Deltaray,
I have a Hotsprings Grandee 220V. I've experienced a few power related issues. Here are my thoughts:
Makse sure the thermostat is set higher than 70 degrees. Should be a no brainer but you never know.
Make sure the breaker for the heater is on. Mine has 2 breakers in the sub panel, one for the heater and one for the pumps and electronics.
Try resetting everything. Turn off the breakers, let it site for a minute, turn the breakers back on.
On mine the cover for the control box is easy to open with a screwdriver. Turn off the breakers and open the control box. Examine the wires that go to the heater. Make sure they are attached.
Mine also has a fuse or two on in the control box. Look to see if they are blown.
The heaters do not last forever. They are pretty simple to replace, especially if you already have experience replacing the circulation pump.
If you are comfortable with electricity it is easy to see if the heater is getting power at the control box. Use a volt meter to check for voltage.
If the circulation pump was not working, the heater could have overheated. Look for a reset button, or if you know the heater is getting power, replace the heater.
When the heater is working you should feel hot water coming out of the drain vent. Your electric meter should also be spinning pretty fast. My Grandee takes a few hours to heat up, but you should feel the hot water almost immediately.
What year/model do you have? 110V or 220V?
I don't know if this is really an answer, but I've had a Coleman 6 person spa in service for about 25 years. Yes, 25 years! I have never had the same problems you are experiencing. We learned early on that any clothing (bathing suits etc) must be very clean and rinsed extremely well before going into the spa. Clothes cleaning detergents left in the fabrics release into the pool water and cause foam quickly. The clarifier is good at flocking these soaps, but they do tend to gum up the works quickly. We never lost a lot of pressure however, just had to clean the filter. We also made a habit of completely changing the water at least twice a year. After draining as much as we could via the drain hose, we used a wet vac to get as much out of any low spots. We then put in just enough water to cover the intake close to the bottom and jogged the pump to cycle clean water through the system and get the dirty water out. Again, drain and vacuum and repeat this process until the water looks clear. We do this with the filter out. This will clean out a lot of crap. About once a year just before we do a drain and complete cleaning, we treat the tub with anti-scale cleanser. This cleans the internal pipes of built up scale and solids. In your case, you may want to do this anti-scale treatment for a few days with the circulator on low and the filter out to purge the internal system. Then go ahead and change the water.
Another test you can do is to see how well the pump pushes water. When you have the small amount in the bottom, just above the intake, jets exposed, turn the jets to full, air off and see if the pump will shoot the water out of the jets with extreme force. Mine will shot water 20 feet away!!!! lol. If you don't have really good pressure, you may have a warn pump impeller. They can be damaged over time by chlorine based sanitizer, I never use chlorine, just bromine based shock etc.
The other item to check is if you have the right filter for your unit. Filters come in so many styles and densities. You may need to find one that allows better flow.
Assuming you find the root cause of your problem, good housekeeping, keeping as much contaminates as possible out of your tub will help. Very clean bathing suits or none at all,(yahoo) no body cosmetics such as body oils, moisturizers, perfumes etc. Keep chemical treatments to a minimum. Do a quick check weekly for PH, Total alkalinity, and bromine level. Adjust them sparingly so not to overdo any chemicals. Once you get everything under control, maintaining it is very easy and quick. Good Luck
Best Answer
The hardest material on earth to heat
With off-grid homes, they sometimes use solar-thermal panels to collect the sun's heat. The early systems circulated the solar heated water through "tanks full of rocks" during daytime, to heat up the "rocks"... and then at night heated their house with the extra heat.
But what are the best rocks? Which material holds the most heat? Scientists did an exhaustive search, looking at every element and compound known to science. It was -- are you ready for it?
H2O. Plain water.
It holds more heat than any other substance, and that means it is also the hardest substance to heat up.
So let's crunch the numbers. Your tub holds 210 gallons. That's 1750 pounds of water. A "BTU" is the energy needed to heat 1 pound of water 1 degree F. So your tub takes 1750 BTUs to do that. (we're not talking BTU/hr, yet).
Your heater is 1300 watts. 1 watt is 3.41 BTU/hour. 1300 watts is 4433 BTUs per hour.
Applying that to 1750 pounds of water should raise the water 2.533 degrees F per hour (4433/1750). In theory.
You say it's doing about that. So the heater is working pretty efficiently.
Plain power sockets are painfully limited
Why is it only 1300 watts? Because the heater has a common plug-in cord that plugs into any ordinary wall socket. (Mind you, the circuit feeding all the wall sockets on it, has a capacity of only 1800 watts to begin with). In order for an appliance to be certified as safe, a common cord cannot draw more than 1500 watts. It needs some for the pump, so that leave 1300 watts. Ouch.
So 120V is a real pain in the keister.
Now, you had an idea to use a dedicated 20A, 120V socket. Again we must derate 20% so we are at 16A @ 120V, or 1920W. OK, that's 6547 BTU, or 3.7 degrees per hour warmup. Added to your 2.5, that's 6 degrees per hour warmup.
Golfclap
Now, the way I'd do that is, they make "tanked" 4 gallon water heaters that take 120V @ 16A. Gosh, there'll be 4 gallons left in the heater, who cares out of 210? So I'd just plumb that "tanked" water heater to the loop going from the regular heater to the hot tub. You'll want those lines to be insulated, so get some snap-around foam pipe insulation.
Mind you, this heater needs to be hardwired, and it should be on a GFCI for your family's safety. Anytime you're doing some sort of horky-dorky "hack" like this, you should always GFCI-protect everything - and that goes triple for anything involving water! I would install a switch-rated GFCI "deadfront" in the receptacle location, and use that to turn the heater on and off. (it has an "OFF" label for "test" and an "ON" label for "Reset").
Now you're playing with power
The 120V is also a limitation... big heaters use 240V. Let's take a water heater, 30A @ 240V. It's only allowed to use 80% of circuit capacity, or 24A, and builders realistically aim for 23A @ 240V. Let's see what that does for us. 23A x 240V is 5520 watts.
Let's plug that in. 5520 watts is 18,800 BTUS/hour. That's 10.75 degrees per hour. That's a little bit better.
Let's take a modern "on-demand" heater with dual 40A circuits - again runs a bit shy of 80% of that, so 31A x 2 = 14,880 watts. That's 50,740 BTUs/hour. So into our 1750 pound tank, that's 29 degrees per hour gain. That's more like it!
If you want the bigger kick of a 23A heater, do exactly the same above thing, but with a 15-20 gallon water heater that is 5500 watts. The GFCI will need to be a breaker and be in the breaker panel. Which means you will need a disconnect switch. This is a statutory Code requirement, but you actually will want it! Because you need to be able to turn off the booster heater when it's at the right temp (or if the main pump is turned off)! So get a really decent disconnect with a knife switch made to throw everyday, not one of those hokey pull-outs.
If you want the super big kick of an on-demand heater, you'll probably need to upgrade your electric service also. That would be insane; you'd be at temperature in 2 hours.
2 hours, right? Water is really, really hard to heat.