Can’t stop brass bolt fitting with fibre washer from leaking

Hi, if you've replaced the fibre washers with new ones and it's still leaking, I'd check a few things before trying any sealants:

Check the washer size and thickness. Make sure they're the correct size and sit properly inside the fittings.
Inspect the sealing faces. Limescale removal may not be enough if the brass surfaces are scratched, pitted or damaged.
Check the alignment. If the pipework is under tension or the fittings aren't aligned properly, the washers may not seal correctly.
Don't overtighten the nuts. This can damage the washers or distort the sealing surfaces.
Check the type of connection. If it's a flat-face union designed to seal with a washer, PTFE on the threads won't normally fix the leak because the washer provides the seal.

I'd recommend posting the photos of the connections, as these would help identify the actual cause. Also, make sure the system is isolated and depressurised before dismantling any fittings.

Hope this helps!

The issue is most likely that the fibre washer isn't sealing properly against the faces of the brass fitting, or the connection is being tightened incorrectly. Simply replacing the washer with another one may not fix it.
Here's how I'd approach it on a heating system.
Isolate the pump and let the system cool. Switch off the heating and electrical supply to the pump, isolate the water supply if possible, and relieve the pressure before undoing any fittings.
Check the sealing faces. Remove the bolt and washer and inspect both mating faces. Look for pitting, scratches, corrosion, old washer material or uneven surfaces. Clean them thoroughly.
Check the washer size. Don't go by the nominal pipe size alone. The washer must fit the sealing face correctly, with the right internal and external diameters. A washer that's too large, too small or too thick can fail to seal.
Check the bolt and fitting. Make sure the bolt isn't bottoming out before clamping the washer, and that the fitting isn't cracked or distorted.
Use the correct washer material. If the original connection uses a flat fibre washer, use the correct replacement specified for that fitting and the heating system. Don't automatically substitute a rubber washer.
Tighten evenly. Make sure the faces sit squarely together and tighten to the fitting manufacturer's specification. Excessive tightening can distort components or damage the washer.

Hi, thanks for the detailed explanation. I understand the problem you’re having with the underfloor heating pump connections. If the pump itself is sound and you have already replaced the fibre and rubber washers, the issue could be with the condition, alignment or seating of the brass connections rather than the washers themselves.
I’d be happy to come and inspect it properly. I can identify exactly where the leak is coming from and advise you on the best solution. If any parts need replacing, I can also let you know what is required before carrying out the work

1. Isolate and drain the relevant section.
2. Remove the bolts and both halves of the fitting.
3. Inspect the two flat sealing faces carefully.
• If there are any pits, score marks or old fibre stuck on, lightly face them with fine emery cloth or a flat file so they are smooth and flat.
4. Use a new fibre washer of the exact original size (measure OD, ID and thickness of the old one if you still have it). Avoid the mixed “selection kit” if possible – buy the correct size.
5. Optional but very effective: put a thin smear of jointing compound (Boss White, Fernox LS-X, or a PTFE paste) on both faces of the washer. This fills microscopic imperfections without being a full PTFE tape job. Many plumbers do this on fibre-washer joints even if the original installation didn’t.
6. Reassemble, making sure the pipe is fully home and the two flanges are parallel.
7. Tighten the bolts evenly in a cross pattern, a little at a time, until the washer is just compressed and there is no gap. Do not overtighten – fibre washers crush and then leak worse.
8. Refill, pressurise slowly and check.
3. If it still leaks
• Replace the fibre washer with a compressed fibre or Klingersil type of the correct size (more durable than cheap red fibre).
• Or switch to a soft copper washer of the correct size (common on some pump flanges).
• In extreme cases the brass faces themselves are no longer flat enough and the whole coupling needs replacing.
You do not normally need liquid PTFE or White Bond on a properly assembled fibre-washer joint, but a thin smear of jointing compound is standard practice and solves most “it still leaks after new washers” cases.

Why It's Still LeakingThe "Selection Kit" Trap: Assortment boxes from places like Screwfix usually contain general plumbing washers. Standard central heating pump washers are uniquely thick (usually 2mm to 3mm) and rigid. If the washers you used are too thin, the brass nut will run out of thread and bottom out before it fully compresses the washer against the mating face.Rubber vs. Fibre: Rubber washers seem like a logical upgrade, but under the high pressure and heat of an underfloor heating manifold, soft rubber can easily warp, squish out of the sides, or split when you tighten the heavy brass nut. Heavy-duty, rigid rubber or thick vulcanised fibre is required.Mating Face Alignment: Heating pumps are heavy and often fixed rigid. If the pipework or the pump dropped even 1mm out of perfect alignment when you took it apart, the two flat faces won't be perfectly parallel. The nut will feel tight, but it will only be squishing one side of the washer.