Air trapped within a closed radiant system is the most common cause of inefficient system performance. Fortunately, it's an easy problem to solve and won't become an issue at all if care is taken during the initial filling process.
Take a moment to study the Expansion and Purge Kit . Normally, hot water enters from the left, travels through the Expansion and Purge Kit (EPK) and enters the Zone Manifold. From there, the water is drawn by the circulator pumps, through each individual zone, then back to the heat source. A closed system circulates the same fluid around and around, and is completely separate from the domestic water supply.
You will notice that there are three valves on the Expansion and Purge Kit...two boiler valves and a ball valve. The first boiler valve is located to the left of the expansion tank. For the sake of this description, we'll call this the fill valve.
The second boiler valve (to the right of the expansion tank) we'll call the drain valve. It is used to evacuate air from the radiant system.
Please note that either of these valves can act as fill or drain. Which function the valve serves depends upon whether the pump or pumps are located to the right or left of the EPK. In other words, you always want to fill the system in the direction of the circulator pumps.
Between the drain and fill valves is a shut off valve. Closing this valve during the filling process will force the water introduced into the fill valve to travel past the pumps, through the floor tubing, into the hot water tank or boiler, then out the drain valve. The close proximity of the drain and fill valves to each other guarantees no air pockets in the system.
A multi-zone closed system should be filled one zone at a time. Using the ball valves located before each circulator pump on each zone, close off every zone but #1. This will force the fill water to travel through all the loops of tubing in that zone. Next, attach a garden hose to the drain valve and run it to a convenient sink, floor drain, outside, or wherever you want many gallons of discharge water to go.
- Close the shut off valve.
- Attach a second garden hose to the fill valve. We've included a brass female to female hose thread fitting to make it easy to connect the male end of your garden hose to this valve. This fitting can be used on either of the boiler valves, depending upon which of the two becomes the fill valve.
- You are now ready to purge the air out of zone #1.
- Using full house pressure, flood the zone.
If you are using a new or empty water heater, you'll be filling the tank during this procedure as well, so expect zone #1 to take the longest to fill. Any remaining zones will only be flushing air out of the floor tubing and the process will be much faster.
Observe the drain hose. Depending upon how large your hot water tank is, several minutes can pass without any water whatsoever discharging from the drain line....only air. Eventually, water will begin flowing, often in spurts and sputters. Be patient. A steady stream of water doesn't necessarily mean that all the air is out of the system. A good rule of thumb is: once it seems that all the air is out of the zone, let a continuous stream of water flow one minute for every 100 feet of tubing in the zone. Sometimes, water can actually flow around a pocket of air, especially in a radiant system where many curves and bends are normal. However, given a few minutes of water flow, even the most stubborn bubble will break up and flow out the discharge hose.
It's also a good idea to listen carefully to the water flowing through the system. In a floor joist system, quite often, as the water passes through the floor the air pockets are audible as they pass through the tubing. In a slab installation, the initial water and air emerging from the slab into the return manifold is quite noisy. Also, listen for any sounds coming from the hot water tank. Your goal is silence. In a properly charged radiant system, no sound whatsoever is audible.
You may also want to run your zone pump during this point of the procedure. If any air is trapped in the impeller, the force of the water now flushing the system will dislodge it. You only need to run the pump for half a minute or so to accomplish this. And remember, cast iron circulator pumps are so quiet you have to touch them to know they're on. Stainless steel pumps emit a very slight, nearly inaudible hum. In either case, if your pumps are noisy, then air is present.
So, when the water is flowing steadily out the drain hose and all audible indications of air in the zone have stopped, you're ready to repeat the procedure with the remaining zones.
Open the ball valve before zone pump #2, and close the ball valve before zone pump #1.
Remember to allow at least one minute of water flow per 100 feet of tubing in the zone and, as in zone #1, be certain that all audible indications of air are absent.
When, after several minutes, the water is flowing steadily from the drain hose, close the valve before zone pump #2 and open the valve before zone pump #3.
Repeat this procedure for all remaining zones.
The last step, once all the zones are flushed, is to close the drain valve on the Expansion and Purge Kit and observe the pressure gauge. As soon as you close the drain valve, the pressure from the house supply entering the fill valve will begin pressurizing the radiant floor system. When the pressure gauge reads 15 psi, close the fill valve. This is your cold system pressure. When the system is hot the pressure will be a few psi higher. A positive pressure in the system will guarantee that any remaining air in the tubing, or any gassing off during normal operation will be purged by the air eliminator.
Note: The cap on top of the air eliminator should always be open during normal operation.
Open the shut off valve between the fill and drain valve.
Your heat source is now ready for firing.
Filling a Closed System with Anti-Freeze
The above procedure should be followed to eliminate all air from the system. When that has been accomplished, one additional step is required.
Determine how much anti-freeze your system requires by adding the total amount of fluid in the tubing (2.7 gallons per 100 ft. of 7/8" Poly ...1.3 gallons per 100ft of 1/2" Pex) plus the volume of water in the hot water heater or boiler.
Determine what percentage of anti-freeze to water mixture is recommended by the manufacture. Some recommend 30 % anti-freeze, other's 50%. The proper mix is also influenced by the degree of low temperature you wish to protect against.
Once the proper amount of undiluted anti-freeze has been calculated, place some anti-freeze in a clean five gallon bucket and use a submersible utility pump to pump it into the system. Keep replenishing the bucket as needed.
All the valves before all zone pumps should be open. This will help diffuse the anti-freeze evenly through each zone.
Since the radiant system has been filled with pure water to eliminate the air, the addition of the measured amount of pure anti-freeze will displace a corresponding amount of water out the drain hose.
A submersible utility pump (sump pump) works well for this application because it will effectively drain all but 1/4" of fluid from the bucket. When the anti-freeze is gone, close the drain valve.
All that remains now is to pressurize the system. Reconnect the house supply to the fill valve (remember to flood the garden hose with water before reconnecting to the fill valve. This will avoid injecting air from the hose into the system), pressurize to 15 psi, then close the fill valve.
Your heat source is ready for firing.
For single zone radiant systems using heat exchangers, or for solar collector loops, the anti-freeze must be mixed in advance because, obviously, there’s no tank or manifold to commingle fluid from one zone into another. By the way, some anti-freeze is pre-mixed, others are in concentrate form. Be sure to check the label before adding water.
However, pre-mixing in these situations is rarely a hassle. In large systems, you pretty much need pure water and the power of house pressure to blast air from large amounts of tubing. But a single zone or a solar collector loop is usually short enough to purge with a small pump.