Heater size for hot tub -20 Celsius

[qwertyjjj]

I am thinking of making an 8 person diy hot tub for use in the winter. However, at our location it can sometimes get to -20 Celsius at night, -10 during the day.
What size heater would I need to maintain a good hot tub temperature in those conditions?

 

[Jimrahbe]

Q,

I am not sure of the answer, but to help those that are, please answer a couple of questions...

Will the hot tub be covered, like a stand alone fiberglass spa with a lid?

Do you plan to heat it and keep it hot, or just heat in when you want to use it.

How many gallons of water will the spa hold?

What kind of heater are you planning to use? Gas, propane, electric??

Our experts should be with you soon.

Thanks,

Jim R.

 

[qwertyjjj]

I'm thinking 1,000 gal, maybe 2,000 (2' x 8')

Electric heat.
Will be covered whilst kept hot.
Maybe kept hot for a week then off until next use maybe a free weeks later (would need a heater to prevent complete freezing).

 

[ajw22]

Please put where you are located In your profile.

When you say electric heat you mean electric resistive heat? Electric heat pumps will not work below 50F air temperature.

 

[qwertyjjj]

I'm not sure about heat pumps, I assumed it was resistive.
We do use heat pumps here for heat in houses that work below -20 Celsius but not sure if the same could be used for hot tubs.

 

[ajw22]

We do use heat pumps here for heat in houses that work below -20 Celsius but not sure if the same could be used for hot tubs.

I think those house heat pumps use a coil buried deep in the ground where it is a constant temperature. That is different then a pool heat pump using the air temperature difference for heating or cooling.

 

[ajw22]

1 BTU will raise 1 lb of water 1 degree farenheit in an hour.

1,000 gallons is about 8,300 lbs and 2,000 gallons is 16,600 lbs.

Raypak has resistive electrical heaters from 19,000 BTU to 92,000 BTU. The 92,000 BTU heater requires three 40Amp 240V circuits and uses 27 KW/hr.

In general bigger is better with heaters. I would say to get the largest heater you have electrical capacity for. The insulation and cover effectiveness will determine how much heat loss you have once you get the water up to desired temperature and how much the heater needs to run to maintain that temperature.

E3T – Raypak

Raypak Pool & Spa, Residential and Commercial Hydronic Products -

www.raypak.com

http://cdn.globalimageserver.com/FetchDocument.aspx?ID=00b2d0f3-21a6-47ec-b62b-bbada851c3d1

 

[qwertyjjj]

I think those house heat pumps use a coil buried deep in the ground where it is a constant temperature. That is different then a pool heat pump using the air temperature difference for heating or cooling.

No, they're the air difference ones, they are a stand alone unit. They don't work much below -20 Celsius but until that point they're still pumping out heat. No coil in the ground.

 

[qwertyjjj]

1 BTU will raise 1 lb of water 1 degree farenheit in an hour.

1,000 gallons is about 8,300 lbs and 2,000 gallons is 16,600 lbs.

Raypak has resistive electrical heaters from 19,000 BTU to 92,000 BTU. The 92,000 BTU heater requires three 40Amp 240V circuits and uses 27 KW/hr.

In general bigger is better with heaters. I would say to get the largest heater you have electrical capacity for. The insulation and cover effectiveness will determine how much heat loss you have once you get the water up to desired temperature and how much the heater needs to run to maintain that temperature.

E3T – Raypak

Raypak Pool & Spa, Residential and Commercial Hydronic Products -

www.raypak.com

http://cdn.globalimageserver.com/FetchDocument.aspx?ID=00b2d0f3-21a6-47ec-b62b-bbada851c3d1

The ones I've seen are 4.5kW heaters, only need one 240v circuit and a very 120 for the pump.
I'll check with your calculation, thanks.

 

[ajw22]

The ones I've seen are 4.5kW heaters, only need one 240v circuit and a very 120 for the pump.
I'll check with your calculation, thanks.

4.5kW heater is going to give you around 16,000 BTUs. It may do a degree or two an hour if heating for your size tub.

 

[qwertyjjj]

4.5kW heater is going to give you around 16,000 BTUs. It may do a degree or two an hour if heating for your size tub.

So, it would take about 3 days to heat up. After that I assume they maintain temperature with a cover fairly easily?
Is there a calculation for heat loss or estimate say on the really cold days? I assume your heart calculation above is at reasonable temperatures?

 

[ajw22]

So, it would take about 3 days to heat up. After that I assume they maintain temperature with a cover fairly easily?
Is there a calculation for heat loss or estimate say on the really cold days? I assume your heart calculation above is at reasonable temperatures?

That does not assume any heat loss from the water to the air. It just considers how long to heat the water.

You then need additional BTUs to replace the water heat lost through evaporation and radiation. You need thermodynamic calculations like illustrated here...

Heat Loss from Open Water Tanks

Due to evaporation the heat loss from an open water tank like a swimming pool may be considerable.

www.engineeringtoolbox.com

 

[cfherrman]

Any natural gas service in your area?

 

[jseyfert3]

So, it would take about 3 days to heat up. After that I assume they maintain temperature with a cover fairly easily?

Water Heating Calculator for Time, Energy, and Power

The calculators on this page compute how long it takes to heat water, how much energy is consumed, and how much heating power is required.

bloglocation.com

30 hours for 45 to 100 °F heating for 1000 gallons, 60 hours for 2000 gallons. Not counting heat losses, obviously. There are three calculators on that page. One for how long to heat at a given heat input (kW), how many kWh used to heat, and the third to calculate the size of heater needed to heat in a specific period of time.

Note that at 2000 gallons, heating from 45 °F to 100 °F uses 270 kWh. That's $30 to heat if you pay $0.11/kWh (which is pretty cheap for electric), again not counting heat loss. So consider the size in gallons in your design.

While I fully support DIY projects, I'm curious why you want to DIY a hot tub? Is it cost? Because if not, you can get really nice pre-made tubs with good insulation, lots of jets, and comfortable molded seats. The 6 seat spa that came with our house:

It seems like it would be very difficult and time consuming to DIY the comfort and relaxation that a pre-made, molded hot tub would have.

When we went to the spa store to grab filters, we looked at the ones they had on the floor. There was one that could fit 8 people in comfort, was deeper than mine in the center, for additional legroom, had tons of jets, height adjustable neck jets, etc.

Any natural gas service in your area?

This. If you have natural gas service, use it. Not sure your prices but where I am in the US electric resistive heat is 4.5x more expensive than an 80% efficient natural gas heater. If you have propane that too is likely cheaper than electric, though not by as much. Additionally, both types of gas will allow you to get a bigger heater and heat your large amounts of water in a reasonable time (hours, not days).