Why can't i get high temps?

Solarwaterheater

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Joined
Nov 29, 2021
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Hello to any one who reads this! Lol.

I built my first solar water heater!
It's 1/2" pex under a piece of Polyethylene terephthalate glycol, or PETG plastic.
Inside the pex I have RV fluid/antifreeze so it doesn't burst at night with freezing temps. I have an Aquastat attached to the pex loop so the pumps won't turn on till the loop temp reaches a user defined value. Currently i have it set to about 85 deg F. The RV fluid is pumped through the loop via a solar powered pump to a plate heat exchanger. On the other side of the plate heat exchanger is the domestic hot water. I have two water heaters connected in series and the first one is turned off. The water from the first (powered off) water heater is what is pumped through the plate heat exchanger. I was hoping to use this setup to heat or at least pre-heat the hot water. I suppose i have half of that goal accomplished because it will circulate and get the water temp in the first water heater up to about 87-90 degrees.

My question to all of you: if i build a second loop and connect it to the first would it get higher temps...
A) if connected in series with the first loop
B) if connected in parallel
Or
C) won't matter, thats the best temp I'm going to get?


Side note, cause i know someone might ask. The piping in the photo also goes to our geothermal hvac system. When it is running it heats the domestic hot water. I have check valves on each side so the geo won't pull water backwards through the plate exchanger and vice versa. Typically, right now in the winter, the geothermal heats the water at night and the solar loop heats it during the day because the geothermal doesn't need to run during the day.

I appreciate any feedback good or bad. And thanks for your help!
 

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sunshine_eggo

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Confirm:

Inlet -> Waterheater 1 (off) -> heat exchanger (from solar) -> Waterheater 2 (on) -> home

A/B) Probably wouldn't matter much, but true parallel would likely see efficiency loss due to more friction and surface area with the additional hoses.
C) unknown, but it sounds like you're just adding more heat exchanger capacity, so you might get a little more depending on how efficient the heat transfer is.

My turn:

D) RV fluid... Pink antifreeze?
E) What's the surface area of the collector?
F) when you say 87-90°F, what temperature rise are you getting.
G) What's the inlet/oulet temp of the RV fluid?
 

Solarwaterheater

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Inlet -> Waterheater 1 (off) and to plate heat exchanger and geothermal unit-> waterheater 2 (on) -> home

D) yes, see photo
E) it's on a 36x36 inch board, so about 9 square feet minus parts where the pex pipe isn't.
F) that's about the max temp it will go to when i look at the thermometers that are in line with the water loop from the first water heater
G) using my infrared thermometer, the inlet pipe temp, right before going into the box with the pex loop is 82-83 right now. Outlet pipe, right out side the box, temp is 87-88.
H) pipe temp of solar loop right before plate exchanger is 87 and after plate exchanger it is 83-84.
 

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Solarwaterheater

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Nov 29, 2021
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The most glaring cause is size, or lack of it. My solar hot water panel is 10x8 ft. Everything else differs too so probably other reasons too.

View attachment 73899
I can agree with your assessment. My concern is that even if i build a second loop I'll only get another 5 degree or so increase in temps. I feel i need to find a more effective method of construction in addition to prob making it larger.
 

circus

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Jul 8, 2021
Messages
271
Ten years ago, after building the pictured collector I started on a second one. Instead of soldering 1/2" tubing to sheet steel (pain in the ***) I modified about 55 premade 48"x4.5" underfloor radiators to snap onto the 1/2 cu tubing. Planned to solder the assemblies to manifolds then paint black. Decided it wasn't needed so I dropped the project.

SHW snap on panels reduced.jpg
 
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Solarwaterheater

New Member
Joined
Nov 29, 2021
Messages
23
Hello to any one who reads this! Lol.

I built my first solar water heater!
It's 1/2" pex under a piece of Polyethylene terephthalate glycol, or PETG plastic.
Inside the pex I have RV fluid/antifreeze so it doesn't burst at night with freezing temps. I have an Aquastat attached to the pex loop so the pumps won't turn on till the loop temp is at least 80 deg F. The RV fluid is pumped through the loop via a solar powered pump to a plate heat exchanger. On the other side of the plate heat exchanger is the domestic hot water. I have two water heaters connected in series and the first one is turned off. The water from the first (powered off) water heater is what is pumped through the plate heat exchanger. I was hoping to use this setup to heat or at least pre-heat the hot water. I suppose i have half of that goal accomplished because it will circulate and get the water temp in the first water heater up to about 87-90 degrees.

My question to all of you: if i build a second loop and connect it to the first would it get higher temps...
A) if connected in series with the first loop
B) if connected in parallel
Or
C) won't matter, thats the best temp I'm going to get?


Side note, cause i know someone might ask. The piping in the photo also goes to our geothermal hvac system. When it is running it heats the domestic hot water. I have check valves on each side so the geo won't pull water backwards through the plate exchanger and vice versa. Typically, right now in the winter, the geothermal heats the water at night and the solar loop heats it during the day because the geothermal doesn't need to run during the day.

I appreciate any feedback good or bad. And thanks for your help!
After reading on some other forums i think i might need to replace the plastic with an old window or some glass. I think i might be losing heat through the plastic.
 

sunshine

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Apr 24, 2020
Messages
510
After reading on some other forums i think i might need to replace the plastic with an old window or some glass. I think i might be losing heat through the plastic.
Yes. Glass has better properties.
With mine I found the glass first, a sliding door panel of about 2m X 1m and a 50m roll of 20mm pex in a oval shape fixed by zip ties. There is gaps between the turns to use the available space. Your tight turns may limit the surface area under direct sun light. If only using one roll having gaps may be better.
 
Last edited:

Solarwaterheater

New Member
Joined
Nov 29, 2021
Messages
23
Hello to any one who reads this! Lol.

I built my first solar water heater!
It's 1/2" pex under a piece of Polyethylene terephthalate glycol, or PETG plastic.
Inside the pex I have RV fluid/antifreeze so it doesn't burst at night with freezing temps. I have an Aquastat attached to the pex loop so the pumps won't turn on till the loop temp is at least 80 deg F. The RV fluid is pumped through the loop via a solar powered pump to a plate heat exchanger. On the other side of the plate heat exchanger is the domestic hot water. I have two water heaters connected in series and the first one is turned off. The water from the first (powered off) water heater is what is pumped through the plate heat exchanger. I was hoping to use this setup to heat or at least pre-heat the hot water. I suppose i have half of that goal accomplished because it will circulate and get the water temp in the first water heater up to about 87-90 degrees.

My question to all of you: if i build a second loop and connect it to the first would it get higher temps...
A) if connected in series with the first loop
B) if connected in parallel
Or
C) won't matter, thats the best temp I'm going to get?


Side note, cause i know someone might ask. The piping in the photo also goes to our geothermal hvac system. When it is running it heats the domestic hot water. I have check valves on each side so the geo won't pull water backwards through the plate exchanger and vice versa. Typically, right now in the winter, the geothermal heats the water at night and the solar loop heats it during the day because the geothermal doesn't need to run during the day.

I appreciate any feedback good or bad. And thanks for your help!
I replaced the plastic PETG with glass. It's almost too late in the day to get a good temperature read. It does seem hotter so far, but not really very much. Starting to think that even if i build a second loop, i won't get much higher temps. If this unit is only producing about 90 degrees max, adding a second loop might increase it 5 more degrees? I do wonder if more time in the sun loop would equal more heat absorption. Maybe the fluid is just moving too fast to really heat up.
 

S Davis

Solar Enthusiast
Joined
Sep 25, 2021
Messages
116
I can agree with your assessment. My concern is that even if i build a second loop I'll only get another 5 degree or so increase in temps. I feel i need to find a more effective method of construction in addition to prob making it larger.
The second loop if in parallel will give you the same temp out to the water heater if constructed the same, if in series the fluid will have double the dwell time and increase the temperature if there is enough energy available from the collector.

One other thing is antifreeze is not as good as water for heat transfer, it has a higher head pressure needing more pump pressure. Only use enough to protect from freezing.
 

Solarwaterheater

New Member
Joined
Nov 29, 2021
Messages
23
Hello to any one who reads this! Lol.

I built my first solar water heater!
It's 1/2" pex under a piece of Polyethylene terephthalate glycol, or PETG plastic.
Inside the pex I have RV fluid/antifreeze so it doesn't burst at night with freezing temps. I have an Aquastat attached to the pex loop so the pumps won't turn on till the loop temp is at least 80 deg F. The RV fluid is pumped through the loop via a solar powered pump to a plate heat exchanger. On the other side of the plate heat exchanger is the domestic hot water. I have two water heaters connected in series and the first one is turned off. The water from the first (powered off) water heater is what is pumped through the plate heat exchanger. I was hoping to use this setup to heat or at least pre-heat the hot water. I suppose i have half of that goal accomplished because it will circulate and get the water temp in the first water heater up to about 87-90 degrees.

My question to all of you: if i build a second loop and connect it to the first would it get higher temps...
A) if connected in series with the first loop
B) if connected in parallel
Or
C) won't matter, thats the best temp I'm going to get?


Side note, cause i know someone might ask. The piping in the photo also goes to our geothermal hvac system. When it is running it heats the domestic hot water. I have check valves on each side so the geo won't pull water backwards through the plate exchanger and vice versa. Typically, right now in the winter, the geothermal heats the water at night and the solar loop heats it during the day because the geothermal doesn't need to run during the day.

I appreciate any feedback good or bad. And thanks for your help!
Ugh. I'm so confused or, really experiencing a lack of knowledge apparently. I put the plastic back on while i wait for the silicone on the glass frame to cure. I went and got this:

( https://www.homedepot.com/p/M-D-Bui...1ea63aec0a0530d5bf12dd719efb05d1294b7ff40e8ba )

to put between the solar loop frame and the PETG plastic. The air temp inside the box is the same air temp as out side the box. I'm 99.9% certain that it is pretty dang sealed for air leakage. The wood on the outside is 120-130. Maybe i should paint everything with the brown deck paint, no black. And take the plastic off, just let it bake in the sun.

I put a solar oven out, 10 feet away and within a few minutes the temp in the solar oven is pushing 175f. ( https://www.amazon.com/dp/B00AECTUZU?tag=solarpowernerd-20&linkCode=ogi&th=1&psc=1 )

I don't understand why the air won't heat up inside the box.
 
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400bird

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Joined
May 23, 2020
Messages
381
I'd set the thermostat higher. With it set to 85f and the relatively small surface area you're not getting much over the thermostat set point.

Is the pump running constantly (well once the sun is out and it turns on the first time)?
 

Solarwaterheater

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Nov 29, 2021
Messages
23
I'd set the thermostat higher. With it set to 85f and the relatively small surface area you're not getting much over the thermostat set point.

Is the pump running constantly (well once the sun is out and it turns on the first time)?
It takes a few tries for the pumps to be on continuously, but eventually they run with out stopping. I'll go change it to 105 or 110 and see how it does today.
 
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400bird

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Messages
381
If that is th case (pump running almost constantly once it gets going), I'd agree with others. You either need more area or a more efficient design. Sounds like more area is the key.

Part of the reason the inside of the box is cooler than the outside surface is that you are constantly pumping in cool water, that cooler water is absorbing the heat out of the box.
 

Solarwaterheater

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Messages
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If that is th case (pump running almost constantly once it gets going), I'd agree with others. You either need more area or a more efficient design. Sounds like more area is the key.

Part of the reason the inside of the box is cooler than the outside surface is that you are constantly pumping in cool water, that cooler water is absorbing the heat out of the box.
Ok, so we are about past the high point of the sun for the day so far. With the aquastat set to 105-110 the pumps come on but don't run continuously, just on and off and the temp in the first water heater doesn't ever get above 65-70. I suppose the next step for me is to just build a second loop and get more surface area as several have suggested. I plan to connect it in parallel, as this will slow the fluid down in the loops for more dwell time but keep the speeds higher in the lines to/from the plate exchanger. I've experimented with painting part of the existing loop with the brown deck paint and that didn't seem to make much difference on the IR thermometer. I've experimented with adding reflective insulation inside the box on the areas that the loop isn't and that didn't seem to make much difference. Since I am going to be draining the system when I add a second loop I am going to try and fill it with a 50/50 mix of water and the pink antifreeze. I'll post an update once that is all complete since this has kind of become a journal for this project. Hopefully it will also help anyone else who wants to try and build something like what I am doing.
 

labeeman

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Jun 2, 2021
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Carencro, LA
Ok, so we are about past the high point of the sun for the day so far. With the aquastat set to 105-110 the pumps come on but don't run continuously, just on and off and the temp in the first water heater doesn't ever get above 65-70. I suppose the next step for me is to just build a second loop and get more surface area as several have suggested. I plan to connect it in parallel, as this will slow the fluid down in the loops for more dwell time but keep the speeds higher in the lines to/from the plate exchanger. I've experimented with painting part of the existing loop with the brown deck paint and that didn't seem to make much difference on the IR thermometer. I've experimented with adding reflective insulation inside the box on the areas that the loop isn't and that didn't seem to make much difference. Since I am going to be draining the system when I add a second loop I am going to try and fill it with a 50/50 mix of water and the pink antifreeze. I'll post an update once that is all complete since this has kind of become a journal for this project. Hopefully it will also help anyone else who wants to try and build something like what I am doing.
How much water are you pumping per hour? It takes one BTU to heat one pound of water one degree. If you have one square meter of collector that will give you 3410 BTU's of heat per hour it has full sun which will happen only for a couple of hours a day. Providing you have no heat losses and the glass will give you some heat loss along with everything else.
 

Solarwaterheater

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Nov 29, 2021
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How much water are you pumping per hour? It takes one BTU to heat one pound of water one degree. If you have one square meter of collector that will give you 3410 BTU's of heat per hour it has full sun which will happen only for a couple of hours a day. Providing you have no heat losses and the glass will give you some heat loss along with everything else.
I am using this pump on both sides of my set up: ( https://www.amazon.com/dp/B00OOED23A?psc=1&ref=ppx_yo2_dt_b_product_details ) This says it will do 3.0 GPM, but I'm assuming that is at full power. Right now both pumps are connected to the same solar panel, but I believe it puts out enough amperage to run them both at full speed when the panel is producing full power. I have toyed with adding a potentiometer in the system to change the pump speed, I need to find a different one though. The one I was using was a 10k and it would only allow current through on the lowest setting.

My main focus is to also get this to perform well in the winter. I am assuming that it won't be a problem to get higher temps in the summer months as it can easily be over 100 here during the summer days.
 

labeeman

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Messages
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Location
Carencro, LA
At the rate you are pumping you can only can only raise the temp 2 degrees if you have a square meter of collector which you do not have. You need to use a PWM speed controller for the pump, not a potentiometer. Your heat exchanger needs to be insulated too.
 
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