Which spanner should you use
on plumbing and heating fittings?
Rounded flats, twisted pipes, slipped grips, cracked solder joints — these are the results of the wrong spanner applied to the wrong fitting with too much force. The right tool makes the job controlled. The wrong one makes it expensive.
Why the tool matters more than most people think
Many fittings are damaged before they ever come loose.
The difference between an open-ended spanner and a ring spanner on a seized brass fitting is the difference between a fitting that comes free and one whose flats are rounded off before it moves. The difference between an adjustable spanner and a correctly-sized fixed spanner on a tight joint is the difference between controlled force and a grip that opens under load and slips off the corner.
Fittings on heating and plumbing systems are often soft metal — brass, copper, aluminium. They do not forgive the wrong approach. Once the flats are rounded, the fitting is significantly harder to remove, and the face below the flats may be damaged in the process. That damage does not fix itself.
The tool decision comes before the force decision. Choose the right spanner for the fitting first. Then decide how much force to apply. In that order. Applying the right force with the wrong tool still damages the fitting.
The spanners — what each one does and where it belongs
An open-ended spanner engages two of the six flats on a hexagonal fitting. It gives good access in tight spaces where a ring spanner cannot fit over the fitting, and it can be repositioned quickly when the fitting needs to turn through an arc in a confined area.
The limitation is the same as the advantage — two-flat engagement. Under high load, the jaw can flex slightly, which lifts off the corners rather than sitting flat on the faces. On a soft brass fitting under significant force, this is where the flats begin to round.
Use the correct size. Do not force a near fit. An open-ended spanner that is one size too large sits on the corners, not the flats. It can round a fitting before it moves it. If the spanner rocks on the fitting, it is the wrong size.
- Good-condition fittings with accessible flats
- Tight spaces where ring spanners cannot reach
- Moderate torque applications
- Holding side where a ring spanner is on the moving side
- Seized or heavily corroded fittings
- Soft brass with already-worn flats
- Situations requiring maximum torque
- Any fitting where you cannot confirm the spanner size is exact
A ring spanner engages all six flats simultaneously. The force is distributed across the full hexagonal profile rather than concentrated on two faces. On a soft or corroded fitting, this dramatically reduces the risk of rounding the flats — the spanner cannot slip off the corners because it is enclosing the fitting completely.
This is the tool to reach for when a joint is seized, when the fitting is soft brass, or when you need to apply significant torque without damaging the fitting. On the holding side of a two-spanner operation, a ring spanner gives maximum control with minimum risk to the component being held.
The limitation is access. A ring spanner must fit over the fitting completely — it cannot be used where pipework, insulation or adjacent fittings prevent it from sliding over the hex. Where access is restricted, an open-ended spanner is the compromise.
- Seized or corroded fittings
- Soft brass and older fittings
- High-torque applications
- Holding side in two-spanner work
- Requires clearance to fit over the fitting
- Cannot reposition easily mid-turn in tight spaces
- Slower in situations requiring rapid repositioning
The adjustable spanner is the most versatile tool in a plumber's bag and the most frequently misused. The adjustable spanner is useful. It is not precise. It adjusts to fit, which sounds like an advantage — and in many situations, it is. The problem is what happens under load.
An adjustable spanner has a worm gear mechanism that opens and closes the jaw. Under sustained torque, that mechanism flexes. The jaw opens slightly. The spanner rides up off the flats and onto the corners of the fitting. On hard steel fittings, this causes a slip. On soft brass, it rounds the flats.
Set it tight — wound down hard against the flats before applying force. Use it for initial loosening where a fixed spanner is not available. Do not use it for the sustained high-torque pull on a seized fitting.
- Non-standard fitting sizes
- Initial loosening at moderate torque
- Where no fixed spanner is available
- High-torque seizure breaking
- Soft brass with worn or corroded flats
- Final tightening on critical joints
Pipe grips work on a different principle from spanners. They grip by biting into the surface with serrated jaws. The harder you pull, the harder they bite. This makes them very effective at shifting things that nothing else will move — and very destructive to anything that needs to remain serviceable afterwards.
The bite marks left by pipe grips on a brass fitting body are permanent. The fitting may still appear to work, but jaw damage can create stress raisers, distort a thin wall or compromise future inspection. A deformed pressure component should be assessed and replaced when serviceability is uncertain. On a fitting that is going straight in the bin, that does not matter. On a radiator valve body that is staying in the wall, it does.
Use pipe grips on the fitting being removed when that fitting is being replaced. Avoid them on the holding side — the body, the valve, the union that is staying in the system — where the bite marks will be on a component that must remain intact and serviceable.
- Fitting has no usable flats remaining
- Fitting is being replaced regardless
- Nothing else will shift it
- Valve bodies remaining in service
- Chrome or plated fittings
- Thin copper or soft aluminium tube
- Anything that must be reused
Two spanners, two jobs
The most important principle in heating and plumbing spanner work is not which spanner to use — it is how many. Most joints need two.
Every threaded fitting connects to something. When you apply rotational force to loosen or tighten a fitting, that force travels through the joint and into whatever is behind it — the pipe, the valve body, the pump housing, the union on the other side. Without a second spanner holding the fixed side, that force goes somewhere it should not.
Goes on the fixed side — the body, valve, or union that must not rotate. Does not move. Absorbs the reaction force from the loosening spanner. Ring spanner preferred. Position it against the direction of rotation so it cannot slip under load.
Goes on the fitting being moved. Applies the rotational force. Open-ended or ring spanner depending on access. Correct size — not one size up. Steady force, not shock. Repositioned as the fitting turns.
Both spanners in position before any force is applied. That is the sequence. Not force first, hold second — hold first, force second.
Quick selection guide by fitting type
| Fitting type | Recommended tool | Notes |
|---|---|---|
| Boiler primary union | Ring + open-ended | Two spanners. Ring on body, open-ended on union nut. Check access first. |
| Pump flange bolts | Ring spanner | All six flats engaged. Cross-pattern tightening. Do not use adjustable. |
| Radiator valve — standard | Open-ended | Two spanners. Hold the valve body. Turn the union nut only. |
| Old brass fittings — soft | Ring spanner | Six-flat engagement critical. Correct size only. No adjustable. |
| Seized union — corroded | Ring + penetrating fluid | Fluid first, wait, then ring spanner. Not pipe grips unless fitting is for replacement. |
| Flexible hose connections | Open-ended | Hand-tight first. Use counterholding where the connected body or valve can rotate — do not assume the hose absorbs all reaction force. |
| Tight access — confined space | Open-ended or adjustable | Extra care — reduced control. Confirm size before applying force. |
| Fitting with no usable flats | Pipe grips — replacement only | Last resort. Fitting is being replaced. Not on the holding side. |
The one mistake that rounds more fittings than anything else
Using a spanner that is slightly too large.
A 24mm open-ended spanner on a 22mm fitting does not grip the flats — it sits on the corners. The moment force is applied, the spanner rides up the corner profile and the flat is gone. The fitting is now harder to remove than before the spanner touched it.
This happens because people reach for the nearest spanner rather than the right one. It happens in cold hands in a tight space with a head torch and a time pressure. It happens when the spanner set is incomplete and the closest size has to do.
Check the size before applying force. If the spanner rocks on the fitting, it is wrong. A spanner that fits correctly sits flush and does not move when you push it onto the flats.
One check that takes three seconds: push the spanner onto the fitting flats and try to rotate it without applying any turning force. If it rocks or shifts on the fitting, it is not the right size. Find the right one before you pull.
The right spanner does not just make the job easier. It decides whether the fitting survives the job.
Ring spanner on seized and soft fittings. Open-ended where access demands it. Adjustable only where nothing else fits — and set it tight before pulling. Two spanners for every joint that has a fixed side. Correct size before force.
The fitting that comes off cleanly needs a new gasket and goes straight back on. The fitting with rounded flats needs a spanner extractor, more time, and often a replacement fitting. The gasket is the same either way. The difference is the tool.