Radiant Heat Temperatures

This article describes actual radiant heat temperature settings and targets.

We describe the normal or correct radiant heat setting temperatures and we explain what happens if the radiant heat temperatures are set too low or too high. And we discuss

• Radiant heat temperature settings at the boiler. Radiant heat temperature settings in the tubing.
• Maximum temperature for radiant-heated floor surfaces.
• Energy efficiencies of radiant floor heating. Heat loss rates into earth below radiant heated floor slabs.
• Operating temperature comparisons for types of radiant heating.

Photo: this Minnesota homeowner indicates the placement of radiant heat tubing in the slab of a new heated garage.

Article Contents

• RADIANT HEAT FLOOR TEMPERATURES (Typical)
• RADIANT HEAT FLOOR OXYGEN DIFFUSION
• RADIANT HEAT FLOOR TEMPERATURE CHECK
• RADIANT HEAT FLOOR IMPACT of CARPETING
• RADIANT HEAT CEILING TEMPERATURES
• RADIANT HEAT CEILING PANEL PRODUCTS
• RADIANT HEAT DRIVEWAY & WALKWAY DEICING SYSTEMS
• RADIANT HEAT CIRCULATOR FLOW RATES
• RADIANT HEAT SUPPLIERS & MANUALS

For help choosing, installing, and adjusting radiant heat temperature controls themselves, see this separate article:

• RADIANT HEAT TEMPERATURE CONTROLS & SAFETY DEVICES

  Here we add details about different operating temperatures for radiant heating systems.

Typical Radiant Heat Floor System Operating Temperatures

Typical radiant floor systems operate at 85 - 125 °F water temperature entering the tubing, and put the floor surface temperature about 5 degrees above the room thermostat set temperature.

Similarly other sources such as the Oregon state energy conservation department gives the typical operating temperature range for radiant heating systems at 85-140 °F (30-60C) though we agree that 140 deg. F. is a bit higher than suggested by other sources.

A radiant heat system manufacturer, Radiant Floor Company, opines that 120 to 135 deg F is "ideal" and most sources we have found discuss typical radiant heat operating temperatures in the 115-135 degree range.

Where PEX tubing is used for radiant-heat under-floor tubing, 125 to 130 °F is typical operating temperature used for water entering the tubing.

Most radiant floor heating systems we've examined run at around 115F water temperature entering the tubing of the radiant section, and you'll see by the temperature gauge at the top of this article that 115 °F was the factory default set temperature on the control of an electric radiant heat boiler we installed.

The control on that boiler's circuit board is shown just above.

But as we explain below, the optimum operating temperature for radiant heat floor systems will vary depending on the insulating or heat-conducting properties of the flooring material itself.

Photo above: adjustment dial for setting the radiant heat water temperature for a radiant heat floor system that turned out to be unusuable: the contractor omitted good insulation under the slab.

You can avoid that fiasco by a review of RADIANT HEAT MISTAKES.

Typical Temperatures at the Upper Surface of Radiant-Heated Floors

Our photo (above) shows our friend Steve and his dog explaining the radiant heat tubing layout in the new garage floor slab of a Minneapolis MN home. Radiant heat tubing was installed under the concrete floor to provide a heated garage.

The open corner garage door design permitted cars to turn into the garage from a narrow alley.

Unless you have a black crayon and an infrared thermometer it may be a bit more difficult to measure the finished-floor surface temperature (what your bare feet would feel when walking on the floor) than it is to just look at a temperature gauge on the hot water heating pipes or heating boiler themselves, but looking at the finished floor surface is one important temperature to note.

The finished floor surface temperature is what the building occupants feel when walking on the floor, affects the rate of heat radiating into the air above the floor, and if too high, can also become a safety concern.

According to CMHC (Canadian Mortgage and Housing Corporation) the finished floor temperature should never exceed 85 F (29 C).

Watch out: Note that for special installations such

as DRIVEWAY & WALK DE-ICING SYSTEMS installation the radiant heat system operating temperatures will be higher than used in an in-building heating system.

Be sure that the tubing you select is rated for use at that operating temperature.

Our photo shows an abandoned driveway de-icing radiant heat system that has been abandoned. Only the manifold remains in place near the home's heating boiler.

Watch out: if you saw 167.8 F at the boiler, that sounds too hot for normal circumstances on a radiant heating system. Running a radiant heat system hotter than necessary can have a range of harmful effects including:

• Shorter heating boiler life
• Shorter radiant heat tubing life
• Oxygen diffusion problems [discussed below]
• Uncomfortably hot or even dangerously hot floor surfaces
• Unnecessarily high heating system operation costs

See PEX PIPING INFORMATION for more details about temperature ratings of plastic tubing. It may be useful to ask why someone has set the temperature of this radiant heating system so high. It could be

• Simply a mistake
• A control not functioning properly
• An improperly installed or designed radiant heat system that is not delivering adequate heat into the occupied space
• A misunderstanding of the controls or a misread of the temperature at the wrong location. See "Where to Check the Temperature ..." at the end of this article.

Typical Radiant Heat Temperatures Under Ceramic Tile

Under ceramic tile a higher temperature might be allowed without risking floor damage (we don't have wood shrinkage for example) and/or might have been set by a homeowner who likes a really warm bathroom floor.

But since ceramic tile is a *better* heat conductor than wood flooring, if anything, one would expect the temperature setting for that floor to be lower, not higher than otherwise.

Radiant under ceramic tile or vinyl tile, lower end of the temp;

Typical Radiant Heat Temperatures Under Finished Wood Flooring

Under wood floors nailed right to the subfloor and tubing below, radiant heat temperatures are typically at the middle of our 85-135 °F. temperature range, or about 105-115 °F. Electric radiant heat systems may warm the floor to just 75 °F.

The radiant heated wood floor shown in our photo (left) was installed asd a retrofit in an older a New York home.

The finish flooring was 3/4" oak over plywood subflooring and was run too-hot, resulting in shrinkage gaps and floor damage. How did this happen?

Attempting to "drive out" tubing odors the installer pushed the floor temperature up a bit too high, taking the blame for gaps that appeared in the flooring as a result discussed at

at GAPS in RADIANT HEATED WOOD FLOORS - caused by running the radiant heat at too-high a temperature.

See also WOOD FLOOR RADIANT HEAT design.

Radiant Heat Temperatures Under Laminate Flooring

Under laminate type finish flooring, keep the temperature under 82 °F or risk floor damage (per http://www.shawfloors.com/)

Radiant Heat Floor Under Carpeting: R-Values & Operating Temperature

While carpeting isn't the most-effective walking surface over a radiant heat floor system, it can work provided you use thin carpet with minimal or no padding. The total R value of any or all materials to be laid over a radiant heat floor should be held to less than R-4 and ideally held to R 2.5 or less.

Under carpet over subfloor, radiant heat will need to operate at the higher end of its temperature range to overcome the insulating effect of the carpeting and carpet padding. Keep in mind that carpet padding or foam backing on carpeting can, by itself add an R-value from R-0.8 to R-2.

Watch out: some radiant heat product manufacturers will void your radiant heat warranty if you install carpeting of any type over their radiant heat system.

Oxygen Diffusion in Too-Hot Radiant Heated Floors

Radiant Floor Company, a radiant heating system manufacturer, raises another interesting point, indirectly:

at temperatures over 140 °F can cause an oxygen diffusion problem and require special "oxygen barrier" tubing in some cases (depending on the boiler design) - otherwise system life may be reduced or the boiler damaged.

Typical Temperatures for Radiant Heat Ceilings

Question: What should my my hot water temperature be so I don't crack my ceilings?

(Oct 15, 2018) Ronald Tabellione said:

I have radiant heat hot water system in my plaster ceilings. What should my my hot water temp be so I don't crack my ceilings.

I have 2 zone heat and a brand new furnace. House built in 50's

Reply: 150-170 degF in the ceiling or wall panels

Radiant heat ceilings generally run in the 150-170 degF. range - thus delivering more BTUs per square foot than radiant floor heating.

Radiant heated ceilings and possibly radiant heat wall panels can operate at significantly-higher temperatures than radiant heated floors, probably because we need to be able to walk on the floor without getting a hot-foot.

The wiring in some electrical radiant heat gypsum board (drywall) panels is rated for 250 degF but doubtless operates well below that.

Here are some details and information sources. I'm collecting more and will add that to this page.

Floor heat temperatures, in comparison with radiant heated ceiling panel temperatures

typical operating temperature range for radiant floor heating systems at 85-140 °F (30-60C) though we agree that 140 deg. f. is a bit higher than suggested by other sources.

The US DOE notes that

Radiant floor heating systems are significantly different from the radiant panels used in walls and ceilings. - www.energy.gov/energysaver/home-heating-systems/radiant-heating#307668-tab-0

Regrettably the DOE doesn't give any different temperature ranges for radiant panel heat in walls or ceilings.

Minimum and Maximum Circulator Flow Fates for Tankless Water Heater Radiant Heat Systems

Reader question: A tankless water heater has a minimum and maximum circulator flow rate however I have not seen this addressed in any radiant heat open or closed designs. Why?

AOsmith has this note in their orders manual.

"The recirculation pump is to provide no less than 2 GPM (7.5 L/min) and no more than 4 GPM (15 L/min) through each activated unit in the system." - On 2021-01-25 by VU -

This Q&A were posted originally at CIRCULATOR PUMP REPAIR FAQs

Here are the recommended flow rates for radiant heat in-floor systems and how is the flow rate adjusted?

VU

Thank you, that's a great question that I'd re-phrase as: What are the recommended flow rates for radiant heat in-floor systems and how is the flow rate adjusted?

I'm not an expert on the question but I can offer some comments and explanation, and I'll research further and improve this text.

Illustration: Bell & Gossett's Sysetm Sizer calculator wheel, one of several tools the company provides free to HVAC installers and designers to permit calculating the required heating water flow rate for a building.

Watch out: Keep in mind that the actual flow rate in GPM that you need to heat a building's occupied space depends on the building properties including square feet, volume, heat loss rate and heating zone or climate.

So there will not be a single "one-flow-rate-fits-all" answer for hydronic heat flow rate needed.

The basic hydronic or hot water heat flow rate calcuation for buildings, widely discussed, is

GPM Required = Heat Load¹ / 500² x DeltaT²

where

1. Heat Load
   
   = the heat loss rateor heating load for the building expressed as BTUs/Hour
2. 500
   
   = a constant representing the weight and volume of water at a standard temperature, expresses as
   
   8.33 pounds per gallon x 60 minutes at 60°F
   
   Note that more-accurately the water temperature must be adjusted from 60°F to the actual higher temperature used for building heating
   
3. Delta t°F
   
   is the difference in water temperature between water leaving the heating boiler and water returning to the boiler - typically designers use t°F = 20°F

• Source: Bell & Gossett Systm Syzer, available free at https://bellgossett.com/selection-sizing-cad-tools/system-syzer/ available as a computer program, an app, and historically as a round cardboard calculator wheel that you can order from your local B&G supplier.

In general, radiant heat floor flow rates are much less than those used other hydronic heating systems employing baseboards or radiators.

• Hydronic heat flow rates in GPM:
  
  typically have a capacity between 0 and over 50 GPM with a typical residential hydronic heat circulator (e.g. Taco 0015e3ECM) running between 0 1nd 16 GPM.
• Radiant floor heat flow rates
  
  in GPM: typically operate between 0.4 GPM and 5.0 GPM [see Taco's RMB table below] and the system maximum flow rate or GPM capacity will run between 0.4 and 15 GPM.

Illustration: the Taco Radiant Heat mixing block used to control flow rate and thus BTU outpout of radiant heat systems.

[Click to enlarge any image]

Water flow rates for Domestic Hot Water Production

Watch out: your question cites minimum and maximum flow rates for tankless water heaters. A tankless water heater is used to produce domestic hot water (for washing and bathign) and is absolutely not intended for use as a building space heater (hyrdonic heating to keep occupants warm).

Do not confuse hot water for domestic use with hot water for heating.

Tankless water heater flow rates are indeed specified by the manufacturer, such as your cited

no less than 2 GPM (7.5 L/min) and no more than 4 GPM (15 L/min)

because at too slow a speed the heater might overheat or might not even turn on its heating element (for safety), while at a too high-flow-rate the tankless water heater can not heat the pasiing water hot enough for domestic use (washing and bathing).

Watch out: also if you are using a tankless water heater as your heat source for a building radiant heating system, make sure that the manufacturer agrees that the tankless water heater you have selected is rated and acceptable for that level of usage.

Water Flow rates for Building Heating Systems

Hot water flow rate through any hydronic or radiant heating system will indeed affect the heat output: flowing faster delivers more BTUs per minute to a given length of radiant tubing or baseboard, but I suspect an accurate equation is far more complex than meets the eye.

We need both temperature and time of contact to conduct heat out of hot water into and thus out of a radiating device.

And in-floor hydronic radiant heat systems are more-complex than a conventional baseboard or radiator heating system in that we also need to keep the floor temperature from rising anywhere near the surface temperature of a conventional radiator or baseboard, lest we burn the occupants' feet or damage the floor itself.

So radiant floor systems use a mixing system that recirculates the same hot water through the flooring loop, mixing in a bit of hotter water from the boiler under the control of one or more radiant loop thermostatic controllers monitoring the temperature of the return water.

That temperature control is in addition to the aquastat on the boiler itself.

So you can see that regulating flow rate rather than thermostatically-controlled hot water mixing much complicates the question of whether or not we should be concerned with varying the gpm rate of the circluator pump itself.

In fact the terms "GPM" and "Flow Rate" do not even appear in many circulator pump IO manuals.

See the various circulator pump manuals provided at

CIRCULATOR PUMP MANUALS

The simplest approach used by heating system installers or designers is to choose a circulator that, at a given temperature range, can deliver sufficient BTUh of heat to the heated area - which of course varies by area size and heat loss rates and the total amount of radiating equipment.

Taco offers products specifically designed for radiant heating systems such as the company's

TACO RMB RADIANT MIXING BLOCK INSTRUCTION MANUAL [PDF] (2015) TACO, INC., 1160 Cranston Street, Cranston, RI 02920 USA Tel: (401) 942-8000 FAX: (401) 942-2360.
TACO (Canada), Ltd., 8450 Lawson Road, Unit #3, Milton, Ontario L9T 0J8 Canada Tele: 905/564-9422. FAX: 905/564-9436.
Website: http://www.taco-hvac.com

from which the table below is excerpted.

[Click to enlarge any image]

Specifications fodr the Taco RMB discussed here include:

Flow Range: 0-15.5 GPM
Head Range (ft): 0-15′
HP: 1/40, 1/25
Max. Fluid Temp.: 185°F (85°C)
Min. Fluid Temp.: 32°F (0°C)
Max. Working Pressure: 125 PSI
Connection Size: ¾” NPT

Additional Taco radiant heat system conrols are found

at TACO HVAC components & controls

I will review our documentation on radiant heat system installation and controls: there are indeed controls on radiant heat systems that will affect flow and temperature, but to date our focus, and that in the documents I've read and systems I've worked-on has been on getting the right temperature rather than the flow rate in gpm;

I will repeat the question and include a reply in the reader Q&A at

RADIANT HEAT TEMPERATURES

If I find enough different material on flow rates for radiant heat I'll spin that off into a new article. Thank you again for asking.

Most hyrdonic heat system circulator pumps do not have an adjustable flow rate, though a group of circulators have a variable speed control, such as the Taco brand - take a look at the conventional hydronic - hot water - heating system flow rates discussed at

CIRCULATORS VARIABLE SPEED

in a section titled

CIRCULATOR PUMP SPEED SELECTION & ADJUSTMENT

We welcome your further questions, comments, suggestions.

Reseach & Support for Heating Load Calculators and Hot Water Heat Flow Rate Calculations

• Bell & Gossett System Syzer Hydronic Heating System Flow Rate & BTU/H Calculators - https://bellgossett.com/selection-sizing-cad-tools/system-syzer/
  
  See details and manuals
  
  at BELL & GOSSETT B&G EQUIPMENT MANUALS & CONTACT
• Carey, George How to Calcuate the Proper Flow Rate for any Hydronic Sysetm, Indoor Comfort, web article (2019) original source: https://www.indoorcomfortmarketing.com/how-to-calculate-the-proper-flow-rate-for-any-hydronic-system also available as archived copy at https://inspectapedia.com/heat/calculate-GPM-Flow-Rate-for-Hydronic-Heat-Carey.pdf
• TACO HVAC components & controls
  

Radiant heat ceiling product manufacturers

Note that some of these radiant heat ceiling or wall panels are entirely electric; other radiant heat panel systems use hot water from a boiler or from a solar heating system.

• Calorique Perfectly Warm Radiant Ceiling Heat, 2380 Cranberry Highway West Wareham, MA 02576 USA Tel: 800-922-9276 Email: info@calorique.com Website: https://www.calorique.com
• Enerjoy Radiant Heating Systems, SSHC, Inc. P.O. Box 769 Old Saybrook,CT 06475 USA, Tel: 800-544-5182 or 860-399-5434
  SSHC, Inc. 1244 Old Clinton Road Westbrook, CT 06498 Email: info@sshcinc.com
  
  Enerjoy produces radiant heat modules - electrical panels that can be installed on a ceiling or possibly a wall.
• Messana Radiant Cooling and Heating ceiling solutions Messana Radiant Cooling 4105 Soquel Dr, Suite B Soquel, CA 95073 USA
  Tel: (855) RAY MAGIC (729-6244) Email: info@radiantcooling.com Website: https://radiantcooling.com/
  
  radiantcooling.com/messana-radiant-cooling-products/ray-magic-radiant-panel/
  
  Products include an EPS radiant panel built into sections of drywall that can be installed on walls and ceilings.
• Thermaray Therma-Ray Inc. 670 Wilsey Road, Unit 6 Fredericton, New Brunswick Canada E3B 7K4 Tel: 866-457-4600 Website: thermaray.com/radiant-ceiling-electric-radiant-heating-system/
  
  Thermaray produces electric radiant heating systems for floors, ceilings, and other applications such as driveway de-icing systems and snow melting.

I am collecting more operating temperature specifications for those radiant heat panels and will add that information, product specifications, etc. here.

Where to Check the Temperature of A Radiant Heated Floor System

Mixed Radiant & Hydronic Baseboard or Radiator Heating Systems

Watch out: often there are mixing and flow controls on the radiant system that limit the actual temperature in the radiant tubing, typically by only introducing new hot water into the loop when temperature requires it - otherwise recirculating most of the loop water without returning it to the boiler.

You will particularly see this if the boiler is serving a mixed-design system, including some heating baseboards and other radiant floor heated sections.

That's because we want the hotter temperatures in the baseboards (hotter is more efficient heat transfer), and cooler temps in the radiant flooring.

So: was this a mixed design system with some baseboards and some radiant? and were there mixing controls on the radiant loop section ?

Question: how can I boost the temperature of our radiant-heated floors?

I have radiant heat floor throughout my house and I have some questions about boosting its output. I’d love to enlist the serves of someone from your team who can answer questions about heat exchangers, optimal temperatrures, max temperatures, etc.

The sooner the better as we are feeling chilly at the moment. - Anonymous by private email 2017/12/11

Reply: (Mod) said: take care not to over-boost radiant heat floor temperatures

I may be able to answer specific questions or I may have a few suggestions but you will be best served by an on-site expert who can look at your equipment, controls, house and its design and points of heat loss.

Watch out: In general, if you boost temperature too high with a radiant heat floor you can find the floors too hot to walk on comfortably; and of course adding rugs to make walking comfortable block the heat output.

• I would take care to keep your radiant heat temperatures in the heating loop under 135°F (57°C).
• The finished floor temperature should never exceed 85°F (29°C).

That is typically managed by adjusting the temperature controls at the mixing valve that maintains the floor temperature by mixing in return loop water with outgoing water from the circulator/boiler.

Too much heat can also damage floors if they're wood, and too much heat may exceed the rating of the radiant tubing that was used, leading to a tubing failure - costly leaks.

My photo shows gaps that developed in a wood floor installed over radiant heat tubing after the radiant heat was driven to temperatures above those recommended by the radiant floor heat system manufacturer.

Start by finding the operating temperature of your system as it is.

Consider also looking over the home to reduce its heat loss, particularly if there are drafts.

I cannot assist on a for-pay consulting basis. We do not sell anything. In order to absolutely assure our readers that we write and report without bias we do not sell any products nor do we have any business or financial relationships that could create such conflicts of interest.

For reducing your home's heat loss these articles might be useful

• HEATING COST SAVINGS METHODS
• HEAT LOSS in BUILDINGS - home

Carpeting Impact on Radiant Heat Floor Systems

Question: does it cost more to operate radiant heat flooring covered by carpeting than by tile?

On carpet - while carpet may impact the performance of radiant heat, does it impact the efficiency? As in, am I spending more money trying to heat the carpeted areas of the house vs the tile?

I guess a bad carpet installed could insulate the floors so much that all the heat gets bled off into the uninsulated parts, but Id think even if I removed all the carpet in favor of LVP for example, Id still be using more gas due to other heat loss issues.

Can carpet cost more to heat? - Anonymous by private email 2019 09 27

Reply: yes

OPINION: Wall to wall carpeting, especially thick or heavy carpeting acts as insulation to slow heat transfer from the tubing and warmed slab up into the occupied space.

The result is that for the radiant heating system to satisfy the thermostat it has to stay on longer. It's a bit over-simplifying but a heat source that has to run longer to satisfy the room thermostat - all other factors being held constant - increases energy consumption and thus heating cost.

One would infer that during that "stays on longer" only if there were ZERO heat / energy losses through pathways that do not transfer heat into the occupied space would there be close to zero additional heating cost. So yeah we're heating something else besides the occupied space.

Even in a theoretically zero-entropy "perfect" heating system with zero heat losses through other pathways than into the occupied space, and thus zero additional heating energy cost (electricity or gas or oil or whatever), nevertheless simply running the circulator would add a little energy cost. And of course that ideal case doesn't exist in the real world.

"The capacity of a floor system depends on the heat exchange between the floor surface and the space (convective and radiant heat exchange coefficient), the heat conduction between the floor surface and the tubes (floor surface material, type of concrete, type of floor system, slab thickness, spacing between tubes) and the heat transport by water (water flow rate, temperature difference between supply and return). (Olesen 2002) Olesen is an ASHRAE fellow.

• Olesen, Bjarne W. RADIANT FLOOR HEATING IN THEORY AND PRACTICE [PDF] ASHRAE Journal July 2002 pp. 19-24. Retrieved 2019/09/27 original source: Universitas Miguel Hernandez, Instalaciones de Climatizacion, offering a masters degree in heating and electrical systems and energy efficiencyt, retrieved 219/09/27 original souirce: http://umh0717.edu.umh.es/wp-content/uploads/sites/482/2015/03/Radiant-floor-heating.pdf

Here is a key remark from that article

For maximum capacity, it is important to avoid floor coverings with a high thermal resistance such as heavy wall-to-wall carpets.

This increases the required water temperature and then also the back losses to ground or unheated space.

Also, higher water temperature reduces the efficiency of the heat generator (condensing boiler, heat pump).

Really? Well other energy losses can dominate heating cost. Very basic things the air leakage rates will dominate the heating load even in a well-insulated home. We've published some studies on this, for example see

AIR LEAK SEALING STRATEGIES

and

HEAT LOSS DETECTION TOOLS

Driveway & Walkway De-Icing Systems Installation

What are the basic design parameters, tubing sizes, operating temperatures and controls used in snow and ice melt systems (SIMs) for driveways, walks, and parking lots? Here we discuss these parameters and include links to detailed SIM design guides from experts in the field.

Question: can I mix 1/2" and 3/4" tubing on the same PEX radiant heat circuit

2019/09/19 Bill said:

I have a radiant floor heating system for two rooms with 1/2" PEX that works fine. When I put it in, I used a manifold with an extra space in case I wanted to add a circuit.

We are going to re pour front steps and sidewalk to the driveway and I thought that that would be a good use of my additional circuit. But I have read that any outdoor application should use 3/4" PEX instead of 1/2".

Can I mix have two circuits with 1/2" PEX and one with 3/4"? Can I just use 1/2" for the outdoor steps and sidewalk and expect it to keep the snow off? (I live in SE Michigan.)

This Q&A were posted originally at PEX PIPING INFORMATION

Reply: yes but watch out for some snafus: Design Parameters for Snow or Ice Melt Systems (SIMs) under Driveways & Walks

Bill

Depending on the heating load for your snow and ice melt system for your walk and driveway, you can use 1/2" PEX tubing. Here is a quote from the Radiant Floor Company who have provided an installation and design manual that we cite in detail below:

The 1/2” ID PEX is also a polyethylene tubing with a very high temperature and pressure rating (180 degrees at 100 psi). It emits about half the heat of the 7/8” PEX, but its bending diameter is tighter.

Using 1/2” PEX for small zones, tight crawl spaces, or snow melt applications makes sense. It has a bending diameter of 15” and should be spaced 8” to 12” on center. - Radiant Floor Company 2007

In a similar but not identical opinion, the Plastics Pipe Institute (cited below), in discussing outdoor de-icing systems notes that Pex Tube size for driveway and walk de-icing and snow-melt systems is typically 3/4 NTS but adds that 1/2 inch and 5/8 inch tubing is sometimes used.

What PEX tubing size you can use on a de-icing system or snowmelt system will be adequate depends on several factors

• the size of the area to be de-iced
• the drive or slab temperature when snow-fall begins and of courses the rate, density and depth of snow or ice on the surface;
• the outdoor temperature range anticipated. Wind velocity and sky temperature and teen outdoor humidity levels are also factors considered when computing the Snow and Ice Melt (SIM) system design loads.
• the design of the outdoor de-icing system (watch out: insulation and tubing placement are absolutely critical)
• the BTUH capacity of the heating boiler that's producing the hot water, and of course the circulator(s) and water flow rates possible
• other simultaneous loads on the system
• the type, placement, and quality of insulation under the drive or walkway
• the proper placement of the PEX or other heating tubing within the concrete or other paved drive or walkway surface - its proximity to surface versus bottom of the slab - detailed
  
  at RADIANT HEAT MISTAKES

I'd start by making sure that the boiler providing hot water to the current and future systems has the needed capacity.

Then, yeah, you MIGHT be fine with 1/2" tubing.

Starting with our article RADIANT HEAT TEMPERATURES you'll see that outdoor or driveway or walkway de-icing systems need higher temperatures than indoor radiant heat flooring. Some details worth noting when choosing tubing for a de-icing or snowmelt system:

• You may need to re-size the hydronic boiler expansion tank when adding radiant heat loops for any purpose as the water volume is also going to increase
• Typically a Snow and Ice Melt System (SIM system) operates at two different temperatures:
  
  an "idle" temperature of 28°F (-2°C) when melt operation isn't needed but the system is in "stand-by" mode,
  
  a melting temperature of 38°F (4°C) when the system is at work; both of these temperatures must be adjustable for variation in local conditions.
• PE-RT is a type of polyethylene tubing that has Raised Temperature-Resistance (CSSA B137.18)
• Your ice or snowmelt system will need to warm the melt-area by applying 051 BTU/lb (Op. Cit.). MacNevin, writing for the PPI, notes also that

That does NOT mean the same boiler can't serve both, but you'll need temperature regulation devices that provide the proper temperature for indoor versus outdoor PEX radiant heat tubing loops.

See the Radiant Heat Guides suggested just below. You will read that experts ( MacNevin 2016) discuss an array of additional controls such as drive or walk surface moisture monitors and other instrumentation that can improve both the efficiency and effectiveness of SIM (snow and ice melt) systems.