Central Heating leak sealer

Central heating system has been losing pressure for a few weeks now. Pressure drop varies from day to day, can be as much as 0.7 bar or as little as 0.3 bar.

Have checked the “overflow” pipe and that is dry as a bone, put a plastic bag over it just to double check. So assume the PRV and expansion tank is OK.

Don’t get clouds of steam out of the flue so assume that the heat exchanger is OK.

Have checked all visible joints and radiators and no leaks there.

So assume that most likely a joint under the floor is leaking. Have checked the ceilings and no visible sign of damp. No sure how much water lose it takes to reduce pressure by 0.7 bar and whether that would show up on the ceiling.

A few year back I put new flooring upstairs so can’t just lift a floor board to check those joints. Did move a radiator at the time and had problems getting water tight compression joints so used silicon jointing compound. Thus quite likely that one of those joints has now failed.

As first recourse going to try leak sealer. Whats the likely chance of it working?

The price seems to vary a lot.

No Nonsense Leak Sealer 1Ltr £2.36
No Nonsense Leak Sealer Tube 310ml £2.99
No Nonsense Central Heating Leak Sealer 500ml £9.19
Sentinel Internal Leak Sealer 1Ltr £9.99
Fernox Heating Leak Sealer Concentrate 500ml £13.99
No Nonsense Central Heating Leak Sealer 310ml Concentrate £14.92
Sentinel Rapid Dose Leak Sealer 400ml £14.99
Fernox Heating Leak Sealer Superconcentrate 290ml £24.99

What is the difference and what is recommended?

Have heard of sealers causing damage to combie boilers so best to drain/flush the system to remove the sealer after it done its work or leave it in to seal any future leaks?

Thanks.

One other question does it damage the boiler if its run on low pressure say around 0.2 bar?

I would recommend a plumber with a heat gun, with heating hot he/she may be able to pinpoint leak, the amount of water it would take to lose 0.7 bar really depends on the size of your system, sounds like quite a lot to lose in 1 day though

Thanks for the reply.

Don’t think get a plumber with heat gun in will help as there are no visible leaks and lifting the floor boards is not an option atm.

As to the size of the installation. it’s a 3 bed mid terrace and has 2 largish double rads ( 1 switched off) and 6 medium/small single rads ( 1 switched off)

Never had much luck with leak sealers but you don't have much to lose. * That said, don't now (but doubt) whether they have nay iimpact on combis.

Having had a mysterious leak in the past, worse than yours, I would be surprised if you couldn't see it by now in the ceilings. Is some of the ground floor pipework under screed?

Thanks for the reply.

Non of the pipe work is under screed. Its all either in the ceiling void or run down the walls but boxed in.

When topping up the system it SOUNDS like a lot of water is being added so also would have expected to see a puddle somewhere or a damp ceiling. Which is why I initially discounted it being a leak. But how much water is actually being added I wonder.

As far as I’m aware there is only 3 ways that the pressure could drop. Having. In my mind anyway, shown that the other 2 are OK just leaves a leak as the cause.

Which leak sealer did you try?

Are the more expensive ones better or are you just paying for the name?
Is the super concentrate more expensive be because its better or just easier to apply?

I used the Fernox one and it stopped-or-near-as-dammit-stopped-it for a couple of weeks, thereafter it started dropping again. When it was warm and on full bore, there was never an issue. But when it cold it would drop like a lead balloon. In the end I replumbed the system from the top down rather than bottom up.

I'm no plumber, so these are just suggestions. Have you a pressure vessel? Could that be taking in water? If the PRV isn't blowing off and pressure vessel is sound, and the gauge isn't dicky, then water is leaking somewhere. Bottom of a wall drop may not show up for a long while. My own leak was huge and it only just wetted the screed. Would be cool if you could bypass sections of the system, even if it means adding a restrictive valve.

As a visual, if you get the pressaure up to normal, then open vent a radiator until the pressure drops to the level you're experienecing through your 'leak' it'll give you some idea of the amount of water loss you should be looking for!

I'm certain one of those leak detector firms could find something, but pretty sure you could replumb new pipework for less anyway!

Internal leak sealers ?? have used them with success ? did a large sealed system a couple of weeks ago , they had a leak either in the screed or on the hot water coil to cylinder ??

used 2 litres of the sentinel stuff left it for a couple of weeks , no improvement , still topping it up twice aday , so used 2 tubes of the fernox F4 ( I think ) seems to have cured it ?? however there is always a risk to useing it , on some condensers they have small water ways , used it ounce & blocked the heat exc !
Maybe if u use it ? try not to tip it or squirt it directly into the boiler ? put it in to the flow maybe so that it dilutes around the system prior to returning to the boiler , May be ???

Thanks for the replies

My leak seems slightly different to yours. Seems that the colder the weather the more the pressure drops, I assume because the heating is on more. Very little loss during the day while heating is not running. Perhaps the hot water is expanding any poor joint gap combined with the increase in water pressure when hot is allowing/forcing more water out than when cold.

Will try draining the system down by half a bar and see how much water need to be released. Sound like a good idea.

Sounds like I will give the stuff sentinal stuff a miss. Was reading a review on screw fix where someone said it seemed a little thin which is perhaps why it didn’t work for you.

It’s a Worcester 28CDI combi boiler so not a condensing type so hopefully the waterways will be large enough not to block.

P.S. Will be add it via the radiator in the bathroom which is quite a long way from the boiler.

Just a small update in case it helps anyone.

I had to drain off about 250ml to drop the pressure from 1 bar to 0.2. So guesstemate that over the past couple of months that this has been going on must have lost about 10l of water. Wonder where it went. Watch out for a new thread on how to cure damp rot

In the end brought the Fernox Heating Leak Sealer Concentrate 500ml. Added that then topped back up to 1 bar, water was still warm at this point. Turned the water temp up to max, normally run at about 2/3 max and turned the room stat up to ensure the water had max possible circulation. About 12hrs later checked the pressure, when cold, and it was 0.8bar. How much of that drop was due to the leak and how much due to the water being cooler I don’t know.

Anyways topped up again to 1 bar and 20 odd hrs later not dropped below the 1 bar mark. So seems either problem cured of the stuff has ceased the pressure gauge

Only problem now is have to find a new hobby as watching a non falling pressure gauge is extremely boring, back to watch the electricity meter spinning I guess.

Hope it's a permanent fix. My experience was leak sealers buy you time, although, obvs, it depends on the size of the leak!

Fingers cross for you!

Deleted.Should've read the posts! Doh!