Annual Vaillant boiler F.75 error

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Hi, I find myself asking for help again with pressure issues on my Vaillant ecoTEC plus 637 system boiler. This seems to be an annual problem!

A few weeks ago, my boiler was serviced and last week I awoke to a cold house and a F.75 error. I let more water in and the boiler, CH and hot water have been fine since, or so I thought! I noticed the other day a dripping boiler overflow pipe, then checked the boiler pressure to find it was 3.2 bar - much too high! At first I thought I may have let too much water into the system after the F.75 error, but the PRV has been constantly dripping for 3 days now and the pressure hasn't changed (despite the obviously water loss from the boiler).

Unfortunately, the guy who serviced the boiler was unavailable, but had previously given me some instructions for recharging the expansion vessel. These involved draining the cool water out of the boiler while the CH flow and return valves were closed, using the boiler bleed valve, then pumping air into the expansion vessel (to 1.0 bar), while the boiler bleed valve was still open. Closing the bleed valve and re-opening the CH flow and return, then turning the boiler on, I was pleased to see the pressure at 1.5 bar. Unfortunately, even at this pressure, water was dripping out of the overflow pipe, which I don't understand. 30 minutes later, it was still dripping and the pressure was now upto 2.5 bar! An hour later and the boiler is warning the pressure is too high at 2.9 bar, with the overflow constantly leaking. So, recharging hasn't made any difference!

When originally letting air out of the expansion vessel, there was no sign of water, so I don't think the expansion vessel is broken, possibly a sensor issue? Also, I was surprised that I didn't need to let more water into the system after re-pressurising as I had drained about half a bucket. It may be that putting more air in the EV has taken up some of the volume of the drained water and that the reduced volume of water is why the pressure was not so high!

My title, says "Annual Vaillant...." as I had EXACTLY the same issue last year, again, just after the boiler had been serviced. However, in that case, re-charging the expansion vessel resolved the issue and I haven't have a single pressure problem with it until now.

It seems a coincidence that I'm getting pressure issues just after a service, but it's possible this is nothing more than coincidence. I can understand possibly having to let more water into the system after a service, especially if the engineer had drained the boiler, but I don't understand why the PRV is constantly leaking and why the boiler pressure is remaining high.

I would appreciate any ideas on what might be causing the issue so I can discuss with my service guy.
 
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But honestly, I have not had the energy to read the whole looooooong post.

Clearly, you have better things to be doing at the weekend, but then why did you even bother to respond? It's a loooooooong post as I wanted to give as much info as possible, rather than being asked 20 questions by 20 different posters. Sorry for being so thorough :D
 
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Because you havent recharged your expansion vessel properly, another problem is a blocked communication hose or pipe but if you had followed all the advice that would be evident, do it correctly stop missing out the important steps
 
Because you havent recharged your expansion vessel properly, another problem is a blocked communication hose or pipe but if you had followed all the advice that would be evident, do it correctly stop missing out the important steps

Hi Ian, thanks for the useful information.
 
For reference, here's what the Vaillant ecoTEC plus 637 System Boiler Installation & Maintenance Guide says about checking the expansion vessel in the section 7.4 Checking the expansion vessel:

• Ensure the boiler CH service valves are closed.
• Release the pressure from the boiler.
• Remove valve cap from expansion vessel charge point.
• Check that the internal charge pressure of the
expansion vessel is between 0.75 and 0.9 bar.
If the pressure is lower than this the vessel should be
repressurised using an air pump.
• Refit the valve cap.
• Repressurise boiler and heating system.

The instructions I followed do all of the above.

As an aside, it's also interesting that the manual states: "Note: It is not necessary to perform this check every year – a check every three years is sufficient." My understanding was this should be done "at least" every time the boiler is serviced!
 

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You shouldn't really be taking the cover off your boiler - it forms the safety room seal and should only be removed and refitted by a Gas Safe Registered engineer

From your description I'm not convinced that you've recharged your expansion vessel correctly, as the pressure in the system should be zero at the end of this process. As Ian has said, the expansion vessel hose should have been checked for blockages. It sounds like your PRV is shot and you need a new one if it's dripping constantly
 
I am absolutely fed up posting this as no one reads what you have to do in order for it to be done properly , you have to drain the boiler and leave a drain point open while you pump up the vessel, the vessel will be full of water and as you pump up the vessel the water has to go somewhere, once the vessel is empty of water then you will now pressurize the bladder to the correct pressure, then you close the drain point and top the water pressure back up , if you do not leave a drain point open you can not possibly completely inflate the bladder, if when you start pumping no water comes out of the drain point then you probably have a blocked communication hose or pipe
 
Milleniumaire, the likely correct long term solution to your regular problem is to install an external expansion vessel.

The 637 is a very powerful boiler with a small vessel, and unless you have a small system (and the boiler is therefore overpowered for your needs), it is likely that the integrated expansion vessel is insufficient for your system.
 
I am absolutely fed up posting this as no one reads what you have to do in order for it to be done properly , you have to drain the boiler and leave a drain point open while you pump up the vessel, the vessel will be full of water and as you pump up the vessel the water has to go somewhere, once the vessel is empty of water then you will now pressurize the bladder to the correct pressure, then you close the drain point and top the water pressure back up , if you do not leave a drain point open you can not possibly completely inflate the bladder, if when you start pumping no water comes out of the drain point then you probably have a blocked communication hose or pipe

... These involved draining the cool water out of the boiler while the CH flow and return valves were closed, using the boiler bleed valve, then pumping air into the expansion vessel (to 1.0 bar), while the boiler bleed valve was still open. Closing the bleed valve and re-opening the CH flow and return, then turning the boiler on, I was pleased to see the pressure at 1.5 bar. ...

Sorry Ian, I'm still not seeing what it is you think I have missed/done wrong. My description of what I did matches your description of what needs to be done! I perfectly understand a drain point MUST be left open while pumping air into the vessel to ensure water can escape. This is exactly what I did.

I can also confirm that when I pumped air into the vessel (with the drain point open) water did drain out, which suggests the communication hose or pipe is not blocked.

You shouldn't really be taking the cover off your boiler - it forms the safety room seal and should only be removed and refitted by a Gas Safe Registered engineer

Hi muggles, I understand, thanks for the advice.

Sorry about the trolls Millenium. It's best to just ignore them, they get a kick out of responses. Remember these are so-called professionals belittling people on a DIY forum. That they even waste their time here says a lot about their mental state.

Anyway - how big is your system? How many radiators, zones etc?


I understand about the trolls, I've been posting on forums for long enough to realise they don't have a life and get their fix out of being unhelpful and provocative.

I'm guessing you are trying to determine if the boiler expansion vessel is big enough for my system, and that's a fair question. The internal expansion vessel is 10 litres and can accommodate a system with upto 100 litres. I've been meaning to do exactly that calculation, so will do it today and try to get a feel for the size of the system. The original system was expanded a couple of years ago when underfloor heating was added as part of a kitchen extension, so it is possible the engineer who installed this didn't consider the impact the larger system would have on the internal vessel. Having said that, apart from this issue and a similar one around the same time last year, in the last two years (since the underfloor heating was added) the system has worked well.

This morning I decided to turn down the heating and hot water dials on the boiler to try to reduce the pressure and this appears to have worked (not surprisingly). The high pressure 3.2 bar flashing warning has now gone and the boiler is operating at around 2.5 bar. I can't really tell if the boiler overflow is still leaking as it's raining, so everything is wet! The boiler reports 62 degrees for the hot water and 55 degrees for the radiators. Although hot, I can now keep my hand on the radiators, whereas before they were too hot to touch! Time will tell if this is sufficient to keep the house at the required temperature.
 
it is easy to see if you have an expansion problem, let the system get cold and set your pressure to 1.5 bar, remove the filling loop hose and then fire the system and go back and check after about 30 mins or so and see what the pressure is now at, it shouldnt go up to more than about 2 Bar if it is at 3 Bar you have an expansion problem, but make sure you remove the filling loop hose before the test
 
Anyway - how big is your system? How many radiators, zones etc?

The 637 is a very powerful boiler with a small vessel, and unless you have a small system (and the boiler is therefore overpowered for your needs), it is likely that the integrated expansion vessel is insufficient for your system.

Hi, I guess my system would be classed as large, even before the underfloor heating was added as I live in a 5 bedroom house with 3 bathrooms (over 3 floors - includes a loft conversion). There are 10 medium sized radiators, one of which is a double, 2 large radiators, 2 towel rails and a small radiator. There is also underfloor heating underneath the kitchen/dining/sitting area. There are two zones; the underfloor heating and the rest of the house.

My very rough estimate is that this may be as much as 200 litres? That sounds high and is obviously twice as large as the boiler's internal expansion vessel is designed to work with. It does make me wonder why an external expansion vessel wasn't fitted when this boiler was installed 6 years ago, although most of these 6 years have been trouble free, with only the odd pressure issues.
 

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