Crystal Clear And Hot Water After 5 Days Away.

[TinyBubbles]

Ok, we took our first vacation away from the tub, after having it for only a week. When we came back the water was as clear as the day we left. However, the temp. was still 102 and I had turned it down to 80. Is this just a testament to a well insulated spa and a good cover? We delayed our orientation because of the trip, so the dealer will be out this Friday to go over everything, but I'm impatient. Should the spa have maintained this temp. or is the spa malfunctioning?

[dave nh]

Ok, we took our first vacation away from the tub, after having it for only a week. When we came back the water was as clear as the day we left. However, the temp. was still 102 and I had turned it down to 80. Is this just a testament to a well insulated spa and a good cover? We delayed our orientation because of the trip, so the dealer will be out this Friday to go over everything, but I'm impatient. Should the spa have maintained this temp. or is the spa malfunctioning?

if your filter was running and it was not cold out, it was likely enough to keep the temp up. A good cover is a +++.

[Got-soaked]

Ok, we took our first vacation away from the tub, after having it for only a week. When we came back the water was as clear as the day we left. However, the temp. was still 102 and I had turned it down to 80. Is this just a testament to a well insulated spa and a good cover? We delayed our orientation because of the trip, so the dealer will be out this Friday to go over everything, but I'm impatient. Should the spa have maintained this temp. or is the spa malfunctioning?

if your filter was running and it was not cold out, it was likely enough to keep the temp up. A good cover is a +++.

What was the ambient temperature? I would be highly suspicious that a hot tub could maintain 102 degrees for fives day with the heater set at 80. That must be some insulated tub

[Aurora4dth]

I was having the same problem with my tub. It is a clearwater and I have only had it hooked up for about 3 weeks. I would set my temp at 90 and it would live at 103. It is indoors so cold is not a factor. Turns out the tub came set with a circ pump that ran 24 hours a day which was keeping the tub hotter than I set the temp. I called the company and they told me how to reprogram it to run only as much as I wanted it to. Now the temp maintains right around where I set it.

[TinyBubbles]

We got back late on Tuesday and had to return the rental car. I had time yesterday to take the cover off. The temperature finally fell 1 degree. Then we used it without turning up the heat and it dropped to 99 by the end. I was concerned that the heat was coming on, but it wasn't. I have asked around and I believe it was in the low 70's while we were gone, 60's at night. It has given me some confidence that we won't get gouged with high electric bills over the winter. It is a fully insulated tub and it has a really good cover. I've heard that good covers make a big difference and I'm a believer now.

[TinyBubbles]

Quick question for hot tub efficienados.....why would the circulation pump running, raise the temperature 13 degrees higher than Aurora had the tub set? Wouldn't the heater also have to be on?

[kengorman]

Quick question for hot tub efficienados.....why would the circulation pump running, raise the temperature 13 degrees higher than Aurora had the tub set? Wouldn't the heater also have to be on?

Not necessarily. Aurora states his 'circ' pump was on 24-hours. For tubs that don't have dedicated 24-hour circulation pumps, they use a main pump to perform circulation/filtration duties. These pumps can be programmed to run a few hours a day up to 24 hours a day.

On my tub, for instance, the filtration pump consumes 4 amps in low speed mode and approximately 13 amps in high speed mode. Assuming the low speed motor runs 24/7, it is putting some amount of heat back into the water.

Is it reasonable to expect the temperature to rise 13 degrees?

We can do some quick calculations to see if we're in the ballpark.

watts = volts * amps

The 4 amp low speed motor running at 240 volts consumes:

240 v * 4 amps = 880 watts

Assuming that 25% of this energy is wasted as heat energy and the rest is going to the motor, we can say that 240 watts of energy is going into the air space. There will be some heat losses and the water won't absort all this heat, but for the sake of argument, let's say that 200 watts go into the water.

A typical 350 gallon hot tub has 1325 liters of water, or 1325 kgs of water.

The specific heat of water is 4190 J / kg degC.

The water increased in temperature from 90 degF (32.2 C) to 103 degF (39.4) for a total increase of 7.2 degC.

To heat the water up the amount of energy required is:

mass * specific heat * delta Temp = 1325 * 4190 * 7.2 = 4 x 10⁷ J

How long would it take constantly running circulation pumps to raise the water temperature 13 degrees?

1 watts = 1 J/s, so the 200 J/s motor would take:

200 J/s = energy / time = 4 x 10⁷ / t.

Solving for t (seconds) equals 2⁵ seconds = 55 hours.

Now, obviously, there are other heat losses - the temperature of the water won't continue to increase past a certain point because it will reach equilibrium. The amount of heat being put into the water will be lost through the the cabinet, the cover, etc.

The numbers seem to validate that it is reasonable to assume that constantly running pumps can raise the water temperature. Other factors to consider include the ambient air temperature. If the temperature is room temp (e.g 78 degF) then the heat losses of the water aren't going to be too great.

My real world experience mimics what Aurora experienced. Initially, my filtration cycle was set to F12, in other words, the tub filtered water 12 hours per cycle at 2 cycles per day = 24 hours. Regardless of what the set temperature was, the tub temperature increased to 105 degrees over a 3-day period. During this time, the heater didn't cycle on once.

Regards,

Ken

[TinyBubbles]

I guess that's good during the winter, but not so great during warmer times. With a dedicated circulation pump and an ozonator, the whole point is to run them 24 hours a day. So, cutting them back during warmer months to keep your temp. from rising, seems like a negative. Your calculations were based on a pump running at low speed, any idea how much difference would be involved with a smaller circulation pump?

[Aurora4dth]

The Temp in my house is ususally at 75 or so. We leave the cover on all the time we are not using it. At first I was a little concerned but after I started to think about it, this means my heater will never run and my circ pump will run about 16 hours out of a 24 hour period. My electric bill shouldnt increase to much at all. In the warmer months if i remove the cover also for a few hours the heat should also decline a little bit. I called a fewe places and asked if my water quality would go down with not running hte circ pump non stop and people seemed to think it would not unless I started running it really short periods like 8 out of 24 hours.

[kengorman]

I guess that's good during the winter, but not so great during warmer times. With a dedicated circulation pump and an ozonator, the whole point is to run them 24 hours a day. So, cutting them back during warmer months to keep your temp. from rising, seems like a negative.

It depends. I have a dedicated 24-hour circulation pump along with a 2-speed pump that runs in low speed during filtration cycles. With the filtration cycle set to 4 hours day (2 cycles at 2 hours each) and the circulation pump running 24/7, my spa doesn't heat up. The heat losses through the cabinet and cover pretty much equal the heat gains from the 2 motors. There are lots of other factors to consider, most notably being how well insulated the tub is and what the ambient air temperature is. Over the last few weeks, the high daytime temps here in the suburban Philadelphia area have been between 70-80 degF.

When my filtration cycles are set any longer, the temps do creep up during this mild fall season. I don't yet have any experience how hot the temps will get if the daily temps make their ways into the mid-to-high 90s.

Your calculations were based on a pump running at low speed, any idea how much difference would be involved with a smaller circulation pump?

My small circulation pump is rated at 1/15 hp which is rated at 1.3 FLA (full load amps). It is also rated for 115v. At full load, it only consumes 149 watts. (115v * 1.3 amps).

That is roughly 6 times less power than the larger 2-speed motor running at low speed. Even at 149 watts, you can assume that most of that energy is going into moving the water and only some of it is being wasted as heat. Further, some of the heat is going to escape the cabinet and only a portion of that will actually heat the water.

Given that my water temp stays fairly constant in 70-80 degree weather, I'm assuming that the heat losses through the cabinet/cover roughly equal the heat generated from the pumps.

Ken

[Tom]

Is it reasonable to expect the temperature to rise 13 degrees? We can do some quick calculations to see if we're in the ballpark.

Ken, there are some oddities in your figures.

The 4 amp low speed motor running at 240 volts consumes:

240 v * 4 amps = 880 watts

Where did this come from? 240*4 = 960! You used 220V in your calculation.

Assuming that 25% of this energy is wasted as heat energy and the rest is going to the motor, we can say that 240 watts of energy is going into the air space.

We can say that, but we'd be using funny math - 880*0.25 = 220W. It's clear that you are back to using 240V. And note that "going into the air space" part. Clearly, this motor is in a well-constructed hot tub that locks the heat in, a sort of heatlock construction if you like. In most hot tubs, that surplus heat would be vented to the outside of the cabinet.

There will be some heat losses and the water won't absorb all this heat, but for the sake of argument, let's say that 200 watts go into the water.

A typical 350 gallon hot tub has 1325 liters of water, or 1325 kgs of water.

The specific heat of water is 4186 J / kg degC.

The water increased in temperature from 90 degF (32.2 C) to 103 degF (39.4) for a total increase of 7.2 degC.

Let's go with all of those.

To heat the water up the amount of energy required is: Q=mc∆t

mass * specific heat * delta Temp = 1325 * 4186 * 7.2 = 4.0 x 10⁷ J

Yup. More common to use kJ for large amounts, but that's 4.0x10⁴ so we won't argue about three decimal places.

How long would it take constantly running circulation pumps to raise the water temperature 13 degrees?

1 watts = 1 J/s,

This is a bit of a typo: 1 watts means 1 W-s (Watt-second) = 1 J/s.

so the 200 J/s motor would take:

200 J/s = energy / time = 4 x 10⁷ / t.

Took a jump here. Ken assumed that 200W of power would be available as 200J/s of heat energy going into the water. May be easier to note that 1W = 3.6kJ/h

Solving for t (seconds) equals 2⁵ seconds = 55 hours.

Lost me there. 2⁵ =32 seconds, which is a far jump to 55 hours. Just a slip; t = 2 x 10⁵ seconds.

Coming at it with larger units, we see that 200W=200*3.6=720kJ/h

Plug that into energy/time gives 720kJ/h = 4*10⁴kJ/t

Solving for t (hours) gives just over 55h, so even if Ken's calculations strike me as odd in places, his result appears to be correct.

The numbers seem to validate that it is reasonable to assume that...running pumps can raise the water temperature.

But of course! We've been saying that for 12 years. We specifically engineered our product to take advantage of waste motor heat. And here you all figured it was just Arctic marketing hype.

[kengorman]

My original text offset by quotes and on a white background.

Tom's responses in blue.

Ken, there are some oddities in your figures.

Yes - thanks for catching those mistakes.

Where did this come from? 240*4 = 960! You used 220V in your calculation.

We can say that, but we'd be using funny math - 880*0.25 = 220W. It's clear that you are back to using 240V.

It should be 960. 240v * 4 amps = 960W. 960wW * 0.25 efficiency = 240W. The 880 was just to make sure you were paying attention.

1 watts = 1 J/s,

This is a bit of a typo: 1 watts means 1 W-s (Watt-second) = 1 J/s.

Agreed. I forgot a hyphen. Instead of typing

1 watts = 1 J/s, I should have written 1 watt-s = 1 J/s.

Solving for t (seconds) equals 2⁵ seconds = 55 hours.

Lost me there. 2⁵ =32 seconds, which is a far jump to 55 hours.

Another typo - it should have read 2 * 10⁵ and not 2⁵.

Solving for t (hours) gives 55.56h, so even if Ken's calculations strike me as odd, I reach the same result.

Great! I'm glad the end results match. We may have gotten there in different ways.

Hey! We've been saying that for 12 years. We engineered our product to take advantage of waste motor heat. And here you all figured it was just Arctic marketing hype.

Tom,

Thanks for keeping me honest.

Ken

[TinyBubbles]

These are my conclusions....non-scientific of course. Obviously, a well insulated tub is very important. I now think that a good cover is very, very important. Since I've been home and opening the cover, I'm able to keep the heat lower. Without opening the lid, that sucker stays warm. While shopping for tubs, every dealer threw in a cover. I, personally, paid no attention to which cover. When my tub was delivered, I was suprised at how nice the cover was compared to some that I had seen. If I was starting my search now, the cover would definately be something I addressed.