# Heat transfer to water through copper pipe

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I'm attempting to extract some additional heat from an existing fireplace and wondering if anyone can help me with the rate of heat transfer from air through a copper pipe to water. Here's what I have....a fireplace with two chambers outside of the firebox where air can be blown to other rooms via duct work.

Rather than blowing air, I'd like to install a copper coil of 1/2" flexible copper tubing and heat water to pump through my in floor tubing in the basement. The question is, if I have 40' of 1/2" copper tubing in the chamber with water flowing through it at a rate of 1-3 gallons per minute (if I set my pump on low speed and only open 1 of 4 loops in the floor it is at 1 gpm, if I open the system up and set my pump on high speed it can push about 3 gpm) can I increase the water temperature from 60 F to 90 F?

I checked the temperature in the two chambers; during an average fire, the one closest to the firebox is at about 400F and the secondary one is at about 200F. The secondary chamber has better access for construction and would be preferred for use, but I don't think 200F is enough heat.

The system is set up so it flows into an open tank, so it intentionally isn't a closed loop system to avoid the chance of boiling waterm high pressure, expolosion, etc.

I'd appreciate an assistance in calculating if this might work, as well as any other suggestions for stripping the heat from the air and getting it to the water (i.e. radiator fins similar to hydronic base boards).

Thanks for any assistance!

Edited by CivilWausau

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Any chance of providing a drawing or something? After reading your post I'm utterly confused as to what this all looks like.

Mind you, I'm not heat transfer guy so I doubt I'll be able to answer it, but to have any chance at all I have to understand the question....

Edited by ewmon

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Any chance of providing a drawing or something? After reading your post I'm utterly confused as to what this all looks like.

Mind you, I'm not heat transfer guy so I doubt I'll be able to answer it, but to have any chance at all I have to understand the question....

I can take a picture of the fireplace and post it, but it'll be a few days until I'm back by the unit to do this. The simple schematic is an air space that maintains heat at 200F with 40' of 1/2" copper inside it, water coming in at 1-3 GPM at 60F and I'm trying to figure out what increase in temperature I could expect to see at the outlet. I also have the second area that is heated around 400F, but it is smaller and less accessible for construction.

Thanks for the input - I'll work on getting a photo out.

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... I checked the temperature in the two chambers; during an average fire, the one closest to the firebox is at about 400F and the secondary one is at about 200F. The secondary chamber has better access for construction and would be preferred for use, but I don't think 200F is enough heat.

Understood, but keep in mind "400F" or "200F" is not heat; it is temperature.

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I would first experiment before installing a final version. I'm glad to see you start with an open system to avoid trouble with steam. Good.

The heat transfer coefficient you want to know will be anything from 400 W/m2K to only 20 W/m2K. Multiply that value with the surface area of the pipe (in m2), and by the temperature difference (in Celsius or Kelvin), and you get a power (in Watts), which can be used to calculate how much your water will increase in temperature - depending on the flow rate.

That heat transfer coefficient is gonna be influenced by practical things like:

- The turbulence in your heating place. In other words: it changes as you change the fire itself.

- The geometry of the coil.

- How dirty the coil got from soot. The heat transfer will get worse as the coil gets dirty. Soot is an insulator.

I cannot give a better estimate than 500 W/m2K to 20 W/m2K... although I would guess that it will be much closer to the low value than to the high value. Start with your water pump at maximum flow, and measure the temperature. Then reduce that flow as long as your water will not be too hot.

Also, your fireplace will probably not produce a constant power (it can vary easily by a factor 2, if not more), so either install a feedback system (something that changes your water flow depending on the temperature), or go for a safe value and keep the water from boiling all the time. Your temperature control is probably gonna be your biggest nightmare, and for me that would be the reason not to try this myself.

I'm sorry that I cannot be bothered to convert all your feet, inches and fahrenheits to some more modern units, like meters and kelvins. You'll have to do the calculations yourself, or convert them for me.

My advice would be to see if there isn't a ready-made wood burning central heating system. It's a lot more costly, but a whole lot more efficient, and safer too.

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It looks like reverse thinking.

Usual systems heat water then hot water circulating in some kind of radiator heats the air.

You are about doing the contrary: using hot air to heat water. Since you plan to do that anyway i would be interested in your results.

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It looks like reverse thinking.

Usual systems heat water then hot water circulating in some kind of radiator heats the air.

You are about doing the contrary: using hot air to heat water. Since you plan to do that anyway i would be interested in your results.

Normally, the water gets heated by a gas flame in your central heating system, or by any other fire (wood, gas, oil, coal, garbage) in case of district heating. So, a hot gas heats water, which heats the air in your rooms. So in fact it is very similar to a central heating.

But building a central heating system, which is essentially what we're trying here, is not as easy as it sounds.

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Normally, the water gets heated by a gas flame in your central heating system, or by any other fire (wood, gas, oil, coal, garbage) in case of district heating. So, a hot gas heats water, which heats the air in your rooms. So in fact it is very similar to a central heating.

But building a central heating system, which is essentially what we're trying here, is not as easy as it sounds.

You mean a flame, as you said earlier, "a gas flame".

The system of the OP is about hot air. Air is not a good heat conductor, air is an insulator. It is like trying to heat a casserole with a hair dryer.

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You mean a flame, as you said earlier, "a gas flame".

The system of the OP is about hot air. Air is not a good heat conductor, air is an insulator. It is like trying to heat a casserole with a hair dryer.

Isn't hot air or a flame practically the same, with only a few hundred degrees Celsius difference?

Heat transfer is a little more complicated than saying that "air is an insulator". The heat transfer coefficient of air and air can differ by a factor 10 or more... depending only on the turbulence of the air, and nothing else!

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Isn't hot air or a flame practically the same, with only a few hundred degrees Celsius difference?

Heat transfer is a little more complicated than saying that "air is an insulator". The heat transfer coefficient of air and air can differ by a factor 10 or more... depending only on the turbulence of the air, and nothing else!

Here I need a true physicist, and an engineer together maybe: I don't know what exactly happens inside a gas chamber. A flame is plasma, but maybe it is not the flame that heats the water, I need outside help...

Air is an insulator means that heat does not transfer well from air to anything else. Air is what is used in a double glazing for insulation. I suppose for heat transfer it is one of the worst material. You can see that also with climate control: if you use it for heating, the system propulses hot air in the room but walls and objects remain cold. If you stop heating the room gets cold in a few seconds.

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Here I need a true physicist, and an engineer together maybe: I don't know what exactly happens inside a gas chamber. A flame is plasma, but maybe it is not the flame that heats the water, I need outside help...

Air is an insulator means that heat does not transfer well from air to anything else. Air is what is used in a double glazing for insulation. I suppose for heat transfer it is one of the worst material. You can see that also with climate control: if you use it for heating, the system propulses hot air in the room but walls and objects remain cold. If you stop heating the room gets cold in a few seconds.

Luckily, I am a chemical engineer, with some experience in heat transfer.

What happens in any heat transfer is that the warmer material has molecules that 'bounce', and 'wiggle' faster. That's what is means to be warmer at a molecular level. So, these hot molecules bounce into the colder material, and transfer some of their energy into the colder material. They lose some, the other material gains some. After colliding, they will bounce and wiggle slower, and the atoms in the copper pipe into which they crashed will move faster. The copper atoms will transfer their heat inwards onto other copper atoms, until the ones in contact with water transfer their energy into the water.

So, a gas, which is very light, has fewer molecules and therefore conducts heat much worse than a solid or liquid. If you want to carry cargo across a river, it will go faster if you have more boats. Same with heat. More molecules generally means better heat transfer. In double glazing, they use air (or another gas) to insulate, because with so few molecules between the two windows, heat transfer will be terrible (and if you want to insulate, terrible is good).

But if you have a convective flow ("wind"), that greatly improves the heat transfer, because although the density of molecules is still low, at least you replace the molecules that cooled down quickly with hot molecules. So, it's not as simple as knowing the material properties or density. Turbulence and convection matter a lot.

The point I tried to make earlier is that a plasma (fire) and air both are low density. It's just that the plasma molecules will move faster, and have more energy to transfer. But it's still essentially a gas. It is still a very low density material.

I maintain that what our OP is proposing is essentially a central heating system. It's just that he wants to burn wood instead of gas.

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Thanks for all the replies. I'll crunch some numbers with the info from CaptainPanic.

I installed a coil (since I had some excess material available) in the upper 200F chamber and was only getting a few degrees increase in temperature - very disappointing. If I shut off the pump and let the water sit, it heats up to 200, but the constant flow is too much for the 40' coil configuration to get a significant increase.

If the calculations don't show much better by changing the temperature from 200 to 400, it'll be time for a shopping trip to simply get a boiler. I needed the pump, valves, etc. to hook up to a boiler regardless, but thought I'd at least try it first using heat that is already available. My goal was to not have the basement be less than 60F when we have fires going (usually at least 4 nights/week).

I know I could get plenty of hot water in the fire box, but I wanted to avoid doing anything inside the firebox due to the risk of steam and explosions or leaks. I know I could get the temperature that way, but it had way too many variables for me to experiment in my living room with.

Does anyone think that using aluminum fins, similar to baseboard radiators for hydronic heat, would make a difference? I was contemplating less length of pipe, with a larger diameter and the fins may be more effective since it'll transfer the air temp better and the water will flow slower with a larger diameter of pipe? Any thoughts?

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Maybe using a car radiator.

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Radiating heat and absorbing heat are (theoretically) the same principle, only in reverse. But...

With a fire burning, you will get soot. And a radiator is an absolute nightmare to clean... in fact, I would say it is impossible to clean. So, yes, aluminium fins, or a radiator setup would work. But only for a little while until it's dirty. As mentioned before, dirt/soot/dust/ash will make the heat transfer worse (by a significant factor). A car radiator is almost like a filter... every little particle will attach to it. I really don't recommend it.

My advice would be to use a geometry used in boilers. They tend to increase surface area by using more pipes, and a very limited amount of fins (as in the picture), or often no fins at all. Also, they often keep the pipes as straight as possible (until the end, where there is a relatively sharp 180 degree bend). Not only is that easier to fit into a furnace, but it's also easier to clean.

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Here's what I have....a fireplace with two chambers outside of the firebox where air can be blown to other rooms via duct work.

It sounds as the air chambers are separated from the firebox, if so then heat first transfers internally through the walls to the air chamber.

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