Last Updated on July 22, 2019 by HVAC

Radiant Floor Heating Design Guide

Designing a radiant floor heat system can be straightforward or very complicated, depending on many factors. This guide is intended as a basic overview of the radiant design process. Many variables are not addressed in this guide.

Step 1  Determine the number of zones.

A heating zone is one or more radiant heat loops (tubing) controlled by a single thermostat. Open areas (garages, basements, etc.) with one or more loops can be controlled by a single thermostat, but homes usually require multiple thermostats to control the temperature in the different areas effectively. Depending on the floor-plan, bedrooms can usually be controlled by a single thermostat located in the hallway outside the bedrooms; the kitchen, living and family rooms can usually controlled by a thermostat located in the center of these rooms; isolated master bedrooms and master bathrooms are usually controlled by separate thermostats.

Heat loss and floor type also play a significant role in determining the zone set-up in homes. Rooms with large heat loss due to numerous windows, poor insulation, northern exposure, etc., usually require a separate zone when adjacent room(s) have significantly different heat loss (fewer windows, better insulation, southern exposure, etc.). Rooms with carpet and hardwood flooring can generally be controlled by a single zone, but need to be zoned separately from rooms with masonry floors (tile, marble, etc.) due to the different temperature of supply water required. You can check out the Aquasana Rhino EQ-300 if you need a great water filtration system. Common in radiant slabs, tubing spacing can be adjusted to compensate for different heat loss or floor type within a single zone, for example, closer tubing spacing for carpeted areas and wider tubing spacing for areas with tile. More zones always allows better temperature control and optimum system performance.

Step 2  Determine the number of tubing loops.

1/2″ Barrier Pex tubing is generally spaced 8″ to 16″ on center in concrete slabs depending on the design temperature, heat loss, floor type, flow rate and several other factors. Most living areas and office areas require 8″ spacing, most basements and non-living areas including garages and warehouses require 12″ spacing. Use closer spacing for the first few rows along the exterior walls. Closer tubing spacing has a higher installation cost, but allows for even heat distribution and helps minimize Hot-Spots and Cold-Spots (important for living areas, not so important for garages).  For sub-floor installations with floor joists on 16″ centers, 8″ tubing spacing is required (two tubing runs per joist cavity). Tubing spacing over 16″ is not recommended.

Radiant heating tubing spacing:

For	6″	Spacing, multiply square footage by	2.00
For	8″	Spacing, multiply square footage by	1.50
For	10″	Spacing, multiply square footage by	1.20
For	12″	Spacing, multiply square footage by	1.00
For	14″	Spacing, multiply square footage by	0.86
For	16″	Spacing, multiply square footage by	0.75

 

Multiply the square footage by the spacing multiplier above to determine the tubing length for each zone. The absolute maximum loop length for 1/2″ tubing due to frictional pressure loss is 400′, but we don’t recommend loop lengths over 300′.  The optimal 1/2″ loop length is 300′, or less. Shorter loop lengths work better because they provide less temperature drop in the loop.

Hot-Spots and Cold-Spots become more noticeable with longer loop lengths.  If your 1/2″ tubing length is over 290 ft (allows 5′ for leaders on both ends of the loop), divide the tubing length by 290 to determine the number of loops required for that zone (round up to whole number). Then divide the zone tubing length by the number of zones to determine the loop length.  Make sure to add at least 10 ft to each loop for connecting to the manifold.

For example: 575 sq ft area with 6″ spacing.  Multiply 575 by 6″ spacing multiplier 2.0:  575 x 2.0 = 1150 ft of tubing required for the zone. Calculate number of loops: 1150 divided by 290 = 3.97,  round up to 4 loops.  Divide tubing length by number of loops for loop length: 1150 divided by 4 = 287.50. The 575 sq ft area requires 1150 ft of tubing with 6″ spacing in (4) 287.50 ft loops.  Each loop needs an additional 10 ft for connecting to manifold (that’s 5′ on each end for leaders). Total tubing required for the zone is 297.50x 4 = 1190 ft.

If you need to install loops over 300′, use 5/8″ tubing.  5/8″ tubing can be used for loops up to 400′. If you are wanting the best system performance available with the smallest loop temperature differential possible, use 5/8″ tubing with 300′ loops

Step 3  Installation supplies.

Secure the tubing every 30″ before pouring concrete. If you’re installing 2″ rigid foam-board under-slab insulation (recommended), use Screw Clips placed every 30″ to secure the tubing.  Screw clips twist into the 2″ foam-board insulation then the tubing simply snaps into the screw clip.  Divide your tubing length (feet) by 2.5 to determine how many Screw Clips to order (275′ loops usually require about (4) 25 Pks. of Screw Clips). 8″ Tubing zip ties can also be used to secure the tubing to the re-mesh or re-bar (available in 100 Pks).

You will need 2 slab risers (protective elbows), for each loop.  Also, you will need a short length of protective sleeve wherever expansion joints or control grooves cross the tubing.  (Protective Sleeve comes in a 10 ft coil).

Radiant Slab Rough-In Packs (PXOB3C3SRP) include Barrier Pex tubing and the supplies used for un-insulated slab installations.  Radiant Insulated Slab Rough-In Packs (PXOB3C3SRPSC) include Barrier Pex tubing and the supplies (including Screw Clips) used for insulated slab installations.

For Sub-Floor HRH systems, install HRTP3a Heat Transfer Plates.

Comap manifolds can be easily configured several ways:  1) Single zone manifold with multiple loops  2) Multi-zone manifold with 1 loop per zone.

The single zone Comap manifold can be used to by itself, or with other single zone Comap manifolds to create a multiple zone radiant system. For a Single Zone radiant floor heat system, the circulator pump is usually installed directly to the Comap manifold and controlled by a 110V thermostat (Single Zone Radiant Packages for Concrete Slabs are set-up this way).

Multiple zone radiant floor heat systems are set-up the same way, just add a zone pump controller and 24V thermostats.  Zone pump controller are sized according to the number of zones, for example, a 3-Zone system requires a 3-Zone pump controller (zone pump controllers are available in 1, 2, 3, 4 and 6 zone configurations).

The multiple zone/single loop manifold uses a circulator pump installed directly to the Comap manifold, with loop actuators (manifold mounted zone valves) attached to each zone on the manifold, controlled by a zone valve controller with 24V thermostats. A zone valve controller can operate up to 5 zones. An alternate multiple zone set-up requires a circulator pump installed in the boiler supply line with a zone valve controller operating multiple motorized zone valves controlling the flow to multiple, single zone manifolds with 24V thermostats. A zone valve controller can operate up to 5 zone valves.

In most situations, both types of manifolds will work fine with one pump and up to 10 loops per manifold. For multiple manifold systems, all one type or a combination of both types, you will need to link the manifolds with your boiler supply and return lines.

Note:  Comap manifold and radiant package item numbers ending with “L” require a 1/2″ crimp tool. Even though Radiant systems are very easy to install, we recommend installation by licensed, experienced professional installers. Radiant systems require electrical wiring, which must comply with the NEC and other local electrical codes, if applicable. All electrical wiring must be preformed by a licensed electrician.

 

Comap Manifolds are the best improvement to Radiant Floor Heat systems since Pex tubing!

The Comap Hydronic Radiant Heat (HRH) Manifold system is modular so you can easily expand the system to as many zones and loops as you need!  Just add one return module and one supply module to the Comap Connection kit for each loop to configure your own manifold. It can be configured as a single zone manifold with multiple loops, or as a multiple zone manifold with one loop per zone. Balancing any radiant system is critical for even heating without “hot spots” or “cold-spots”…Comap manifolds have balancing valves to regulate each zone, or loop, for the best possible heat distribution!

They also have a thermometer and an automatic air vent on both the supply and return manifolds. The manifolds assemble easily without tools and you can add as many as you need.

The Comap single zone configuration (pictured above as a 3 loop manifold, is used for large open areas that need multiple loops to heat the entire area with one thermostat…large living rooms, garages, basements, warehouses, these floor plans, etc. For a simple set-up, use a line voltage thermostat wired direct to the circulator pump to control the flow to the manifold. For systems with more than one single zone manifold, a Zone Pump Controller is used to control the hot water flow to the manifolds. Each loop (Comap module) has a balancing valve to regulate the flow.

The Comap multiple zone configuration is used to control the temperature in each room.  A zone valve controller is used to control the loop actuators installed on the Comap return modules.  The zone valve controller also starts the circulator pump when a thermostat calls for heat.

The Comap Connection Kit is the backbone of both systems and includes 2 Comap 1″ FPT manifold inlet adapters with ball valves, 2 manifold fill-and-drain adapters, 2 automatic air vents, 2 thermometers (supply water temp. and return water temp.) and 2 manifold mounting brackets.

Add supply modules and return modules, one of each per zone or loop, to the Connection Kit to complete the basic manifold assembly (A return module with integral flow meter, HRCMFLM, is also available for single zone manifolds).  You will need (2) 1/2″ Loop Fittings for each module to connect 1/2″ BPex tubing loops to the manifold (for professional installations, we recommend the HRXL3 1/2″ crimp loop fittings.)  The manifold has an inlet and outlet to plumb to your heat source (boiler, water heater, etc.)

A Zone Pump Controller is the brain of radiant systems that controls 1, 2, 3, 4 or 6 zones (for single or multiple manifold applications).  When a thermostat calls for heat, the zone pump controller sends a control signal to start your boiler and circulator pump or motorized zone valve so hot water will flow. Domestic hot water (DHW) priority mode can be used on zone one for systems that use a common heat source for both heating and DHW.

The Zone Valve Controller  is the brain of the multiple zone manifold system that controls the zone valves  or loop actuators (use additional zone controllers for systems with more than 6 zones). When a thermostat calls for heat, the zone controller sends a 24 volt control signal to start your boiler and/or circulator pump and opens a Loop Actuator, or motorized zone valve, so hot water will flow. LED’s indicate active zones and the domestic hot water (DHW) priority mode can be used for systems that use a common heat source for both heating and DHW.

Thermostatic mixing valves –  are used to control the temperature of the hot water supply to the manifold. This is necessary when the design temperature is significantly lower than the supply water temperature from the heat source (most radiant systems require supply water in the 90 F to 125 F range). The thermostatic mixing valve installs in the plumbing between the manifold and the boiler.  It blends return loop water from the manifold(s) and supply water from the boiler to a manually set temperature. Thermostatic mixing valves are required If using a std domestic hot water heater as your radiant heat source.  Includes detailed installation instructions.

Circulator Pump Assembly connects to Comap manifold for easy installation.  Includes circulator pump. 1″ OEM flange set and a Pump Mount Kit for Comap manifolds . 1″ FPT connections for plumbing to boiler supply line. For potable systems, use one of the best bronze corrosion resistant pumps that are available).

Expansion tanks are required for all hydronic radiant heat systems.  An internal air charged diaphragm provides an air-cushion to the system. The size of the expansion tank is determined by heat source size. Expansion tanks install in the heat source supply or return plumbing and are available with an automatic fill valve to maintain minimum system pressure.

For a complete Radiant Heat System, add BPex Tubing and installation supplies. The absolute maximum loop length for 1/2″ tubing is 300′. The recommended loop length is 290′, or less. Shorter loop lengths work better because they provide less temperature drop in the loop. Hot-spots and Cold-Spots become very noticeable with longer loop lengths.

Oxygen Barrier Pex Tubing is designed specifically for radiant systems, including closed hydronic under-floor heating systems with ferrous components (cast iron boiler and pumps, geothermal systems, etc.)

Barrier Pex (or Pex) tubing is generally spaced 6″ to 16″ on center in concrete slabs depending on the design temperature, heat loss, flow rate and several other factors. Most living areas require 6″ to 8″ spacing and most garages and basements require 10″ to 16″ spacing. Closer tubing spacing and shorter loop lengths allow for even heat distribution and helps minimize Hot-Spots and Cold-Spots due to excessive temperature drop in the loops. For 6″ spacing, you will need 2 feet of tubing for every square foot of heated area, 8″ spacing requires 1.5 ft per sq ft, 12″ spacing requires 1 ft per sq ft and 16″ spacing requires .75 ft per sq ft.

Radiant heat tubing layout

Radiant Heating Questions…

Can I use a std. Water Heater for radiant floor heat?

Yes, but check with your local code enforcement department (some require boilers only) and make sure your water heater is sized correctly for the area you are heating.  You’ll also need a properly sized expansion tank and a thermostatic mixing valve.

How many square feet can I heat with a std. water heater?

There are many variables that dramatically affect heat loss, but here are some general guidelines, based on a new construction radiant slab, very well insulated (walls, ceiling and under slab), in a mild to moderately cold winter climate area using a thermostatic mixing valve:  A standard 50 gallon electric water heater with (2) 4500 watts elements and a 20 gallon per hour (gph) recovery rate can usually heat about 250 sq ft.  A standard 40 gallon gas water heater with a 40,000 BTU burner and a 40 gph recovery rate can usually heat about 500 sq ft.  These square footages can generally be doubled for “partially heated” areas where a 50 to 60 degree inside temperature is acceptable on the coldest days (very common for garages, workshops and storage buildings).

What’s the difference between Manabloc manifolds and Comap manifolds?

Manabloc and Comap manifolds are used for completely different applications.  Manabloc manifolds are used as a distribution panel for potable water plumbing systems (like an electrical panel, but for water instead of electricity). Comap manifolds are used for hydronic radiant floor heat systems.

What is the difference between the HAR and HCM crimp tools?

HAR series crimp tools are “bolt-cutter” style crimpers that require two hands to operate.  The HCM series CRIMPMAKER crimp tools are compact and feature an angled head for hard-to-reach crimps.  HCM crimp tools also have an  “open-stop” that allows one-handed crimping.  HCM crimp tools are a popular choice for professional installers.  Both types of crimpers are heavy-duty, professional quality tools.

For sub-floor radiant packages you will also need tubing clamps or heat transfer plates, depending on your application.

Do the Screw Clips install above or below the re-mesh in my concrete slab? 

Screw Clips twist into the foam-board insulation.  The tubing snaps into the screw clips. Then, wire mesh (remesh) and/or rebar is laid over the tubing. When you pour the concrete, pull the wire mesh to the middle of the slab. Be careful not to pull the tubing up with the remesh when you pour. Many people prefer to use fiber-reinforced concrete so they don’t have to install wire mesh.

What is the difference between Pex and oxygen barrier BPex?

BPex is standard Pex that is manufactured with a special external coating that prevents available oxygen from permeating the tubing.  Available oxygen in the water is what causes rust to form on ferrous radiant system components.  We recommend BPex tubing for all radiant systems.  If you use standard Pex tubing for your radiant system, instead of the recommended BPex tubing, you will need to install corrosion resistant system components (stainless steel, brass or bronze pump, boiler and fittings).

What is the best length for the radiant tubing loops?

The absolute maximum loop length for 1/2″ tubing due to pressure loss from friction is 400′, but we don’t recommend loop lengths over 300′.  For optimum performance, keep your radiant loops short.  Shorter loop lengths work much better because they provide less temperature drop in the loop.  Hot-spots and Cold-Spots become very noticeable with longer loop lengths.  We stock BPex in 300′ and 500′ coils that can be cut to the exact length for shorter loops.  300′ coils work great for 290′ loops with a 10′ leader, or (2) 140′ loops.  500′ coils work great for (2) 240′ loops with 10′ leaders.

Should I use crimp fittings or compression fittings?

Crimp fittings are best!  They are permanent, economical and can be used throughout the radiant system.  They allow concealed splices and, if properly crimped, provide the most reliably connection available.  Using crimp fittings allows you to use all the tubing you purchase without having to worry about ordering the exact length needed for each radiant loop. The crimp tool required can be purchased for a little over $100 or rented.

The alternative, compression fittings, are approved for use in accessible locations only and cannot be used in concealed areas (behind walls, in concrete slabs, etc.).  Compression fittings need to be monitored for slow leaks.

Why should I use a Comap manifold for my radiant system?

Comap manifolds are modular so you can easily expand the system to as many zones and loops as you need.  Just add one return module and one supply module to the Comap Connection kit for each loop to assemble your own manifold.  It can be configured as a single zone manifold with multiple loops or as a multiple zone manifold.  Comap manifolds have balancing valves on all supply and return modules to regulate each loop for the best possible heat distribution.  The manifolds assemble easily without tools and you can add as many as you need.  They also include a thermometer on both the supply and return manifolds and come with an automatic air vent and fill/drain adapter.

Do I need to install the rigid foambiard insulation under my concrete for radiant heat?

It is not absolutely required, but it will greatly increase the efficiency and performance of your radiant floor heat system.   We recommend 2″ foam-board insulation under all radiant concrete slabs.  Most builder supply yards stock it or can order it for you.  There are also many foil-faced bubble type insulations available and the manufacturers claim they work well.

Can Comap manifolds handle multiple zones? I need a radiant system with one heating zone for the basement (several loops) and 4 zones for my main floor (3 zones with 1 loop each and 1 zone with 2 loops).  Can a Comap system handle that? 

Absolutely.  You need 2 manifolds, one single zone, three loop manifold for your basement and one three zone, four loop manifold for the main floor.  The zone for your main floor that needs two loops can be configured two different ways.  1)  Link the two loop actuators together so one thermostat will control both, or   2)  Use a three zone, three loop manifold and “Tee” the second loop from the controlled loop.

Can I use Pex tubing instead of the more expensive BPex or PAX tubing?

Yes, if your boiler is corrosion resistant and you use a stainless steel or bronze circulator pump and fittings, but the  small additional cost for BPex is well worth the money. We recommend BPex or PAX tubing for all radiant systems.

What are screw clips?

Screw Clips are used to secure the tubing before pouring your concrete slab.  They twist into the 2″ foam-board insulation under the concrete and the tubing simply snaps into the screw clip.  We recommend Screw Clips placed every 30 inches.

Can programmable thermostat be used for radiant systems?

Programmable thermostats can be used, but consider this:  Radiant systems are designed to maintain a constant temperature.  It can take quite a while to warm a room from a cold start and the system is going to use a lot of energy to heat the room.  Digital thermostats are more precise than standard mercury-bulb thermostats and may cause the system to cycle too frequently.  Mercury-bulb thermostats are generally the best choice for radiant systems.

What is a thermostatic mixing valve?

A TMV is used to control the temperature of the hot water supply to the manifold.  It is necessary when the design temperature is significantly lower than the supply water temperature from the heat source.  The TMV installs in the plumbing supply and return lines between the manifold and the heat source.  It blends return loop water from the manifold and supply water from the heat source to a manually set temperature.

What is the difference between a zone valve controller and a zone pump controller?

A zone valve controller is used to operate motorized zone valves or loop actuators.  When a thermostat calls for heat, the zone valve controller will open the motorized zone valve or loop actuator and start the circulator pump.  A zone valve controller is used for systems with one circulator pump and multiple zones.

A zone pump controller is used to operate zone pumps.  When a thermostat calls for heat, the zone pump controller will start the appropriate circulator pump and send a control signal to start the heat source.  A zone pump controller is used for systems with multiple circulator pumps supplying multiple single zone manifolds.

Note:  Comap manifold and radiant package item numbers ending with “L” require a 1/2″ crimp tool.  Even though Radiant systems are very easy to install, we recommend installation by licensed, experienced professional installers.  Radiant systems require electrical wiring, which must be comply with the NEC and other local electrical codes, if applicable.  All electrical wiring must be preformed by a licensed electrician.