Radiant floor heating zone manifold (C) D Friedman R Arlyck Radiant Heat System Controls, Definitions, Terms

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Radiant heating system key components, terms, definitions:

This article describes key controls, devices, & terms used with radiant heating systems. Our page top photo illustrates the heating boiler, circulators & mixing controls for a multi-zone radiant floor heating system installed during reconstruction of a home in Tivoli, NY.

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Radiant Heating System Controls & Components

Radiant heat system controls (C) Daniel Friedman

A radiant heating system used in buildings is comprised of heating elements embedded in building surfaces and a heat or energy source for those elements.

Electric radiant heat, popularized in the U.S. in the 1950's was installed in ceilings, using either flat electrical resistance heaters atop ceiling drywall or in special gypsum-board panels containing electric resistance heat wiring.

Similar systems were occasionally installed in poured concrete floors or under tile floors. But as most widely used in modern installations, radiant heat is installed in or beneath building floors using flexible tubing heated by water or a water-antifreeze mix that is in turn heated by a heating boiler.

Our photo (above left) illustrates a four-circuit radiant heat installation and shows the supply and return manifolds at left and the mixing valve (gray control, upper right) in a Tivoli, NY home. Photo courtesy Ralph Arlyck, Timed Exposures.

To make a radiant heat floor system work well, in addition to normal controls found on any heating boiler (varying by boiler type: electric, LP or natural gas, oil fired, or possibly passive solar-heated) additional controls are usually found including these:

Radiant Heat System Air Eliminators & Air Purgers

Radiant heat zone pressure gauges (C) Daniel Friedman

The air eliminator, typically part of an expansion tank and air purge kit, is a device used to vent air from the radiant heat tubing and piping system.

Without an automatic air purger on a hot water circulating heating system it is easy for water flow through the heating tubing to become blocked or air-bound.

The problem of air-bound heating systems is not unique to radiant heating, nor are its solutions, but as we explain below, in radiant floor heating applications it's easy to mistake an air-blockage in the system for other heating system problems.

In our photo (above left) you can spot the air scoop/air purge device - the green device near photo center. on the top right of that air scoop you can see the brass-colored automatic air bleed valve that removes air collected in the air scoop or piping. At middle right of the photo you can see pressure and temperature gauges installed on the system.

Spirovent air purger on a radiant heat system (C) Daniel Friedman

Air bleeder valves are used with an expansion tank and air purge "kit" to manually remove air from the radiant heat tubing during installation.

While these components are found on other hydronic heating systems, often a factory-packaged radiant heating system installation will include a factory-designed plumbing package that includes all of the necessary fill and drain valves, expansion tank, air purge or air eliminator valve.

Our photo (left) shows a Spirovent air purger eliminator installed on a radiant heat floor system.

Watch out: while just about every hydronic-type radiant floor or ceiling heating system includes one or more air purging device, air in the tubing loops can reduce heat output or even prevent sections or entire heating zones from delivering heat. Air in the tubing may accumulate during a system shut-down, from leaks, or other defects.

See AIR BOUND RADIANT HEAT where we present a case history that documents diagnosing & repairing air-bound tubing loops in one zone of a three-zone radiant floor heating system.

Details about air bleeders and air purge devicves are at

Radiant Heat System Antifreeze

Antifreeze is often included in the heating fluid mix for radiant heat installations. Installers may use either propylene glycol (non-toxic food-grade antifreeze, recommended where a tankless coil is used on a heating boiler and in some other cases), or ethylene glycol (automotive type and highly toxic antifreeze).

Details are at ANTIFREEZE for BOILERS.

Radiant Heat Circulator Pump(s)

A circulator pump is used to force heated water through the circuits of the radiant heat tubing. As with other hydronic heating systems, design may use a single circulator pump and multiple zone (or circuit) control valves or it may use multiple circulator pumps, one for each heating zone. To provide zone heat control, each zone has its individual room thermostat that in turn operates the zone valve or circulator pump.


Radiant Heat Adapters, Couplings, Headers & Zone Manifolds

Zone mainfold radiant heat (C) Daniel FriedmanSpecial fittings are required to connect the radiant heat tubing (typically rubber or polyethylene or PEX) to copper piping that is required close to the heating boiler itself.

A header or manifold is a connecting device that feeds multiple radiant heat tubing loops from a single larger (usually copper) pipe that is in turn fed with hot water from the heating boiler. Each radiant heat tubing loop, routed through a particular heating zone or floor area, is referred to as a circuit.

A header may be used at either or both the supply and return ends of the radiant heating system piping. The advantage of using a header is the greater ease with which flow can be controlled and balanced among the various loops or zones in the radiant heat system.

A zone manifold for radiant heating systems (photo above left) combines the manifold itself, used to connect radiant heating circuits to the hot water supply with a pre-packaged assembly of circulator pumps, flow balancing valves, pressure testing gauge, and controls.

Radiant heat detals - courtesy of Entran radiant heating

Our photo shows that there are four heating zones (we see four zone valves grouped on the left side of the manifold). [Click any InspectAPedia image to see an enlarged, detailed version.]

Our sketch at left shows a simple diagram, courtesy Entran radiant heating, illustrating the pressure test kit connections to a radiant heating circuit.

But the system in our "real world" photograph (above left) is trickier than that. Look at that left-most zone and you'll see that its feed splits off into two sub-circuits or loops, both on the same zone;

The next-to-leftmost zone valve feeds four circuits, and so-on.

Most likely the installer did this either to distribute heating circuits within individual rooms or to try to balance circuit resistance. Each circuit has an individual balancing valve or control - all of which we observed were in the full "open" position.

Radiant heat mainfold (C) DanieL Friedman

Radiant heating control manifolds or "slab manifolds" provide from one to six individual circuit connection points.

Manifolds for radiant heat circuit connections may be wall-mounted near the heating boiler or some installers actually place the manifold into the concrete slab itself, though it must be protected from actual contact with the concrete (to avoid corrosion issues and to assure access for inspection, maintenance, repair, adjustment).

We were a little worried when we saw this Two Harbors MN radiant heat tubing manifold arrangement as it was not obvious that a copper or brass manifold had not been buried in the concrete - but, then, the installation was incomplete.

Perhaps there were only a series of 90's in the concrete and the manifold was yet to be installed.

Radiant Heat Pressure, Pressure Testing, Pressure Gauges

Radiant heat zone pressure gauges (C) Daniel FriedmanA closed type radiant heating system typically operates at 15 psi and never over about 27 psi - a pressure close to that at which the TP valve on the heating boiler would open.

An open type radiant heating system (see below) operates at house water pressure, typically between 20 psi and 70 psi.

The piping in a radiant heat system should be pressure -tested for leaks before the system is placed in service, particularly as finding and fixing a leak later can be so troublesome.

The manifold or header assembly installed to connect the heating boiler to the several radiant heat tubing circuits will typically include a pressure gauge that can be monitored to check the system for leakage before it is placed into service.

A pre-acceptance pressure test is quite simple: a specific pressure is set in the system, the boiler is left off, and the pressure is monitored for at least 24-hours. If pressure does not drop then the presumption is that the system is not leaking.

Radiant Heat System Recovery Rate Definition

The recovery rate for radiant heating is defined as the length of time necessary for the heating system to raise the water temperature back from its cooled-off state (while heat was off or hot water was not circulating because a room thermostat was satisfied) up to the normal operating temperature of the system.

Cool water from the radiant heat tubing may be at a temperature of 65-50 °F. (during the heating season) when the room thermostat next calls for heat, and may involve 20-40 gallons of water/antifreeze-mix at that temperature. The boiler has to then re-heat that water volume back to its output temperature (180F) or if a water heater is used water is typically heated back up to around 125 °F. The length of time required for the heater to achieve this temperature rise is defined as the recovery rate.

Details are at Water Heater Recovery Speed Comparisons

Radiant Heat Circuit Resistance & Flow Balancing

Radiant heat flow balancing valves (C) Daniel FriedmanIn an ideal radiant heat system installation, all of the individual circuits would be of the same length, thus giving the same flow resistance, making balancing heating easy.

But often this is not the case and heating circuit lengths may vary. The radiant heat manifold will include flow balancing valves to permit necessary adjustments so that heat is not inadvertently distributed unevenly in the building when multiple heating zones are calling for heat at once.

Flow balancing valves are used on most hydronic heating systems, not only for radiant heating.

Our photo (above-left) shows two flow balancing valves on two individual radiant heat circuits. Interestingly the right-hand valve is in the fully-closed position while the one at left is in the fully-open position.

Radiant Heat Safety Devices, Controls: Temperature/Pressure Relief Valves, Mixing Valves, Temperature Controls

Radiant heat mixing valve (C) Daniel FriedmanThe water circulated through radiant heating system tubing is almost always required to be at a lower temperature than the normal output temperature of the heating boiler itself.

There are several reasons for the dichotomy: a heating boiler is generally more efficient when operated at higher temperatures; radiant heated floors would be uncomfortably hot and might also be damaged if water circulating through the radiant heat tubing is too high in temperature.

In our photo (left) we have circled the mixing valve installed between the cold return side and the hot feed size of the heating circuits.

Typically water leaving a conventional heating boiler is at 160-180 deg F.

The mixing valve mixes a small amount of very hot water from the heating boiler into the cold or returning water at the end of the heating loop, so that the temperature of water sent onwards through the radiant floor heating loop will be re-heated but not raised to an excessive temperature.

In addition, for some heating loops and heating conditions, the returning hot water (at the end of the heating loop) is still warm enough to re circulate, sending additional heat into the occupied space, without having to re-heat the water by turning on the boiler.

Closed vs. Open Radiant Heating System Designs

Thermolec radiant heat boiler - abandoned due to Dreamworks SNAFUs (C) InspectApediaIn a closed system radiant heating design, a closed loop (or loops) of piping circulate heating water through radiant heat tubing installed (typically) in or beneath the building floors, arranged in one or more heating zones or circuits.

The system is "closed" because hot water (or water and antifreeze mix) is contained within dedicated heating piping and the heating boiler itself.

Our photo (left) illustrates a Thermolec electric boiler installed to supply a radiant heat floor slab system. Sadly the system never worked and had to be abandoned, not from a problem with the boiler but because of improper tubing depth and incomplete slab insulation. Details are at RADIANT HEAT MISTAKES

In an open system radiant heat design a portion of the hot water supplied a domestic water heater, used to supply hot water for washing and bathing, is routed through radiant heat tubing installed in one or more floor circuits where it is used to provide heat to the occupied space before continuing onwards to provide hot water for plumbing fixtures like sinks, bathtubs or showers.

While some radiant heat designers refer to this as an "efficient" system because we've avoided installing a separate heating boiler for heating building floors, in our OPINION the design may violate the warranty of your water heater and by increasing its duty cycle may also shorten its life. In addition, in areas where water is high in mineral content there may be an added risk of ultimately very costly mineral clogging of radiant heat tubing or piping in the building.

Radiant Heat Piping & Control Troubleshooting & Repair Q&A

Reader Question: figuring out the control valves on radiant heating piping systems

Heating piping & valve layout (C) InspectApedia JCHi there, thanks for your informative site.

I've combed your sited for help on one specific thing but found no answers.

They being on the on/off valves for my heating system. I've attached a picture of it below- what Im referring to is the two red shut off valves- one on top between (now off) where water flows upstairs to the radiant flooring and the other below (between where the water flows to then basement heat)- also in off position.

My question is are these suppose to be in a off position or on position during regular heating? - J.C. 11/20/2013


I need to see more of the piping in and out of the boiler and entering and exiting the room.

Are your baseboards or radiators all getting hot. Some? None?

What type of heating system are we looking at: baseboards, radiators, radiant?

Reader follow-up:

Radiant heat piping, valves, controls (C) InspectApedia JCOn the right leads to the upstairs top floors radiant flooring.  On the left is the return supply.  I currently have both water valves closed.  It seems to be working by heating up the house-  but When they are closed this seems to only work for a day or two and then the floor-house cools down as if the heat is turned off?  I know this could be even an underlying issue wit h a part and might not even have to do with the two water tee shut off valves

[Click to enlarge any image]

Attached is the best i can do. On the right leads to the upstairs top floors radiant flooring. On the left is the return supply. I currently have both water valves closed.

It seems to be working by heating up the house - but when they are closed this seems to only work for a day or two and then the floor-house cools down as if the heat is turned off?

I know this could be even an underlying issue with a part and might not even have to do with the two water shut off valves .

But Im also worried by having them closed im doing damage to something by not letting water pass through the right way. The temp and psi pressue of the heater istelf has been constent and working. Sorry if to vague- thanks though for the reply and getting back to me nonetheless. Jake


You need that valve shut, Jake, as far as I can make out. It seems to be a bypass valve. Based on noticing a red dot I see on the mixing valve that usually indicates the valve's "HOT" side, I think I've marked the water flow direction on an edited version of your photo immediately above.

[Click the labeled image above to see text details explaining the piping layout, bypass, mixing and zone valves.]

If I'm right about the water flow direction the system is laid out in a common pattern for radiant heat flooring systems, though in general we prefer to put the circulator and zone valves on the return side or cooler side of the heat piping loop where they may have a longer life. But since radiant heat systems flow cooler water through the valves, at least those fellows care less than in other hydronic heating system designs.

In any event, even if I'm wrong in my guess about direction of water flow, that valve between the zone valves and the return lines should normally be shut. That manual bypass is there for the service tech as an over-ride for the thermostatically controlled mixing valve labeled and shown in your photo at lower right.

Your system has a separate bypass and mixing valve (as I note in my edited photo) that tempers the hot water leaving the boiler with fairly warm water returning from the zones - a common practice especially where radiant heat floors are used but one also used in some other hydronic heating systems.

Finally, the fact that your system heats adequately at times and not others is basis for a service call to see what's going on with the controls.


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