How to Calculate Water Coil Length.

JJLL · Post by **JJLL** » Mon. Jan. 28, 2008 11:30 am

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.

WNY · Post by **WNY** » Mon. Jan. 28, 2008 12:03 pm

Buy a boiler unit, you probalby need more than just a coil....if you looking at whole house heating. That's becuse they would probably be too big for a regular stove to get the correct amount of BTU input/output.

JJLL · Post by **JJLL** » Mon. Jan. 28, 2008 12:31 pm

I understand that a boiler unit would be best, but it still doesn't answer my question.

How do the manufacturers of coal boilers calculate their coil size? There should be some kind of calculation? My radiant system consists of about 95 gallons of water. Most boilers don't exceed 180 degrees. So I know that my max output shouldn't be over 180 degrees.

How long of a coil would I need to heat 90 gallons of water effectively? And yes, I understand recovery rates too

I have a 1/4 hp motor on my current boiler. I've tapped into my lines already and I need to slow the flow down to achieve a better heat recovery rate.

/me is getting long winded.

Is there even a way to calculate the amount of coil? Maybe thats a better question?

WNY · Post by **WNY** » Mon. Jan. 28, 2008 1:43 pm

Found this, pretty technical...

http://www.pfonline.com/articles/109902a.html

JJLL · Post by **JJLL** » Mon. Jan. 28, 2008 2:59 pm

Hmmmm thanks VNY, thats pretty close to what I need. I'm going to keep looking, if I find anything more I'll post for all to see.

BigBarney · Post by **BigBarney** » Mon. Jan. 28, 2008 5:20 pm

JJLL,

In a design of this type you don't usually use a single coil because you cannot get enough surface area to extract the

heat needed to heat this large volume of water in a reasonable time.A multi-coil is needed with each leg sized to

extract a portion of the total load, each calculated to a percentage of the load,and the available energy.This will

involve a little bit of deviation because the coal fire is not constant,so size will be approximate.In a single coil the

water will only heat to a limit,then at a certain distance down the coil,and the rest of the length will be wasted.

In small coils of 1/2" or 3/4" tubing the area of 18.85 sq" or 28.28 sq" take a lot of footage to gain a large amount of

BTU's.

BigBarney

jpen1 · Post by **jpen1** » Mon. Jan. 28, 2008 7:42 pm

I think you are barking up the wrong tree so to speak. I have a coil in my channing feeding my oil boiler just to heat the domestic hot water and it is barely enough to even do this the coil I have would be the equivalent of about 15' of 3/4 pipe's surface area. For hydronic heating a boiler is a must as you won't get enough BTU's out of any coil you can fit in a stove to offset the cost of it's manufacture and hookup. I built my own coil and even with my wholesale pipe cost for the stainless and no out of pocket cost for the manufacture I am at over $350. If you have to buy a ready made coil like that double that cost. Plus to be even marginally effective for hydronic heating you would need 3 to 4 times the area and at 3 to 4 times the cost of my coil as well. You can buy a used boiler for the same money and it will be much more effective as well as much more efficient.