The best automatic air vents, compared.
Top pick: Cylinder air vent 1/2 Best value: Altecnic Robocal 15 mm – Highest score per pound at 75: 15 mm compression at about £10.69, with 4.5 from 116 ratings. Updated 22.10.2026.
An automatic air vent is a small float valve that lets free air out of a sealed heating circuit and shuts when water rises. In the UK you fit it upright at a high point on 15 mm compression pipe, on a 3/8 inch or 1/2 inch BSP port, or as the specific part inside a boiler. A hygroscopic radiator bleeder is a different mechanism from the float vents in this set. The order for connection, pressure and shut-off is in the [automatic air vent buying guide](/hub/diy-supplies/automatic-air-vent/sub/automatic-air-vent-buying-guide). Work inside a gas boiler belongs to a Gas Safe registered engineer. You fit it upright, where you can see it, not above something a drip would ruin, and not where it can freeze. Flamco says do not insulate the vent and do not use it on closed drinking water or steam. Screwing it fully tight stops a weep and also stops the venting. Flamco's fitting note is half a turn open while filling, then hand-tight. Caleffi's note is one full turn open and left there. If water still comes out, replace the vent.
An automatic air vent is a small float valve that lets free air out of a sealed heating circuit and shuts when water rises.
38 products checked · 61 hrs invested · +22,800 readers
Swipe for more
1-2 of 11
Personal buying guide
Question 1 of 2
What joint do you already have?
The unbranded cylinder vent ZL-7313R is £8.84 with 4.7 from 31 ratings. The title says 1/2 inch while the bullets say connections 3/8 x 1/2, and none of the short reviews confirm which thread arrived. The listing does state 6 bar and 110 °C, and the body is brass with a chrome finish at 150 g. The UK texts in the file are a sentence each: easy to fit, tidy, good value, with no word on the cap or on leaks. A non-UK line says air left a hot-water radiator. That star line looks strong beside the Flamco, but 31 brief ratings do not settle the thread. Buy it only after you have measured the port against both printed sizes.
Pros
Cons
Best for: You have checked the port against both size lines and accept a short review file.
Less suitable: You need one agreed thread, or you are choosing from the star line alone.
Specifications
Common questions
Is the connection 1/2 inch or 3/8 inch?
The title says 1/2 inch. The same listing says connections 3/8 x 1/2. Match it to the port you have.
What limits are printed?
Maximum working pressure 6 bar and maximum temperature 110 °C. The body is brass.
Does it name a brand?
No brand is given. The model number on the listing is ZL-7313R.
The HTS listing is sold against Vaillant automatic air vent reference 104521 at £11.50, with 4.7 from 11 ratings. The manufacturer named on the page is HTS, not Vaillant, and no GC number, thread, pressure or temperature is printed. Three short UK reviews say it worked and was as good as the original, and one used a little lubrication to insert it. A 12-month warranty is a listing claim, not something those reviews test over a year. It is a boiler-case part, not a 15 mm pipe vent. Order it only when 104521 is the reference on your Vaillant and you accept a pattern part with eleven ratings.
Pros
Cons
Best for: Your Vaillant calls for AAV 104521 and a Gas Safe engineer will fit the pattern part in the case.
Less suitable: You need a 15 mm or 3/8 inch pipe vent, or a box made by Vaillant.
Specifications
Common questions
Is this a genuine Vaillant part?
The listing names the manufacturer as HTS and the model as (AAV) 104521. It does not say Vaillant made it.
Will it fit every Vaillant boiler?
The bullets say it is for Vaillant boilers. No GC number is listed, so the page does not prove a match.
What did reviewers say about fitting?
Three short UK reviews say it worked. One said a little lubrication made insertion easier.
The Flamco Flexvent 27750 is a 3/8 inch brass float vent with a shut-off, listed at £16.79 with 4.5 from 1,255 amazon.co.uk ratings. Reviewers fit it at towel-rail high points, loft primaries, manifolds and air-separator outlets, and several say they hear air leave on refill and then stop bleeding that circuit. The shut-off is the practical difference in this set: people swap the head later without a full drain. One buyer reported that one of six leaked and needed another drain-down, and another arrived with the red cap missing, so a check before you fill is worth doing. The listing weight of 10 g and the oversized centimetre line are not credible for a brass valve, so ignore those catalogue fields. A reviewer also thought the thread was 1/2 inch while the title says 3/8 inch, which is the mismatch to rule out before you order. In this capture it is the top pick when you already have a 3/8 inch port and want the shut-off included.
Pros
Cons
Best for: You have a 3/8 inch heating port and want a shut-off so the head can come off later.
Less suitable: You need 15 mm compression, or you cannot confirm the port is 3/8 inch.
Specifications
Common questions
Does this Flamco include a shut-off?
Yes. The listing title is a 3/8 inch float vent with a shut-off valve, part 27750.
What pressure and temperature are listed?
The listing says it suits systems at 6 bar with a peak of 10 bar, and below 90 °C with a tip of 120 °C.
Is it 1/2 inch?
The listing is 3/8 inch. One reviewer thought they had received 1/2 inch, so check the port before you order.
The Altecnic Robocal 502649 is a 15 mm compression brass vent at £10.69, with 4.5 from 116 amazon.co.uk ratings. The listing states 6 bar maximum venting pressure and 10 bar maximum working pressure, and reviewers fit it on loft pipe and in place of a leaking plastic vent. Several say radiator and pump noise dropped once the system was refilled. The plastic cap has to be loosened slightly or it will not vent, and the box does not include a manual. No shut-off is stated, so one reviewer added a separate isolating valve upstream. Temperature is not printed on the amazon.co.uk page. In this set it is the best value when your pipe is 15 mm compression and you will remember the cap.
Pros
Cons
Best for: Your high point is 15 mm compression and you will crack the cap open after fitting.
Less suitable: You need a shut-off in the box, or the port is 3/8 inch or 1/2 inch BSP.
Specifications
Common questions
Will it fit 15 mm pipe?
Yes. The listing is a 15 mm compression auto air vent, part 502649.
Does the listing include a shut-off?
No shut-off is stated. One reviewer fitted their own isolating valve before it.
How is the cap used?
A reviewer says the red or plastic top has to be unscrewed a little and left open so air can escape.
The Wärmer System 7313B is a 15 mm compression brass cylinder at £7.99, with 4.0 from 45 ratings. Reviewers call it slim and say it lets air out, and one stopped constant radiator bleeding for the first two weeks. The listing gives no pressure and no temperature, so you cannot check a seal against a printed limit. The red top has to be unscrewed a little and left open, and one reviewer found water came out if it was even slightly open, which turned theirs into a manual vent. Guidance in the pack is thin. It is the cheapest 15 mm body here, with a shorter track record than the Altecnic at £10.69.
Pros
Cons
Best for: You want the lowest 15 mm price in this set and will watch the cap after filling.
Less suitable: You need a printed pressure limit, or you want a vent you can leave closed.
Specifications
Common questions
What size pipe is it for?
15 mm compression, as stated in the title and the size field.
Does it vent with the cap closed?
Reviewers say the red top has to be unscrewed a bit and left open. One said water comes out if it is left even slightly open.
What is the maximum pressure?
The listing does not give a pressure or a temperature.
The mizikuu pack is three 1/2 inch radiator bleed valves at £10.99, nickel-plated brass, with 3.8 from 52 ratings. The listing states 6 bar with a 10 bar peak and 90 °C with a 120 °C peak, and it claims a floating seal. The two UK texts in the file are thin: one is a one-word approval, and one says there were no instructions and the buyer was not sure they worked. Leak comments, including packs where every valve dripped and knobs that needed a spanner, come from outside the UK in this dump, so they are not a UK sample. The title is a radiator bleeder, not a 15 mm pipe vent for a loft high point. Treat the dial as something you must set, and do not assume it replaces a Flamco on a 3/8 inch port.
Pros
Cons
Best for: You are replacing 1/2 inch radiator bleeders and will test the dial on one radiator first.
Less suitable: You need a pipe float vent, or you want UK reviews that describe a dry automatic seal.
Specifications
Common questions
How many are in the pack?
Three 1/2 inch air vent valves.
What limits are printed?
Up to 90 °C with a short peak of 120 °C, and 6 bar with a peak of 10 bar. The body is nickel-plated brass.
Do the UK reviews explain the dial?
One UK reviewer found no instructions and was not sure they worked. Leak comments in the stored file are from outside the UK.
The Regun listing is a G 1/2 male brass vent at £12.29, with 3.8 from 23 ratings. Two of three UK reviews say it was easy to fit and did the job, and one called the price good. A third UK review says water leaks out of the air vent. The page claims high temperature and home or industry use, but it prints no pressure and no temperature number. The attribute block also says compression, which conflicts with the G 1/2 male title, and the dimensions field prints 10 x 10 x 10 mm against a 140 g valve. There is no shut-off. At more than the Altecnic, the missing rating and the leak report are the reasons to leave it unless you have confirmed a G 1/2 port and can isolate it.
Pros
Cons
Best for: You have a G 1/2 port, an isolator nearby, and you will watch the outlet on refill.
Less suitable: You need a printed pressure limit, or your pipe is 15 mm compression.
Specifications
Common questions
What thread is it?
G 1/2 male, also written as 1/2 inch. The attribute block also says compression, which conflicts with that.
Does it include a shut-off?
No shut-off is stated.
Do the UK reviews mention leaks?
One UK review says water leaks out of the air vent. Two others say it was easy to fit and did the job.
The PlumbEZ 313352 is a slimline 15 mm compression brass float vent at £9.99, listed to 10 bar and first available on 2 October 2025. The amazon.co.uk line is 3.4 from 4 ratings, and only two UK texts are in the file. One calls it good value. The other found the float was not assembled, so at about 2.5 bar of hot water only the plastic cap stopped the water. The bullets say it should sit upright on headers, risers, towel rails and manifolds, and they claim glycol blends without a percentage. Temperature is not stated, and no shut-off is included. With one assembly fault in a sample of four, it sits behind the Altecnic on the same 15 mm connection.
Pros
Cons
Best for: You want a spare 15 mm slimline and will check the float is seated before you fill.
Less suitable: You want more than a handful of ratings, or you will not open the body to check the float.
Specifications
Common questions
Will it fit 15 mm copper pipe?
The listing says a 15 mm compression connection for copper or barrier pipe.
How should it sit?
The listing says upright or vertical at high points.
What did the reviews report?
One reviewer called it good value. Another found the float was not assembled, so only the plastic cap stopped water.
The ERGAOBOY OP-866 is a single 1 inch BSP brass vent at £9.49, with the same 1.6 MPa, 150 °C and mesh-filter claims as the half-inch pair. The star line is 3.0 from 4 ratings. The only written review in the file is not from the UK: it says the valve would not seal on a solar mat, water followed the air, and the top union leaked. Three of the four ratings have no text. A 1 inch port is uncommon on a domestic radiator or 15 mm heating pipe, so the size itself narrows who can use it. The solar claim on the listing is not supported by that review. Treat the pressure figures as claims until you have a UK report of a dry seal.
Pros
Cons
Best for: You have a 1 inch BSP port and will pressure-test the union before you leave it.
Less suitable: Your system is 15 mm or 1/2 inch, or you wanted a UK review of a dry seal.
Specifications
Common questions
What size is the thread?
1 inch BSP (DN25), model OP-866, one piece.
What limits are claimed?
1.6 MPa and 150 °C, with a mesh filter. Those are listing claims. 1.6 MPa is 16 bar.
What is in the written review?
The only review text says it would not seal on a solar mat, water followed the air, and the top union leaked. That review is not from the UK. Three of the four ratings have no text in the saved listing.
The ERGAOBOY OP-864S is two 1/2 inch BSP brass vents at £10.49, and the listing claims 1.6 MPa, 150 °C and a mesh filter. The amazon.co.uk line is 1.0 from 1 rating. That single UK review says it does not vent by itself, holds air until you bleed it by hand, and then lets water out. One review is not a repeated pattern, but it describes the opposite of an automatic vent. The rank in this capture sits far down the Central Heating System Accessories list. Leave it unless you have already decided a 1/2 inch BSP pair is worth inspecting before it goes on a live system.
Pros
Cons
Best for: You want a spare pair of 1/2 inch BSP threads to inspect before they go on a live system.
Less suitable: You need a vent that buyers say releases air on its own.
Specifications
Common questions
How many valves are included?
Two. Model OP-864S, 1/2 inch BSP (DN15).
What limits are claimed?
1.6 MPa and 150 °C, plus a mesh filter. Those figures are listing claims. 1.6 MPa is 16 bar.
What did the review say?
The only review says it does not vent automatically, holds air until bled by hand, and then lets water out.
The Fdit listing is a G 1/2 brass air vent at £11.56, and the raw amazon.co.uk page has no customer-review line and no review text. The title says an air vent valve, one attribute says check valve, and another line says copper while the material line says brass. High temperature is claimed with no figure, and there is no pressure. The weight field is 40 g. With nothing from buyers to confirm that it vents and then seals, it is a thread-size guess rather than a heating spare you can compare with the Regun, which at least has 23 ratings and a leak report you can read. Leave it unless you can return it unused if the thread or the seal is wrong.
Pros
Cons
Best for: You only need a G 1/2 body and you can return it if the thread or the seal is wrong.
Less suitable: You want any UK review evidence before you drain a system.
Specifications
Common questions
What thread is it?
G 1/2, also written as 1/2 inch.
What is it made of?
The listing says brass. One attribute line says copper.
Is there a review score?
The raw page has no customer-review line and no review text.
Shelf prices and stars are the amazon.co.uk snapshot of 19 October 2026 already stored for these eleven listings. This is not a laboratory test. The score uses stars, review volume, listing fitness, UK review sentiment, and price against that evidence. The highest score is the top pick. Best value is the highest score per pound, which is a different listing. A high star line from a handful of reviews cannot lead.
Stars
20%The amazon.co.uk rating divided by 5, then scaled to 20 points, to the nearest whole number. A listing with no star line scores 0.
Review volume
15%A larger stored review count scores more, up to 15 points. One to four reviews score 2. Five hundred or more score 15. No reviews score 0.
Listing fitness
25%A single clear connection, a pressure figure, a temperature figure, a shut-off in the box, and a named float or boiler vent. A size clash or a missing joint scores less.
UK review sentiment
25%Repeated UK praise with rare faults scores highest. A known cap caveat, short praise only, a UK leak, or no UK review text scores less. This is not a laboratory test.
Price against evidence
15%A clear joint, 50 or more ratings, under £12, and no dominant defect scores highest. A dearer vent can still score when the shut-off and the review sample are there. A report that it does not vent scores low.
Buy the connection before the brand. An automatic air vent on a UK heating system is a 15 mm compression float vent, a 3/8 inch or 1/2 inch screw-in vent, or the part inside a boiler, and they are not substitutes. A 3/8 inch Flamco does not replace a 15 mm nut. The cap has to stay able to pass air. A shut-off under the head is what lets the next swap avoid a drain, and only the Flamco in this set includes one. Work inside a gas boiler belongs to a Gas Safe registered engineer.
An automatic air vent is a small float valve that lets free air out of a sealed heating circuit and shuts when water rises. You fit it upright, where you can see it, not above something a drip would ruin, and not where it can freeze. Flamco says do not insulate the vent and do not use it on closed drinking water or steam. The cap has to pass air. Screwing it fully tight stops a weep and also stops the venting. Flamco's fitting note is half a turn open while filling, then hand-tight.
The same search also returns through-wall fans and grilles. A 15 mm compression nut is not a 1/2 inch BSP port, and a 3/8 inch Flamco does not replace a 15 mm nut. Measure the joint you already have.
Fully tight stops the venting. Fully open can let water out. Flamco's note is half a turn open while filling, then hand-tight. Caleffi's Robocal note is one full turn open and left there. If water still comes out, replace the vent.
The HTS listing is a pattern part sold against obsolete Vaillant 104521. Genuine Vaillant is now 0010033857 and must match the boiler's GC number. Refill without inhibitor, and work inside the boiler case, are separate mistakes. Boiler work belongs to a Gas Safe registered engineer.
The first check is the joint you already have. In this set that is 15 mm compression, 3/8 inch, 1/2 inch BSP or G, or a boiler part number. A 3/8 inch Flamco does not replace a 15 mm nut. The Altecnic Robocal 502649, the Wärmer System and the PlumbEZ are the 15 mm compression rows. The Flamco Flexvent 27750 is the clear 3/8 inch float vent, and it is the only one here with a shut-off in the box.
Working and venting pressure, and temperature, should come from a manufacturer sheet where one exists. Domestic sealed systems usually sit near 1 to 1.5 bar cold. That is a rule of thumb. Altecnic 502649 is 6 bar venting, 10 bar maximum working, 0-115 °C, glycol to 30 percent. Flamco Flexvent is 6 bar operating, 10 bar peak, from -10 °C, 90 °C continuous and 120 °C peak, glycol to 50 percent. The amazon.co.uk page for the Altecnic does not print the temperature.
Three typical situations, each with a recommendation from this comparison.
You have a cold high radiator or a gurgling pump, and the high point is 15 mm compression you can drain and refill. The Altecnic Robocal 502649 is best value at about £10.69, with 4.5 from 116 ratings. The cap has to be left slightly open. No shut-off is stated, so add an isolator if the next swap must avoid a drain. Restore inhibitor after a refill.
You are replacing a known 3/8 inch vent and you want the head to come off later without a full drain. The Flamco Flexvent 27750 is the top pick at about £16.79, with 4.5 from 1,255 ratings and the shut-off in the box. It does not replace a 15 mm nut. Check the port before you order.
The vent is inside a gas boiler, so the work belongs to a Gas Safe registered engineer. The HTS listing is sold against obsolete Vaillant reference 104521 at about £11.50, with 4.7 from 11 ratings. The page names HTS, not Vaillant, and prints no GC number. Genuine Vaillant is now 0010033857 and must match the boiler.
The top pick is the Flamco Flexvent 3/8 inch at about £16.79. It scores 89, the highest here: a stated float vent with a shut-off, and 4.5 from 1,255 ratings. A reported leak on one of six, and a missing red cap, sit in the review notes. Best value is the Altecnic Robocal 15 mm at about £10.69. It scores 75, the highest score per pound in this set, with 4.5 from 116 ratings. The listing states 6 bar venting and 10 bar working pressure, and no shut-off in the box. The Wärmer and the unbranded cylinder score more points per pound than the Flamco, and less than the Altecnic.
Altecnic Robocal 15 mm
Highest score per pound at 75: 15 mm compression at about £10.69, with 4.5 from 116 ratings.
Editorial
Peter F. is the editor for DIY & tools at productscompared.co.uk. Comparisons in this category are checked against amazon.co.uk listings, review evidence and public UK sources.
Guides on what to look for, how to use a product, and common mistakes.
Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 19 October 2026.
This page contains affiliate links to amazon.co.uk (tag pcuk26-21). If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are the amazon.co.uk figures already stored for this set. This is not a laboratory test.
