sizing radiant heating PEX tubing based on heat losss calculatio

Brad White_203

regarding those books and publications, do you mean that you have them and tried to put them to use, or that you tried to obtain copies of them?

The RPA has both "Radiant Basics" and "Radiant Precision" books (the latter being recently updated) and both authored by Mr. Siegenthaler.

I would also suggest that you get a Rad-Pad (available on this site too), to get a sense of the "what ifs".

Now, some very basic, general advice: With staple up systems, yes, use plates, you got that. But you also have a limitation on spacing, roughly 8" OC if your joists follow any rational pattern. This of course can vary if you have TJI's or older construction or your joists were laid out on a Friday PM or Monday AM if you catch my drift..

The circuit lengths: With 1/2", most manufacturers recommend not exceeding 200 LF, sometimes more, maybe 250 and of course less is OK too. I would not go over 200 myself. If your coverage is 8" then your SF coverage divisor is 0.667 so a 200 SF room divided by 0.667 will need 300 LF. You are looking at two 150 foot circuits in that example. Make sense?

You also have to expect to use a higher water temperature because you are driving the heat through sub-floor and finished floor plus any area rugs or carpeting. Just accept that. The good news is, even if that higher resistance hurts that way, it also means a more even surface temperature because of "spread-out".

Basically, once your spacing is set (and it may be set for you is my point), temperature is your controlling variable.

Now, the "heat loss per SF" issue. This is not just dividing the BTU per Hour heat loss by the SF of the room. No, rather you divide that BTU number by the floor area you can actually use as a radiant floor. Sometimes this is the same (such as a living room). Sometimes it is less, such as an office, laundry room or, toughest of all, kitchens with all cabinets and built-ins.

More to it, but might that get you started?

Jim_88

sizing PEX tubing length based on heat loss

If you were doing a stapple-up job using PEX w/ aluminum plates, I understand the first thing you do is a heat loss calc for each room, but now I'm lost. From what little I could find (books, websites, asking my wholesaler, etc.) the next thing you do is calculate the heat loss per square foot. At this point I don't know what to do. I assume there are charts and graphs "out there" that will give me the length of PEX needed for each zone- based on, for example: 1/2inch PEX, distance between tubing, desired floor surface temp, floor covering type (R factor), and max floor temp, but every source on "HOW TO..." doesn't give me what I want: the length of 1/2 inch PEX required per zone. And I have tried "IPEX Manual". "Wirsbo manual" Siegenthaler's "Manual of Hydronic Heating" (I really couldn't find it in here-at least to the point where it is obvious), as well as a number of web sites. Any suggestions anyone?

Jim_88

Sorry, when I said "tried" I mean I 'tried' to read it looking for a clear explanation.
By the way, what is a "Rad-Pad"?
And you are absolutely right about the "available" square footage- the master bed is taken up by the queen-size bed, that's why I had to explain to my wife that I will probably need some kind of additional radiator panel for this room,as well as the master bath.
So, if I understand you, I take the "available" square footage, divide by .667, and that gives me my tube length, and if that number is greater than 300 (or 250 preferably) I use two loops for that zone, using 1/2 inch pex at 8inches on center w/ plates. That sounds easy to remember. Thanks so much.

Brad White_203

Rad Pad

The Rad Pad is a unique slide rule which covers many generic radiant heating applications.

For beds, that is always a good question, but if there is space below (i.e. not a platform bed), I have yet to worry about it. Sweater boxes though....

But I think you grasp the basics (very basic). All advice is qualified and using a panel radiator is sometimes necessary.