Extremely simplistic HP pool heater

theoldwizard1 · 10-30-14, 10:38 AM

I understand the basic refrigeration/heat pump thermodynamics (I had 1 class in thermo back in college about 40 years ago. I don't think the basics have changed ). I have done a lot of reading here and learned/re-learned a lot, but I still have no "hands on" experience. (yet !).

My neighbor has an in-ground swimming that can only be used about 3 months of the year due to our local climate. The gas fired pool heater died a long time ago. He also has about a 2.5 A/C unit.

IN THEORY, if we could plumb a refrigerant/water heat exchange between the compressor and the condenser and then pipe the water from the pool filet through the other side of the HX it would heat his pool and cool his house, correct ?

I want this as simple as possible, so I am thinking full manual control. If there is no cooling demand in the house, no pool heating occurs. Even if there is cooling demand in the house, if the filter pump is not turned on, no pool heating occurs.

Other than all of the issues with plumbing both the pool water and the refrigerant to and from the HX, what is wrong with this idea ? My biggest concern is damaging the HX when there is no water flow and the compressor is running.

Sidebar for a newbie :

Can some one link to a discussion on commercial tube-in-tube coil versus plate HX ?
In an application like a pool heater, where you have a theoretical "infinite sink" how do you size the HX by the size of your wallet ?
How do you calculate the additional refrigerant required ?

F357 · 10-30-14, 11:20 PM

I'm no expert, but...

Quote:

Originally Posted by theoldwizard1

IN THEORY, if we could plumb a refrigerant/water heat exchange between the compressor and the condenser and then pipe the water from the pool filet through the other side of the HX it would heat his pool and cool his house, correct ?

Yep. I did something very basic like this several years ago with a window unit. Basically pumped pool water over the condenser and caught some of it to go back into the pool. It worked quite well, aside from looking like a disaster and leaking water all over.

The air inside will be colder, and it will probably use less watts, too!

Quote:

Originally Posted by theoldwizard1

My biggest concern is damaging the HX when there is no water flow and the compressor is running.

That would be my concern also. You don't want to run it if there is no water flow.

Quote:

Originally Posted by theoldwizard1

[*]How do you calculate the additional refrigerant required ?

It will have to do with the pressures and temperatures you see in your system. You will need a cheap set of A/C gauges for charging to read when it is full.

For a heat exchanger, you could use something as simple as a length of copper tubing submerged in water. (30 feet?)

stevehull · 10-31-14, 06:34 AM

A simple coil of copper would work and could be put in easily.

There are a significant number of threads that describe this situation. Go for it.

Steve

theoldwizard1 · 10-31-14, 08:16 AM

Quote:

Originally Posted by stevehull

A simple coil of copper would work and could be put in easily.

There are a significant number of threads that describe this situation. Go for it.

I have read several of those but I have not found the answers to my questions/issues.

Quote:

Originally Posted by theoldwizard1

My biggest concern is damaging the HX when there is no water flow and the compressor is running.

Sidebar for a newbie :

Can some one link to a discussion on commercial tube-in-tube coil versus plate HX ?
In an application like a pool heater, where you have a theoretical "infinite sink" how do you size the HX by the size of your wallet ?
How do you calculate the additional refrigerant required ?

WyrTwister · 10-31-14, 05:39 PM

How does the energy cost of this compare with the red neck pool heater of a length of black poly pipe & a pump , as a solar system ?

Seems like the problem with what was described is , as the weather gets cooler , the more pool heat will be needed , while less house cooling will be needed ?

God bless
Wyr

jeff5may · 10-31-14, 06:27 PM

IMHO you should not have a problem doing this at all. By the time it is warm enough outside to require air conditioning, there would be no chance of frost. I recommend running a coaxial heat exchanger as a desuperheater. Due to the chlorinated pool water, a cupronickel or stainless steel refrigerant tube is necessary for longevity.

https://surpluscityliquidators.com/v..._TON_COAX_COIL

The pool plumbing could be tapped into like another jet and the hx could be setup like a solar drainback system. All that would be needed is some sort of vacuum breaker at the high point in the plumbing. When the pool water was not being circulated, the ac unit would rely on the original condenser. When the pool pump was running, the added hx would heat pool water when the compressor was active. The hx wouldn't burn up when it was idle, but when it ran it would dramatically increase the efficiency of the ac unit.

theoldwizard1 · 10-31-14, 07:10 PM

Quote:

Originally Posted by WyrTwister

Seems like the problem with what was described is , as the weather gets cooler , the more pool heat will be needed , while less house cooling will be needed ?

You are 100% correct ! This is why it has to be an addon to an existing system and it has to be done inexpensively.

We we are located, pool heat could be used well into June.

theoldwizard1 · 10-31-14, 07:19 PM

Quote:

Originally Posted by jeff5may

IMHO you should not have a problem doing this at all. By the time it is warm enough outside to require air conditioning, there would be no chance of frost. I recommend running a coaxial heat exchanger as a desuperheater.

https://surpluscityliquidators.com/v..._TON_COAX_COIL

Something like that is exactly what I was thinking of ! I'll have to remember the proper terminology - coaxial coil HX.

Quote:

Due to the chlorinated pool water, a cupronickel or stainless steel refrigerant tube is necessary for longevity.

That is a good tip !

Quote:

The hx wouldn't burn up when it was idle, but when it ran it would dramatically increase the efficiency of the ac unit.

This is my biggest concern.

How do you calculate the additional refrigerant required ?

jeff5may · 10-31-14, 07:42 PM

With a capillary tube system, you charge by superheat. With a TXV you charge by subcooling. I would do the charging while the added hx was inactive. All manufactured units have both initial charge weight figures and superheat/subcool recommendations and specs in the install manual. Lots of them have temp/pressure charts listed for a range of common operating conditions also.

With a retrofit or mod into an a/c only unit, I wouldn't even worry about it if you're going to have a short length of plumbing connecting the coax exchanger. I would charge by weight, then take a superheat reading. If it was close to factory specs, I would let it ride. If I had time to burn, I might try to optimize the charge. Even then, not much extra refrigerant would be needed, maybe an ounce or three.

The main thing with this scenario is indoor airflow. If you increase the efficiency of the outdoor unit much, you may end up with a very cold evaporator. Especially with a cap tube system, the reduction in high side pressure could freeze it quickly if there's not enough air moving through it.

theoldwizard1 · 10-31-14, 08:05 PM

Quote:

Originally Posted by jeff5may

With a retrofit or mod into an a/c only unit, I wouldn't even worry about it if you're going to have a short length of plumbing connecting the coax exchanger. I would charge by weight, then take a superheat reading.

Newbie here !

Yes, the plan would be to to mount the HX as close as possible to the compressor outlet.

I don't understand "charge by weight" and where do you take "a superheat reading" ? (Is that the temp coming out of the compressor ?)

10-30-14, 10:38 AM	#1
theoldwizard1 Apprentice EcoRenovator Join Date: Oct 2014 Location: SE MI Posts: 105 Thanks: 3 Thanked 12 Times in 9 Posts	Extremely simplistic HP pool heater I understand the basic refrigeration/heat pump thermodynamics (I had 1 class in thermo back in college about 40 years ago. I don't think the basics have changed ). I have done a lot of reading here and learned/re-learned a lot, but I still have no "hands on" experience. (yet !). My neighbor has an in-ground swimming that can only be used about 3 months of the year due to our local climate. The gas fired pool heater died a long time ago. He also has about a 2.5 A/C unit. IN THEORY, if we could plumb a refrigerant/water heat exchange between the compressor and the condenser and then pipe the water from the pool filet through the other side of the HX it would heat his pool and cool his house, correct ? I want this as simple as possible, so I am thinking full manual control. If there is no cooling demand in the house, no pool heating occurs. Even if there is cooling demand in the house, if the filter pump is not turned on, no pool heating occurs. Other than all of the issues with plumbing both the pool water and the refrigerant to and from the HX, what is wrong with this idea ? My biggest concern is damaging the HX when there is no water flow and the compressor is running. Sidebar for a newbie : Can some one link to a discussion on commercial tube-in-tube coil versus plate HX ? In an application like a pool heater, where you have a theoretical "infinite sink" how do you size the HX by the size of your wallet ? How do you calculate the additional refrigerant required ? Last edited by theoldwizard1; 10-31-14 at 08:12 AM..

10-31-14, 06:34 AM	#3
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	A simple coil of copper would work and could be put in easily. There are a significant number of threads that describe this situation. Go for it. Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990

10-31-14, 06:27 PM	#6
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	IMHO you should not have a problem doing this at all. By the time it is warm enough outside to require air conditioning, there would be no chance of frost. I recommend running a coaxial heat exchanger as a desuperheater. Due to the chlorinated pool water, a cupronickel or stainless steel refrigerant tube is necessary for longevity. https://surpluscityliquidators.com/v..._TON_COAX_COIL The pool plumbing could be tapped into like another jet and the hx could be setup like a solar drainback system. All that would be needed is some sort of vacuum breaker at the high point in the plumbing. When the pool water was not being circulated, the ac unit would rely on the original condenser. When the pool pump was running, the added hx would heat pool water when the compressor was active. The hx wouldn't burn up when it was idle, but when it ran it would dramatically increase the efficiency of the ac unit. Last edited by jeff5may; 10-31-14 at 06:44 PM..

10-31-14, 05:39 PM	#5
WyrTwister Master EcoRenovator Join Date: Jun 2013 Location: USA Posts: 543 Thanks: 6 Thanked 44 Times in 38 Posts	How does the energy cost of this compare with the red neck pool heater of a length of black poly pipe & a pump , as a solar system ? Seems like the problem with what was described is , as the weather gets cooler , the more pool heat will be needed , while less house cooling will be needed ? God bless Wyr

10-31-14, 07:42 PM	#9
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	With a capillary tube system, you charge by superheat. With a TXV you charge by subcooling. I would do the charging while the added hx was inactive. All manufactured units have both initial charge weight figures and superheat/subcool recommendations and specs in the install manual. Lots of them have temp/pressure charts listed for a range of common operating conditions also. With a retrofit or mod into an a/c only unit, I wouldn't even worry about it if you're going to have a short length of plumbing connecting the coax exchanger. I would charge by weight, then take a superheat reading. If it was close to factory specs, I would let it ride. If I had time to burn, I might try to optimize the charge. Even then, not much extra refrigerant would be needed, maybe an ounce or three. The main thing with this scenario is indoor airflow. If you increase the efficiency of the outdoor unit much, you may end up with a very cold evaporator. Especially with a cap tube system, the reduction in high side pressure could freeze it quickly if there's not enough air moving through it.