Boiler adding air into system when hot?

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I previously made a post here about low heat output of the boiler - this has been mostly rectified with a balance however two radiators still only get hot at the top and not at the bottom.

Edit: Forgot to mention the boiler is a 26kW Viessmann Vitodens 100-W WB1B :)

When the heating/hot water/both starts after sitting for some time, the pump hums away quietly as the boiler begins to heat water. If the heating dial on the boiler is turned only half way (about 3/6), the boiler stays at quite a low power output and slowly heats up the water fine to around 55°C. Set any higher, the flow temperature on the display will start dropping after reaching around 40°C, followed by maximum power, temperature rapidly climbing and the burner turning off for 2-3 minutes. This will happen for around 20 minutes until reaching a stable temperature.

When the flow temperature surpasses about 55°C on the boiler's display, no matter what, the pump (in the airing cupboard) will become noisy, sounding like it's pumping gravel, or like running a bathroom tap. Every time I have stopped and bled the pump out of its bleed screw after this happened, I can hear air come out before the trickle of water, so I think it's air. The noise only stops when either the whole system has been turned off for a while, maybe 2 hours, or if the pump runs for a few minutes with the boiler burner off (there is no pump overrun but it gets quieter when the burner cuts out while warming up). I don't think it's overpumping, it still happens even if the pump speed is set to 1, and whether hot water is on or not.

I've checked the F&E tank and it's got plenty of water in it, and the cold feed valve is not blocked. The end of the vent pipe from the boiler is sat slightly under the water level but the top of the U shape is at least 2 feet above the water level. There is a thin layer of sludgey-slimey stuff coating the tank, and occasional bubbles come out of the vent pipe.

I believe the 3-port valve is also functioning correctly, the right pipes are getting burning hot to the touch.

Also not sure whether it is worth mentioning, but the boiler only ever reaches a temperature of ~67 celsius, modulating itself down to 1/5 on the display. I can also hear what sounds like tiny bubbles of air in one downstairs radiator when it's set this high. It's not weather-compensated (no sensor box outside), so is this normal? I would expect to be able to aim for around 70 flow and 50 return, with a 20 degree drop on the radiators.

We had a heating engineer in to look at it but he seemed to be in a bit of a rush, just tweaked the radiator valves for a bit, turned up the pump and said the downstairs ones were getting hot and left. It sounds like we probably need someone to inspect the boiler to me, unless there are any other possibilities for these issues? Any help would be greatly appreciated.
 
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Did you have the system cleaned properly before it was fitted?
Was a filter fitted on the return flow?
 
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Did you have the system cleaned properly before it was fitted?
The boiler was fitted in 2014 by the previous owners (to replace the original 2004 one) - the invoice they left says it was flushed with magnacleanse and inhibitor was added before fitting. It's been annually serviced every year since but I'm not sure if any more inhibitor has been added since 2014 in these services.

Was a filter fitted on the return flow?
There's a magnaclean on the return, yes.
 
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what you you get out when you clean the magnet?

scrape it into a jar if you can.

is there any mud in the F&E?
 
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what you you get out when you clean the magnet?

scrape it into a jar if you can.
I've cleaned out the magnaclean, not sure how clean I'm supposed to get it but took the sleeve off and scraped off all the crud and gave the rest a good rinse and a wipe down with plenty of tissue. I've attached some photos of the filter and the stuff I scraped off.

PXL_20210120_185927496.jpg PXL_20210120_185932421.jpg PXL_20210120_194910429.jpg PXL_20210120_194926198.jpg

is there any mud in the F&E?
There's just a thin layer of what looks like the same sludge type stuff as what came off the filter in the dish, no mud
 
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That is a fair bit of sludge, inhibiter should be checked every year. Maybe you have built up quite a bit.
Also if you have a leak your system will keep topping up with new water.

Drain system, see how dirty water comes out, refit and get a couple of bottles of F3, put it in and leave for a week with boiler on, drain refill and put in a bottle of F1.
It may clean it out quite a bit or you may need a powerflush I think.
 
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that's quite a fair amount of black sludge. Do you know when it was last cleaned? If it has not been done since the boiler was installed, it's understandable.

Put it back and have another look in a month or so. If there's only a smear, that's OK.

It should be sufficient to check it around once a year, there should be nowhere near as much as you found.

The black iron oxide arises when the steel radiators corrode in the presence of air.

As you have an open-vented system, air may enter from the F&E tank, it will be much worse if you have a leak and water is constantly being topped up from the tank. If you put a half-full glass jar or something under the fill valve, so that the valve drips or squirts into it, you will know it has been topping-up because the jar will fill to the brim with fresh clean water. A photo would be useful.

Air can also enter if there is a leak on the suction side of the pump.

The inhibitor contains an oxygen-scavenging chemical, but this will get used up if more keeps entering. Also, if it is being topped-up, the chemical will leak away and be diluted.
 
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p.s.

if you're looking for a leak on the suction side of the pump, it will probably be on one of the joints with big brass nuts, and you may may find marks of water leaking out, probably green stains or limescale, hanging off the joint or running down the pipe.

There may also be harmless green stains round soldered joints, if you clean these off with a nylon pan scourer and a damp rag, you will soon know if they are leaking, because the stains come back.
 
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The last service was at the end of 2019, so it's due another - maybe I should ring up the business who did the services and ask if they generally clean out magnacleans when they do it?

I imagine the sludge has been coming from one or two of the upstairs radiators which seem to build up quite a bit of air in them. I just checked, and the one nearest the pump just let quite a bit out even though I last bled it maybe a week ago at most. I think there were about 3 radiators pretty full of air when we moved in last month.

I only know about one leak - on the flow side of the radiator in the conservatory, there is a very very slow drip from the TRV - so slow that I've never seen it drip, and each drip actually evaporates from the dish underneath it before the next so it looks dry (though without the dish it leaves a tiny wet patch on the carpet). However having the lockshield on the return side completely closed seems to make no difference.

I'll try putting a glass jar below the cold feed tomorrow morning, and take a photo of the tank while I'm back up in the attic.

As for a leak on the suction side of the pump - annoyingly, I can't really follow the pipe. The boiler is in the utility room which was built as an extra bit of the bottom floor of the house sticking out, so it has its own attic space. The copper pipes from the boiler go into this separate attic, connect to plastic pipes at a pair of valves and run across as plastic pipes into the gable end wall of the house, and the next time I can get to them is in the airing cupboard as copper pipes coming out of the floor. Given the layout of the house, the pipes from the airing cupboard must run under the floor at the top of the stairs. From the bits I've seen, I haven't spotted any leaks, but I'll take another look tomorrow. Thanks for the tip about the solder joints too.

As for draining/refilling, I've never done this before - how difficult or time consuming is it? We were already thinking about getting a powerflush when we can get a heating engineer to do a service so if it's likely that it doesn't help that much would it be better just going straight for that instead of spending what I imagine is several hours on two days to drain/refill twice?
 
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Get a proper independent tech in.
Yeah, probably will do. Just thought I'd see if it was something I could have sorted by myself after the bloke who said he'd come round last week didn't turn up or ring up!
 

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