## Freeze prevention for a hot tub – page 4 gas and sand

First, you need to make a few decisions: Is this to be autonomous, or will you require the homeowner(s) to take action outdoors during a winter blackout? Do you want to assure that it’s protected from freezing, or is a halfway measure acceptable?

If you want to assure that it’s protected from freezing, you can’t just guess at the amount of heat required. Calculate the surface area exposed to the weather (including the bottom) and the effectiveness of the insulation. ("R" value) That, and the temperature difference required (I suggest a ΔT of 40-50°F, depending on which part of Massachusetts it’s in) will tell you how much heat is required.

Making a few rough estimates, I calculated that a few thousand BTU/hour (several hundred watts) are required for long-term protection, and that it will take a few days for the water temperature to drop to freezing. Several more days would be required for the tub to freeze solid, but you can’t rely on that because the water in the pipes and pump body won’t benefit from the thermal inertia of the ton of water in the tub.

First, you need to make a few decisions: Is this to be autonomous, or will you require the homeowner(s) to take action outdoors during a winter blackout? Do you want to assure that it’s protected from freezing, or is a halfway measure acceptable?

If you want to assure that it’s protected from freezing, you can’t just guess at the amount of heat required. Calculate the surface area exposed to the weather (including the bottom) and the effectiveness of the insulation. ("R" value) That, and the temperature difference required (I suggest a ΔT of 40-50°F, depending on which part of Massachusetts it’s in) will tell you how much heat is required.

Making a few rough estimates, I calculated that a few thousand BTU/hour (several hundred watts) are required for long-term protection, and that it will take a few days for the water temperature to drop to freezing. Several more days would be required for the tub to freeze solid, but you can’t rely on that because the water in the pipes and pump body won’t benefit from the thermal inertia of the ton of water in the tub.

(I’m being deliberately vague because I don’t know any of the actual parameters)Hopefully we are shooting for only being able to deal with a week maybe two weeks without power – with unknown low temperatures. Major disaster and anticipated longer outage – maybe better just drain the tub if it is going to be cold enough to freeze up if you don’t otherwise have sufficient standby power to allow it to run normally.

so there’s perhaps your answer. if there is no NG then get a LP tank big enough to heat water on lowest setting for 30 days. you can create a closed loop and use gen power to turn a water pump to recirc the water around when needed. if the gen dies then it is possible to use city water pressure to turn a small water pump.

a solar solution is also viable. ~300watt of panel + 4 12v (two in series for 24v then parallel the two sets). you can use sun during day to power a pond heater in the tub to keep it hot and should remain above freezing when no sunlight, and then you can use a low temp sensor to have batts kick on to power the pond heater if needed.

i have a 7×7 hot tub, built in a in-ground vault. under the slab is ~35ft of 2" PVC (surface area is key, so parallel paths), my tub has jandy valves so when its 100F+ outside i turn valves so that the water circulates underground through the PVC (approx 4ft underground) where the soil is cool, thus removing heat from the water (which stays too hot even w/o heater on, due to daily heat soak).

one could do the same to keep a tub from freezing, use solar power (or the generator) to power a small water circ pump, bury a coil of pex or something to make a heat exchanger (bury ~4ft below frost line), the heat from the ground will always be above freezing, etc. if a coil is not doable then even a ~50-100gal poly tank buried in the ground will work, fill that tank with water and some propylene glycol, circulate it to a smaller coil that is in the tub water, or you could even just use a small transmission cooler and small DC fan underneath/inside the tub skirting, small solar power (or gen) can run fan (fan mounted on cooler) and circ pump.

or, propylene glycol to serve as the protection is power dies for extended period, you’ll just need to drain it later before next use, etc. but this requires the glycol to at least circulate, which you could still do with a small gen before everything else in house gets turned on, etc. add glycol, run pumps for ~1-2min, turn off breaker to tub, power on the house. but this still requires a body to drain some water 1st, then add lots of glycol, so perhaps not a ez solution at all.