 3 phase. The small commercial rate isn't dependent on service size but rather based strictly on usage. We pay close attention to demand and I run my Blanchard off a 125kW generator which we already had, since the grinder's 60hp motor can zing the meter pretty good.

For an example of how demand charges can vary, there's a small utility serving about 3 counties just north of here. Their use rates are slightly less per kW, but their demand charges are 2.5 times as much as ours would be from Progress Energy. A 3 phase service on their system will generally have demand charges which exceed the use charges by a fair bit.

You'd have to crunch the numbers to see where you stand, but my gut feeling would be that the instantaneous heaters will likely cost you more than you'll save if they affect your demand charge.

If you haven't already done so, it would be worthwhile to find out if your utility has something similar to the small commercial rate like we have. They won't necessarily run out to tell you about it on their own, but it represents a big savings over time as compared to demand charges starting with the first kW.

Suppose you have a bank of 20 12 V car batteries, 240 V. Maybe 80 AH rating. These are charged from your 3 phase line. Current limited charger to keep your peak down. Use DC to the water heated.

If your lathe is CNC you already are using DC internal, maybe at 325 VDC. If its manual put a vector drive in the system and again feed it DC.

Then also add solar panels, both electric and liquid.

Do some analysis on costs with these methods.

.

Talk to your electric supplier. I split my electric service to get a 480V service for part of the operation. I was then able to drop the demand charge off my main service. There was no net savings however, because the base rate increased per kwh. It went from about $.12/kwh to $.15/kwh while I dropped about a $150.00 demand charge. My demand charges were refigured monthly.

If the demand charge premise is based on them "reserving" enough power for you, is it at all feasible to use their idea and "store" the power yourself?

I've got no real handle on high voltage parts and pieces, and someone who does is probably laughing right now, but the power company can store power, so is it do-able on a smaller scale? If you could trickle power into the storage device, and then handle high demands on your side of the meter....

But hey Sean, you've made me a bit less jealous of your "house"; I remember when you first posted about it and were trying to get rid of tile.

Rob