How long does it take to heat your spa?

[RDspaguy]

It’s 240 v

Ok. I don't really have time for a lesson in basic electrical theory right now. Believe whatever you want.

 

[jseyfert3]

You are most likely burning more electricity heating it from ambient temperature to 100 than if you would just leave it on and let it maintain.

That's a common myth. Assuming insulation is the same between leaving it on and letting it cool when not in use, and your heat source is constant (i.e. not an air source heat pump) it's always more energy efficient to turn off the heater when not in use. This applies to everything. Letting a spa cool between uses saves energy over keeping it hot. Letting a house cool in the winter when not home saves energy over keeping it at time. Etc. This is because the standby losses are linearly related to the temperature difference between the item of interest (in this case the water temp) and the ambient temp. The higher the temp difference, the higher the standby losses. So by letting it cool the standby losses are reduced over keeping it at temp.

Q=cp * d * V * (Tf - Ti)

joules of heat transferred = isobaric specific heat capacity measured in joules per gram Kelvin difference * density measured in grams per cubic metre * volume in litres * (final temp - initial temp in centigrade)

isobaric specific heat capacity of water = 4.18 joules per gram Kelvin difference
density of water = 1 gram per cubic meter

For my 1643L spa, with incoming water temp of 6.5C, target temperature of 38C, this is 215,149 joules of heat transferred = 215.149 megajoules, divide by 3.6 for kWh = 59.764kWh.

For a 4kW heater, divide by 4 = 14.941 hours to transfer that heat assuming zero heat loss through cover/sides/plumbing. So consider your resulting number the absolute minimum amount of time to heat. At $0.22/kWh, electricity cost is $12.97, and at $1.58/cubic metre of water, water cost is $2.60.

I prefer this : Water Heating Calculator for Time, Energy, and Power


@Sjde 50 °F/11 hrs is 4.3 °F/hr. This doesn't account for standby losses but they should be minimal for a well insulated tub with a tight fitting cover in good condition. If you are getting roughly ~4 °F/hr with your 4 kW heater on 375 gallons than everything is working appropriately. But you said 2-3 per hour so this doesn't seem like everything is working as good as it should.

I found out ours is a flow through type heater, 60 hertz and 600 watts.

6000? That's not what it appears to be from the blurry zoomed in pic (it appears to be 4 kW). Can you post the pictures asked for already in this thread and we can help you figure this out?

Tried to enlarge the tag on the heater, looks like it may be 4kW. All of the info is on that tag if you can get a better pic of it.

I've been waiting for the pics, but I need to see the wiring connections at the breaker too. You will have to remove the faceplate, there is usually a screw at the bottom. Turn off the main breaker in the house before removing.

Post a picture of the wiring diagram and equipment area so I can see what's what.

 

[Joe kipp]

@jseyfert3 Thanks for the link. We filled and started our new hot tub on Monday and it matched the time from your calculator almost exactly. We're seeing about 4 deg of change per hour.