# How to calculate water coil length.

Posted By JJLL, Jan 28, 2008 at 5:42 PM

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1. #1

### JJLL New Member 2. ```NULL ```

Jan 28, 2008
4
0
Loc:
USA
Hello, I've been trying to find out how much coil I will need to hook up to a hydronic heating system (hot water/radiant heat).

I can easily find information on domestic hot water (hot water tanks/storage) but I'm looking for full house, radiant systems.

Can anyone give me a rough idea or direction on how to calculate the length of pipe/coil needed to heat X amount of gallons of water?

Thank You.

2. #2

### Eric Johnson Mod Emeritus 2. ```NULL ```

Nov 18, 2005
5,875
147
Loc:
Central NYS
Welcome to the Boiler Room, JJLL.

I'm not clear on what you're asking. Usually a copper coil is used as a heat exchanger to get heat into and out of a hot water storage tank. You size the coil to the output of the boiler. Is that what you're asking?

Or, are you asking how much infloor radiant tubing you'd need to heat a given space?

Or are you talking about a coil for heating domestic hot water in a boiler?

3. #3

### JJLL New Member 2. ```NULL ```

Jan 28, 2008
4
0
Loc:
USA
Let me explain my situation....

I have a very old boiler. I can easily access all of the water lines. What I'm thinking of doing is putting a number of coils (or one large one) inside of my stove in order to heat the water thats running into the boiler. I was going to tap into the cold water return so that the motor on the boiler would pull the water through the system. My biggest challenge is simply figuring out how much/how long of a coil I would need in order to heat a 2,800 sq ft. home. I've working on this system many times and from draining the system I know that I have about 110 gallons of water running through it.

Any suggestions? How to boiler manufacturers determine the size of their coils? Thanks for your help

4. #4

### Eric Johnson Mod Emeritus 2. ```NULL ```

Nov 18, 2005
5,875
147
Loc:
Central NYS
I think you would be ahead to pick up a used wood-fired boiler and pipe it into your system. It would cost less than doing what you're proposing, and it would almost certainly be safer and more efficient.

Most boilers don't use coils or pipes. The pressure vessel is the heat exchanger. Wood gasifiers based on the Seton design do use black iron pipe running through refractory, and it's not a whole lot of pipe.

In any event, I definitely wouldn't use copper inside any firebox.

5. #5

### wdc1160 New Member 2. ```NULL ```

Dec 28, 2007
356
0
Loc:
Lafayette IN -BoilerMakerCountry
If I understand your trying to make a boiler. I know there are other guys on here who have done it. They have even made gasfiyers.

6. #6

### JJLL New Member 2. ```NULL ```

Jan 28, 2008
4
0
Loc:
USA
Yes, I guess you could say I am trying to make a boiler. I've bent schedule 40 pipe before so I can easily make the coils. I just need to know how long

As for cost, for me it's simply the price of the pipe that I'm bending. If any of you have links to people who have made their own boilers, maybe that would help.

7. #7

### Eric Johnson Mod Emeritus 2. ```NULL ```

Nov 18, 2005
5,875
147
Loc:
Central NYS
Here you go, JJ.

Keep us apprised of your progress. Sounds like fun. Don't forget a dedicated pressure relief valve on the stove piping.

8. #8

### WRVERMONT New Member 2. ```NULL ```

Oct 29, 2007
104
0
Loc:
Northern Vermont
I started my journey towards gasification by building coils to fit in the firebox my big old stove. I started by using sweated copper, then copper again then welded stainless. I found that 45 ft of 3/4" stainless could warm my 864 sq ft radiant slab fairly well if I had a serious fire going. Output temp was around 105 degrees. If the fire was really cooking I could get output temps of 130degrees. My retrofits were very educational. In my opinion this approach could make sense in a few situations, however what a "real (could be homebuilt?) boiler achieves is truly awesome.

9. #9

### Bob Rohr Minister of Fire 2. ```NULL ```

Jan 9, 2008
1,037
157
Loc:
SW Missouri
just be careful with those homemade internal heat exchangers. There is a lot of energy released when water flashes to steam, should the pump stop, or a power outage. A gallon of water is like a stick of dynamite when it flashes to steam. we have all seen pictures of exploding water heaters blasting through walls and roofs when the flash to steam.

I'd rather see the coil outside on the back of the stove, for instance as opposed to inside. Be sure it has a low pressure 30lb. or less pressure relief with adequate capacity to release the energy should something go wrong.

hr

10. #10

### JJLL New Member 2. ```NULL ```

Jan 28, 2008
4
0
Loc:
USA
I do have a pressure relief valve already installed (it was the first thing I did). If the pressure goes over 30 psi, the system drains.

As for putting the coils on the outside of the stove, I wouldn't even bother. I've done it already and it didn't work AT ALL. I was really confident that it would at least preheat the water before it went into the coils that are inside my stove. But sadly, it was a waste of time and money. The coils on the outside of my stove were literally strapped to the sides of the stove and I got nothing. The reason was that I have a circulating motor thats pushing the water. The water simply wasn't spending enough time in the pipe. I can slow the motor down, but that means buying a variable speed motor and controller. I'd rather not spend the money for that yet.

As I'm learning, this kind of work is both fun and experimental. Safety is key.

I'm learning that to heat the amount of water that I need in my radiant system (roughly 100 gallons) I need between 40-50 feet of 3/4" pipe. I'm still doing more research. With any luck, I can post some pictures soon.

Thanks again for all of your help.

11. #11

### WRVERMONT New Member 2. ```NULL ```

Oct 29, 2007
104
0
Loc:
Northern Vermont
Agreed Sparks, Be Careful. Place the pressure relief valve in an effective location. Safety First.