Solar water heat exchanger

[gettinthere]

I have a 6 gallon hot water tank. It has the heat exchanger circuit using the engine coolant to heat the water in the tank.

Considering making a solar water heater to replace the engine as the source of the water heating. A coil of copper tubing inside a "greenhouse" box in the sun. The tubing heats the water in the sun and circulates thru the heat exchanger, heating the water in the tank.

The water in a solar collector of about 2' square can get very hot very quickly in full sun. My questions are about circulation and water pressure. I'm guessing the system would have about 1 to 2 gallons of water that needs to be circulated. A small 12 volt pump can do that. The bigger question is how much expansion will the water do when heated? I'm sure I will need a pressure relief valve which if at the top of the circuit can also be the fill point when removed. Any thermal expansion experts here? The 6 gallons of water will absorb a certain amount of the heat, lessening the thermal expansion. But the specifics are all guesswork at this point. I'd like to minimize the level of guessing.

 

[dlochner]

Add a small expansion tank with an overflow. When the water expands, the tank holds the extra water, when it starts to cool off the water drains back into the tubing. You will always want some water in the tank to keep air out of the lines.

1 GALLON ALUMINUM EXPANSION TANK w/7# CAP

www.suremarineservice.com

 

[gettinthere]

Add a small expansion tank with an overflow. When the water expands, the tank holds the extra water, when it starts to cool off the water drains back into the tubing. You will always want some water in the tank to keep air out of the lines.

1 GALLON ALUMINUM EXPANSION TANK w/7# CAP

www.suremarineservice.com

$377!!! Ouch!
Thinking maybe a simple tank at the very top of the loop would allow water to rise and fall with the heat. Wouldn't even need a pressure relief valve, the water could just overflow out of a hole in the top.

 

[dlochner]

$377!!! Ouch!
Thinking maybe a simple tank at the very top of the loop would allow water to rise and fall with the heat. Wouldn't even need a pressure relief valve, the water could just overflow out of a hole in the top.

It needs to be sealed with pressure relief. If water is allowed to leave the system, it will induce air into the system that may cause an air lock and nothing will work. When the water enters the tank, the air will compress and force the water back down as the water cools, this keeps the air out once the system is bled.

 

[Stu Jackson]

But the specifics are all guesswork at this point. I'd like to minimize the level of guessing.

That's a good attitude.

The issues involved are both the one you raised about pressure, but also and more importantly ones about heat exchange which include the volume or the water being transferred and the temperature differences that impact that heat transfer.

If you're trying to heat 6 gallons in your water heater, 1 to 2 gallons being circulated simply won't do it. You should compare the amount of water in your freshwater cooled engine, for starters, it's a lot more than 1 - 2 gallons. And it's temperature is above 180F because its under pressure.

i spent 40 years as an HVAC engineer and did these kind of heat transfer things all the time, but I retired and intend to stay that way! The basic equation includes temperature differences and volumes.

Either find someone willing to do it, or do some basic heat and water transfer research on your own and you'll find out how to do it.

That being said, you aren't the first skipper to come up with idea and all those before you have been dissuaded from doing so for good reasons: basic heat transfer.

 

[PaulK]

It might be cheaper and easier to use one of these: Amazon.com
Drain the sun-heated water into your water heater and it will have much less work to do.

 

[dlochner]

It might be cheaper and easier to use one of these: Amazon.com
Drain the sun-heated water into your water heater and it will have much less work to do.

Or one of these.

Nemo Equipment 32010 | Fisheries Supply

Save More on Your Nemo Equipment #32010 NMO 32010 HELIO LX PRESSURE SHOWER BLK/APPLE GREEN at Fisheries Supply. Excellent Customer Service, Ready to Ship. Marine Supplies Since 1928!

www.fisheriessupply.com

 

[gettinthere]

This is where I got the idea...

145d within minutes. He did one with copper tube, same size, 175d. That is flow through from a garden hose, not a continuous loop that would stay warm/hot just about all the time. So a similar temp as engine coolant.

 

[Johnb]

The solar heaters mentioned in posts 6 and 7 above would work but for what it is worth I have used:

1 A translucent 2.5 gallon watering can filled with water and placed next to the pool for post swim showers. Comes complete with lifting handle and sprinkler on the spout. Be careful - the water in the spout gets very hot.

2 At Burning Man, a 5 gallon translucent water container placed on its side in the sun. In about 2-3 hours the water can get too hot to use directly. Leave the equivalent of about a quart empty.

I also predict the idea in post 8 will work, just going to have to circulate the water.

 

[gettinthere]

BTW, I'm looking at this because I want a whole house hot water system. Not interested in the shower bag setup.

 

[Stu Jackson]

BTW, I'm looking at this because I want a whole house hot water system. Not interested in the shower bag setup.

Then why not Google:

solar hot water system

Lots of information there for what you want.

Why do people come on boating forums and ask basic 101 questions about systems not even related to boating that they know nothing about that Google can answer?

 

[dlochner]

Why do people come on boating forums and ask basic 101 questions about systems not even related to boating that they know nothing about that Google can answer?

Because HI is better than AI?

 

[gettinthere]

Then why not Google:

solar hot water system

Lots of information there for what you want.

Why do people come on boating forums and ask basic 101 questions about systems not even related to boating that they know nothing about that Google can answer?

I have searched google. There is not much information about heating a hot water tank on a boat with solar. Yes, a boat. The complete water system for drinking and bathing on a boat is considered "whole house". Mine has 3 sinks and 2 showers and is my home. If you read the first sentence of my first post you would probably be able to figure that out.

 

[JamesG161]

If you read the first sentence of my first post you would probably be able to figure that out. []

Here is your answer.

Pressure relief is done by your Marine unit, assuming you did not modify it.

This took me 5 minutes to find.

Your friend,

Jim...

 

[JamesG161]

Here is the Data sheet for MH-38

and owners manual.

 

[LloydB]

If you want recommendations by experienced users you probably need to state your expected use by amount and time of day. Solar probably wouldn't be much help for a long hot shower first thing in the morning for instance or even a short shower probably.

 

[gettinthere]

Here is your answer.

View attachment 218343

Pressure relief is done by your Marine unit, assuming you did not modify it.

This took me 5 minutes to find.

Your friend,

Jim...

Well son of a biscuit! None of the porn sites I searched had that.
However, this does not appear to be a heat exchanger using the engine cooling circuit common on many boat hot water tanks. Maybe that is not necessary, it certainly would keep it simpler. Without an expansion tank I wonder how much water I would loose out the pressure relief valve.

 

[JBP-PA]

If you already have a pressurized water system, you probably already have an expansion tank. You can get them cheaper than $377 if you want plastic.

Shurflo 182-200 | Fisheries Supply

Save More on Your Shurflo #182-200 Accumulator Tank - 20 PSI at Fisheries Supply. Excellent Customer Service, Ready to Ship. Marine Supplies Since 1928!

www.fisheriessupply.com

I see no reason why your plan wouldn't work. A solar powered pump would automatically turn off at night.

 

[gettinthere]

Well son of a biscuit! None of the porn sites I searched had that.
However, this does not appear to be a heat exchanger using the engine cooling circuit common on many boat hot water tanks..

They do have this as an option on the website. Very interesting.

Thank you very much!

 

[dlochner]

If you already have a pressurized water system, you probably already have an expansion tank. You can get them cheaper than $377 if you want plastic.

Different critters. The $377 expansion tank is to provide a space for the heated water to expand into. Any pressure generated in the tank is from the heated water. As it cools the water drains out of the tank and back into the system. The one you referenced is designed for cold water. There is a pressurized air bladder in the tank that compresses further when the pumps puts more water into he tank. It then provides pressurized water to the system allowing the motor to be off until the pressure in the tank drops at which point the pump comes back on and refills the tank.

For the solar water heater to be effective, the water must be quite hot, not sure how the plastic pressure tank would stand up to the heat.