tankless water heaters and rates

[mivey]

I don't mean to be the harbinger of doom, but eventually, residential rates will be billed on demand and eventually hourly (or less) loads (some places already do this). Just something to think about as we move into the future.

The instant hot water devices have taken demand need that used to be stored in the tank and have put it onto the supplier's lines. This is an added cost to the supplier that has been overlooked in the majority of cases. There will be a day of reckoning when the costs eventually get allocated back down to the rates.

Some of the fundamental principles of rates are:
Reflect costs / revenue goals
Fair
Reasonable
Stable
Understandable
Non-discriminatory
Provide proper price/usage signals

Adding demand costs to the utility system, and not being charged, is not a proper price signal and will eventually be fixed. The customers who add these demands would be charged separately or else the principles of fair & non-discriminatory would be violated. Once the means to bill properly are in place (i.e., advanced metering), these issues will be addressed. My guess would be within the next 10-20 years. The smart grid is getting smarter.

If it were my house, I would allow for future means to use other systems. It will make life easier if you have to go back in the future to use something different, like a storage tank.

There are many examples of energy storage systems where the system is charged off-peak so the energy can be used on-peak. Distributed energy storage will be a part of our future and, in fact, is already here.

For now, enjoy the free ride.:smile:

 

[Buck Parrish]

So if every body didn't get home around the same time of day. Then it would not be a problem.
I can remember installing on demand water heaters 20 years a go. Appollo made them.
We would disconnect the heat elements in the water heater. Yet still use the thermostats. We would wire it so the thermostat would turn the on demand water heater on. The old water heater would be used for a hot water storage. We did this with the use of a circulating pump.

 

[mivey]

So if every body didn't get home around the same time of day. Then it would not be a problem.

It would solve the problem with generation but not the over-sized equipment located only at the houses with electric instant water heaters.

This bigger equipment would have less utilization, but would still have to be paid for.

Not sure how the gas suppliers feel about it, but they would have some of the same issues, but not near as many.

 

[Rewire]

our utility is regulated so any rate changes must pass the legislature who fear the voting public.

 

[mivey]

our utility is regulated so any rate changes must pass the legislature who fear the voting public.

Which ones would they fear more: the high demand customer who is saving money by pushing some of his costs onto his neighbors, or the customer who is subsidizing part of someone else's savings?

Most entities frown on subsidies, especially when they are brought to light.

 

[charlie]

our utility is regulated so any rate changes must pass the legislature who fear the voting public.

I have not bothered to look it up. However, I was told that if we find a tank-less WH in service in a residence, they are automatically put on a commercial rate to cover the cost of installing larger or express secondary and possibly a larger transformer. :smile:

 

[jrannis]

I have not bothered to look it up. However, I was told that if we find a tank-less WH in service in a residence, they are automatically put on a commercial rate to cover the cost of installing larger or express secondary and possibly a larger transformer. :smile:

I too think that it is unethical to install electric tankless water heaters and I will not. But, until all of my children are through collage, I will keep quiet and continue to repair meter sockets and overloaded load centers as they come along.

 

[charlietuna]

I have not heard any intention to bill residential electrical rates including demand meters. Electrical meters are not cheap! How does the electrical utilities company know or even care what type of loads you are using in a residence?? They just want to know the service size! The "possible" high amperage rate an ELECTRIC TANKLESS WATER HEATER can require depends upon the hot water asked for it to produce at any one time. Most of the time a tankless electric heater will not draw any more amperage as a tank type heater! The demand factor incorporated in the tankless heater is high since there is a remote possibility that a maximum hot water creation does exist! Of course this is done in steps to control power requirements. If two people occupy a house, chances are they will never use any more demand power "with" a tankless as they would with a tank type! Remember, 45 percent of a tank type heater's energy escapes due to storage, and many people have these tanks in conditioned air spaces which caused additional operation of A.C. systems to remove this heat outside the conditioned air space. This alone reduces overall demands on utility systems..

 

[charlie]

1) . . . How does the electrical utilities company know or even care what type of loads you are using in a residence? . .

2) . . . The "possible" high amperage rate an ELECTRIC TANKLESS WATER HEATER can require depends upon the hot water asked for it to produce at any one time. . .

3) Most of the time a tankless electric heater will not draw any more amperage as a tank type heater! . .

4) The demand factor incorporated in the tankless heater is high since there is a remote possibility that a maximum hot water creation does exist! . .

5) . . . If two people occupy a house, chances are they will never use any more demand power "with" a tankless as they would with a tank type! . .

6) . . . This alone reduces overall demands on utility systems. . .

1. The same way we 'catch' welders and other power quality problems, someone calls in a voltage complaint.
2. If several small ones are installed, I agree. However, a large one to supply multiple appliances such as the washing machine, dish washer, and two showers would have a large circuit.
3. I agree if you have several installed.
4. Yep.
5. HMMM, we have had the washing machine, dish washer, and two showers going at the same time on occasion. More often, we do have the washing machine and two showers going at the same time.
6. My WH is in the garage with an extra blanket wrapped around it and the pipes insulated.

 

[mivey]

Remember, 45 percent of a tank type heater's energy escapes due to storage

As I have told charlietuna before about this energy escape number, it is an exaggeration at best. You must verify what the salesmen are telling you as they will tell you anything.

Example: A 2 bedroom home will use about 50 gallons per day. For a 50-gallon tank, 33 ft^2 area, with an inlet water temperature of 60 deg F, and heating to 120 deg F (60 deg rise), and using all the water at the very last minute of the 24 hour day (so we can give charlietuna the maximum energy escape).

We can find the final temperature by:
Final Temperature = TA + (TH - TA)*e^[-h*A/(g*8.34*R)]
where
TA = Ambient temp
TH = Hot temp
e = 2.71818...
h = hours
g = gallons of water
R = Insulation value

For an R-16 (normal efficiency), we get a final temperature of 113.3, meaning a 6.7 deg drop or 11.2% loss of what we put in.

For an R-24 (high efficiency), we get a final temperature of 115.4, meaning a 4.6 deg drop or 7.6% loss of what we put in.

This is a long way from what charlietuna has been told.



Knowing that 8.333 BTU will raise 1 gal of water 1 degree F, let's verify the heat loss with a simple heat loss equation of H = A*(T-TA)/R
where
H = heat loss in btu/hr
A = surface area in ft^2
T = water temperature
TA = Ambient temperature
R = insulation value in hours*deg F*ft^2/btu

Plot the temperature drop for each of the 24 hours to get the following:
Hour 00: btu loss = 0.00, Temp = 120.00
Hour 01: btu loss = 82.50, Temp = 119.80
Hour 02: btu loss = 82.23, Temp = 119.60
Hour 03: btu loss = 81.96, Temp = 119.41
Hour 04: btu loss = 81.69, Temp = 119.21
Hour 05: btu loss = 81.42, Temp = 119.02
Hour 06: btu loss = 81.15, Temp = 118.82
Hour 07: btu loss = 80.88, Temp = 118.63
Hour 08: btu loss = 80.61, Temp = 118.43
Hour 09: btu loss = 80.35, Temp = 118.24
Hour 10: btu loss = 80.08, Temp = 118.05
Hour 11: btu loss = 79.82, Temp = 117.86
Hour 12: btu loss = 79.55, Temp = 117.67
Hour 13: btu loss = 79.29, Temp = 117.48
Hour 14: btu loss = 79.03, Temp = 117.29
Hour 15: btu loss = 78.77, Temp = 117.10
Hour 16: btu loss = 78.51, Temp = 116.91
Hour 17: btu loss = 78.25, Temp = 116.72
Hour 18: btu loss = 77.99, Temp = 116.53
Hour 19: btu loss = 77.73, Temp = 116.35
Hour 20: btu loss = 77.48, Temp = 116.16
Hour 21: btu loss = 77.22, Temp = 115.98
Hour 22: btu loss = 76.97, Temp = 115.79
Hour 23: btu loss = 76.71, Temp = 115.61
Hour 24: btu loss = 76.46, Temp = 115.42

 

[mivey]

And for those who are keeping up, that is at most about 0.56 kWh (about 0.25 kWh if you have a heat pump).

This is about $10-$20 per year. You can use that number to weigh against the other costs.

 

[mivey]

trying to match the sales pitch

trying to match the sales pitch

More info for those playing at home:

For a water inlet temperature of 60 deg F (ground temperature) and an ambient of 70 deg F (water heater indoors), the losses go down.

In 24 hours, the temperature drops from 120.0 to 116.2, you lose 6.34% of what you put in, or 0.46 kWh/day (4.6 cents/day $16.97/year) with a conventional WH or or 0.21 kWh/day (2.1 cents/day $7.71/year) with a heat pump WH.

To lose 45% of the heat you put in, it would take 235.34 hours (9.8 days) of an R-24 water heater sitting idle. This is 33 cents for a conventional WH and 15 cents for a heat pump WH.

If it sat idle for a year, the water temperature would be 85 degrees and you would have lost 58.3% of what you put in 365.25 days earlier.

Maybe that is where the salesmen are getting the 50% number. The problem is, we don't heat the water and let it sit for a year, as most of the time we use it. If we have an over-sized tank, then maybe we can start talking about heating water that we don't use and maintain at a set temperature.

So on that sales pitch, let's go back to the 2-bedroom home and put in an extra, unneeded, 50-gallon tank. Let's assume the temperature will be held at 119-120 deg F. About every 6.13 hours, the thermostat will kick on and we will replace the lost heat to get us from 119 to 120 degrees (8.333 Btu/gallon)

We know we will lose 6.341% on our 1st 50-gallon tank. We need to find out how long we need for the extra tank to lose the remaining 43.66% of energy input to get us to 45% input loss for the two tanks combined.

We reach that point in about 271.5 hours, or 11.3 days.

Moral of the story: don't put in two 50-gallon water heaters in a 2-bedroom home.

In fact, if you were to use 50 gallons of hot water in one peak hour, you would really only need a 40-gallon water heater.

 

[iwire]

mivey, I am finding your posts about water heaters fascinating but I do have a question. :smile:

Wouldn't the size and shape of the storage container effect the rate of heat loss?

 

[mivey]

mivey, I am finding your posts about water heaters fascinating but I do have a question. :smile:

Wouldn't the size and shape of the storage container effect the rate of heat loss?

Yes, the surface area makes a difference. If we had a ball-shaped container, it would have the least surface area per gallon of water.

I looked up the dimensions of a Marathon unit and it has a cylindrical shape with a round top. A 50-gallon tank is about 35.2 ft^2 and an 80-gallon is about 59 ft^2 (Ruud dimensions).

So for a 60 deg F inlet temp, 60 deg F rise, 70 deg F ambient, 24 hour period, R-24 unit, you lose 6.746% of the input energy for a 50-gallon unit and 5.43% of the input energy for a 105-gallon unit.

So one big tank looks to be more efficient than two smaller tanks, depending on how far you have to push the water down the pipe to the utilization point.

 

[mivey]

Spherical tank

Spherical tank

You could have 50 gallons in a 34 in dia sphere (28 for the tank with 3 inches of insulation) for a loss of 4.9% in 24 hours as compared to the 6.75% for the cylindrical tank with round top.

 

[iwire]

Yes, the surface area makes a difference.

So that leads to my next question. How did you get your loss numbers without the knowledge of the shape? I am not doubting you, I am only trying to learn here. :smile:

 

[charlietuna]

Very interesting Mivey ! But a few things don't make sense. The greater the surface area of the tank should equate to greater total heat loss due to the laws of heat transfer. Also , the greater the difference in inside and ambient temps, the greater the heat transfer. It seems you are using a decreasing inside temp. ,when it maintains a relatively constant temp.. Now i have installed a "heat recovery system" in my last house. It was very efficient to the point that i could actually turn off the gas eight months out of the year and never knew the difference. This was a 30 gallon gas tank type unit with three people using it! And it also was helpful in reducing the actual current drawn by the A/C compressor as the head pressure was reduced whenever the heat recovery pump was running. This was like increasing efficiency in both directions. Now i am living in an area where this type arrangement could only be use about six months out of the year.

I recently helped a buddy install a temporary service for his future retirement home on a five acre lot. I met the power company's field engineer on the site prior to setting the equipment. Due to the size of the future home, the engineer suggested bringing the primary close to the house, set a pole with a transformer, which would minimize the length of the secondary service drop to his house. I really don't see how the power company, in this case, could ever find out he had a tankless electric heater. Also considering that his primary 2 pole 60 amp breaker would provide, say 60 percent of his normal hot water and an occasional added 2 pole 30 amp load for, "guessing" 30 percent, those times that he had two showers and the dishwasher on at the same time. And then the last 2 pole 30 amp breaker load for, guessing 10 percent of those times he had two showers,the dishwasher,wash machine and vanity, in use! My own experience with power companies, they upsize a residential transformer when the original starts blowing fuses or burns up! We have traced power quality problems to undersized transformers which they will upgrade when confronted. I have never heard of a demand meter being set on a residence. But maybe this is something new, but i can't see them enforcing tankless water heater penalty loads by monitoring a residential load ?

 

[mivey]

So that leads to my next question. How did you get your loss numbers without the knowledge of the shape? I am not doubting you, I am only trying to learn here. :smile:

I used some information I had that listed the tank surface area of a 50 gallon tank as 33 ft^2 and 37 ft^2 for an 80 gallon tank. The dimensions will change for the amount of insulation, of course. You can find tank dimensions on the manufacturer's web pages, but they may not tell you the R-Value as they usually just list the efficiency rating.

So you don't have to look it up:
Cylinder area = Pi*Dia*(Height+0.5*Dia)
Cylinder Volume = 0.25*Pi*Height*Dia^2

Sphere Volume = 1/6*Pi*Dia^3
Sphere Area = Pi*Dia^2

 

[mivey]

Very interesting Mivey ! But a few things don't make sense. The greater the surface area of the tank should equate to greater total heat loss due to the laws of heat transfer. yes

Also , the greater the difference in inside and ambient temps, the greater the heat transfer. yes

It seems you are using a decreasing inside temp. ,when it maintains a relatively constant temp. no. the inside temperature stayed the same, it was the water temperature that was dropping.

Now i have installed a "heat recovery system" in my last house. It was very efficient to the point that i could actually turn off the gas eight months out of the year and never knew the difference. Neat

This was a 30 gallon gas tank type unit with three people using it! And it also was helpful in reducing the actual current drawn by the A/C compressor as the head pressure was reduced whenever the heat recovery pump was running. This was like increasing efficiency in both directions. Now i am living in an area where this type arrangement could only be use about six months out of the year. I have read about some heat recovery systems that recover some of heat from the waste water. Sounds like something we may all have to do one day.

I recently helped a buddy install a temporary service for his future retirement home on a five acre lot. I met the power company's field engineer on the site prior to setting the equipment. Due to the size of the future home, the engineer suggested bringing the primary close to the house, set a pole with a transformer, which would minimize the length of the secondary service drop to his house. I really don't see how the power company, in this case, could ever find out he had a tankless electric heater. Also considering that his primary 2 pole 60 amp breaker would provide, say 60 percent of his normal hot water and an occasional added 2 pole 30 amp load for, "guessing" 30 percent, those times that he had two showers and the dishwasher on at the same time. And then the last 2 pole 30 amp breaker load for, guessing 10 percent of those times he had two showers,the dishwasher,wash machine and vanity, in use! My own experience with power companies, they upsize a residential transformer when the original starts blowing fuses or burns up! We have traced power quality problems to undersized transformers which they will upgrade when confronted. I have never heard of a demand meter being set on a residence. But maybe this is something new, but i can't see them enforcing tankless water heater penalty loads by monitoring a residential load ?

The power company really doesn't care if you have a tankless system or not, if it does not impact the system. They only care what happens at the delivery point. It could be a welder or something else that causes a high peak and larger service demands. We know for a total electric, 2500 sq-ft home, the demand is going to be about 30 kW and we would set a 25 kVA transformer. If a 2500 sq-ft home required a 50 kVA or 75 kVA transformer, it would indicate that they have abnormal service requirements.

 

[mivey]

It seems you are using a decreasing inside temp. ,when it maintains a relatively constant temp. no. the inside temperature stayed the same, it was the water temperature that was dropping.

Just realized "inside" meant inside the tank but I was thinking inside the house.

In #10, the house (ambient) temperature stays the same, but the temperature inside the tank drops. I was not adding any more energy to the system for the scenario where all of the hot water was used at the stroke of midnight. I was comparing to the initial input to get the 60 deg rise.

In #12, I calculated maintaining a 119-120 degree temperature for the extra tank.

To show the 24 hour detail as in #10, and maintaining 119*120 degrees, you get the following for a 50-gallon tank, 33 ft^2 area, with an inlet water temperature of 60 deg F, and heating to 120 deg F (60 deg rise), 119 degree minimum:
Initial Btu in = 25,000.00
24 hour Maintenance Btu = 1,429.54
Total Btu input = 26,429.54
24 hour Btu Lost = 1,635.11
% Loss/Input = 6.187%

as shown here:
Hour 00: Btu loss = 0.00, Temp = 120.00, Btu added = 25000.00
Hour 01: Btu loss = 68.75, Temp = 119.84, Btu added = 0.00
Hour 02: Btu loss = 68.52, Temp = 119.67, Btu added = 0.00
Hour 03: Btu loss = 68.30, Temp = 119.51, Btu added = 0.00
Hour 04: Btu loss = 68.07, Temp = 119.34, Btu added = 0.00
Hour 05: Btu loss = 67.85, Temp = 119.18, Btu added = 0.00
Hour 06: Btu loss = 67.62, Temp = 119.02, Btu added = 0.00
Hour 07: Btu loss = 67.40, Temp = 120.00, Btu added = 476.51
Hour 08: Btu loss = 68.75, Temp = 119.84, Btu added = 0.00
Hour 09: Btu loss = 68.52, Temp = 119.67, Btu added = 0.00
Hour 10: Btu loss = 68.30, Temp = 119.51, Btu added = 0.00
Hour 11: Btu loss = 68.07, Temp = 119.34, Btu added = 0.00
Hour 12: Btu loss = 67.85, Temp = 119.18, Btu added = 0.00
Hour 13: Btu loss = 67.62, Temp = 119.02, Btu added = 0.00
Hour 14: Btu loss = 67.40, Temp = 120.00, Btu added = 476.51
Hour 15: Btu loss = 68.75, Temp = 119.84, Btu added = 0.00
Hour 16: Btu loss = 68.52, Temp = 119.67, Btu added = 0.00
Hour 17: Btu loss = 68.30, Temp = 119.51, Btu added = 0.00
Hour 18: Btu loss = 68.07, Temp = 119.34, Btu added = 0.00
Hour 19: Btu loss = 67.85, Temp = 119.18, Btu added = 0.00
Hour 20: Btu loss = 67.62, Temp = 119.02, Btu added = 0.00
Hour 21: Btu loss = 67.40, Temp = 120.00, Btu added = 476.51
Hour 22: Btu loss = 68.75, Temp = 119.84, Btu added = 0.00
Hour 23: Btu loss = 68.52, Temp = 119.67, Btu added = 0.00
Hour 24: Btu loss = 68.30, Temp = 119.51, Btu added = 0.00

It looks odd when you compare the two because I did not require that the final temperature be brought back to 120. If we do that, we get:
5.976% loss for the floating case with a one-time reheating cycle at the end
6.139% loss for the minimum 119 deg case with many reheating cycles