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New Construction and PEX Radiant Heat
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1. What do I need?
To properly size most components related to your underfloor heating system we highly recommend a heat loss
calculation for your project if this is your primary heat source. Why? Heat loss is a critical
step, as we can estimate the average output of a radiant floor at 25 BTU’s per square foot
but windows, doors, insulation, and degree days all make a major impact on getting you
just what you need.
The most common sizing mistake with in floor heat is in oversizing. This not only makes the new
radiant heating system cost more to install, but also forces it to operate inefficiently,
break down more often, and cost more to operate. Oversized heating equipment also often
creates uncomfortable and large temperature swings in the house plus it will short cycle
the hot water boiler and run outside the design parameters costing you more money.
We are not in the business of selling equipment that you don’t need and a little work up
front can save you thousands of dollars in costs over the life of your system.
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2. How do I calculate my heat loss?
- Have your architect or builder supply it to you as in many states like NH or CA it is required.
- Calculate it yourself using software like
HVAC Calc which sells for $49.00.
- Or use one of the two different rough guides below.
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Insulation Type and Climate Zone
(NOTE: We highly recommend that you do a heat loss calculation and provide the information
below as a starting place)
- No insulation in walls, ceilings, or floors; no storm windows; windows and doors fit
loosely .... 60 to 100 BTU's per Sq. Ft.
- R-11 insulation in walls and ceilings; no insulation in floors over crawl spaces; no storm
windows; doors and windows fit fairly tight .... 50 to 60 BTU's per Sq. Ft.
- R-19 insulation in walls, R-30 in ceilings, and R-11 in floors; tight-fitting storm windows
or double pane windows .... 29 to 35 BTU's per Sq. Ft.
- "Energy Star Rated" house with R-24+ wall insulation, R-40 in ceilings, and R-19 in floor;
tight-fitting storm windows or double pane windows; vapor barrier sealed carefully during
construction .... 20 to 25 BTU's per Sq. Ft.
- SIP or Earth-sheltered house with little exposure; argon filled windows, and R40+
insulated .... 10 to 15 BTU's per Sq. Ft.
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Climate Zone
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Heating Sq. Footage by Climate Zone for a pre-1970's House
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ZONE 1
Houston, TX
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ZONE 2
Los Angles, CA
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ZONE 3
St. Louis, MO
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ZONE 4
New York, NY
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ZONE 5
Minneapolis, MN
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15 - 25 Btu's
per sq. foot
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25 - 30 Btu's
per sq. foot
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30 - 40 Btu's
per sq. foot
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40 - 50 Btu's
per sq. foot
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50 - 60 Btu's
per sq. foot
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Outdoor Design Temperature
The Outdoor Design Temperature (ODT), also referred to as the 2.5% design day temperature,
is not the coldest day ever, but rather a temperature that is achieved 97.5% of the time.
Examples:
ODT
Chicago = - 8°F
Denver = 1°F
Minnesota = -12°F
Washington = 17°F
Simply multiply the appropriate factor above by your home's total heated square footage to arrive
at your approximate required heating capacity. For example, if you live in Zone 3, your
home is well insulated, and you have 2000 heated square feet, the equation will look like this:
2000 square feet of "Energy Star" grade new construction but with lots of windows =
35 BTU's per sq ft.
70,000 Btu Load
Then, to calculate the output on a hot water boiler, multiply its efficiency rating by its listed input
rating for the actual Btu output of heat. For example, if a hot water boiler has a listed input rating of
100,000 Btu's and an efficiency rating of 70%, it will produce:
87,000 Btu input of Embassy BMS 10/20 Boiler
X .86 efficiency =
73,000 Btu actual output
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3. Existing Heating System
- All hot water boilers sold in the U.S. must have a rating plate. Check the rating plate and get the:
92,000 Btu Input of your Hot Water Boiler
X .80 efficiency of your Boiler
73,000 Btu actual output
- Count the total linear footage of baseboard in the home. Multiply this number by 600BTU’s.
This will give you the BTU output at 180°F. This number should be close to the Boilers
actual output.
There are a few ways to calculate heat loss. Use the above information to get a rough idea.
We strongly recommend that you
download a heat loss calculator. Why? Because windows and doors make a huge difference
to the heat load of your home. Once you have an idea of your requirements, we will
be able to get you a
quote.
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4. Ways to install Radiant PEX with an existing floor
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Under Floor Packages
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Over Floor Packages
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Radiant Slab
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5. Radiant Slab on Grade
For residential slabs we recommend 1/2" PEX tubing to be 12" on center. Along walls
with lots of glass or high heat loss the PEX should be 6" to 9" on center on the
outside walls for the first 2 feet, and 12" on center everywhere else.
When figuring the over all length of tubing you will need you divide any 6" spacing area
by .5, divide any 9" spacing area by .75 and any 12" spacing area by 1. This will
give you the over all length of the PEX needed in the slab. You will need to add the length
of tubing needed to get up to the pex manifold.
Typically pex manifolds are mounted 18" to 24" off the slab.
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6. Installing PEX Tubing
Following good piping practices the maximum length of each 1/2" PEX tubing runs should
be no longer than 300 feet (300-foot maximum is code in many places). When the pipe loops
exceed 300 feet you need to use larger circulators (pumps) to maintain this temperature
drop. With larger circulators initial cost is higher and they usually require twice as much
electricity to run. Most good radiant installers try to limit piping loops to below 300 feet.
There are many correct ways of installing in floor heat PEX within a slab. The best way is tying the PEX
to the reinforcing mesh or rebar. When attaching the PEX Tubing to wire reinforcing mesh or rebar it
is recommend that a zip tie be used every 2 feet of PEX Tubing.
Another way to install PEX in a slab is attaching the PEX Tubing to ridged insulation.
The use of insulation screw clips or large plastic staples is common.
We recommend an insulation screw clip or staple every 2 feet if installing the tubing
over insulation only (no wire mesh). If you use 2" polystyrene insulation it is
recommended that you use a 6 mil. polyethylene moisture barrier.
Installing the manifolds and keeping the lines under pressure (air or water pressure) for
the concrete pour is highly recommended and required by code in many locations.
In Floor Slab Packages - Click to Review:
- Under Slab Heating Packages - 250 feet
- Under Slab Heating Packages - 500 feet
- Under Slab Heating Packages - 750 feet
- Under Slab Heating Packages - 1000 feet
- Under Slab Heating Packages - 1250 feet
- Under Slab Heating Packages - 1500 feet
- Under Slab Heating Packages - 1750 feet
- Under Slab Heating Packages - 2000 feet
UnderFloor Heating System Packages - Click to Review:
- Underfloor Heating System Packages - 165 feet
- Underfloor Heating System Packages - 320 feet
- Underfloor Heating System Packages - 480 feet
- Underfloor Heating System Packages - 640 feet
- Underfloor Heating System Packages - 780 feet
- Underfloor Heating System Packages - 940 feet
- Underfloor Heating System Packages - 1120 feet
Slab Shield Insulation and Underfloor insulation - Click here to review:
- Under Slab Insulation - Slab Shield
- Underfloor Heating System Foil Insulation - Pre-Cut Insulation
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7. Insulation
Insulation is always needed with any radiant heating system and especially needed under slabs.
Why, if the soil has any moisture in it the moisture will wick away the heat at a tremendous
rate making your system inefficient.
Today many radiant slabs being installed with insulation only around the perimeter. Their
belief is that you should store the heat in the ground for use later. One problem with this
notion is that a large portion of the heat is absorbed into the ground and never warms your
home. Why do you want to pay to heat the ground? Slab Insulation is important for the entire
slab.
We recommend
Slab Shield Insulation
which was designed specifically for under slab
applications. Manufactured using two separate layers of 1/4" polyethylene foam with a pure
aluminum center. This product is available in 4' x 63' rolls for easy application. It is simply
unrolled and taped together (this is necessary for a complete vapor barrier to be achieved).
With Slab-Shield there's no time wasted installing 4' x 8' foam boards. With a puncture
resistance of 92.9psi you can work and walk on top of it without it crumbling apart.
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8. Here's a rough idea of what it will cost
Below are some pricing guidelines. These numbers are higher than most proposals, but can act
as a "stand-in" as you're creating your construction budget.
- High Efficiency (87%+) hot water boiler: $2,500 to $4,500
- Tankless Water Heater as a heat source: $1,200 to $1,700
- Per zone controls: $250.00 ea. zone
- Slab on grade Radiant: $1.20 per square
- Wood Underfloor Radiant: $1.70 Per square
- Myson radiators: $260 per 5000 BTU
People consider radiant heating for its superior economic and comfort advantages. But with
energy prices rising 35%+ this year, whichever efficient system you choose, you’ll appreciate
the cost savings!
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