Over the years, due to the non barrier PEX underfloor heating installed by the previous owner, the ancient cast iron boiler has turned into rust and sludge which has deposited inside the UFH loops... time to clean the mess.

This contraption mixes water with compressed air and injects it:

The water inlet is taken before the house's pressure regulator, so it comes in at 7 bar, which compensates for the pressure drop in the red PEX pipe. With only 3 bar supply it would have been necessary to use 20 or 25mm pipe to get proper flow.

It is followed by a 4-way valve to allow flow in both directions.

Besides the compressor (which I already had) and the flex hoses (which came out of the old heating system) this is not expensive to implement. Probably less than €100.

It worked wonderfully. It pulled a ton of crap from the loops.

Compressed air helps a lot. I rinsed with with water until it came out clear, then turned on the air: brown stuff immediately came out, and kept coming out. I let it run for several hours, reversing flow direction every five minutes, one UFH loop at a time.

Air bubbles create a sort of water hammer effect which works really well for dislodging crap. Also air flows through pipes easier than water, so an air/water mix can flow at surprisingly high speeds through UFH loops, several meters per second at least. I used a feed pressure of 2.5 bar.

Drain pipe must be firmly secured. It did its best to get out of the washing machine U-bend.

Once this is done, it's time for chemicals.

I got a 200l polyethylene bin from a local farmer, and reused the filter and pump from the old groundwater heat pump system. The previous contraption is now partially disassembled, keeping only the air injector.

UFH loops are flushed with compressed air, then the bin is first filled with Sentinel X-800 cleaning agent diluted to spec in 40°C water, then the pump turned on. Again, reverse flow every five minutes and do each UFH loop in turn.

Feed pressure at the pump outlet is 3-4 bar depending on length of UFH loop being cleaned, flow is 2-3 m3/h water plus air, so velocity in the UFH loops should reach 5-10 m/s. The whole thing is shaking and kicking.

Water came out clear at the previous step, but the cleaning agent still pulled a lot of crap out of the pipes.

These flakes are quite large (the fitting is 1" for scale), I wonder how that got through the circulator...  Well, it died, maybe that has something to do with it...

This is what the filter caught:

...I only put the filter in line when I switched from plain water to Sentinel cleaning agent, so who knows how much of this stuff got flushed into the sewer.

Some of these flakes are large enough to match the internal diameter of the UFH pipes, I guess the compressed air helped break them up.

Next, I rinsed with clean water plus air.

The reverse osmosis rig is currently filling the big blue tub with demineralized water, so tomorrow I'll pour some inhibitor in it and use the pump to flush and fill the loops.

Conclusion: I'm very happy with this DIY powerflushing stuff. It makes a mess of course, but I save 5-800€ per manifold, and I have 3 of these manifolds. Not bad for assembling a bunch of fittings, setting a timer to switch valves every five minutes, and mopping the floor afterwards.