# Diagnosing sealed heating system losing pressure ?



## flanajb (6 Dec 2011)

Our boiler has started losing pressure at quite an alarming rate. We are having to top up using the filling loop at least once a week. When we do have to top up it only takes a few seconds for the pressure to return to what it should be.

No here is my probelm.

I took the front off the boiler to have a look at the heat exchanger and I can see that it has been leaking at some point from a flange. Very annoying, as this is a manufacturing defect, but being out of warranty there is nothing I can do.

Now what I want to be 100% sure about before spending 2-2.5k  getting a new boiler fitted is that it is actually the boiler and not a leak in the actual system. 

As the flow and return to the boiler can be isolated from the rest of the system, I was planning on isolating the boiler and then performing a pressure test (without draining the system) by making up an adaptor with pressure gauge and valve that I can connect to one of the radiators. Having connected the air input valve I can then pump air into the system to a given psi and then leave it to see if the pressure drops.

My only concern with doing this is whether the water in the system will affect the test.

I can then do the same with the boiler by using the pressure gauge on the boiler and opening up the filling loop.

If anyone has a better suggestion, then please do let me know as I am reluctant to get a plumber in as the ones I have spoken too so far seemed to have about as much knowledge as me on the best way to work out where the fault is.

Thanks


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## tsb (6 Dec 2011)

I suppose there could be 3 possibilities.
1- you have a leak in the boiler. Top the boiler up, isolate the boiler from the rest of system and see if the pressure drops.
2- Your expansion vessel has lost its charge . To check this just tie a plastic bag onto the pressure release pipe, which should be a short length of 15mm pipe sticking outside. Use the boiler as normal until you need to top up the boiler then check the bag and see if there is any water in it.
3- there is a leak in the rest of your system. If after doing the other two tests, this is the only option


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## flanajb (6 Dec 2011)

tsb":2chvvisc said:


> I suppose there could be 3 possibilities.
> 1- you have a leak in the boiler. Top the boiler up, isolate the boiler from the rest of system and see if the pressure drops.
> 2- Your expansion vessel has lost its charge . To check this just tie a plastic bag onto the pressure release pipe, which should be a short length of 15mm pipe sticking outside. Use the boiler as normal until you need to top up the boiler then check the bag and see if there is any water in it.
> 3- there is a leak in the rest of your system. If after doing the other two tests, this is the only option


Thanks tsb. Doing options 1 and 2 will as you say either rule out or confirm 3


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## bosshogg (6 Dec 2011)

As you seem well up to the task, you can get leak sealant for closed loop radiator heating systems, now how you get it in is different. If you have a ladder type towel rail with a top plug, simply pouring it into that would work! If not it will depend on what boiler and set up you have, one with a scour valve that you can drop some water out of the system would allow room for the sealant, if you know what I mean.
Try looking for the leak(s) yes there may be more than one, at the pressure relief valve which can often leak, give each inlet to a radiator, a very small tighten feeling for any small slack in the threads, do the same to the outlets (although I have never found these ness. myself) you are trying to identify for any looseness in the olives.
Most plumbers merchants will stock a sealant, failing that some general builders merchants might have it.

Hope this helps...bosshogg


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## flanajb (6 Dec 2011)

bosshogg":1ecvk977 said:


> As you seem well up to the task, you can get leak sealant for closed loop radiator heating systems, now how you get it in is different. If you have a ladder type towel rail with a top plug, simply pouring it into that would work! If not it will depend on what boiler and set up you have, one with a scour valve that you can drop some water out of the system would allow room for the sealant, if you know what I mean.
> Try looking for the leak(s) yes there may be more than one, at the pressure relief valve which can often leak, give each inlet to a radiator, a very small tighten feeling for any small slack in the threads, do the same to the outlets (although I have never found these ness. myself) you are trying to identify for any looseness in the olives.
> Most plumbers merchants will stock a sealant, failing that some general builders merchants might have it.
> 
> Hope this helps...bosshogg


Thanks bosshogg. Last time I put inhibitor into the system I drained a small amount of water out from down stairs and fille the towel rail from the top as that is the highest rad in the house.

Does leak sealer gum the whole system up though and run the boiler ?


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## Eric The Viking (6 Dec 2011)

[wrote this before Bosshog's posting - but this still has a bit of relevance]

Important question: what make of boiler is it? 

Adding a little and expanding on TSB's comments, there are several places I'd look first:

1. The pressure relief valve on the boiler (I think this is TSB's #2 above). These can easily leak, especially if they've been used to quickly de-pressurise the system at any point (e.g. during a service) crud stops them closing properly. Check by going outside to the other end of its pipe to look for drips/water, as TSB says.

2. The isolating valves immediately next to, or part of the boiler. At least one particular brand leaks terribly. Many installers fit separate service valves close by, and never touch the boiler valves, as disturbing them can cause problems. [later] should say - check them cold, as they're so close to the boiler that small leaks evaporate immediately, so you can't see them.

3. The air release valves, near the pump inside the boiler, usually (although ours has a second one at the top of the HW tank, close to the highest point). If they get crusty they can fail to release air properly and/or weep.

4. The expansion vessel: In normal use they keep the operating pressure fairly constant, and it should stay that way for months. If they lose pressure (but aren't punctured) the bladder inside squashes up and they don't do anything, so the operating pressure will fluctuate wildly - up when hot and too low when cold. This may cause the pressure release valve to open, venting water to outside, so next time the system tries to start from cold the initial pressure is too low. 

If there is a leak somewhere, and the vessel is OK the pressure will take longer to drop. The vessel prevents any need for the pressure relief valve to operate and the system won't lose water faster than the actual leak.

5. The parts of the boiler itself. You've already mentioned evidence of past leaks. Three-way valves were notorious in combis, although I haven't owned one for years and they may be better now.

If none of the above, you do have a leak somewhere else. 

If it's the boiler, it may still be repairable cost effectively - depends a lot on who made it.

You do have Fernox in the system, don't you?

E. 

PS: Just yesterday I replaced some el-cheapo Toolstation "corrosion proofer" with Fernox, as the former wasn't doing its job at all. The result has been a transformation - far less boiler noise and evidence of gassing (= corrosion activity). Wish I'd done it months ago!


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## tomatwark (6 Dec 2011)

Our boiler has been doing this recently although not to the same extent as yours.

When we had it serviced a couple of weeks ago it turns out that the pressure vessel has failed, and we are just waiting the the replacement to be fitted.

But when we had this problem last year I discovered that one of the radiators had developed a pin hole at the bottom, once I replaced that it was fine

Our boiler is 15 years old but inside looks as good as new so I am having the plumber replace the pressure vessel as I don't want to replace the boiler until we do our kitchen extension when it will have to be moved anyway.

The vessel for our boiler is about £70 plus fitting. which is a small simple job, what the plumber did say was if it was going to really expensive the other way would be to remove the one from inside the boiler and locate it somewhere else in the system as a standard vessel is about £35, but would involve more work to install it. So on balance we decided that the original part was the way forward for us.

It does help that the plumber I use is the one I recommend for kitchen fits so I don't get the sucking of teeth and looking at the ceiling before a magical theory and price as given.

Tom


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## flanajb (6 Dec 2011)

Eric The Viking":2kjmsu4b said:


> 4. The expansion vessel: In normal use they keep the operating pressure fairly constant, and it should stay that way for months. If they lose pressure (but aren't punctured) the bladder inside squashes up and they don't do anything, so the operating pressure will fluctuate wildly - up when hot and too low when cold.
> 
> You do have Fernox in the system, don't you?



That is one observation that I have noticed. When the boiler is cold the and I set the pressure = 1 bar. When the boiler is hot it goes up to nearly 3 bar. That does seem like a big difference ?

I did put some fernox in some time ago. Should you keep adding it periodically ?

Can I also use fernox and leak sealer in together ?

Thanks


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## Eric The Viking (6 Dec 2011)

1. That does sound like your pressure vessel. Before you chuck it though, they're ordinary Shraeder (tyre) valves. You can check it with a tyre gauge and pump it up with a bicycle pump. Try washing-up liquid on the valve - if it blows bubbles the valve isn't sealing. Yuo should be able to get a spare (pennies) from an old-fashioned bicycle shop - unscrew with fine needle-nosed pliers (or the proper tool). 

Pros will tell you correctly, but I *think* you need about 0.8 - 0.9 bar cold pressure. It might be the expansion vessel alone that's causing the problem. If you have one in the boiler too, check that one's pressure - if they're significantly different, one of them won't be doing anything. 

Most modern sealed systems have a working range between one and two bar. If yours is old like mine, and was once an open system with a header tank and (two!) gert big cast iron boiler jobbies in the scullery, I think 2 bar is probably being a bit unkind to it. I think 1.25 bar is sufficient and puts less stress on everything generally. If I'm wrong I welcome correction on this! 

2. Spent a while reading Fernox bumf over the w/e before buying some: they say all their products are compatible with each other, and you *can* add boiler silencer or leak sealer without diminishing the effectiveness of the corrosion inhibitor.


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## flanajb (6 Dec 2011)

Eric The Viking":12s2foau said:


> 1. That does sound like your pressure vessel. Before you chuck it though, they're ordinary Shraeder (tyre) valves. You can check it with a tyre gauge and pump it up with a bicycle pump. Try washing-up liquid on the valve - if it blows bubbles the valve isn't sealing. Yuo should be able to get a spare (pennies) from an old-fashioned bicycle shop - unscrew with fine needle-nosed pliers (or the proper tool).
> 
> Pros will tell you correctly, but I *think* you need about 0.8 - 0.9 bar cold pressure. It might be the expansion vessel alone that's causing the problem. If you have one in the boiler too, check that one's pressure - if they're significantly different, one of them won't be doing anything.
> 
> ...


Boiler is only 5 years old 

Will the manual state what the cold pressure of the expansion vessel should be or do I just set it to 1 bar so that when the water heats and expands it can overcome the force of the diaghram in the expansion vessel ?


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## Eric The Viking (6 Dec 2011)

It usually says on it what the cold pressure should be.

What make is the boiler?


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## flanajb (6 Dec 2011)

Eric The Viking":2sk3163x said:


> It usually says on it what the cold pressure should be.
> 
> What make is the boiler?


It's a gas Trianco boiler. Trianco started making gas boilers and then stopped shortly after. I wonder why


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## sparkymarky (6 Dec 2011)

a bungalow - 0.5 bar - a 2 story house 1.0 bar - three story town house 1.5 bar is a good general rule of thumb, the air pressure in the expansion vessel must match the wet system pressure to allow for the correct rate of expansion.
water expands by around 4% between 20-Oc and 80-Oc, water cant be compressed however air can be, so a expansion vessel is used on sealed systems to keep the pressure stable. if the water pressure is @ 1.5 bar and there is 1 bar air pressure in the vessel then it will of reduced the room for expansion by around 50% so on a 300 liter system 4% expansion will be needed = 12 liters so correct pressures will need to be set in the system and expansion vessel, though with the system at 1.5 bar there would only be =6 liters of room for expansion thus causing the pressure to rise.
most gas combi boilers and some oil combi boilers are only supplied with 10 liter expansion vessels this can cause the pressure to rise from say 1 bar when cold to 2 bar when hot if the expansion vessel is under sized, if this occours then an aditional expansion vessel will need to be fitted into the heating circuit.
hopes this makes sense and helps someone.
cheers, mark.


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## Eric The Viking (6 Dec 2011)

Yup. Mine has an external expansion vessel as well as in the boiler. 

I have run it at 1bar, but I suppose it ought to be a bit lower really, as it's measured at the boiler, which is at the top of the house (3 floors).


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## flanajb (7 Dec 2011)

I had a quick look over the boiler today and found the following

1) Expansion vessel pressure was virtually flat so have inflated to 1 bar
2) We had the exhaust gases checked by a Gas Safe registered plumber 2 weeks ago. Only trouble is. He did the test, but then left the bung out of the flu on top of the boiler. I have just refitted it :roll: 
3) Air purge valve on boiler was leaking so have twisted it 1/2 a turn as it was loose
4) Outside expansion pipe was dripping. Most likely due to number 1


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## flanajb (7 Dec 2011)

sparkymarky":2pymkvjn said:


> a bungalow - 0.5 bar - a 2 story house 1.0 bar - three story town house 1.5 bar is a good general rule of thumb, the air pressure in the expansion vessel must match the wet system pressure to allow for the correct rate of expansion.
> water expands by around 4% between 20-Oc and 80-Oc, water cant be compressed however air can be, so a expansion vessel is used on sealed systems to keep the pressure stable. if the water pressure is @ 1.5 bar and there is 1 bar air pressure in the vessel then it will of reduced the room for expansion by around 50% so on a 300 liter system 4% expansion will be needed = 12 liters so correct pressures will need to be set in the system and expansion vessel, though with the system at 1.5 bar there would only be =6 liters of room for expansion thus causing the pressure to rise.
> most gas combi boilers and some oil combi boilers are only supplied with 10 liter expansion vessels this can cause the pressure to rise from say 1 bar when cold to 2 bar when hot if the expansion vessel is under sized, if this occours then an aditional expansion vessel will need to be fitted into the heating circuit.
> hopes this makes sense and helps someone.
> cheers, mark.


I think this might be the problem. Our boiler has a 10 litre expansion vessel, which is most likely why it goes from 1 bar when cold to 3 bar when hot. The bodger plumber who fitted the boiler should have known that.

I get the impression that just because they are gas safe registered, that counts for diddly squat and many of then are incompetent.

So pressurizing the expansion vessel to one bar has just made my problem worse. Boiler pressure gauge now goes 1 bar cold to nearly 3.5bar (in the red zone). I suspect the expansion vessel was deflated so as to get the pressur gauge looking ok.

I think I might do the Gas Safe course myself and fit my own boiler. At least that way I can be sure it is done correctly.


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## Eric The Viking (7 Dec 2011)

If the pressure relief (vent) pipe is still dripping outside tomorrow, try venting the system through the pressure relief valve a couple of times (usually they're on a ratchet - twist the knob and they open against the spring). It sometimes washes out the crud that's making it leak. It's worked with ours 

Incidentally, if it's a condensing boiler there should be a pipe to a drain, taking condensate away. That will drip in normal use, but it should look like a plastic overflow pipe, not ordinary 15mm pipe.


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## flanajb (7 Dec 2011)

Eric The Viking":1w6h38b6 said:


> If the pressure relief (vent) pipe is still dripping outside tomorrow, try venting the system through the pressure relief valve a couple of times (usually they're on a ratchet - twist the knob and they open against the spring). It sometimes washes out the crud that's making it leak. It's worked with ours
> 
> Incidentally, if it's a condensing boiler there should be a pipe to a drain, taking condensate away. That will drip in normal use, but it should look like a plastic overflow pipe, not ordinary 15mm pipe.


Thanks Eric, I will get the manual out to see where it is located.


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