How do I use a hygroscopic air vent?
July 01, 2015

How do I use a hygroscopic air vent?

Some models of Caleffi's automatic air vents are equipped with a hygroscopic safety vent cap including the 5020 Series MINICAL and 5022 Series VALCAL.  It's referred to as a safety cap because it will seal off the vent if water gets into the cap area. This feature protects against potential vent spit and is perfect for locations that are not easily accessible and for locations where dripping water is absolutely unacceptable.

Here's how they work: inside the cap are a number of very thin, washer-shaped discs made of a cellulose fiber - a natural plant-based material that expands by 50% immediately when wet. This expansion seals the vent opening. Once the discs dry out, they shrink down again to allow air passage.

The hygroscopic cap MUST BE HAND TIGHTENED in order to work properly; do not leave it partially unscrewed as is done with a standard vent cap. Standard caps must be left partially unscrewed to vent air and they provide an air-tight seal when tightened. The hygroscopic cap is different; it is not designed to provide an air-tight seal and if left partially unscrewed it will leak water!

Caleffi utilizes the same hygroscopic technology in the 5080 Series manual vent which is perfect for radiators and high points in piping systems. Hand-tighten the 5080 for automatic air release operation or open it to fast-release air (or water) for example when filling/purging the hydronic system.


The hygroscopic products from Caleffi are a great problem-solver and are extremely popular! Check them out and, as always, let us know if you have any questions!


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Blog post comments
July 03, 2015

In order to prevent air from escaping from an AAV with the hygroscopic cap, such as when doing an air test, do we need to remove all the discs, or is there a trick to closing it off?

Kevin Freidt
July 07, 2015

Nick, you'll need a different cap if tight close off is required; the brass R59119 or plastic R56214 will do. You cannot achieve tight close off by removing the discs in the hygroscopic cap.

January 04, 2016

I have a hygroscopic vent valve on a residential heating system. from what I read on the website I should be able to manually turn the cap and bleed the system. It feels like I am going to break it if I continue. the system has air in it and this is the only bleed. can you tell me if there are different types and how they work. I have attached a picture of it for your review.


In reply to by pat

January 06, 2016

Yes, you should be able to manually unscrew the cap about 1/2 turn to manually vent the air. When closed, the hygroscopic discs will automatically bleed air (if the discs are not clogged). If the knob will not turn it is probably stuck due to excessive lime scale buildup. We suggest replacing it with a Caleffi 5080.

June 02, 2016

Can I install the 5080 in an upright orientation, or must it be installed on its side? Thanks.

June 03, 2016

Upright is fine; the 5080 can be installed vertical or on it's side.

June 13, 2016

When I was a kid, we were always having to bleed the upstairs convectors, which were tapped off the loop that ran through the large-tube Tuttle & Bailey baseboards downstairs. Before I moved out, I replaced the bleeders with float valves and all was well. It was only then that someone told me that the old ones were hygroscopic, and it occurred to me that we were probably over-tightening them. I'll bet a lot of people do the same thing and wonder why they have no heat.

June 30, 2016

The Caleffi hygroscopic vents don't seal completely, even when tightened. As long as they are dry, they'll allow air to escape. Maybe the ones you had years ago were different?

vincent monaco
November 08, 2016

is the cap # R59681 used on vent # 502343A?
and, why does # 501502A have a female threaded port on top?
thank you

November 09, 2016

Yes, the R59681 Hygroscopic cap does fit the 5023 Series air vent, in fact the 5023 comes with a R59681. The large 501502A has a female 3/8" straight thread on top where a discharge tube can be connected, if needed.

susan jeffrey
March 14, 2017

I have a hygroscopic vent valve on a residential one-pipe steam radiator to protect against vent spit. However, since I have installed the hygroscopic vent valve, only 6 fins of a 16 fin radiator will heat up. Does the hygroscopic vent valve prevent a residential radiator from heating completely? It is brand new, so the discs are not clogged. Thank you, susan

April 07, 2017

The hygroscopic air vent will not prevent the radiator from heating completely. As long as it is not clogged up, it will release air and keep the radiator full of water.

April 21, 2017

I want to use the 5080 hydroscopic air vent on top of my hot tub pump. Every time I drain and refill my tub air will get trapped in the pump and I have to take panels off and purge the air out. Its a real hassle. How well does this work with chlorine? Is there a plastic version available?

Cody Mack
September 12, 2017


Thanks for your question!

None of our products have been tested for use with chlorine. They have been tested to be used with water and mixtures of water and propylene glycol.

I wish I had a direction to point you in for such a product but I don't know much about this type of application.

Best Regards,
Cody Mack
Applications Engineer
Caleffi North America

Ian Harper
November 29, 2017

Is there a minimum temp for these to work? as we have them on a jacketed hot water pipe system ( runs at 45 - 50 degrees C) and have to manually bleed them to get air out, have had too since they were installed.

December 04, 2017

No, there is no minimum temperature required. They should bleed air slowly, through the fiber wafers, as long as they are dry. if wet, no, because they'll expand to close the air path. Suggest disassemble and check the wafers?

vincent monaco
December 04, 2017

on the 502234A vent, I've been on installations where; 1) the cap is missing wafers, 2) the cap screws down tight, 3) the cap does not screw down tight, but spins at the bottom of it's travel.
How many wafers should there be in each cap?
Why would some caps screw down tight and others not?
thank you

Greg Tubbs

In reply to by vincent monaco

December 20, 2017

The R59681 Hygroscopic vent cap should thread on to the air vent until it stops. If the cap continues to spin there may be a problem with the threads on the air vent or inside the cap. The caps are supposed to come with the Hygroscopic discs in them (8 brown, 1 red) If these are missing from the cap when you get them please let us know. We can be reached at (414) 238-2360 ( Monday through Friday) or we can be reached via email at

vincent monaco
December 04, 2017

how many wafers are there in each cap (# R59681) on the # 502234A air vent? Why might some of these caps tighten down fully and others spin infinitely, after full travel on the threaded stem?

Brian Willenbring
December 10, 2017

If the 5080 vents air automatically and the 5022/5023 also vents air automatically, why would I not just use the 5080 everywhere in my hydronic system? Is it simply because the 5080 will fit where the 5022/5023 will not, and because the 5022/5023 does not require as much maintenance?

Greg Tubbs

In reply to by Brian Willenbring

December 20, 2017

The 5080 Automatic Hygroscopic air vent is commonly used in panel radiators or in high points of a system that have a vertical stand of piping where air frequently gets trapped. If you install this in a horizontal run where less air congregates, the discs will not dry out very often to let air release. This is where float type air vents are a better option. Water pushes the float up sealing the vent. When air fills the vent body, the float is displaced, opening the vent port and allowing air to escape. This is the best option for a part of the system where smaller amounts of air collect out of solution and there is more water than air. There are different types of air vents fro different parts of the system. When they are applied correctly, air will be removed more effectively.

February 05, 2018

I have a domestic hot water return line that appears to be getting air bound. Are these vents acceptable for use in domestic water systems? Do they carry any kind of NSF certification or acceptance stamp?

Greg Tubbs
February 23, 2018

Hello Gerry
We currently do not have a low lead air vent available for a potable water application such as a domestic hot water recirc.
The NA52640A plumb vent should be available later on in 2018. This will be our low lead offering.

March 30, 2019

Today I installed a 5080 on my pool filter, which accumulates air. I know it was stated earlier that they haven't been tested with chlorine. My question is: How long should it take the wafers to dry to the point of releasing air? I had it running for a while and when I turned it to manual, air bled out. Is it not working or is it a matter of drying time?


In reply to by Bob

April 01, 2019

The natural fiber wafers in the 5080 typically dry out enough to start slowly releasing air after a few hours. With your application, however, drying time and air release are unknown because we do not recommend the 5080 on pool water.

Mark deangelo
October 08, 2019

My air vent was spraying water out of the cap the other day. I twisted the cap tight and it stopped but I need to know if it needs to be replace or if there is another issue wrong with my system. Can you help? I have a residential hot water boiler for a 1,600 sq ft home.

October 28, 2019

Hi Mark,
Your vent needs to be cleaned; there is dirt in the valve mechanism and it's not closing tight. Isolate it, remove and clean it out. Depending on the type of cap on your vent; you may want to replace the cap.

P Townsed
November 15, 2020

Will the Altecnic hygroscopic automatic air vent work on a domestic radiator if the system has already been treated with a leak sealant product such as Fernox F4


In reply to by P Townsed

November 19, 2020

The unit may work my concern is that the leak sealant looks for oxygen and then collects to stop a leak. This may hinder the performance of the hygroscopic wafers over time by collecting and closing off the automatic venting. There has not been testing to identify if they are compatible.

January 08, 2021

Hi l just buy some of the hygroscopic valves.can l use them in the horizontal or have to be just in the vertical??


In reply to by Luis

January 11, 2021

Luis, Any auto air vent that is a body and float design has to be installed in a vertical application. The only Hygroscopic auto air vent that can be installed in a horizontal position is the Ca;effo 5080 Hygrocal. That is the 1/8" hygroscopic air vent that has manual air venting and auto air venting via the internal hygroscopic discs, this model does not have the internal float.

December 24, 2021

Do these work with steam radiators ?(5080)


In reply to by Sheila

December 28, 2021

Sheila, The Caleffi Hygrocal 5080 is approved for use with hydronic heating systems and low pressure steam systems. They have a maximum working temperature of 212 F.

April 18, 2022

Is is possible to order the G1/2" (British Standard Pipe Parallel Thread (BSPP)) variant of this sold overseas? Caleffi part number 508041. My issue is that I have already bought, installed, and begun to utilize my European made radiators that utilize G1/2" (BSPP) threads and I would love to utilize this hygroscopic vent for my high point radiators. However, as you can imagine, it is difficult to convert BSPP to NPT - or find another comparable air eliminator/vent. My other question would be that if I am able to solidify the G1/2" BSPP variant of this vent, are the hygroscopic discs interchangeable between the 1/8" NPT model sold here in North America and the G1/2" BSPP variant sold in Europe?


In reply to by Ryan

April 26, 2022

Ryan, In the North American market we offer that air vent as model number 508013A, it is 1/8" MNPT connection. The European market offers it in other connection sizes. The replacement cartridge is available under part number 508100A. This would fit any of the models available.