Replies

1. 
   Piffin | Dec 18, 2003 04:24am | #1

   Have you designed this system yourself?

   Having loops of differing lengths is not too much trouble. They just need to know how long each loop is, which is why the PEX is numbered every so many feet. With that info, they can figure the balancing act at the manifold by openning one floe valve more or less than others.

   But the reason I asked if you have done the design thus far, is that I was advised that they don't like to go much over 250' with each loop. Friction in longer runs cuts down flow rates and that impedes heat delivery. using 5/8" helps with longer runs but 1/2" is the more common with stapleup because the fins are made for 1/2".

   .

   Excellence is its own reward!

2. 
   Mongo | Dec 18, 2003 08:53am | #2

   I'll ask the same question piffin asked...who did the design?

   If you're using 1/2" tubing, your loops are a little long. Actually, they're about 50% too long. Rule-of-thumb (and yes, thumbs do bend, however...) is for 1/2" PEX, the max loop length would be about 330', for 5/8ths you can do up to 500'.

   There are other variables, though. How much of a pressure drop you suffer (tube size/GPM/fluid type/fluid temp), the velocity of the water running through the tubing(BTU required/tubing spacing/llop length), and the circulating pump.

   There are myriad details to designing a good system, and your post doesn't come close to offering enough info for us to give decent feedback. If you post the numbers you're sure to get good answers.

   Piffin gave good advice regarding balancing the system. Some manifold manufacturers have very simple formulas to get the manifold settings ballparked the first time out. They're simply based upon the length of the loop that you're adjusting compared to the length of the longest loop on that manifold. Very simple.

   Now, after all that...<g>

   If the system was properly designed (assuming 5/8ths tubing to handle your long runs), if the heating unit can handle the extra BTUs required by the extra loop, and if the loop that you want to add on is not excessively longer than the longest loop that the system was designed for (meaning the circulating pump can keep up) then you should be able to add another loop with no problems.

   One other common misconception. In general, a larger diameter tubing does not provide more heat output. It simply provides less resistance to fluid flow, making life easier on your circulating pump.

   Don't you love long answers that don't actually answer the question?<g>

1. 
   DavidThomas | Dec 18, 2003 07:04pm | #3

   "One other common misconception. In general, a larger diameter tubing does not provide more heat output. It simply provides less resistance to fluid flow, making life easier on your circulating pump"

   It doesn't provide much more heat per linear foot.  But, because you can move more GPM  in the larger diameter tubing, the loop can deliver more BTUs.   And/or there be less temperature drop fomr start to finish.

   I agree with the loop length thoughts Mongo and Piffin give for staple-up.  But in a slab, with its greater heat conduction, I'd like to see loops of 200-250' for 5/8" and 150-200' max with 1/2"

   Rather than worry about "balancing" between loops*, I'd look at another issue.  The water will come back cooler than it went out, right?  So where you route it first will put out more BTUs/linear foot.  Several places you might want the greater heat/sq ft are: along exterior walls, bathrooms, or under carpet (since it insulating).   Closer spacing of the tubes is another way to acheive that.  Not every bay has to have one length.  Bays near exterior walls, etc could get two lengths, for instance. 

   * The modest difference you describe in loop length would make very little difference in the GPM in each loop.  There will be more difference between the beginning and end of each loop.David Thomas   Overlooking Cook Inlet in Kenai, Alaska

3. 
   csnow | Dec 18, 2003 07:17pm | #4

   The short answer is "no", because you can use valves to balance the flow to the loops.

   Just to add to the excellent advice/info you have already received;

   Some manifolds have flow gauges and thermometers on them to help with your 'balancing act'.  These are just so very cool! [I might add].

   The physical layout of the tubing and the heatloss characteristics of the structure are other important factors to consider, particularly with those long lengths.  {edit: whoops, Dave beat me to this point, nevermind}

   
   

   ---

   Edited 12/18/2003 11:19:37 AM ET by csnow

1. 
   ruffmike | Dec 18, 2003 11:26pm | #5

   Here is more info-system from radiantdirect.com,loops can be 500 according to them using 1/2"pex,I'm trying not to freak out here.zone manifold by calefi/thermostat control by zonemaster2000 by nix/circulating pump nbf22 by bell and gosset/ondemand boiler by trinity,combination w/domestic,flow rate 45 to130 degrees 4.5gpm.

   my loops were laid out by me,zones decided by me and radiantdirect rep.The house is two stories,bedrooms are one first floor.1 zone for masterbed and bath in slab.2 zone for two beds and one bath.3 zone for living dining kitchen bath and guest bed(this is the zone originally asked about).

   I am confident this system will heat my house as in ca. heat is more of a convenience than requirement.Thanks for allfeedback Ive got the pro's attention here.Once again I am limited to online time so I my be slow to come back.thanks,Mike

1. 
   DavidThomas | Dec 19, 2003 12:30am | #6

   "my house in CA"

   Having grown up 15 miles away, I know that your design temps are much milder than mine (-40F).  However I think you mentioned that the ground floor has RFH in a slab.  If so, please describe (4" slab-on-grade, 1.5" lightweight concrete, etc) and also the loops in the slab (500' of 1/2" (hope not) or 250 of 5/8" or whatever)

   Staple-up is low mass and doesn't need to be hit as hard.  A 4" slab is massive and has a lot of thermal inertia.  You need to get a lot of BTUs into it to move its temperature.  An analogy would be moving a pulling a tricycle with dental floss and moving a Mack Truck with 1" rope.  They both work but are not interchangable.

   Being on (between, no less!) the water, you've got pretty moderate temperatures (at least until those east winds in October fire season).  I've got to respond to mild (25F) dropping to cold (-20F) therefore 45F within 16 hours of time.  Your worse case is a sunny mild winter day (60F plus solar input) to a clear, still night (35F) so only 25F in 12 hours.  So you still have to move the concrete temp around somewhat quickly.  (Or put another blanket on the bed.)David Thomas   Overlooking Cook Inlet in Kenai, Alaska

1. 
   ruffmike | Dec 20, 2003 01:45am | #7

   Thanks for your input,the 2 zones in the slab are 250 and 275 in length 1/2" pex installed 18"on center.The slab is 5" with vapor barrier and 1"stryrofoam underneath.I know I didn't do my homework on the science side,but  I believe or believed the system could easily provide the comfort needed.Piffin's point about loops only being 250' has me concerned but I can't see redoing the staple-up at this point.Am I in a bad way here?Mike

1. 
   DavidThomas | Dec 20, 2003 02:54am | #8

   I'm not terribly concerned about the staple-up.  It is low-mass and if it has some kind of aluminium fins, it is probably going to be just fine in your climate.

   That slab has a lot of mass. And not a lot of tubing.  Consider the 275 foot loop.  At 18" spacing, it covers 412 sq ft of 5" concrete (+perimeter curbs?).  Call it 90,000 pounds with a Cp of 0.22 or so.  To raise it one degree you need to add 19,800 BTUs.   So if the heat demand increases from no much to 20 BTU/hr/ft, the floor needs to heat up about 10 degrees (more if covered with wood or carpet).  200,00 BTU just to raise the slab temp, plus (desireable) losses to the rooms during that time.

   If you are pumping 3 gpm of water and expect a 10F temp drop on the loop, that's 15,000 BTU/hour.  So 13 hours to the slab to temperature (actually a lot longer because its is losing heat.  13 hours can be a long time to wait for heat.

   Max heat output, if you accept a 25F temp drop, would be 37,000 BTU/hour for that loop or 90 BTU/sq ft.  For a 30F day outside you'd need (this was a Victorian, right?) about 40 to 120 BTU/sq foot.  The lower range if you have tightened things up, blown in insulation, etc.  If the structure hasn't been made more energy efficient in the last 80 years, then the RFH described is going to have trouble keeping up on a cold day.

   Repost with a natural gas bill (in therms or 100 cu ft) for a winter month.  That will capture an average heat load and we can fudge factor from that to a worst case design number. David Thomas   Overlooking Cook Inlet in Kenai, Alaska

2. 
   ruffmike | Dec 20, 2003 10:28pm | #9

   David,here is some more backround ,we bought the house 4 yrs. ago.It was a 900 sq.' high basement cottage with the bay window torn off.I had it jacked up to allow habitable space down stairs,putting the bedrooms down to keep the traditional front stairs.The foundation is a 2 pour(the slab done after the foundation) because of the cribs.The ground floor is 8' with all new framing and windows,insulation,ect..I plan on fibreglass insulation on first floor and possiblly cellulose in existing upstairs and attic(blown in walls from attic-balloon framing)I used to install celulose in the 70's,just out of highschool and always figured I'd use it myself someday.I have concern though about cellulose in walls without any tar paper(my old house).and I lean to the fibreglass downstairs because of ease of installation.Anyways, my last gas bill is last winter with just the 900sq',no insulation,leaky windows(new windows upstairs too) and a lousy floor furnace.so I doubt it would be relevant(plus I'd have to find one).

   So,by my thinking,the slab is isolated from the curb(foundation?),house will be relativily tight,probablly 30 feet of the 275 loop is the return to the boiler.I am keeping my fingers crossed,as I figure I am stuck with whats in the concrete.In any case,I've got a direct vent gas fireplace (17,000btu) in my masterbed(the 275 loop).so we'll be warm-let the kids freeze.

   Some one not familiar with the bay area might think this was a teardown house or a waste of money but when complete we wil have a 600k+ house for about 275k.I have also replaceed the bay(2 story),removed collapsing chimineys and replaced them with "solarplaces"-skylights with art glass,all new plumbing and electric with 200 amps(got to have a hot tub-someday).Anyways thank you for the help,I hope to be arround here awhile,Mike

2. 
   greggo | Jan 07, 2004 03:54pm | #10

   Well now I;m looking into radient ,Could you tell me the manufacture or brand of those manifolds you were talking about?

   Some manifolds have flow gauges and thermometers on them to help with your 'balancing act'.  These are just so very cool! [I might add].

   Thanks Greg

1. 
   BowBear | Jan 07, 2004 07:19pm | #11

   Rehau manifolds have flow guages. The temp guage is separate.An ex-boat builder treading water!

1. 
   greggo | Jan 08, 2004 04:58am | #14

   Thanks Bob:this radient  stuff is looking  better and better

2. 
   csnow | Jan 07, 2004 07:23pm | #12

   Some links to manifolds:

   http://www.wattsradiant.com/professional/manifold.html

   http://www.appliedradiant.com/hydro.htm

   https://www.mvsupply.biz/hrh_manifolds.htm

   http://www.aimradiantheating.com/store/page35.html

1. 
   greggo | Jan 08, 2004 04:52am | #13

   Thanks csnow:nice stuff.I'm like a kid in a candy store!