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High boiler modulation after pressure loss


Adsibob

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I have mentioned here before that our Viessmann Vitodens 200W 35Kw system boiler occasionally loses pressure. It happens every 8 weeks or so. @Nickfromwales recommended flushing the system with a special sealant and I’ve asked my installer to do this… still waiting.

 

 Yesterday night the boiler recorded a low pressure fault at 1124pm. We didn’t notice till 9am this morning when we woke up and realised we’re had no heating and the hot water was warm.

 

 I’ve topped up the pressure to 2.1, and this caused the boiler to start firing up again. I noticed that the flow temp slowly went up from 20C to 65C, it took about 10 minutes to get there. I also noticed that the modulation of the boiler was 98% to 100%. This has slowly come down to 55% as the boiler has been running over the last 40 minutes or so, but that is still much too high for normal. Even with every UFH zone on and our 2 rads and HW switched on, the boiler doesn’t go over 50%. At the moment, wet only have the HW, the two towel rads and about half the UFH on.

 

Am I just being impatient, and will the modulation fall to around 35 or 40% eventually, which is where I’d expect it to be for this level of demand?

 

Can anyone explain why this is happening Is it to do with the boiler running less efficiently until the whole system pressure is balanced?

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In the end the modulation got back down to my usual reading in the 30s very quickly.  I was just panicking (I’m an expert at that).

3 hours ago, Nickfromwales said:

Do you have a magnetic filter? You can dose via that DIY with ease. 

Yes, we’re have a magnaclean fitted. Why would I need to dose more water? I’ve already topped up the pressure by turning two little valves that my plumber told me were for topping up.

 

 Weirdly, now that the house and HW have been heated and the boiler is off, it’s showing the actual flow temp as 74C. Not sure why that is.

Edited by Adsibob
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22 minutes ago, gaz_moose said:

take the lid off, decant some of the water in it and pour in sealant. 

Might be a good idea to turn the 2 isolation taps off first....LOL.

 

Turn the 2 isolation taps 1/4 turn to the off position, firmly shut, and get an old towel at the ready. Under the mag filter should be a drain off ( maybe not but have a look ). After turning off, and hopefully draining vis the sump, unscrew the lid and remove.

Withdraw the magnet from the assembly and clean the plastic shaft of debris. Keep the magnet away from anything you like / is sensitive to magnetic fields ( like your wallet / cards etc ). Reinsert the magnet into the lid / pot aperture and put it safely to one side.

 

With the filter now empty, reinstall the mag / internals as you found them, and fill up the vessel with the leak sealer chemical, until full to the brim. Screw the lid on until as hand tight as you can, and then give it a final 1/4 turn or less with the filter spanner.

Reopen the Stilton vales and run the heating. DO NOT vent the filter as you'll just let the chemical out.

 

Run the heating for 4 hours and then repeat so you can get the rest of the chemical in ( as you can't get a full 1L in in one go ;) ).

 

Happy days.

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Thanks. You are assuming a level of familiarity with these things that is a bit ambitious.

 For example, what does this mean:

5 hours ago, Nickfromwales said:

Reopen the Stilton vales

And how do I:

5 hours ago, Nickfromwales said:

NOT vent the filter

 

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3 hours ago, Adsibob said:

Reopen the Stilton vales

PMSL. That typo is staying for all eternity. ffs 🤣

 

3 hours ago, Adsibob said:

NOT vent the filter

 

I shall call on an expert.......AH!, here's one. 

 

1 hour ago, Spreadsheetman said:

Reopen the isolation valves and don’t let the air out the air bleed valve in the top of the magnaclean.

Yup. 

 

And then help yourself to as much cheese as you like whilst waiting for the system to warm up again.

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If you are losing pressure from the system I would look for the cause of this. Adding leak sealer is just putting a band aid over the wound. Leak sealer has always been considered a last resort. When topping up the system pressure 2.1 bar is generally too high. Anything from 0.7 to 1.2 bar is normal system pressure when cold. Has your heating man checked that non of the air vents are leaking, expansion vessel pressure is good (is the expansion vessel sized correctly for the system volume), prv is not letting by etc. Are there any weeps inside the boiler or on any of the pipework and joints. Check all the above before adding leak sealer.

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15 minutes ago, Adsibob said:

what would this look like?

Water dripping / leaking, or evidence of it doing so but evaporating before it gets to form a drip.

 

On 26/02/2023 at 10:38, Jimmyjazz said:

If you are losing pressure from the system I would look for the cause of this. Adding leak sealer is just putting a band aid over the wound. Leak sealer has always been considered a last resort. When topping up the system pressure 2.1 bar is generally too high. Anything from 0.7 to 1.2 bar is normal system pressure when cold. Has your heating man checked that non of the air vents are leaking, expansion vessel pressure is good (is the expansion vessel sized correctly for the system volume), prv is not letting by etc. Are there any weeps inside the boiler or on any of the pipework and joints. Check all the above before adding leak sealer.

Well aware of this, and agree it's a last resort. But if it's a joint under a floorboard etc then this is the way to cure it without pulling the house apart. No apparent or obvious leaks / drips yet reported by the OP so I rules out the obvious.

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Update: installer has identified three small leaks across two of our four UFH manifolds. I'm not sure if these are "new". The water marks may have been there from day one, but he tightened the joints on three spots and said he was pretty sure this would fix it. So for the moment there is no need to flush the system with chemicals.

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  • 2 weeks later...

No pressure loss since the installer tightened a few joins on 1st March, but today I observed the modulation shoot up to 98% again when we ran the heating and hot water for the first time in 48h as we had been away. Just wondering how the boiler knows this is the case:

 

On 25/02/2023 at 11:00, Nickfromwales said:

the boiler is going higher than normal to get you back up to the regular ambient temperature.


I had assumed the boiler is just given binary inputs from each of the thermostats controlling our various zones and HW demand valve. But if @Nickfromwales is right, the system is more intelligent than that, and knows how much colder each zone is from target temp, and not just that a zone is colder. But how?

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