A/C compressor or heat pump used as hot tub heater?

Rickyd

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@freedom_in_4low I'm hoping you could weigh in on this as an engineer. I'm just an installer/maintenance guy.
I have several functional heat pump takeouts from customers and a few ac compressors as well.

I was googling alternative hot tub heating methods today and stumbled across heat pump hot tub heaters using 1/4 of the power. I know the easy answer is just buy a purpose built heat pump but wonder how hard it would be to rig something up with what i have?

What would the ramifications of losing all that heat from the high pressure side to a heat exchanger before an orifice or txv have? Thanks in advance for entertaining this hair brained scheme
 
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.I wonder how many BTUs it takes to maintain the hot tub temperature?

That will be dependent on tub insulation,ambient temps and other stuff. Like how much you use it.

I found this formula for btu's-

The amount of BTU's necessary to heat water is given as: BTU's = 8.33 x number of gallons x number of °F temperature rise.
 
I've been in the mountains with no service so I'm just now seeing this.

So would the intent be to pipe a hot tub heater upstream of the indoor unit so you'd heat the hot tub AND the house during winter? Or are you just wanting to rig up a standalone heat pump that heats the hot tub?

If the former, you could pretty easily suck all the heat out and not have any left for the house. Might be doable with a diverting valve so you can choose whether you want to heat the house or the tub.

I'll be on the road all day so it's hard to run numbers until I get home but I can address the general concepts at play.
 
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I've been in the mountains with no service so I'm just now seeing this.

So would the intent be to pipe a hot tub heater upstream of the indoor unit so you'd heat the hot tub AND the house during winter? Or are you just wanting to rig up a standalone heat pump that heats the hot tub?

If the former, you could pretty easily suck all the heat out and not have any left for the house. Might be doable with a diverting valve so you can choose whether you want to heat the house or the tub.

I'll be on the road all day so it's hard to run numbers until I get home but I can address the general concepts at play.

My intent is to rig up a unit solely for the hot tub using either a heat pump or refrigeration compressor.

I'm leaning towards refrigeration parts from something like a walkin fridge as they can be run with a minimum of electronics and are easily configured.
I'm unsure how I'd keep the heat pump and all its electronics happy.But, if i could just run a heat exchanger on the high side during heat mode and rig a water temp sensor in place of air temp that would be a nice self contained unit?

Ideally a heat exchanger that allowed the high side coil to contact the water with a titanium coating to survive the hot tub corrosion would be simplest. I'll need to investigate hot tub heat exchangers
 
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My intent is to rig up a unit solely for the hot tub using either a heat pump or refrigeration compressor.

I'm leaning towards refrigeration parts from something like a walkin fridge as they can be run with a minimum of electronics and are easily configured.
I'm unsure how I'd keep the heat pump and all its electronics happy.But, if i could just run a heat exchanger on the high side during heat mode and rig a water temp sensor in place of air temp that would be a nice self contained unit?

Ideally a heat exchanger that allowed the high side coil to contact the water with a titanium coating to survive the hot tub corrosion would be simplest. I'll need to investigate hot tub heat exchangers

I would lean toward the heat pump simply because the defrost controls are already built in. I don't know a lot about your climate but I suspect it would at least occasionally get cold enough to build ice on the outdoor coil and at that point will need to stroke the reversing valve to run in cooling mode to thaw it out...or if the ambient stays above 32 you could turn the compressor off but run the fan instead.

If you want to use the refrigerated walk-in parts, just make sure the compressor comes from a cooler (like 30-40F) and not a freezer, because freezer compressors move a lot less mass flow so they usually have smaller motors that will overheat in a medium temp application. Some circuitry with a delay timer and a strategically placed klixon could probably do what you need for the defrost.

If you find a hot tub heat exchanger and can post the specs on it along with the expected flow rate on the water side I can help figure out what compressor size it would best match up with.
 
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Ok, looks like these would work

https://www.hotspotenergy.com/titanium-pool-heat-exchangers/
Screenshot_20231125_151030_Chrome.jpg
 
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The coolest I've seen is 14* here. I do have a functional 30k fujitsu heat pump sitting in my garage right now
 
I can't imagine those would have enough BTUs to heat a hot tub sufficiently. I have a 500 gallon in ground hot tub that uses a 400,000 BTU heater and it works it when it's cold.
 
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The coolest I've seen is 14* here. I do have a functional 30k fujitsu heat pump sitting in my garage right now

Yeah that's definitely cold enough. Hard to say the cutoff exactly but it's safe to say any time it's running in an ambient below 40 it's going to need to run a defrost.
 
I can't imagine those would have enough BTUs to heat a hot tub sufficiently. I have a 500 gallon in ground hot tub that uses a 400,000 BTU heater and it works it when it's cold.

In that case may I recommend this 35 ton heat pump that will produce 125F water down to -10F ambient, made in USA and with control software written by yours truly! 🤣

Screenshot_20231125-215314.png


That's gotta be gas, right? That would pull more than double what most residences have for their entire electrical service at 230V. Surely people use hot tubs with electric heaters which would likely be no more than 50-60A, bringing it down to a more achieveable 40,000 BTU/hr, right?

In the end it's all about your expectations for how long it should take to heat up, how cold it can be outside and still be usable, and how well it holds while you're in it with the cover off.
 
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In that case may I recommend this 35 ton heat pump that will produce 125F water down to -10F ambient, made in USA and with control software written by yours truly! 🤣

View attachment 476979

That's gotta be gas, right? That would pull more than double what most residences have for their entire electrical service at 230V. Surely people use hot tubs with electric heaters which would likely be no more than 50-60A, bringing it down to a more achieveable 40,000 BTU/hr, right?

In the end it's all about your expectations for how long it should take to heat up, how cold it can be outside and still be usable, and how well it holds while you're in it with the cover off.

That's awesome. Defiantly overkill for a hot tub. Mine is gas heated, and doubles as the heater for the pool. It's oversized for the hot tub alone, but on cold days when the water is down in the 30s and the wind is blowing it takes an hour to get it warm. I'm guessing that the electric units rely heavily on the insulation and cover to keep them warm.
 
I can't imagine those would have enough BTUs to heat a hot tub sufficiently. I have a 500 gallon in ground hot tub that uses a 400,000 BTU heater and it works it when it's cold.

I'm hoping one could maintain something like 90* for me while keeping the normal heating element as backup. Smaller refrigerant lines definitely aren't going to reach many hundreds or thousands of degrees.
 
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I'm hoping one could maintain something like 90* for me while keeping the normal heating element as backup. Smaller refrigerant lines definitely aren't going to reach many hundreds or thousands of degrees.

Ooo I need 99-103 depending on the temperature outside at the time. 90 is a bit chilly.
 
Ooo I need 99-103 depending on the temperature outside at the time. 90 is a bit chilly.

I agree. I'm looking to constantly heat a smaller 80"×80" tub cheaply.then probably set a timer for higher temps. Pouring a pad with insulation under it,more spray foam on the tub and other things are all up in the air.

I'm jealous of your pool/hot tub! I love going back and forth between the hot and cold water
 
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