A/C Heat Exchanger to Heat Pool - DIY Questions

Don_Q

Member
Oct 30, 2019
6
Orlando, FL
Hey all... after struggling to find any contractor in Central Florida that would tackle installing an FHP Hotspot or any type of HVAC heat pump exchanger, I decided to venture this on my own. Well, after starting and not finishing, I have questions. Hopefully someone here could help clarify?

For cost, I went with a Doucette Industries device, which is pretty much the same thing without the pretty blue enclosure and all of the accessories (and apparently customer support):

The first problem... zero instructions and zero customer support. The kept referring to page 3 of the brochure and kept telling me any HVAC guy could figure it out. However, my HVAC started the job but couldn't figure out after several days of working on it and promptly quit, leaving my without AC at the moment.

His issues were the below:
  • First, he installed a manual switch to the air compressor fan. This way I can manually shut off the compressor fan when the pool is being heated.
  • Second, we intercepted the refrigerant line coming out from the house evaporator, and routed it to the pool heater.
  • Then from the output of the pool heater, re-connected to the refrigerant line that was going to the condenser.
  • At this point, whenever we turn on the HVAC unit to normal operation, it freezes the coils.
Pictures here:
Does anyone know what he did wrong?
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
16,227
Northern NJ
My experience as an unknowledgeable consumer is whenever my AC freezes up it turns out to be low refrigerant.

With the rerouting of the refrigerant line it would need a different amount. Has the proper amount of refrigerant been added? How did you know what the correct amount is? You sure some has not leaked out?
 

jseyfert3

Bronze Supporter
Oct 20, 2017
677
South-Central WI
EDIT: Nope. I got off on the wrong track. I was right the first time. Let me think about this a bit and I'll post again. There are a few things I see wrong, but they may or may not be dealbreakers.
 
Last edited:

Brad_C

Well-known member
Nov 15, 2018
143
Perth, Western Australia
Looks like you've connected the pool loop in the wrong spot. It has to go between the compressor and the evaporator, taking the place of the condenser. You appear to be pumping already condensed liquid through that loop.
That'll do 2 things.

1) it won't reject much heat into the pool water as even with the condenser fan switched off you'll end up rejecting a considerable amount of heat through the condenser just by convection.
2) You've starved the evaporator coil of refrigerant. The refrigerant charge is calculated to fill the evaporator, the line supplying it and back some up into the condenser to allow for subcooling. You've just significantly increased the size of the liquid line, so it'll need a small metric crapton of additional refrigerant, which I'll lay odds hasn't been accounted for.

The pdf you linked has a neat little diagram on how you might hook it up. It would appear from the photo you also attached that you have nothing like that configuration. Note the 3 way valve and several non return valves in the diagram.
This is all predicated on your inside evaporator coil having the TXV fitted there. If it's in the outdoor unit then you need a serious rethink as you'll be cooling the pool, probably icing up the coil and causing all sorts of injury.

So if your AC has the TXV on the coil, you can get your AC working by adding additional refrigerant such that the evaporator is appropriately filled. You'll be putting a little bit of heat into the pool.

You really need that loop right at the output of the compressor to catch the condensation and de-superheating to get a worthwhile amount of heat out of it.
 

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
2,564
Stuart/FL
Here's what looks wrong to me:
  • First, he installed a manual switch to the air compressor fan. This way I can manually shut off the compressor fan when the pool is being heated.
  • Second, we intercepted the refrigerant line coming out from the house evaporator, and routed it to the pool heater. Instruction diagram shows connection on the refrigerant line coming out of the compressor not evaporator in the house. Are you sure you described this correctly? Looks like he actually did the connection to the small uninsulated refrigerant which would be the correct line.
  • Then from the output of the pool heater, re-connected to the refrigerant line that was going to the condenser. Instruction diagram shows this connected to the output of the condenser. Can't tell from your photos if this is where he made the connection. Also diagram shows several check valves and a new 3-way expansion valve. I can't see any of this in your photos.
  • At this point, whenever we turn on the HVAC unit to normal operation, it freezes the coils. Did the tech check the charge and discharge pressures so he knew how much refrigerant to add. Are you sure he charged with Puron?
If your tech did the connections properly and had the right amount/type of refrigerant then the new 3-way valve could be failing. I don't see this in the new piping. All I can see in your photos is that he installed the new coil in the high pressure line with no 3-way valve and no check-valves. This won't work, everything needs to be installed as shown in the brochure. Can you show photos of each component? Did all of the components come with the kit and do you have any parts left over? I'll also ask @swamprat69 to comment. He's a very knowledgeable HVAC expert.

Chris
 

jseyfert3

Bronze Supporter
Oct 20, 2017
677
South-Central WI
Went to bed before I finished my post, and it looks like it's mostly been answered, but I'll throw in my two cents and diagrams anyway.

Okay, so first off, I'm not an HVAC guy. We did cover all sorts of heat cycles, including heat pumps, in my thermodynamics classes in college.

Firstly, let's look at a normal AC system. This consists of four main parts, so far as the refrigerant is concerned. These are the compressor, condenser, expansion valve, and evaporator. The compressor takes vapor and compresses it. It is now very hot, and is piped through the condenser, where it is cooled and condenses by a fan drawing air across it (the condenser fan, which you called the compressor fan). This liquid refrigerant, now closer to ambient temp, is piped into the house. Once at your HVAC system, it is pushed through an expansion valve. This causes it to rapidly drop in temperature, and some of the liquid becomes vapor. The rest of the liquid evaporates in, well, the evaporator, which is the set of coils inside your HVAC unit (what makes the air in your house cold). Now completely vapor, the refrigerant makes its way back to the compressor in the outdoor unit to start all over again.
Normal AC.png

Now let's go over how the manufacture said to install this. I referenced the Hotspot FPH brochure as well, as it was a little more descriptive and has the exact same setup. They need you to intercept the refrigerant line between the condenser and compressor and install a 3-way valve. You then need to tap the line after the condenser and add a check valve. One connection on the 3-way valve goes to the evaporator (suction) line, which should be the bigger, insulated line coming out of the ground. The other connection runs to the pool heater. After the pool heater is another check valve, before the two lines tee together and run into the house as normal.

According to the Hotspot FPH brochure, what the 3-way valve does is swap refrigerant flow between either the condenser or the pool, based on a thermostat that measures the pool temp. It also evacuates either the condenser coil or the pool heater coil, whichever isn't being used, likely so the overall system charge is not changed much. That's what the connection to the suction line is for, along with the check valves. Note that the 3-way valve (and check valve) doesn't have to be inside the AC Unit as I've drawn, but you will need to tap lines inside the AC unit to install it, as you need to tap between the compressor and condenser, and that line doesn't come outside of the AC unit.
Manufacture Instructions.png

From what you've said, and the pictures you've posted, I believe this is how you actually connected things. While not remotely like the manufacture outlines, if it is actually how you installed it it should work (again, I'm not an HVAC guy, and this isn't ideal, which I cover below). If this is how it was installed, then your only "issue" is that the wrong type or amount of refrigerant was put into the system after it was connected.
DIY mod.png

Now, what's actually wrong with how you did it? You didn't put the pool heater on a 3-way, pool water thermostatically controlled valve (along with the check valves). The first issue with this:
1) it won't reject much heat into the pool water as even with the condenser fan switched off you'll end up rejecting a considerable amount of heat through the condenser just by convection.
The condenser is still in the loop, and has a ton of fins. You'll loose a good bit of heat even with the fan off, before the refrigerant makes it to the pool heater. Additionally, I wouldn't be surprised if the compressor wanted some of that airflow to help keep from overheating, in which case shutting off the fan is a bad idea, but I'm not certain on that.

Second issue: Lack of heat regulation in your pool. The primary purpose of the 3-way valve is to switch between pool heating and normal AC operation. This is based on pool water temp so you don't, for example, heat your pool when it's 100 °F out and your pool water is already 95 °F. Now, most of the heat is lost in the condensing process, so the pool heat gain is minimal if the condenser fan is running, but there will still be some. I think the biggest issue with your setup is that the manual fan switch may cause the compressor to overheat, especially being surrounded by hot condenser coils (but I'm not an expert, not sure if this would be the case. It seems logical though). If the 3-way valve was installed, then when not heating your pool, the condenser coils would be evacuated, so they'd be cool, and the condenser fan would be running still, so the compressor would stay cool. Actually it would stay cooler than normal, likely leading to better efficiencies.

The other thing is that instead of being a fully automatic setup, the manual switch means you have to physically switch the system from heating the pool to not heating the pool.

My uneducated summary is that your setup should, in theory, work, though not as well as if it was installed as designed with the 3-way thermostatically controlled valve and check valves. It will cause the compressor to run hotter than designed, with potential negative effects. It's likely that the tech put the wrong or too little refrigerant into your system. If he charged it with the amount specified, it's likely underfilled, and underfilled systems freeze up the evaporators (another bit the 3-way valve takes care of, since it evacuates the unused coil, so the total amount of refrigerant shouldn't change). He should have charged it using pressure gauges instead.

Clever system though, when installed as directed. I may consider adding it to our house. Of course the obvious problem for me is peak AC usage is also the hottest time of the year, and we may not want additional heat at that point, so the cost/benefit may not be worth it. Likely better for the OP, being in Florida, as I imagine you use AC year-round down there. For myself, I'll look into putting a data logger for AC run time and pool temp throughout the year, then next winter compare the data and see if it would help with our particular location/pool/temps.
 

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Last edited:

Joshii

Well-known member
Jul 15, 2013
240
A couple of us on here have the Hotspot system and are big fans. See this thread Hotspot FPH AC heat reclamation pool heater - a review!

Bottom line is, for an uncovered pool, in cool climates, this system will keep your pool 5-8 degrees F hotter than it normally would be.

I hear the objection that the system only provides heat during the warm months when you don't want it. Personally, I never don't want it! I would keep my pool 90 if I could. I dump as much heat as possible into it from April to October.

(Of course, I don't want it enough to actually keep a solar blanket on the pool, but that's a different story)
 

Don_Q

Member
Oct 30, 2019
6
Orlando, FL
I am having a different HVAC tech out. I'll ask him to check this first. Thanks!!

My experience as an unknowledgeable consumer is whenever my AC freezes up it turns out to be low refrigerant.

With the rerouting of the refrigerant line it would need a different amount. Has the proper amount of refrigerant been added? How did you know what the correct amount is? You sure some has not leaked out?
 

Don_Q

Member
Oct 30, 2019
6
Orlando, FL
I now see that! BTW, no 3-way valve and no non-return (check) valves. Is the 3-way valve a manual valve or how is it operated?

Looks like you've connected the pool loop in the wrong spot. It has to go between the compressor and the evaporator, taking the place of the condenser. You appear to be pumping already condensed liquid through that loop.
That'll do 2 things.

1) it won't reject much heat into the pool water as even with the condenser fan switched off you'll end up rejecting a considerable amount of heat through the condenser just by convection.
2) You've starved the evaporator coil of refrigerant. The refrigerant charge is calculated to fill the evaporator, the line supplying it and back some up into the condenser to allow for subcooling. You've just significantly increased the size of the liquid line, so it'll need a small metric crapton of additional refrigerant, which I'll lay odds hasn't been accounted for.

The pdf you linked has a neat little diagram on how you might hook it up. It would appear from the photo you also attached that you have nothing like that configuration. Note the 3 way valve and several non return valves in the diagram.
This is all predicated on your inside evaporator coil having the TXV fitted there. If it's in the outdoor unit then you need a serious rethink as you'll be cooling the pool, probably icing up the coil and causing all sorts of injury.

So if your AC has the TXV on the coil, you can get your AC working by adding additional refrigerant such that the evaporator is appropriately filled. You'll be putting a little bit of heat into the pool.

You really need that loop right at the output of the compressor to catch the condensation and de-superheating to get a worthwhile amount of heat out of it.
 

Don_Q

Member
Oct 30, 2019
6
Orlando, FL
I see that now as well, the 3-way needs to be installed between the compressor and the condenser. It is as you read it... incorrectly so. We have no valves whatsoever, just straight up intrecepting it between the evaporator and the compressor.

Here's what looks wrong to me:
  • First, he installed a manual switch to the air compressor fan. This way I can manually shut off the compressor fan when the pool is being heated.
  • Second, we intercepted the refrigerant line coming out from the house evaporator, and routed it to the pool heater. Instruction diagram shows connection on the refrigerant line coming out of the compressor not evaporator in the house. Are you sure you described this correctly? Looks like he actually did the connection to the small uninsulated refrigerant which would be the correct line.
  • Then from the output of the pool heater, re-connected to the refrigerant line that was going to the condenser. Instruction diagram shows this connected to the output of the condenser. Can't tell from your photos if this is where he made the connection. Also diagram shows several check valves and a new 3-way expansion valve. I can't see any of this in your photos.
  • At this point, whenever we turn on the HVAC unit to normal operation, it freezes the coils. Did the tech check the charge and discharge pressures so he knew how much refrigerant to add. Are you sure he charged with Puron?
If your tech did the connections properly and had the right amount/type of refrigerant then the new 3-way valve could be failing. I don't see this in the new piping. All I can see in your photos is that he installed the new coil in the high pressure line with no 3-way valve and no check-valves. This won't work, everything needs to be installed as shown in the brochure. Can you show photos of each component? Did all of the components come with the kit and do you have any parts left over? I'll also ask @swamprat69 to comment. He's a very knowledgeable HVAC expert.

Chris
 

Don_Q

Member
Oct 30, 2019
6
Orlando, FL
Oh boy! This was extremely insightful and thorough. I (a layman) completely understand it. Mechanical Engineers have a way of describing things very well (made an assumption). I'm an electrical engineer and I struggle with breaking things down to laymen. Question, you mention the 3-way valve then you call it a "thermostatically controlled valve", are they the same thing or different?

At the sake of asking a stupid question, are the 3-way valve (or thermostatically controlled valve) and the check valves available at Home Depot or would I need to go to a HVAC supply house?

Went to bed before I finished my post, and it looks like it's mostly been answered, but I'll throw in my two cents and diagrams anyway.

Okay, so first off, I'm not an HVAC guy. We did cover all sorts of heat cycles, including heat pumps, in my thermodynamics classes in college.

Firstly, let's look at a normal AC system. This consists of four main parts, so far as the refrigerant is concerned. These are the compressor, condenser, expansion valve, and evaporator. The compressor takes vapor and compresses it. It is now very hot, and is piped through the condenser, where it is cooled and condenses by a fan drawing air across it (the condenser fan, which you called the compressor fan). This liquid refrigerant, now closer to ambient temp, is piped into the house. Once at your HVAC system, it is pushed through an expansion valve. This causes it to rapidly drop in temperature, and some of the liquid becomes vapor. The rest of the liquid evaporates in, well, the evaporator, which is the set of coils inside your HVAC unit (what makes the air in your house cold). Now completely vapor, the refrigerant makes its way back to the compressor in the outdoor unit to start all over again.

Now let's go over how the manufacture said to install this. I referenced the Hotspot FPH brochure as well, as it was a little more descriptive and has the exact same setup. They need you to intercept the refrigerant line between the condenser and compressor and install a 3-way valve. You then need to tap the line after the condenser and add a check valve. One connection on the 3-way valve goes to the evaporator (suction) line, which should be the bigger, insulated line coming out of the ground. The other connection runs to the pool heater. After the pool heater is another check valve, before the two lines tee together and run into the house as normal.

According to the Hotspot FPH brochure, what the 3-way valve does is swap refrigerant flow between either the condenser or the pool, based on a thermostat that measures the pool temp. It also evacuates either the condenser coil or the pool heater coil, whichever isn't being used, likely so the overall system charge is not changed much. That's what the connection to the suction line is for, along with the check valves. Note that the 3-way valve (and check valve) doesn't have to be inside the AC Unit as I've drawn, but you will need to tap lines inside the AC unit to install it, as you need to tap between the compressor and condenser, and that line doesn't come outside of the AC unit.

From what you've said, and the pictures you've posted, I believe this is how you actually connected things. While not remotely like the manufacture outlines, if it is actually how you installed it it should work (again, I'm not an HVAC guy, and this isn't ideal, which I cover below). If this is how it was installed, then your only "issue" is that the wrong type or amount of refrigerant was put into the system after it was connected.

Now, what's actually wrong with how you did it? You didn't put the pool heater on a 3-way, pool water thermostatically controlled valve (along with the check valves). The first issue with this:

The condenser is still in the loop, and has a ton of fins. You'll loose a good bit of heat even with the fan off, before the refrigerant makes it to the pool heater. Additionally, I wouldn't be surprised if the compressor wanted some of that airflow to help keep from overheating, in which case shutting off the fan is a bad idea, but I'm not certain on that.

Second issue: Lack of heat regulation in your pool. The primary purpose of the 3-way valve is to switch between pool heating and normal AC operation. This is based on pool water temp so you don't, for example, heat your pool when it's 100 °F out and your pool water is already 95 °F. Now, most of the heat is lost in the condensing process, so the pool heat gain is minimal if the condenser fan is running, but there will still be some. I think the biggest issue with your setup is that the manual fan switch may cause the compressor to overheat, especially being surrounded by hot condenser coils (but I'm not an expert, not sure if this would be the case. It seems logical though). If the 3-way valve was installed, then when not heating your pool, the condenser coils would be evacuated, so they'd be cool, and the condenser fan would be running still, so the compressor would stay cool. Actually it would stay cooler than normal, likely leading to better efficiencies.

The other thing is that instead of being a fully automatic setup, the manual switch means you have to physically switch the system from heating the pool to not heating the pool.

My uneducated summary is that your setup should, in theory, work, though not as well as if it was installed as designed with the 3-way thermostatically controlled valve and check valves. It will cause the compressor to run hotter than designed, with potential negative effects. It's likely that the tech put the wrong or too little refrigerant into your system. If he charged it with the amount specified, it's likely underfilled, and underfilled systems freeze up the evaporators (another bit the 3-way valve takes care of, since it evacuates the unused coil, so the total amount of refrigerant shouldn't change). He should have charged it using pressure gauges instead.

Clever system though, when installed as directed. I may consider adding it to our house. Of course the obvious problem for me is peak AC usage is also the hottest time of the year, and we may not want additional heat at that point, so the cost/benefit may not be worth it. Likely better for the OP, being in Florida, as I imagine you use AC year-round down there. For myself, I'll look into putting a data logger for AC run time and pool temp throughout the year, then next winter compare the data and see if it would help with our particular location/pool/temps.
 

jseyfert3

Bronze Supporter
Oct 20, 2017
677
South-Central WI
Oh boy! This was extremely insightful and thorough. I (a layman) completely understand it. Mechanical Engineers have a way of describing things very well (made an assumption). I'm an electrical engineer and I struggle with breaking things down to laymen. Question, you mention the 3-way valve then you call it a "thermostatically controlled valve", are they the same thing or different?
Thanks. I am a mechanical engineer, so you guessed right. :)

What I meant is that the 3-way valve is controlled by a temperature sensor that monitors your pool water temp and a controller unit. So yes, the 3-way valve is what I meant as the "thermostatically controlled valve".

At the sake of asking a stupid question, are the 3-way valve (or thermostatically controlled valve) and the check valves available at Home Depot or would I need to go to a HVAC supply house?
I am not certain on the three way valve. I did just now notice something. The FPH Hotspot includes the heat exchanger (what you have), plus the digital temp controller, AC sensor relay (not sure what that is), temperature sensor for measuring pool temp, the 3-way heat recovery valve, and a pump-out line restrictor (not sure what that is). These are included as standard components. They also mention in the component list that they include an installation manual. Between these two things, it appears you unfortunately were comparing prices between one component and a complete, ready to install system, so it wasn't a fair price comparison. :(

It also notes that the check valves are usually supplied by the AC tech, at $25/each. There's some other things not included that I'm not certain what they are for, since again I'm not an HVAC guy and FHP Hotspot doesn't list install instructions on their website, just a brochure plus a component list: https://www.hotspotenergy.com/pool-heater/Pool-Heater-Components-List.pdf

All that said, it appears FHP sells the heat recovery valve, controller, temp sensor all separate, so you may be able to buy it from them. Heat Recovery Pool Heater | Compare To Solar Pool Heater | HotSpot Energy LLC

Like I said earlier, what you have installed already should work in theory, with the following four things:
  • It's going to be less efficient than using the 3-way valve
  • It could potentially overheat the compressor, though I'm not an HVAC guy so don't quote me on this
  • It will need the correct charge of refrigerant
  • It needs to be manually switched back to cooling your house if you pool gets too hot in the middle of summer
Because of this, if I were you, I may consider trying to charge the system to the correct new capacity and see if it works. On the other hand, I might also consider throwing out the Doucette Industries heat exchanger and buying the whole system from FHP Hotspot. That would hurt financially given what you've already spent on this, including labor, but the FPH Hotspot comes with (nearly) everything, including an instruction manual. Or maybe you could convince FHP to sell you everything except the heat exchanger, including instructions?
 

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
2,564
Stuart/FL
You can get AC parts from Johnson Supply or online. But I'd develop a brief scope of work for an AC tech and get prices from each to fix the installation. Our new mechanical engineer has supplied nice diagrams you can use for the scope. Get them to break down into labor and each part. Often they get a very large discount from suppliers and that way you're not "in the middle" of the work. Any delays you cause from wrong part are on your nickel.

Chris