how to size a boiler and pipe sizes?


Old 09-24-14, 07:44 PM
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how to size a boiler and pipe sizes?

i am just curious but how does one size a boiler for a fan coil system? do they calculate just the house heat needs or do they add up the max temp requirements of all the fan coils and DHW tank? say everything ads up to 200,000 btu but the house only needs 100,000 btu, how would a person figure this out? not all the fan coils would be on st the same time ether. and with 10 zones calling 2 to 3 gallons per minute how does one prevent the pipe from running dry on the main boiler loop?
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Old 09-25-14, 04:06 PM
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It's a complicated set of questions you've asked... one that pretty much requires a textbook to answer.

Lemmee see if I can lay some 'groundwork' ...

First, it's never good practice to install more air handler BTU than the home requires. (unlike with baseboard and radiators... if you install more than you need, you can heat the home with cooler water) This is because air handlers NEED the hotter water. If you run cooler water through them the air comes out FEELING cold and drafty.

So, if the home requires 100K for example, there should be 100K of air handlers installed, and 100K of boiler.

Depending on the air handler size, 2-3 GPM may not be enough. They need a pretty good flow through them.

About pipe flow, it's not exactly linear, the numbers below are more or less 'ballpark'. Could be a little more or less:

1/2" pipe is good for 1.5 GPM

3/4" 4 GPM

1" 7.5 GPM

1-1/4" 16 GPM

1-1/2" 25 GPM

Basically speaking it the VELOCITY of the water in the pipe that needs to be controlled, if it's too SLOW you will have problems moving air out of the pipes, if it's too FAST the system will be noisy and the pipes may erode from the inside out, causing pinhole leaks years down the road.

with 10 zones calling 2 to 3 gallons per minute how does one prevent the pipe from running dry on the main boiler loop?
You can't 'run the pipes dry'. It's a CLOSED system. Your pipes need to be big enough to handle the flow requirement and not move the water too fast, or too slow.

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