Heater Sizes?

[TubbinSoon, now we be tubbin]

There seems to be about 3 sizes of heaters; 4 KW, 5 or 5.5 KW and even a 6 KW. What is going on with the different sizes? Is this due to the circulation pumps having different flow ratings and may or may not over heat? Volume of water that needs to be heated? Test results that indicate that smaller or larger size is needed to maintain or reach temperature?

[HotTubMan]

A 5.5 kw heater will maintain temperatures better than a 4 kw, all other things being equal. A 5.5kw will also use more energy and sometimes requires a greater size GFCI.

For example, Beachcomber used to use 4 kw heaters standard. A 750 (with a 4.5, 3.5 bHP and 4 kw) used to require a 50 Amp GFCI. Now with the 5.5 Kw the same tub requires a 60 Amp GFCI. Why did they change? Most likely because after 30-40 minutes in -10 degree celcius the tub would start to lose heat.

[Bubbles]

How can an element that provides the exact same surface area, heat more?

[Chas]

A 5.5 kw heater will maintain temperatures better than a 4 kw, all other things being equal.

But all things are seldom equal. Some tubs recycle the heat from the jet pumps, others don't. Some are well insulated, some are poorly insulated. Some are larger (more surface area) others are smallish. What I'm saying is: YMMV. There are lots of tubs out there that will work great with a smaller heater. I have sold tons of convertable spas - plug in, 1500 watts - and since our brand doesn't have a blower, and we live in a mild climate, they work great. The larger tubs need more heater, and some folks want 6000 watts just because they read somewhere that it's better.

A 5.5kw will also use more energy and sometimes requires a greater size GFCI.

Not always. A 5.5kw heater will run a lot shorter time than a 1.5kw - but it will heat to the same exact temp, and cost the exact same amount - with one difference: if the pump is large, having it run for a longer period will make the spa with the 1.5kw heater cost more to heat than a similar spa with a 6kw heater. A spa with a 1.5kw heater can take all day and all night, so again: if the pump is big, that spa will cost more to heat. Spas with continuous circulation don't do that, they cost the same to heat with any size heater, but may not stay hot in cold weather, and the bigger heaters will change the temp faster.

For example, Beachcomber used to use 4 kw heaters standard. A 750 (with a 4.5, 3.5 bHP and 4 kw) used to require a 50 Amp GFCI. Now with the 5.5 Kw the same tub requires a 60 Amp GFCI. Why did they change? Most likely because after 30-40 minutes in -10 degree celcius the tub would start to lose heat.

That's possible. They also may have decided to go with a 60 amp breaker because they are easier to find now than they were several years ago. Also, they may have simply decided to not have two different heaters to mess with. HotSpring has a 4kw and a 6kw - so that the spas with the bigger pumps can still work on a 50 amp breaker. I haven't seen any difference in the heating or heat-keeping ability of my Vista with the new 4kw heater, as compared to my Grandee which had the 6kw heater.

[Brewman]

the 5.5 kwh heater will use 5500 watts of power each hour the heater is on.  The 4 kwh will use 4000 watts during the same time period.  
Whether one costs more to operate than the other would depend on how long either one runs each day to maintain the set water temperature.  That's a function of the overall design of the spa.
Brewman

[Chas]

Good point, and I'm sorry if I didn't make that clear: a 1500 watt heater will run an hour and do the same work a 6000 watt heater can do in a quarter of an hour - for the exact same cost on your power bill. The pump may have to run the extra 45 minutes, and that may cost you more.

To answer the question posed by Bubbles:

How can an element that provides the exact same surface area, heat more?

An element can put out 1500 watts on 110 volts, and then put out 6000 watts if wired to 220. Not all elements can do this, of course. Some elements have a higher watt-density than others too: for example, some elements are little short things which still put out 6000 watts while others are big, robust designs only putting out 4000, or even 1500 watts. Size matters as to longevity and efficiency, but is not directly related to wattage output.

Above is a domestic water heater element, which is used in many  low-end spa systems. The pictures don't really show the big difference in size: compare the wire connectors and power cord to get a better idea. The threaded bulkhead fitting above is 1" in diameter, the flange below is 2x4" in size.

[wmccall]

When BSing with a CAL spa salesman, he mentioned they have models with dual elements a 1.5kw and a 5kw.   I believe he said the 1.5 will come on when the spa is only a degree or two below the set point and the big one when the spa is more than that from the set point. Just a gimmick?

[Gary]

How can an element that provides the exact same surface area, heat more?

Resistance

Gary

[Chas]

When BSing with a CAL spa salesman, he mentioned they have models with dual elements a 1.5kw and a 5kw.   I believe he said the 1.5 will come on when the spa is only a degree or two below the set point and the big one when the spa is more than that from the set point. Just a gimmick?

Well, if you have a 5kw heater, I really don't see why you would want or need to run a smaller element. If you have to run the low speed jet pump as Cal Spa does, it would mean a longer run to get the tub back to temp. And, I can't imagine that running the smaller heater element would completely offset cooling from the full use of the pumps and jets, and deffinitely not the blower. Now, offering a small amount of heating could extend the jet time a little bit while allowing a smaller (say 50 amp) power supply to the tub.  I don't know if it would be very efficient, nor do I know if it truly works that way in a cold climate, or at all, I would love to hear from anybody who has some first-hand experience with this system  

By way of comparison: HotSpring spas will run the full capacity of heat while running all jet systems, on any of our 220 or convertible models. Some of our models offer a 4KW heater with larger pumps, others offer 6KW with smaller pumps. All can run all systems at the same time, on a 50 amp breaker. That simply means this: full heat is available any time: jets on or off.

[Spa_Tech]

How can an element that provides the exact same surface area, heat more?

You know, its funny. I was just explaining this very thing to JA. I see that Chas has pretty much beat me to the explanation of Watt density so I'll forgo the post.

On certain elements as Chas also pointed out, the more voltage you apply the greater the output. Specifically, if you double the voltage, you quadruple the output-  

1500 watts @ 120v goes to 6000 watts @ 240v...

As for the Cal Spa guy- If it is as he described, it seems to be a total gimmick in my opinion- Id suspect this may have been a misunderstanding of a convertable system- one that would allow you to have a choice of 30 or 50 amp operation... But the dual element thing, like the dual wet end pump, in my estimation, defies logic.

[SerjicalStrike]

Lets say you have a 350gal tub.
@120v the 1.5kw heater draws 12.5amps
@240v the 4kw heater draws 16.667amps
@240v the 6kw heater draws 25 amps

1gal water=8.33lbs*350= 2916.666lbs
1BTU=watts*3.41214
BTU/lbs= change in degrees/hour

1500*3.41214=     5118.21/2916.666=  1.75degrees/hr
4000*3.41214=   13648.56/2916.666= 4.68degrees/hr
6000*3.41214=    20472.84/2916.666=7.02degrees/hr

a 1.5kw@ 120v will heat the tub @ 1.75 degrees/hr
a 4kw@240v will heat the tub @ 4.68degrees/hr
a 6kw@240v will heat the tub @ 7.02degrees/hr

If you are losing more heat than you are producing, your heater will never be able to maintain a certain temp.

[TubbinSoon, now we be tubbin]

If it is a warm environment then it might make sense to use only 12 amps to hold temperature? Sort of like a variable speed pump, we could have several steps of heating depending on if we can hold temperature. Only add what we need? It seems that a variable heater would work well with either two speed pumps or a small versus larger pump to maintain heat rate. Saw another heater last night that was rated 1.5/5 KW or something like that. Looked at too many brochures and web pages to remember.

Having more choices could be useful feature unless it complicates the control and heater selection and/or makes cost higher?

Wonder if this heater question is similar to the pump size issues I see raised on the web. The small circulation pumps are hazardous? Or a good feature? Are there economies to be gained by owner/manufacturer or is using the main pumps advantageous? Why do the smaller pumps get branded as one of the biggest maintenance issues. Does a smaller motor wear out faster? Does a smaller pump wear out faster? Or is there a difference in quality of hardware used?

It appears that a different design is pounced on and mauled by the big players here. So if you don't do like we do, your design is no good? Or if it is our idea it is great but if your idea it is a gimmick? Most comments were pretty good.

Tell me more about the circ pump debate.

[wmccall]

As for the Cal Spa guy- If it is as he described, it seems to be a total gimmick in my opinion- Id suspect this may have been a misunderstanding of a convertable system- one that would allow you to have a choice of 30 or 50 amp operation... But the dual element thing, like the dual wet end pump, in my estimation, defies logic.

Not a misunderstanding on my part, that is how he presented it, exactly as my post said, fill your tub up with cold water the big heater will bring it up to temp and the little one will keep it there.

[HotTubMan]

If it is a warm environment then it might make sense to use only 12 amps to hold temperature? Sort of like a variable speed pump, we could have several steps of heating depending on if we can hold temperature. Only add what we need? It seems that a variable heater would work well with either two speed pumps or a small versus larger pump to maintain heat rate. Saw another heater last night that was rated 1.5/5 KW or something like that. Looked at too many brochures and web pages to remember.

Having more choices could be useful feature unless it complicates the control and heater selection and/or makes cost higher?

Wonder if this heater question is similar to the pump size issues I see raised on the web. The small circulation pumps are hazardous? Or a good feature? Are there economies to be gained by owner/manufacturer or is using the main pumps advantageous? Why do the smaller pumps get branded as one of the biggest maintenance issues. Does a smaller motor wear out faster? Does a smaller pump wear out faster? Or is there a difference in quality of hardware used?

It appears that a different design is pounced on and mauled by the big players here. So if you don't do like we do, your design is no good? Or if it is our idea it is great but if your idea it is a gimmick? Most comments were pretty good.

Tell me more about the circ pump debate.

Well I have already been corrected in this thread once, but I am going to stick my neck out on this one. Having a circ pump CAN improve heater efficiency, and allow you to maintain temperature more effectively.The thinking here being that the the slower moving water will absorb more heat than the higher velocity water from a massage pump on high or even low speed.

OK Chas, SpaTechs, let me have it!!

[SerjicalStrike]

Also, the laing circ. pump is water cooled.  It transfers most of its heat back into the water directly.