Efficient Heating of Water

[Netopia]

Here's the scenario.

[This is real life, not a supposition]

I have a large gas fired water heater (80 Gallons). I have a family of six. Energy costs are skyrocketing.

I have insulated the the tank and the pipes leading into and out of it for 6 feet.

Now here is the question. I can do one of the following:

Turn the water temp down to what is comfortable for a shower, which means the water isn't stored/heated to a higher temp than what is actually needed. We have a dishwasher with it's own heating element, so that's not a factor. But in this scenario, the person taking the shower uses almost 100% hot water and very little cold, which means that more cold water is added to the tank and must be heated, but once heated, it doesn't have to be heated to as high a temp.

The other scenario is to turn the water up to much higher (just shy of scalding). Water has to be heated to a higher temp and maintained there, but when people use the shower a MUCH smaller volume of water is used, so the cold water coming into the tank doesn't lower the overall temp of the water in the tank as much and it takes less energy to (I would think) to get it back to the correct temperature.

If it were just a matter of heating a quantity of water to a certain temperature, then the lower temp would obviously use less energy. This is different though since the lower the heated temp, the more cold water gets added to the reserve.

There is the distinct possibility that I'm getting mixed up with my rationale, and thus one answer or the other might be obviously right and I'm just missing it. What are your thoughts? Which way should use less gas?

Joe

 

[wwswimming]

fewer gallons of water, at the high temperature, with more energy losses (per gallon) when the water is in transit from the heater to the shower.

vs. more gallons of water, at the lower temperature, with less energy loss (per gallon) when the water is in transit.

good question ! 2 ideas -
* try it each way for a week or a month, if you can persuade family members to time their showers.
* when designing a new house, put the heater, the kitchen plumbing, and the bathroom plumbing in a cluster, to minimize heat losses. seems ridiculous, except most houses were designed for oil at $20 a barrel, & nat. gas similarly low.

 

[CycloWizard]

The answer, of course, is: it depends. If your system were perfectly insulated (i.e. no heat flux through the insulation), then raising the water heater temperature higher would be a better option. This is because you would always have hot water available, since you would only ever use a small fraction. However, if your insulation is imperfect, which it is (no offense, just the nature of the beast ), then heat loss will be proportional to the temperature of the water in the tank.

That said, it still may be more efficient to raise the temperature higher and use less water. I'll tell you my reasoning in a little while... Late for a meeting!

 

[Netopia]

You guys see the quandry! Thanks for the resonses thus far.

The one thing I DO have in both scenarios is availability of hot water. I had the HUGE 80 Gallon tank put in a couple of years ago, knowing that there would be three teens in the house and two adults (and a little one) and that the old "domestic hot water coil" in the boiler unit just wasn't cutting it. So 80 Gallons is WAY MORE than enough... probably somewhat wasteful... but gas was so cheap (about $15 per month for hot water)... and then, that same year, it started to go through the roof!

I look forward to more thoughts on the subject.

Joe

 

[Shoal07]

We have a dishwasher with it's own heating element, so that's not a factor.

Actually, that is a factor. The dishwaster uses much more energy to heat the water then your waterheater does. Generally, water that is hot enough for the diswasher so it does not have to use its heating coil will save you more money.

 

[sao123]

Originally posted by: Netopia
You guys see the quandry! Thanks for the resonses thus far.

The one thing I DO have in both scenarios is availability of hot water. I had the HUGE 80 Gallon tank put in a couple of years ago, knowing that there would be three teens in the house and two adults (and a little one) and that the old "domestic hot water coil" in the boiler unit just wasn't cutting it. So 80 Gallons is WAY MORE than enough... probably somewhat wasteful... but gas was so cheap (about $15 per month for hot water)... and then, that same year, it started to go through the roof!

I look forward to more thoughts on the subject.

Joe

without some numerical values to do some calculations on its hard to tell, but common sence says that hotter is more efficient... provided your hot water tank is properly insulated.

 

[Netopia]

Originally posted by: Shoal07

We have a dishwasher with it's own heating element, so that's not a factor.

Actually, that is a factor. The dishwaster uses much more energy to heat the water then your waterheater does. Generally, water that is hot enough for the diswasher so it does not have to use its heating coil will save you more money.

Hmmm.... installation instructions on the dishwasher specifically say to hook the inlet to the cold water supply. Also, though you may indeed be right, it would be hard for me to quantify because the dishwasher uses an electric element for heating and the water heater is gas.

Joe

 

[Mark R]

In general, the hotter the water, the larger the heat loss from the water tank and from the piping.

In both of your scenarios, the total amount of heat energy needed at the shower is roughly the same (same volume of water overall, at same overall temperature). But in the case of the hotter water supply, losses in the pipes and from the tank will be greater.

For maximum efficiency, you should set the water heater temperature as low as possible. But, this should NOT be lower than 60 C (140 F) as below this temperature there is a risk of potentially dangerous bacterial growth.

A technology that has recently come to the domestic market are 'condensing' heaters. If the water in the boiler is less than 130 F (55 C) then the steam (a by product of gas combustion) which normally carrys away about 20% of the heat, can condense and transfer its heat to the water. Unfortunately, you can't just lower the temperature on a conventional heater and get this benefit:

1) You have to ensure that the water is stored at > 60 C for health reasons (so a condensing heater needs a countercurrent heat exchanger - whereas many simple heaters use a concurrent design).
2) The steam from gas combustion contains acids, and is highly corrosive (especially at high temerpatures) which means that the heat exchange surface can't be made of normal materials such as copper or cast iron - stainless steel or incoloy are required.

The ideal system to install is a 'condensing combination' boiler - this operates in condensing mode, and provides space heating in the conventional way (but at slightly lower temperature). Additionally, there is no water storage tank, water is heated on demand by a high power burner. Whereas conventional domestic heating systems might only achieve 70-75% efficiency, a domestic condensing boiler can exceed 95%.

 

[Netopia]

Hmmm.... I think I have mine set to around 115-120F right now. Did not know about bacteria growth. Doesn't the chlorine in the water supply take care of that?

Joe

 

[Peter]

Nope, the Legionella bacteria survive very well in lukewarm water - and if this water is then used for showers, you get some nasty lung disease. Not Good. Whenever a public pool is suddenly closed for no apparent reason, it's because of those.

The answer is: Solar panels. Even in mild climate, they provide quite good heat for at least half the year round. (Thermal panels, not electrical ones, mind.)

 

[pinion9]

Besides, setting your hot water very high is dangerous. You said you have a little one? What happens when he/she turns on the hot water and gets burned? Or if you have a guest over who turns on water it gets 180 degree water on their hands?

Also, consider this: less hot water use means water will stay in your uninsulated pipes for much longer, losing more heat.

You could get a water heater that provides hot water on demand. They usually only put out 3-4 gpm, but that is good enough for a shower.

 

[Netopia]

Well... the bacteria thing sort of settles it. Don't want any sickies in the house just on the off chance of saving a few dollars!

As for the heat being too high... the delta I've been playing with is only between ~115-140.

Joe

 

[PowerEngineer]

I'm with Mark R on this one.

Putting aside the bacteria issue, the rate of heat loss from your water heater will be roughly proportional to the difference between the inside water temperature and the outside air temperature (regardless of the how well insulated the tank is). So this essentially means that a water heater set to a mildly hot water temperature will lose less energy.

The other question boils down (pun intended) to the amount of energy it takes to raise water temperature, and more to the point if it changes with water temperature (i.e. does it take a different amount to energy to raise water temperature from 32 to 33 degress than it does from 102 to 103?). The answer is that the amount of energy is roughly the same regardless of starting temperature. This means (among other things) that you need to add the same amount of energy to your shower water to get that comfortable temperature. You can do it by adding that energy through a small amount of really hot water or a larger amount of mildly hot water (with your cold water adjusted to yield the same rate of flow). The bottom line is that there is no energy penalty for using the mildly hot water setting when hot water is used.

EDIT: I forgot to mention that "heat pump" technology can also be used to pre-heat water before it goes into the hot water heater. I don't remember much about the experimental units I saw years ago, but they were pretty small and probably warmed water in a separate tank just ahead of the regular water heater. Maybe we'll see more interest in these if energy prices remain high.

 

[gsellis]

Another technology is replacing it with a "tankless" system. Have not seen a comparison, but it is hot water on demand. Therefore, you do not have the system "idle" time where it is trying to keep the water at s temp (we had a timer on our electric.) Many B&B's that I stayed in in England had electric water heaters in the shower which were an on demand system (always made me wonder about if it was a good idea )

 

[Netopia]

This tank is only about 3 1/2 years old. At the time, I looked at on demand systems capable of continuous delivery in the volumes a family my size might need, and they were 3-4 times more expensive. At the (then cheap) price of gas, we would never have recouped the cost, so we opted for a more conventional setup.

Joe

 

[WackyDan]

I'll be putting in an on demand heater soon.

They aren't that expensive. One for a family is $900 + installation of you don't/can't do it yourself. They save you 30%+ off the heating water portion of your gas bill- So they pay for themselves fairly quickly and add some appeal to the home if you decide to sell.

Another option is a controller, and you'd have to check to see if your tank is compatible or can be retrofitted with one. Much like a programmable thermostat for your furnace, a controller ensures that the tank doesn't cycle during the sleeping hours and some of the daytime hours when you don't need the hottest supply of water. Why heat water at 2am?

You could do solar water radiators, and that would offset your day use, but that means a new tank/retrofit.

Other options? Low flow shower heads, and screaming at the top of your lungs to your teenagers....

 

[JediJeb]

I have been looking into solar as well, one option is to run the incoming water line out through a solar collector then into a conventional water heater. During the day the solar panel supplies hot water to the water heater so that it only has to make up the difference in temp if the incoming water is too cold. If laundry and dishes are done in midday you will save alot on the cost of heating the water. There are more and more vendors for thist type system out there now especially with the cost of gas rising, so just search and see what is out there, you may be suprised.

 

[Netopia]

I've been thinking about two things for AT lately, and this thread is the basis for one of them. With the universal rise in energy costs, some sticky threads on energy efficiency, alternative technologies, and ways to reduce costs would of value.

The other thread I was thinking of was an AnandTech recipes thread. My church puts out a cook book every so often where people in the church send in their favorite recipe for whatever category (desserts, casseroles, meats... etc) and it's compiled into a book. With over 140K registered users from all over the world, I bet we could get some pretty interesting dishes. Hmmm.... maybe I'll go start that thread in OT.

Joe

 

[WackyDan]

Originally posted by: Netopia
I've been thinking about two things for AT lately, and this thread is the basis for one of them. With the universal rise in energy costs, some sticky threads on energy efficiency, alternative technologies, and ways to reduce costs would of value.

The other thread I was thinking of was an AnandTech recipes thread. My church puts out a cook book every so often where people in the church send in their favorite recipe for whatever category (desserts, casseroles, meats... etc) and it's compiled into a book. With over 140K registered users from all over the world, I bet we could get some pretty interesting dishes. Hmmm.... maybe I'll go start that thread in OT.

Joe

Sounds like a good idea Joe... Count me in.

 

[DrPizza]

Originally posted by: PowerEngineer
I'm with Mark R on this one.

Putting aside the bacteria issue, the rate of heat loss from your water heater will be roughly proportional to the difference between the inside water temperature and the outside air temperature (regardless of the how well insulated the tank is). So this essentially means that a water heater set to a mildly hot water temperature will lose less energy.
Newton's Law of Cooling
The other question boils down (pun intended) to the amount of energy it takes to raise water temperature, and more to the point if it changes with water temperature (i.e. does it take a different amount to energy to raise water temperature from 32 to 33 degress than it does from 102 to 103?). The answer is that the amount of energy is roughly the same regardless of starting temperature. This means (among other things) that you need to add the same amount of energy to your shower water to get that comfortable temperature. You can do it by adding that energy through a small amount of really hot water or a larger amount of mildly hot water (with your cold water adjusted to yield the same rate of flow). The bottom line is that there is no energy penalty for using the mildly hot water setting when hot water is used.

EDIT: I forgot to mention that "heat pump" technology can also be used to pre-heat water before it goes into the hot water heater. I don't remember much about the experimental units I saw years ago, but they were pretty small and probably warmed water in a separate tank just ahead of the regular water heater. Maybe we'll see more interest in these if energy prices remain high.

I had the same answer... edited, giving Newton credit.

Oh, and for what it's worth, water heaters really don't use that much energy, especially compared to the energy it takes to heat a house. (at least, in the Northeast.) I use propane for my clothes dryer and cooking, coal for heating the house, and oil for hot water, thus it's automatically broken up for me as to what I pay for what type of heat. It would take a LONG time to recoup a $900 investment for on-demand heat for water, if it saved 30%. That means, I'd have to spend $3000 on hot water to hit the break-even point. That's well over a decade. (And, another for what it's worth - my oil burner on my hot water tank can outpace demand... I can't run out of hot water.)

 

[rancherlee]

I like my Peak/off peak electric setup for my heating, water heater, and central air. Check into it if your local Electric company offers it or not. I cost me roughtly the equivant of 85 cent a gallon propane to heat my house and electric hot water heater and I get that cheap electric rate about 80% of the day. The other 20% the water heater/central air uses regular rate electricity and my furnace runs on propane.

 

[russell2002]

My 2c,

It makes no difference. As the amount of energy (heat) been used is the same, if its a large amount of cool water or a small amount of hot water.

Hot water would of course cool down faster when the shower was not in use but when been used I think its the same. Except as stated for a minor difference due to your system.

 

[soydios]

As a general thing for water heaters, make sure you flush them out periodically. If you don't, minerals form on the bottom of the tank, effectively insulating the water from the heating element. Drain and flush the tank to wash out the minerals, and it will take less energy to heat up your water.

The cooler water would lose less heat through the insulation, so it would use less energy to keep it at the set temperature.

 

[Evadman]

With a family of six, you need to get a tankless water heater.

Lets assume your sower water should be 100 degrees, and that your cold tap is at 40 degrees.

If you have 80 gallons of water at 100 degrees, and use 100% hot water for your shower, then at 5 GPM, you only have 16 minutes worth of showering time before you ar out of water (not counting recovery time). At 140 degrees (don't put it at 140 BTW that is inviting disaster) then you have 40 degrees per gallon of stored energy to raise the tempeature of the cold tap to 100. That means 2 gallons of hot and 1 gallon of cold nets 3 gallons of ~106 degree water. So now you have the equivelent of (80 * 1.4) = 114 gallons of 100 degree water.

Anyway, the engergy loss comes in 2 places. The tank, sitting there at 140 degrees loosing heat to ambient, and in the transit from the tank to the showerhead, which can drop more than 15 degrees easily in a 3/4" copper pipe.

If you had a tankless, you would have all the hot water you needed for generally one shower and one sink in use at the same time. Witha family of 6 you probably need a bigger tankless heater. Either way, you will only be loosing heat while in transit, not for storage. with a family of 6 assuming you use all 160 gallons of hot water a day you would pay for the new tankless heater in < 2 years.

 

[concernedsophist]

I do not want to hijack the thread, but this bacteria thing seems like a deal killer for any system that preheats the water, like a solar hot water system. I have never heard of this problem for solar hot water. Is anything done in solar water heating to combat the bacteria? If so then maybe the OP could use that solution to preheat safley.