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Retrofitting with a PEX Radiant Floor Heating System
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1. What do I need?
To properly size most components related to your radiant underfloor system we highly recommend a
heat loss calculation for your project if this is your primary heat source. Why? Heat loss is
critical step, as we can estimate the average output of a radiant floor at 18-25 BTU's per square foot
but windows, doors, insulation, and degree days all make a major impact on getting you just
what you need. Remember, Heat Losses can vary from house to house, city to city and state to state.
We have seen heat losses range from 10 Btu per square foot to as high of 120 Bty per square foot for
2 houses in the same area. So - be careful.
Taking the time to do a heat loss calculation will pay for itself many times over. The most
common sizing mistake is in oversizing. This not only makes the new floor 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 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 for Free.
- Or use one of the three 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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Out Door 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 gas boiler, multiply its efficiency rating by its listed input
rating for the actual Btu output of heat. For example, if a 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 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. Installing PEX Tubing in an existing floor
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Under floor with staple up with heat transfer plates
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Underfloor heating systems with PEX with slide brackets
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Over floor with ThermalBoard, over the sub floor with 3/8 inch pex tubing
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Over floor with sleepers made of 2X4 with tubing and mortar over the sub floor
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New pour radiant Slab
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Thin pour over existing concrete
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5. PEX 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 pex 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 manifold.
Typically pex manifolds are mounted 18" to 24" off the slab.
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6. Installing PEX Tubing
Following good pex piping practices the maximum length of each 1/2" Radiant Heat PEX tubing run 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 hydronic pump circulators initial cost is higher and they usually require twice as much
electricity to run. Most good radiant under floor heating installers try to limit piping loops to below 300 feet.
There are many correct ways of installing PEX within a slab. The best way in a radiant floor heating system
is to tie the PEX to the reinforcing mesh or rebar. When attaching the PEX to wire reinforcing mesh or
rebar it is recommend that a zip tie be used every 2 feet of PEX.
Another way to install PEX Tubing 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 PEX 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 radiant pex 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.
Under 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 Insulation - Pre-Cut Insulation
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7. Insulation
Insulation is always needed with any radiant heating systems / application and especially needed under concrete 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 under floor heating systems in cement 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?
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 feet by 63 feet 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 foil insulation there is no time wasted installing in 4 foot by x 8 foot foam boards.
With a puncture resistance of 92.9 psi 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
- Middle Efficiency (80 to 82%+) hot water boiler: $1,500 to $2,500
- 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 PEX Radiant Heating Systems for the 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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