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Boiler problem losing pressure

675 Views 10 Replies 3 Participants Last post by  HotRodx10
My heating system is losing pressure and now requires topping up daily via the filling loop in order for the boiler to work. I have a Vaillant Eco boiler with a large tank in the loft. The filling tap is located next to the tank in the loft. There is a pressure gauge next to both the tank and the boiler. The last Engineer to have a look gave up and just left the filling tap on slightly to maintain pressure (for some reason it fills to 1.5 bar and then stops filling so the PRV never kicks in). I have resorted to simply leaving the filling tap on slightly but since it stops at 1.5 bar you would not know it is even on (I know this is not ideal, not least because the inhibitor will have been thoroughly diluted). If I turn off the filling tap, pressure drops to zero within a few hours.

I can't find any leaks anywhere in the system and the PRV is not leaking which I think rules out both the PRV being faulty and also the expansion chamber as this would present via the PRV right? This, I have read, leaves the possibility that the Heat Exchanger has a pinhole or crack through which system water is escaping via the condensate pipe. This theory is possibly supported by the fact that recently the condense hose was leaking water and had to be replaced (A new hose and trap have been installed) - could this leak have been caused by a problem with the Heat Exchanger and hot escaping water?

I want to confirm whether the issue is in the boiler (i.e. the heat exchanger) or somewhere else in the system. To do this, I believe I can top up the pressure, then isolate the boiler by shutting off the flow and return valves. After a few hours, if the pressure in the boiler has dropped I will know the issues lies there, but if it hasn't and only drops when I turn the valves back I will know the issue is elsewhere in the system?

Four questions?

1) Does the Heat Exchanger seem the likely culprit given what I have said?
2) Am I understanding the check procedure correctly?
3) How do you shut off the flow and return valves on the boiler (is it just a case of rotating 90 degrees with a screw driver)?
4) Does anyone see any other issues or anything I may have overlooked from the details I have given
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with out seeing the valves. What you have said, sounds right.
When the boiler is off, does it still lose pressure?
Do you mean when then boiler is turned off or when the heating is off? Ive not turned the boiler off to check... But im pretty sure it loses pressure when the heating off. It will mintain pressure when heating is on (probably because the water is hot so even it is losing water pressure doesnt drop much), then it drops quick when heating is off.
If it loses pressure when the heating is off (the pressure continues to drop after it cools down), and there's not air in the system, it's leaking water somewhere. If it loses pressure during the heating cycle, it could be blowing off pressure via a PRV or something similar in the system. Just trying to help narrow down the possibilities. If you haven't done it yet, I suggest running a test where you bring the pressure up where you can monitor it after it's cool, and then see if it drops when the heating is still off for a while.
I've checked and its leaking pressure when the heating is off. It must be losing water then, but could this still be from the heat exchanger into the consensate pipe (even when heating off)? I cant find a leak anywhere to explain such a rapid pressure loss.
...could this still be from the heat exchanger into the consensate pipe (even when heating off)?

Probably. The lines through the heat exchanger stay pressurized, and they're typically thinner and lighter than the main lines. If you can't find a leak anywhere else, it's probably in there.
Ok great (well the probem isnt great but your help is ha)... i think I have the answer... likely to be a new boiler unless I can get a reasonable quote!
Can you get a look at the outlet of the condensate drain to see whether there's water coming out?
Well the condensate pipe comes out of the wall and then it runs into another wider pipe which eventually takes it under the decking en route to the mains so I cant get a good look at it without some hassle... however, where it comes out of the wall it almost immediately meets a joint just before it enters the other wider pipe and would you believe it... that joint has a drip on it... i guess its possible the drip could be the condensation that is supposed to be there butbitbwas dripping when the heating was off... a smoking gun perhaps!
If you have access to the rest of the system, and it's not anywhere along there, that's all that's left. You might be able to limp along for a little while, continuing to add water, but it's unlikely the leak will stay small for very long. Probably should look at replacing the heat exchanger or the whole boiler sooner rather than later. If it's been leaking for a while, be prepared to repair or replace whatever catch pan, etc. feeds the condensate drain. Not much that's steel stands up the the combination of air, water and heat for very long.
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