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radiant loop lenghts (4 Posts)
radiant loop lenghtsI am installing radiant tube in a staple up system with aluminum panels and i wanted to have all equal (8) 250' loops with 1/2" tubing per floor, however due to beams that can not be drilled and also some of the layout criteria dictates that some of my loops will be in excess of the 250' desired maximun lenght. What are the downfalls of exceeding 250', maybe 2 loops that weill be 285'+/-.
Self balancing actIf your loop lengths are equal in length +/- 10% then the loops will have balanced flow as long as the layout patterns are pretty consistent between them.
275' Would be the long end of the 10% rule. If you have flow meters on your manifold then its not as big of a deal.
Excessive loop lengths have higher headloss which takes more pump, and more fiddling to get even flow to all loops if required. Plus a higher temp drop across a long loop verses a loop that is shorter with the both having same flow rates given the fact that both loops are covering the same heatloss area. Again flow meters make this easier to adjust.
Back in the day when manifolds did not have visual flow meters this rule was very important for installers to follow and following now still makes life easier. I would call it just good piping practice.This post was edited by an admin on February 6, 2011 4:07 PM.
it is far more importantto isolate rooms on their own loops than it is to equalize loop lengths. of course, this requires balancing for each loop. but it's worth it.
If you don't know what your loop max really is, then downsides could be uneven floor heating, larger pump requirements, or higher water temperature requirements if you are underpumped. extreme cases could fail completely but that's unlikely at 285'.NRT.Rob
Radiant tube lengthsYou can only have equal flow and equal tube lengths if the exposure and therefore the resulting heat loss is the same for each loop.
Tube length limits are a function of both heat load and head loss. If you keep your loop lengths the same length throughout then you need to adjust your tube spacing to match the heat output required by exposure and the resulting heat loss. If you don't do this then you will need to adjust the flow on your loops to control your delta t and even though the loops are the same length the flow required by each loop will be different.
As you mentioned this being a staple up job, and you can't adjust your tube spacing, I would make sure that I knew the flow rate for those longer loops with no more than a 15F delta T and see what that does to your pump selection. Don't forget that head loss rises exponentially with flow so you get a double whammy when you start lengthening loops. Longer length requires faster flow to keep the delta T down and adds more travel distance as well.
If you have a small load it doesn't make that much difference but here in Canada, if this is your primary heating, it is difficult to make a case for loops more than about 220' in a staple-up.