Heat exchanger size for hot tub

[jpowell1979]

I have a 7 person, 600g. hot tub that I'm installing outside in Fairbanks, Alaska. Winter temps down to -50. I certainly don't want to pay 18 cents kWh to heat it when I have a Greenwood wood boiler. I have built the surround out 2 inches with rigid insulation and have a 5" insulated cover so heat loss will be down to a minimum. The tub is 20 feet from the boiler manifold and I will be running some left over 1" insulated pex out to the tub and installing a 007 taco pump for circulation.

Anyone have any suggestion on heat exchanger size and any other ideas on how to make this work? Timers, aquastats?

 

[Bad Wolf]

I’ll try to get some pic’s for you tonight. I made a nofossil style heat exchanger from some 4” PVC . I used about 15’ of ½” copper pipe folded back and forth inside the PVC. I put it in series with my existing electric heater and disconnected the heating element. So now instead of turning on the 240 vac element it turns on a 120vac relay that in turn turns on the circulating pump. My heating system treats it like another zone.
I haven’t been through a winter yet but it seems to heat up the tub faster than the electric heater did. My tub is 7’ by 7’, 500-600 gallons