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Author Topic: Question about tubing in concrete.  (Read 1214 times)

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Offline RickV

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Question about tubing in concrete.
« on: July 12, 2014, 08:32:09 AM »
I am getting ready to break ground on building a new house that will have a full basement. I want to place pex tubing in the concrete for radiant heat. I know that I will need multiple loops for even heat distribution but how many? Is there a tried and true pattern that is more efficient? what size tubing? The foundation will be 30' x 40'. Our average winter temps are low to mid 20's during the day and teens at night. Occasionally we will drop to -10 range. The boiler will set around 50 -75 feet away from the house half way between garage and house. I haven't decided what boiler to use yet, most likely a Central. My plan is to use radiant in the basement and garage floor, and a exchanger in the heat pump. Thanks in advance to everyone.

Online sprucebunny

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Re: Question about tubing in concrete.
« Reply #1 on: July 12, 2014, 10:55:55 AM »
You need about a lineal foot of 1/2" Pex per square foot of floor, either concrete or wood. About 230 feet of tubing per run. A manifold per zone.
I did back and forth. It's easier not to step on.

You should do a www search for sizing a heating system for your area. Some dealers have heat loss calculators so you can enter square feet of windows, wall thickness/type etc.
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Offline snowstorm

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Re: Question about tubing in concrete.
« Reply #2 on: July 12, 2014, 12:22:15 PM »
why would you want heat in the basement floor? i put heat in the floors and with that the basement temp stays around 65 and winter is much colder up here

Offline scsmith42

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Re: Question about tubing in concrete.
« Reply #3 on: July 12, 2014, 01:27:56 PM »
I'm a little bit south of you and have radiant in the slab in my shop.  Because of the heat loss through the edge of the slab, I ran my tubes closer together at the perimeter than in the middle.  Each run is around 300' of 1/2" pex.

The perimeter runs are 12" apart, then I increased to 18", and then to 24".  The bulk of the shop is 24" spacing.  The slab is 6" thick with insulation underneath; the tubes are positioned 4" down in the slab.

It stays quite comfortable with no cold spots. 

Where the tubes exit the slab I ran them through 90 degree PVC conduit elbows in order to protect them.  The conduit elbows also assist with the transition from horizontal to vertical.


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Offline dave_dj1

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Re: Question about tubing in concrete.
« Reply #4 on: July 12, 2014, 04:28:29 PM »
250' per run max. Make sure you insulate under the slab with pex on top of insulation. You will need a mixing valve to cool boiler water down to usable in floor radiant heat. It may be wise of you to hire a pro to assist, hate to see all that money and hard work go to waste if done incorrectly.
I'm a general contractor so I've been around it a time or two.
good luck,

Offline Holmes

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Re: Question about tubing in concrete.
« Reply #5 on: July 12, 2014, 06:25:55 PM »
 12" on center tube spacing will work fine.  Keep you circuit lengths to less than 300', 250 is better. 40' down 40' back 40' down 40' back 40' down 40' back = 240' that would be 1 circuit and it covers a 6' width of the cellar. You will need a feed manifold and a return manifold.
Put down blue or pink insulation board [ 1,2" layer] or [ 2, 1" layers] with 6" wire mess grid on top, tie the tubing to the wire mesh. The tubing can stay at the bottom of the concrete it does not have to be pulled up into the concrete.
 You  will probably need to run 100 degree water or less thru that tubing to keep the cellar warm.
 If you mess up and skip a little in width or length or floor coverage don't worry it will work fine.
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Offline garret

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Re: Question about tubing in concrete.
« Reply #6 on: July 28, 2014, 06:13:24 AM »
Installed system myself in a pole building 3 y ago.  Couldn't be more pleased with performance and efficiency. Very easy to get good results, just don't cut corners in preparation.

Just a few suggestions:

1.Ditto on the 250' max length PEX.  I used 5/8" at 280ft x 4 circuits, less lead loss, smaller pump.  Parallel circuits head loss essentially equal to head loss of maximum length circuit (like parallel resistors).  1 linear ft/sq ft more than adequate btus even at 100 F water temp.

2. DON'T SKIMP on INSULATION.  2" blueboard foam minimum and vapor barrier.  Good drainage under slab as moisture will rob heat.  Insulate edge of slab as well.

3.  Download and design system using LoopCAD.  Free trial version worked perfectly for me.  Designs based on analysis of heat loss and creates various loop configurations and helps with placement of returns and manifolds.

4.  Run tubing through ~ 12" lengths of 1" plastic conduit at slab joints.

5.  Make sure to include a way to fill/purge air in manifold plumbing.

6.  Accurately map location of tubing so that you may place anchors later without incident.  Can also be done using radiant thermometer.
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Offline LittleJohn

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Re: Question about tubing in concrete.
« Reply #7 on: July 28, 2014, 11:42:57 AM »
Main things to remember, start with hottest water where the coldest temperatures will be seen (think outside walls/exterior doors/windows).
I generally like to stick with 6" spacing for 2' on all exterior walls; then go to a 12" spacing, maybe 9" OC if I bought to much pipe.

Foam, under slab is never a bad idea; I am about 95% certain in codes up here in MN.

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