Oh well,
Sometimes too much information causes just troubles: I went to a local DIY store to buy phosphoric acid. As I spoke to the sales rep I was informed that if the material in the Volvo Penta heat exchanger is aluminium and copper I should use oxalic acid instead of phosphoric acid.
The reason is, according to the person I spoke to, precious metals such as copper and aluminium will be damaged by phosphoric acid. Hence his recommendation for oxalic acid.
Now this is confusing as we know BB concentrate contains phosphoric acid but we do not now what else is in the mixture. Maybe they have added some chemical to prevent the phosphoric acid to ruin the aluminium and copper of the heat exchanger?
Conclusion: 10% oxalic acid seems to be a very safe choice to clean heat exchangers but do we have any real life experience wether it does the job or not?
Some info I found congerning the appropriate dilution proportions:
8.7 Cleaning in place - SWEP
"...The choice of cleaning solution depends on the problem, but a weak acid is a good start. This could be 5% phosphoric acid or, if the exchanger is cleaned frequently, 5% oxalic acid. The cleaning liquid should be pumped through the exchanger. For optimal cleaning, the flow rate of the cleaning solution should be at least 1.5 times the normal flow rate. Preferably, the flow should be in a back flush mode, which has a better chance of dissolving the scale because it attacks the deposits from the opposite direction.
After cleaning, the heat exchanger should be rinsed carefully with clean water. A solution of 1-2% sodium hydroxide (NaOH) or sodium bicarbonate (NaHCO3) before the last rinse ensures that all acid is neutralized. One way to get an indication of the appropriate rinse time is to test the pH of the liquid at the outlet from the heat exchanger. A quick and easy method is to use litmus paper. The pH should be 6-9."
and:
https://service.daikin.com/newweb/air_conditioner/Members/tech/news/PDF/E-06033.pdf
"[Correct cleaning method]
1. Clean the heat exchanger with the following cleaning agent according to the cleaning procedure.
Cleaning agent to remove scales.
Prepare a solution made from a 5% diluted acid, such as formic acid, citric acid, oxalic acid, acetic
acid, or phosphoric acid. Do not use any of them at a dilution of 10% or over. Fill the heat
exchanger with the solution at 50°C to 60°C once. Then circulate the solution with a pump for a
maximum of approximately two hours. The cleaning period may vary depending on the removal of
dirt and cleaning condition of the hydrothermal exchanger.
Cleaning agent to remove oil and organic fouling
Prepare a solution made from a 3% caustic soda. Do not use any of them at a dilution of 4% or
over. Fill the heat exchanger with the solution at 50°C to 60°C once. Then circulate the solution
with a pump for a maximum of approximately two hours. The cleaning period may vary depending
on the removal of dirt and cleaning condition of the heat exchanger.
2. Neutralization
After cleaning the heat exchanger, circulate a 1% to 2% sodium hydroxide or bicarbonate of soda for
15 to 20 minutes for neutralization.
3. Rinsing
Sufficiently rinse the interior of the water heat exchanger with clean water after the above
neutralization work. "