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Turning Down The Temp Vs Leaving It Set


trucker11

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For the past couple of weeks, I have been turning down the temp to 90 overnight and then the next afternoon, turning it up to 102 for the evening soak. I am wondering if anyone has any insight or knowledge as to what the most efficient use of electricity is--keeping it at 102 at all times, or turning it down and then turning it back up.

I live in NJ with moderate cold climate. (I don't need to address all the possible ways to reduce my electricity. I will do all those things too, just wanted to know about this question)

thanks

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For the past couple of weeks, I have been turning down the temp to 90 overnight and then the next afternoon, turning it up to 102 for the evening soak. I am wondering if anyone has any insight or knowledge as to what the most efficient use of electricity is--keeping it at 102 at all times, or turning it down and then turning it back up.

I live in NJ with moderate cold climate. (I don't need to address all the possible ways to reduce my electricity. I will do all those things too, just wanted to know about this question)

thanks

You could save yourself a few cents per month by doing that.....but why? If your trying to conserve a few cents maybe a Hot Tub is a bit extravagent? You know of plenty of ways to save a few cents that don't involve turning your tub down and up everyday. You deserve it, leave it at the set temp and don't sweat the small stuff.

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I tend to think if your using the spa daily then your probably costing yourself more money (though likely only slightly). Your spa needs to then heat 12-14 degrees, causing your heater to run 3+ hours solid instead of just a few minutes every few hours. If your the type that the "newness" has started to die down, and only get a chance to soak on the weekends, you may see some savings by turning it down during the week.

Most of this is subject to brand (insulation, cover, etc) and proper usage (avc's closed, cover on, etc).

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Heat travels from hot to cold. The greater the temperature difference the faster the transfer. As your spa cools, it will lose less heat (fewer BTU's) per minute or hour. e.g at 100 degrees it will be losing more heat per hour than at 90 degrees.

Neutons law of cooling : The greater the temperature difference the faster the heat exchange.

Assuming you heat your spa to 100 degrees, and it cools to 90 degrees when you turn on the heat... The system will need to run for less time to bring the temp back up to 100 than it would if it continually kept the water at 100 degrees.... But as Roger said, you're probably only saving pennies a month doing this.

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Heat travels from hot to cold. The greater the temperature difference the faster the transfer. As your spa cools, it will lose less heat (fewer BTU's) per minute or hour. e.g at 100 degrees it will be losing more heat per hour than at 90 degrees.

Neutons law of cooling : The greater the temperature difference the faster the heat exchange.

Assuming you heat your spa to 100 degrees, and it cools to 90 degrees when you turn on the heat... The system will need to run for less time to bring the temp back up to 100 than it would if it continually kept the water at 100 degrees.... But as Roger said, you're probably only saving pennies a month doing this.

I would have to disagree and say that it would be cheaper to keep the temperature @ 100 degrees rather than giving the heater more work to gain another 10 degrees every night. The tub will stop running once the desired temperature is reached. Not much heat is lost and energy used if you have a sufficient enough insulation

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I read this post expecting to see the exact opposite answers. When using gas heat, it is usually cheaper to lower the temp. during times of non-use and raise it when needed. It is the opposite with electric heat. It's less expensive to maintain a constant temperature. I am in the custom pool industry and we use gas heaters on pools with spas. The spas spill over into the pools, so customers do not maintain soaking temps in their spas. The simply turn up the heat when they want to use the spa. They can afford to do this with gas heaters, but not electric, the cost and time involved would be too high. Gas heaters have a much quicker recovery time. On pools with no attached spas, electric heaters are cheaper to run, since there is no need for sudden temperature boosts. With home furnaces, you can turn down your gas furnaces at night or during the day if you are away and raise them when you are home, in order to lower costs. With electric heat pumps this is actually more expensive and you should leave your thermostat at a constant setting. I remember doing experiments in school to see if cold water or warm water reached the boiling point faster than the other. They both reached the boiling point at the same time. So I'm not sure about the 90 degree water heating faster. It's definately an interesting question trucker. I would think that you are using more energy by lowering the temperature every day. I could understand the benefit of lowering it if you are going to be away for several days or more. What type of spa do you have? Is it full foam insulation or thermopane?

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It probably mostly comes out in the wash, and the answer is going to depend on your cost of electricity, outside temperature and how well insulated the tub and cover are.

I bet lowering the temperture 1 degree all the time would provide more savings that putting it up and down.

TinyBubbles, isn't that mostly because a) gas is generally a lot cheaper than electric and B) gas heaters are usually more powerful so you can really wait until the last minute to heat the water, as with an on-demand hot water heater. Nominally you have to put the same amount of energy into the water to raise the temperature no matter what method you are using. However, the power used by the heater compared to the amount that actually goes to heating the water depends on the efficiency of the system.

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Im sorry, I still dont agree. If the spa is at 100 or 90 and its 30 degrees outside, its still going to drop at near the same rate regardless, the curve isnt that steep. Once the spa has cooled to the new setting your still paying very close to the same amount to maintain 90 as you are 100 AND you still need to heat it 10-14 degrees to use it.

10-14 degrees, depending on the kW of the heater 1.5-6.0 for most home use spas, can take anywhere from 2-6 hours of continuous use of your most expensive component to get up to using temp.

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I'm on the east coast whoneeds, and around here, electric is much cheaper than gas. I assumed that was everywhere, based on all the stories on the national news about the soaring cost of heating homes with gas. Maybe on the west coast it's different. My home is total electric, my mother's house has a gas furnace and hot water heater. My electric bill is much less, almost half, of her gas bill alone. That's why our pool customers without spas use electric heaters. Much cheaper to operate. But, if you have a spa, you need the gas heat, with it's fast recovery, to jack that baby up to 104 degrees in a couple hours in the dead of winter. It's like cooking on a gas range or electric. Gas range the heat is instant, with electric you have to wait for the burner to heat up.

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Im sorry, I still dont agree. If the spa is at 100 or 90 and its 30 degrees outside, its still going to drop at near the same rate regardless, the curve isnt that steep. Once the spa has cooled to the new setting your still paying very close to the same amount to maintain 90 as you are 100 AND you still need to heat it 10-14 degrees to use it.

10-14 degrees, depending on the kW of the heater 1.5-6.0 for most home use spas, can take anywhere from 2-6 hours of continuous use of your most expensive component to get up to using temp.

But, and lets make some assumptions here, if it takes 8 hours for it to cool that 10 degrees from 100 to 90. Theres 8 hours of no heater use. Once 90 is reached you are correct, very minumal difference to maintain 90 or 100. Now lets talk about that reheat time of 2-6 hours. If it is 2 with a 5.5-6 KW heater then how much would the heater run to maintain 100 during those 8 or so hours of cooling, versus no heater use when the temp was allowed to drop to 90?

There's alot going on here and as I said for the few pennys saved either way, if we can afford a spa we deserve to have it ready for use at any time. We work hard enough for a few pennys to not matter. Next time you go out to the bar, have one drink less and your saving more!

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Im sorry, I still dont agree. If the spa is at 100 or 90 and its 30 degrees outside, its still going to drop at near the same rate regardless, the curve isnt that steep. Once the spa has cooled to the new setting your still paying very close to the same amount to maintain 90 as you are 100 AND you still need to heat it 10-14 degrees to use it.

10-14 degrees, depending on the kW of the heater 1.5-6.0 for most home use spas, can take anywhere from 2-6 hours of continuous use of your most expensive component to get up to using temp.

But, and lets make some assumptions here, if it takes 8 hours for it to cool that 10 degrees from 100 to 90. Theres 8 hours of no heater use. Once 90 is reached you are correct, very minumal difference to maintain 90 or 100. Now lets talk about that reheat time of 2-6 hours. If it is 2 with a 5.5-6 KW heater then how much would the heater run to maintain 100 during those 8 or so hours of cooling, versus no heater use when the temp was allowed to drop to 90?

There's alot going on here and as I said for the few pennys saved either way, if we can afford a spa we deserve to have it ready for use at any time. We work hard enough for a few pennys to not matter. Next time you go out to the bar, have one drink less and your saving more!

That is an affirmative!

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It's clearly less expensive from an energy usage standpoint to cycle it. There's really no debate about that but I'd rather not bring in thermodynamics.

The tough question is how much could you save? I have no idea about that. Keep in mind that you have to open your door twice to go outisde and fool with it unless it's on a timer. If your use is more intermittant then cycling becomes more of an advantage.

From a mechanical maintenance standpoint it is generally better to cycle equipment fewer times and run for longer durations.

Some utilities have deals where the electric rates are cheaper at night if you set up the right type of metering. In that case it could be cheaper to heat the heck out of it in the middle of the night to minimize your loads during the day. Combine this with dishwashing and laundry and you could see some savings. Heating it as I suggest here will use even more energy to maintain the same soaking temp at time of usage but might be a net savings.

NW

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Well, I contacted my power company. I figured, who better to answer this. They confirmed that you will use less energy by maintaining a constant temperature. They recommend 102 degrees, instead of the 104 degree maximum as a way to conserve energy. Not a problem for me, I keep it at 100 degrees. They, also, suggest shortening filter cycles. They said if you have a continuous pump for circulation, that's fine, that is what it's designed to do, run constantly. They suggest using a thermal blanket. Also, try not to set filter cycles to run between 6 am and 5 am or 5pm to 9pm, these are peak demand times.

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I will let you know in another month....as the first month of my spa I have kept it on ECONOMY MODE (heats to set temp during my set cleaning cycles...I have two of about 3 hours each once in morning and once in evening). I timed the evening one to coincide with my daily dip in the spa and with the cover with night time temps in 40-50s It is not dropping more than a couple degrees in evening to heat up. Then in morning nobody is soaking it is running again just before noon and using sun on cover to help heat the tub or so is my thought.

Two other modes are SLEEP and STANDARD, with SLEEP only keeping temp within 20* of set temp....which might be okay if you dont use it during summer or in winter to keep it from freezing but then it is not keeping it up to soaking temps so not applicable in this case.

Standard keeps the temp constant within couple degrees all day and night whether on cleaning cycle or not. I am going to run it on that next month as comparison so time will tell.

I am thinking with good insulation the economy mode should be better as the temp drops are not significant t cause huge draw on heater to get it back up.

Good luck and am thinking it is brand-model dependent as much as anything.

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I'm on the east coast whoneeds, and around here, electric is much cheaper than gas. I assumed that was everywhere, based on all the stories on the national news about the soaring cost of heating homes with gas. Maybe on the west coast it's different.

In LA-LA land it certainly is. Even during the winter, with gas heat running instead of electric A/C our gas bill is less than 1/3 of our electric bill. In the summer when we are just running the stove, dryer and hot water heater the gas bill is less than the average lunch.

At any rate, it's the laws of physics plus the realities of engineering and a bit of economics that makes the difference as to what is the "best" choice. Sure if you let the tub go down to a steady state temperature and sit for a while, it will cost less. If you are adding heat back in as fast as you are letting it go, you probably accumulate more inefficiencies and it may actually cost more, but that will depend on the exact details of the engineering of your system.

I don't want to have to plan ahead to soak, so we leave ours at a constant temp.

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The utility company is dead wrong if they say maintaining a constant temp uses less energy.

The only way that would be possible is if the operating components and heater are somehow more efficient at 100F than a lesser temperature. Better enough to offset the energy savings.

If this were really the case then there would be no reason to have a home thermostat on a timer unless you wanted to vary the temp during the day to suit your own personal requirements.

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Here's sort of an analogy that might help people's thinking. It's not perfectly accurate physics but I think it is accurate in how it reflects the gross characteristics of the problem.

The temperature of water is basically a measurement of how much heat energy is stored in the water. Your power cost is basically a measurement of how much electrical energy (or gas energy) you have used to keep the water "topped up" with that amount of heat energy. So you can think of the tub as a bucket full of a liquid that represents energy content. You want the bucket to be full to a specific level when you soak, the equivalent of 101 degrees Farenheit. But the bucket is leaky, so picture it as a bucket with a hole in the bottom. In order to keep the bucket full to the level you want, you have to put heat in at some point. So imagine you have a tank (your power budget) that holds a lot more energy than the leaky bucket that represents the hot tub. To make that leaky bucket full to the right level once a day, you have to pour energy from the tank in. You can either pour it in continuously at the same rate the water is leaking out of the bucket, or you can let the bucket drain dry and refill it whenever you want to use it.

Let's say that the leak is at the rate of a half bucket per day. If you want to soak every day, you are putting the same amount of energy in no matter if you top off the energy constantly or all at once (unless, of course, you spill more energy while transferring it from the tank one way or the other.) If you soak every day, you have to put half a bucket of energy in every day and it costs you 7 x 0.5 = 3.5 buckets of energy a week.

However, if you only want to soak once every four days, you will use far less of the energy in the tank by refilling the bucket only once every four days, because the bucket will empty out in two days and you can wait two more days before you need to take any energy out of the tank. This will cost you one bucket every four days, or an average rate of one-quarter bucket per day. This scenario costs 7 x 0.25 = 1.75 buckets of energy a week.

If you soak every other day, you break even either way, because you are putting a bucket of water in as soon as it gets empty and it's one bucket every two days which comes out the same as the every-day figure.

Of course, the truth is that the size of the leak in your bucket varies depending upon the model of hot tub, outside temperature, etc. etc. And the amount of energy you spill when you transfer energy from the tank to the bucket represents inefficiency in the heating system and that's also an unknown. So unless you go out and measure or otherwise determine what these figures are for your specific situation, you can't really decide offhand if you are going to save money one way or the other.

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Here's sort of an analogy that might help people's thinking. It's not perfectly accurate physics but I think it is accurate in how it reflects the gross characteristics of the problem.

The temperature of water is basically a measurement of how much heat energy is stored in the water. Your power cost is basically a measurement of how much electrical energy (or gas energy) you have used to keep the water "topped up" with that amount of heat energy. So you can think of the tub as a bucket full of a liquid that represents energy content. You want the bucket to be full to a specific level when you soak, the equivalent of 101 degrees Farenheit. But the bucket is leaky, so picture it as a bucket with a hole in the bottom. In order to keep the bucket full to the level you want, you have to put heat in at some point. So imagine you have a tank (your power budget) that holds a lot more energy than the leaky bucket that represents the hot tub. To make that leaky bucket full to the right level once a day, you have to pour energy from the tank in. You can either pour it in continuously at the same rate the water is leaking out of the bucket, or you can let the bucket drain dry and refill it whenever you want to use it.

Let's say that the leak is at the rate of a half bucket per day. If you want to soak every day, you are putting the same amount of energy in no matter if you top off the energy constantly or all at once (unless, of course, you spill more energy while transferring it from the tank one way or the other.) If you soak every day, you have to put half a bucket of energy in every day and it costs you 7 x 0.5 = 3.5 buckets of energy a week.

However, if you only want to soak once every four days, you will use far less of the energy in the tank by refilling the bucket only once every four days, because the bucket will empty out in two days and you can wait two more days before you need to take any energy out of the tank. This will cost you one bucket every four days, or an average rate of one-quarter bucket per day. This scenario costs 7 x 0.25 = 1.75 buckets of energy a week.

If you soak every other day, you break even either way, because you are putting a bucket of water in as soon as it gets empty and it's one bucket every two days which comes out the same as the every-day figure.

Of course, the truth is that the size of the leak in your bucket varies depending upon the model of hot tub, outside temperature, etc. etc. And the amount of energy you spill when you transfer energy from the tank to the bucket represents inefficiency in the heating system and that's also an unknown. So unless you go out and measure or otherwise determine what these figures are for your specific situation, you can't really decide offhand if you are going to save money one way or the other.

Another perfect case for that electric meter available at your local Hot Tub dealer. This will give you the definitive answer, those pennys are not worth the trouble.

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Here's sort of an analogy that might help people's thinking. It's not perfectly accurate physics but I think it is accurate in how it reflects the gross characteristics of the problem.

The temperature of water is basically a measurement of how much heat energy is stored in the water. Your power cost is basically a measurement of how much electrical energy (or gas energy) you have used to keep the water "topped up" with that amount of heat energy. So you can think of the tub as a bucket full of a liquid that represents energy content. You want the bucket to be full to a specific level when you soak, the equivalent of 101 degrees Farenheit. But the bucket is leaky, so picture it as a bucket with a hole in the bottom. In order to keep the bucket full to the level you want, you have to put heat in at some point. So imagine you have a tank (your power budget) that holds a lot more energy than the leaky bucket that represents the hot tub. To make that leaky bucket full to the right level once a day, you have to pour energy from the tank in. You can either pour it in continuously at the same rate the water is leaking out of the bucket, or you can let the bucket drain dry and refill it whenever you want to use it.

Let's say that the leak is at the rate of a half bucket per day. If you want to soak every day, you are putting the same amount of energy in no matter if you top off the energy constantly or all at once (unless, of course, you spill more energy while transferring it from the tank one way or the other.) If you soak every day, you have to put half a bucket of energy in every day and it costs you 7 x 0.5 = 3.5 buckets of energy a week.

However, if you only want to soak once every four days, you will use far less of the energy in the tank by refilling the bucket only once every four days, because the bucket will empty out in two days and you can wait two more days before you need to take any energy out of the tank. This will cost you one bucket every four days, or an average rate of one-quarter bucket per day. This scenario costs 7 x 0.25 = 1.75 buckets of energy a week.

If you soak every other day, you break even either way, because you are putting a bucket of water in as soon as it gets empty and it's one bucket every two days which comes out the same as the every-day figure.

Of course, the truth is that the size of the leak in your bucket varies depending upon the model of hot tub, outside temperature, etc. etc. And the amount of energy you spill when you transfer energy from the tank to the bucket represents inefficiency in the heating system and that's also an unknown. So unless you go out and measure or otherwise determine what these figures are for your specific situation, you can't really decide offhand if you are going to save money one way or the other.

Very reasonable analogy, except for one thing. If you apply Neutons law to it, when the bucket is full, it will be leaking out the bottom faster, than when it's less than full. Think of it like, as the water level drops, and there's less pressure on the bottom of the bucket, it will be leaking slower. The further the water level drops, the slow the water will leak out.

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