Air in the hot water coil

[camerart]

Hi,
I have an old back boiler, which is working ok, but every couple of years, the hot water loop gets air in it and the hot water slowly stops getting hot.

Because of the original poor fitting of the heating system any bubbles have to work their way up, through the overflow pipe into the tank.

I have a car oil removal pump, that I use for pulling out the bubbles, which I did yesterday. I don't know where the air is getting in.

When I took the top off the tank, there was a thick film of jelly like gunk, that I scraped off, I presume it's inhibitor. Here's a photo:

What is the film?
Cheers, Camerart.

 

[footprints]

Cannot see it usually it is bacterial growth often caused by addatives.

 

[camerart]

Cannot see it usually it is bacterial growth often caused by addatives.

Hi F,
I don't think it is hard enough to block the pump, so I assume that as most of it is out, things are ok?
Do you think the tank needs more additive?
C.

 

[JohnD]

Scoop out the tank into a bucket, sponge the tank clean, wipe the tank and lid and valves etc inside with bleach to temporarily reduce the spores, drain and rinse the system, and give it a chemical clean, on final refill add inhibitor and a bottle of Fernox Biocide AF10 which will prevent future growth until it gets too diluted. The Biocide you may have to order from a plumbers merchant as it is not widely stocked.

https://www.wolseley.co.uk/product/...MI9sqf0czv6QIVD-ztCh3LnAH5EAAYASAAEgKMkPD_BwE

 

[footprints]

As JohnD says above.
I cannot figure how you are using an oil removal pump to clear air from the hot water coil
A photo of the pipe set up going into the cylinder might help, are you sure you do not have a primatic cylinder? If so additives are not a good idea.