I have to order a hot water coil heat exchanger to be placed in my forced air furnace. All my pipeing is 1.25". My furnace is 80,000 btu. Most coils I have looked at have 1" ports. How do I size the coil in relation to plenum size? My plenum is 20.5" x 20.5".
HOT WATER COIL QUESTION?
- Thread starter jimdeq
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jimdeq said:
The reality of the issue is that the coil needs to be sized to the heating load/aor flow and the plenum should be adapted to fit the coil. As for the 1-1/4" lines and 1" connections, just adapt from your tube to the coil unless you want to custom order a coil with 1-1/4 ports. Look for a coil that will deliver the required heat using a water temp of 140*. This will allow more swing in the water temp while still providing adequate heat and also give you a higher air temp rise across the coil. Nice warm air from the registers = high WAF. Also bear in mind that the introduction of a coil in the air stream can have very negative effects on the amount of air flow. The rule of thumb is oversize if in doubt.
FYI in case it is of help-
https://www.hearth.com/econtent/index.php/forums/viewthread/25090/
https://www.hearth.com/econtent/index.php/forums/viewthread/24899/
The coil that I bought and that's shown there starts moving a LOT of heat as soon as there's any hot water flowing through it, and lets the air flow through very well
You don't necessarily need that specific size, but I second heaterman's comments
https://www.hearth.com/econtent/index.php/forums/viewthread/25090/
https://www.hearth.com/econtent/index.php/forums/viewthread/24899/
The coil that I bought and that's shown there starts moving a LOT of heat as soon as there's any hot water flowing through it, and lets the air flow through very well
You don't necessarily need that specific size, but I second heaterman's comments
Is there a certain amount of fins or a certain diameter of internal pipeing I should be looking for ? I also noticed that some coils are 2.5" thick and some are 3 or 3.5".
jimdeq said:
The "box" that houses the coil may come in all thicknesses. The primary parameter that I'd pay attention to is whether it is a coil that is two rows of tubes thick or four rows thick- those are the main two flavors. There a re few in three rows thick but those are less common and I would tend to be particularly careful before going with one of those, since, as far as I know, that is a variation that is built mostly to cut manufacturing cost, rather than to maximize any net performance.
Four passes give not only more surface area to transfer heat from water to air, but also the additional runs give more opportunity to take fullest advantage of something called "countercurrent flow" which maximizes heat transfer by setting things up and running them so as to keep maximum practicable differential between air and water at each and every point as they pass each other on their ways past each other. Let's say you install your coil like mine, with the blower underneath, pulling air from the house's cold air returns, and the air coming out of the coil then goes to your hot air registers. You want to feed the hot water in the top of the coil, and take the water (that's been cooled by the airflow through and heat exchange by the coil) out the bottom of the coil. Seems counterintuitive until you get your head fully wrapped around it, at which point it makes perfect sense. Your kidneys and other organs operate and rely on this same principle of countercurrent flow and your body would not work without it. After you read the stuff in the next couple of paragraph, use the advanced search function here on hearth to search past boiler room posts for "countercurrent flow" and you'll find some discussions, explanations, and reports from people who switched to it after having it the other way, and found that it did make a difference.
Internal piping diameter is something that engineers would fine-tune if it were a system that was being designed for maximum performance at minimum cost. Too big, and you've bought more metal than you needed to spend $ on, which you obviously would not want to do if you were doing this in mass scale and trying to maximize profit- but there is virtually no performance penalty (that I know of) for having gone a size or two too big on the diameter of the tubing within the coil (I went with 5/8 although it probably would have been OK to do 1/2 in my application and maybe [but probably not] 3/8]. Too small- whether you're building one or a thousand of these, and you've created a situation where you've got a lot of resistance to the flow of the water, at which point it's going to be hard to move enough water and the heat that it is transporting-- and/ or you're going to need to use a much bigger pump to push the water through the skinny tubing, at which point you will have increased up front cost to buy the bigger pump and increased operating cost forever more to run the bigger pump.
My own situation/ philosophy is that the only thing that's harder to stomach than spending an incremental amount more cost up front, or time to locate the proper stuff or design/ install it well ..... is having to go back and yank things out, at which point you have scrap, and have to go buy what you'd have been better off buying in the first place, and you've now spent WAY more than the difference you were looking at up front -- or having a system that just barely works, and not so badly that you can ever justify replacing it, but not so well that you are ever pleased with it.
Dunno if I am verging on pedantic, but I hope that that's helpful. This stuff stands out in my mind because I figured it out by way of reading about it until it all fell into place and made sense (though it was kind of fun, but that's just because I have a seriously nerdy streak- I read Siegenthaler's Modern Hydronic Heating as bedtime reading- and liked it)(I do sometimes read other things that more normal people would enjoy, too).
pybyr, Thank you very much for taking the time to give me a detailed description of what I need to look for. I will check with precision coil because I have not been able to find 1/2" or 5/8" headers. Thanks
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