Freeze Prevention for a Hot Tub

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kwired said:
I've been there, power is out during cold weather, one doesn't even think of hot tub once power is out. Most outages are only a few hours at the most. it is when that unusual condition happens and it is say 10 days when that hot tub will get you. If it isn't too extremely cold it likely pulls through without freezing. By the third or fourth day I might think about the hot tub, it is then I will place something in the plumbing area to provide some heat - or maybe even drain the tub. The tub I have is a small one and just draining it does remove enough water from critical places you shouldn't need to worry about any residual water causing damage if it freezes though.
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maybe no electric at all. open drain just slightly, run a hot water garden hose to fill just slightly. or loop hose around in tub and then have it hang over the side, allow hot water to slowly go through the hose.
or even a garden hose loop that goes from hot water tank into a loop in tub and then back inside to a util sink where you can control the flow, if hot water is going out and cold water is coming back, should keep the tub water from freezing.
most skirted tubs are not insulated behind the skirting (or barely), typically only the cover is some type of insulator.
it might take a few days just for tub water to dip down to low temp threshold, which is probably above freezing.
or if possible to run pump, dump propylene glycol into the water and run low speed pump for a min or two, drain later (check with maker if a propylene glycol is safe for tub internals)

or, make sure gen is big enough to power the tub.

i like the last solution best ;)
 
Thanks for all the ideas. The one i like best is putting a small heat source below the tub. Either a lightbulb or some type of small strip heater. All the other ways of heating the water in the tub might not get the plumbing warn underneath. The idea of sizing the generator for the house load plus the tub is just too large of a generator. It would consume so much gasoline it would be more difficult to keep the gen fueled.
 
How about if you ran the hot tub off the generator, but only for 15 minutes or so per day, while you pretty much shut off all the house loads?
 
gadfly56 said:
1. City water
2. Gas-fired water heater, with a standing pilot.
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OK. But city water isn't likely to be hot.
And a gas fired heater may have some electrical control.........and if you have no electricity.............
Actually, the very notion of a hot tub freezing is beyond my horizon.
 
Besoeker said:
OK. But city water isn't likely to be hot.
And a gas fired heater may have some electrical control.........
Actually, the very notion of a hot tub freezing is beyond my horizon.
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During tropical storm Sandy, we lost power for 3-1/2 days. The hot water heater worked like a charm throughout. When the house is pushing 55°F, a hot shower is a true blessing.
 
i still have hot water without poco, gas fired heater and city water, always pressure and hot. obviously an issue if the home is not setup that way, etc.

and cold city water still is not a bad idea, it is well above freezing, but that means you still need to keep tub drain open a tad and then balance the input so it does not overflow, or, make a garden hose loop in the tub and then let the end of hose stay on ground, meter the water flow so that you dont use too much city water.

make the gen bigger, leave tub alone at a min set temp ;)
 
mkgrady said:
Is there a way to keep a hot tub from freezing during a power outage utilizing a portable generator? I'm looking at powering a home with a 7.5KW portable during any power outages. It will all be done to code with an interlock, inlet, signage, inspection and all the rest. I have done several of these already.

Of course the HT needs a 50 to 60 amp circuit to fully function so the generator will not be large enough for that. I would expect it to be able to run just the pump and not the heat but i don't even know if that is an option on a hot tub. If not, are there other ways of keeping the thing from freezing. I'm picturing running a cord from a GFCI receptacle and plugging in some kind of submersible heater that might draw only a few amps.
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I'm thinking Wesson oil?
 
FionaZuppa said:
make the gen bigger, leave tub alone at a min set temp
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It was right about new years day 2007 when we lost power for 10 days after an ice storm. I have a 8000 watt portable generator. If I would have fed gas to that thing for a month at that time would have amounted to about a $900 electric bill compared to the usual $150-200 bill from the electric company at that time of the year. It wasn't extremely cold -hot tub (which I didn't have one at that time) would have likely made it through those 10 days without freezing. Adding a 60 or 100 watt lamp inside the mechanical compartment definitely would have been enough heat to not worry about much. If outage were going to be reaching 30 days - I would have drained the thing, and we would have went to town and stayed with friends or relatives.
 
kwired said:
He has city water and a gas water heater. If that gas WH has a standing pilot it needs no electric power. Ignition is via the standing pilot and combustion draft is via natural convection.
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Yes, that's what I thought probably FZ meant but
"city water, always pressure and hot" didn't quite read that way
 
Enough with the light-bulb jokes already. (or Appalachian Engineerin' proposals, whichever the case may be)

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 it's to be autonomous, and upsizing the generator isn't an option, I recommend installing a strip heater under the hot tub, with a thermostat set for 40°F, on its own independent circuit which will be live during generator operation.
(and the main hot-tub circuit needs to be interlocked with the generator circuit)

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)
 
drcampbell said:
Enough with the light-bulb jokes already. (or Appalachian Engineerin' proposals, whichever the case may be)

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 it's to be autonomous, and upsizing the generator isn't an option, I recommend installing a strip heater under the hot tub, with a thermostat set for 40°F, on its own independent circuit which will be live during generator operation.
(and the main hot-tub circuit needs to be interlocked with the generator circuit)

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)
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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.
 
kwired said:
Hopefully we are shooting for only being able to deal with a week maybe two weeks without power - with unknown low temperatures. ...
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Because the small water-filled spaces (pipes & pump bodies) will freeze within a day or two, the same preparations are required for riding out a two-day outage as for a week or a month.

The low temperatures are far from unknown; they're published by ASHRAE*. Boston, for example, can expect temperatures at or above -14°C to occur 99% of the time, (all but 3.65 days out of the year) making that a reasonable temperature for designing heating equipment.

These data were published a while ago and do not take global climate change into account. Current and future winters (and summers) are likely to be more severe.
*American Society of Heating, Refrigeration & Air-Conditioning Engineers, www.ashrae.org

ashrae-data-massachusetts.jpg
 
drcampbell said:
Because the small water-filled spaces (pipes & pump bodies) will freeze within a day or two, the same preparations are required for riding out a two-day outage as for a week or a month.

The low temperatures are far from unknown; they're published by ASHRAE*. Boston, for example, can expect temperatures at or above -14°C to occur 99% of the time, (all but 3.65 days out of the year) making that a reasonable temperature for designing heating equipment.

These data were published a while ago and do not take global climate change into account. Current and future winters (and summers) are likely to be more severe.
*American Society of Heating, Refrigeration & Air-Conditioning Engineers, www.ashrae.org

View attachment 20209
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I agree but conditionally. If the outage occurs in December, January or February (around here) that chance of being cold enough to be an issue increases. If the tub were up to normal operating temp when the outage starts and the outdoor temp stays above maybe 20 deg F, and the outage doesn't last for more then a couple days - probably not going to cause a problem.

Wind chill will complicate this if tub is subject to it.

Pumps and some piping are the first places that will freeze - it doesn't take all that much heat inside the mechanical area to make a huge difference though. If it is constantly below maybe 10 deg F and or with wind chill effects - it might freeze at least some components anyway.
 
kwired said:
He has city water and a gas water heater. If that gas WH has a standing pilot it needs no electric power. Ignition is via the standing pilot and combustion draft is via natural convection.
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correct.

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 7x7 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.

creating a system using small LP tank is also possible.

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.

the problem still is not clear to me. but, i think the best solution is one that works when say nobody is there for the length of the power outage, this way the tub is fully protected with no gen power and nobody around to do anything, etc.

many ways to skin this cat, some easier than others. :thumbsup:
 
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