Pump Size, And Energy Use Questions...

[drewskie]

I know that a tub with one pump will use less energy than a tub with two or more pumps, but is the difference very significant? Also, does a 110 tub use less energy than the 220? I have heard very different opinions on this.

[poptones]

My tub uses either, but it's presently wired for 110 only. The pumps are gonna use 110, it's the heater that uses 220. When you double the voltage you quadruple the power dissipation, which means my 1000W heater becomes a 4000W heater. What's this going to do to the BTU output?

And the heating ability is directly related to the DIFFERENCE in temp. That means the 4kW heater is going to heat much, much better and so will run less time. If the tub is well insulated, it would save power because it would run only a short while every few hours when it's cold outside. If it's not well insulated it's still going to run a lot. Mine is pretty well insulated but it's already clear to me I need to wire it for 220 before winter gets here. These cool nights it runs quite a bit even though it stays heated from the sun all day.

I don't know that a tub with one pump will always use less energy. All the pumps are not running all the time and it makes a difference how much they are loaded down.

[Dr. Spa]

In the past (and still today on some spas), tubs that were convertible from 110 to 220 ONLY converted the voltage to the heater (pumps and controllers remained on 110). As an example a 1.5 kw heater converted to 220 volts became a 6kw heater. 4 times the heating ability. HOWEVER, while it had 4 times the heating ability, it ALSO used 4 times the power. Basically, it's a wash. The only difference is that at 1.5 kw, the pump moving water through the heater had to run 4 times as long. This typically was a negligible cost as the pumps were very small or running on low speed (and some had/have a continuous circulation pump that runs all the time any ways).

[poptones]

But what about when it's cold outside? Thermostats have about a two degree hysteresis which means, barring losses due to insulation and evaporation it will take 1200x6 or 2400BTU just to heat a 200 gallon spa those two degrees. If it's a 6x7 spa at 100 deg there's also 210x42 sq ft evap heat loss, or another 8,820 btu/hr loss and that's just when it's closed - open the lid to a small breeze and now we got 20,000 btu/hr.

What I mean is, I live in the south and it is almost always at least 60 degrees or higher during the day. This time of year its more like 80 and gets down to 55 or so at night. The difference between keeping my tub at 102 vs 100 seems to fairly heavily impact the amount of time it runs each night and that's just with an average delta of maybe 35 degrees. What happens when that delta widens to 65 and my tub is already running half the night? I don't know that it does, but I do know that it never catches up when I get in at night - it runs the entire time. I can open the cover and it's off, which means the thermostat is off, but when the lid is open and I'm in it, the 1kW heater never is able to dump enough heat into the water to catch up. This difference in temps I expect in the winter is roughly the same in btu as the diff between having the lid on and having it off now.

So realistically, how important is it to have that extra power? How far north does one need to be before the 110 powered tub is no longer practical for winter time? I'm in Northern Mississippi, it's only nearly October, and I'm already concerned.

[Dr. Spa]

How do you figure a 8,820 btu/hr loss with the insulating cover CLOSED?

[poptones]

Evaporative loss with 0 m/s air movement and 100 deg fluid temp is 205 btu/hr/sq/ft. 205x6x7 (my tub size) equates to 8820. That's assuming a nominal ph of course - as it gets more acidic the losses rise a bit due to faster evaporation.

And yes, I realize that with the cover closed MOST of the water does not really "evaporate" (though some will make it past the seal). But the water is STILL evaporating inside the cover - it just condenses on the cover and falls back into the tub. It is still cooled in the process however, and so contributes to evaporative losses.

I'm really not trying to baffle anyone with bs, I'm just trying to explain how I've come to the conclusion I need to have my own system rewired. I'm genuinely interested in some experienced practical input on this, because I want to know just how much I should be concerned about getting that 220 drop installed before it gets much colder. I have everything to wire the drop, but I'm going to have to buy a 220v gfci and pay the spa tech to replace the 120v pigtail with one for the 220.

[Dr. Spa]

I'm trying to figure out your clacs.

1 gallon of water weighs 8.34 pounds

1 BTU is the amount of energy required to raise one pound of water one degree F

200 gallons of water weighs 3340 pounds

So to raise 200 gallons 2 degrees takes 6680 BTU's

How do you figure 2400 BTU's?

Then you have another 8820 BTU loss through evaporation??? I'm having an even harder time with this. Where is this heat going? It cant be going into the air space between the surface of the water and the cover. Yes, when you close the cover that space is heater by the water, but that temp can't be more than the water temp. So once it reaches the same temp as the water, your loss is done. That 8820 BTU's per hour has to be escaping into the atmosphere to be lost, and I don't see that happening through the cover.

If you put a floating blanket on the spa it virtually eliminates evaporation, Evidenced by by the fact that the underside of the cover remains dry. 8820 BTU's per hour is over 200k BTU's per day. NO WAY is a covered spa losing that much heat to "evaporation". Is it?

[hot_water]

I'm trying to figure out your clacs.

1 gallon of water weighs 8.34 pounds

1 BTU is the amount of energy required to raise one pound of water one degree F

200 gallons of water weighs 3340 pounds

So to raise 200 gallons 2 degrees takes 6680 BTU's

How do you figure 2400 BTU's?

Then you have another 8820 BTU loss through evaporation??? I'm having an even harder time with this. Where is this heat going? It cant be going into the air space between the surface of the water and the cover. Yes, when you close the cover that space is heater by the water, but that temp can't be more than the water temp. So once it reaches the same temp as the water, your loss is done. That 8820 BTU's per hour has to be escaping into the atmosphere to be lost, and I don't see that happening through the cover.

If you put a floating blanket on the spa it virtually eliminates evaporation, Evidenced by by the fact that the underside of the cover remains dry. 8820 BTU's per hour is over 200k BTU's per day. NO WAY is a covered spa losing that much heat to "evaporation". Is it?

In the first place, evaporation rate is not fixed, it depends on the relative humidity in the air space (water vapor pressure divided by saturation vapor pressure at the air temperature) taken right next to the water surface. More importantly, with the cover on, a spa approximates a closed system. The air in the space between the water surface and the inside of the cover (assuming no leakage) will become (virtually) saturated, which means 100% relative humidity, or close to it. Yes, molecules of water from the water surface will achieve sufficient energy to change phase, taking the latent heat of evaportion with them as the leave the liquid surface. But a closed system is in equilibrium - meaning that gas phase molecules in the air space will also re-enter liquid phase, giving that energy back up as the go from gas phase to liquid phase. If your cover was perfectly sealed, it would be a zero-sum situation. The practical matter is that there is some leakage, but with a good cover (makes a tight seal that you can feel as a vacuum holding the cover down when you remove it) it's pretty darn little.

If the cover is making a good seal, you'll notice very little water loss -- i.e., level won't drop much -- when the spa is idle with the cover on. If you're not losing much water, you're not losing much energy via latent heat of evaporation. A cover of poor design could still be losing heat through other heat trnsfer mechanisms, just not evaporation. The bottom line in all this is that if you see your water level dropping (and you don't have shell or plumbing leak) then you have a poorly sealed cover that is costing you money.

On the other hand, MOST of the heat loss in a spa happens when you're using it (cover off). Atmospheric relative humidity is much lower than that in the air space when the cover's in place, so you get a lot of water loss. Which is why your heater comes on and operates much more while you're in the spa than when it's idle with the cover on. This is also why your coffee cools down so fast... you can keep it warm much longer by covering your cup...with anything. Try it.

All this business at the water's surface, and of therefore the cover, is, by the way, far more important to your energy bill than the cabinet insulation.

[poptones]

OK, well let's try it this way: last night I was out there about 90 minutes. I keep my tub at 102, but before the first hour was up it was 100 indicated on the spa, and it stayed there until I left, at which time it was still running. I came in about 1:30-2 am. I happened to be back out there at 6am and it was off and settled, however it's not exactly reassuring to know it may "only" take 4 hours to reclaim that two degrees - and the more troubling part is it's only about 60 at night now. When it hits 40 what happens? That only increases the delta 50% (102-60 vs 102-40) so does that mean I'm only looking at an additional 1 degree drop in the same amount of time? And if it only takes 8000-1000 btus to do this why on earth cannot even a 1kW heater keep up?

[ps558]

I know that a tub with one pump will use less energy than a tub with two or more pumps, but is the difference very significant? Also, does a 110 tub use less energy than the 220? I have heard very different opinions on this.

if you add or have a second pump and use them at the same time the energy use will go up because you are using a second piece of equipment.

As far as 110 or 220 they use the exact same amount of power because the electric company charges by watts.

So if you have a 1 hp pump connected to 110 volts using 12 amps that is 1320 watts

take the same pump connected to 220 volts pulling 6 amps that is 1320 watts

you can divide the watts by 1000 to give you kw rating then times it by your electric rate.

rule is lower voltage higher amps- higher voltage lower amps. If the electric company charged by amps it would cheaper. Amp rating will differ a little between 110 or 220 but not enough to notice on a electric bill

[hot_water]

OK, well let's try it this way: last night I was out there about 90 minutes. I keep my tub at 102, but before the first hour was up it was 100 indicated on the spa, and it stayed there until I left, at which time it was still running. I came in about 1:30-2 am. I happened to be back out there at 6am and it was off and settled, however it's not exactly reassuring to know it may "only" take 4 hours to reclaim that two degrees - and the more troubling part is it's only about 60 at night now. When it hits 40 what happens? That only increases the delta 50% (102-60 vs 102-40) so does that mean I'm only looking at an additional 1 degree drop in the same amount of time? And if it only takes 8000-1000 btus to do this why on earth cannot even a 1kW heater keep up?

I'm not undertanding your concern too well.

You came in at 2 am. right? Presumably you put the cover on when you came in, right? When you visited the spa at 6 am, it was off. How do you know it didn't turn off at 2:15 am? There's a *huge* difference when the cover is on. No cover = large heat loss.

Not trying to be dense here, but I don't see that you have a reason for concern. Heaters can struggle to keep up when it's cool outside, relative humidity is low, and the over is off. When you put the cover on, it should pick up a couple of degrees pretty quickly. For 200 gallons, true 4 kW, with a good cover, I would think about 15-20 minutes in 60 degree weather. If you're spa is really taking hours to pick up 2 degrees with the cover on, there's something amiss.