combi boilers and powerflushing

right a bit of background first.
6 years ago I bought a house with a recently installed combi boiler from a tight-fisted landlord.
Whilst under his ownership, he had the vented system boiler & HW cylinder ripped out and replaced with a sealed combi system boiler due to a failed gas valve. He did not bother having the existing radiators & radiator pipework power flushed even though this invalidated his new boiler warranty.
The then new boiler is a Glow Worm 30cxi.
His wife then soon divorced him and he sold his house cheaply to me as a quick sale to pay off his soon to be ex-wife as he wanted to make sure his wife got as little as possible from him.
Now under my ownership we have suffered from a particular symptom which is oscillating hot and cold water at the hot water taps, which is worst at the bath.
This is because the Domestic Hot Water Flat plate heat exchanger (DHW FPHE) within the boiler keeps on clogging up with magnetite sludge.
This is causing the boiler return water temperature to rise, reducing Delta T (the temp diff between boiler flow and boiler return) to reduce making the fan and burner switch off and then switch back on repeatedly when a Hot tap is opened for a bath.
Its normally a job I can do myself to clear out blockages but the layout of the boiler is such that you have to remove a 4 inch section of gas pipe to be able to get the DHW FPHE out for cleaning, and then once back in, reinstate the 4 inch gas pipe, test the whole house gas pipework for soundness using a U tube manometer at the gas meter.
Obviously I am not gas safe registered and if I did the work and if anything went wrong, the insurance company would refuse to accept a claim. So I get a gas Safe engineer in at 150 quid a time to clean out the DHW FPHE. this has now happened 2 times in 5 years.
I have now got a Liffe lime beater on the incoming main water and a Magnaclean on the boiler return pipe for the past 4 years to try and catch the magnetite from the rads before it gets into the boiler in order to stop a repetition of the problem.
The radiators and their associated pipework are not blocking up as obviously all pipework to and from rads are either 22mm or 15mm.
The DHW FPHE has tiny millimetre sized channels so its not surprising that it gets blocked up in preference to the radiators.
The problem has now reoccurred again for the 3rd time and I now have a tenant in this house so I need to get the problem sorted once and for all so I don't end up having to find a new tenant if the current tenant gets fed up of the problem.
Now what I want to know is if I get a power flush engineer in, will they be able to power flush the DHW FPHE in-situ by attaching their kit to the combi boiler's pump? The focus I require is on finally cleaning out the DHW FPHE as well as the rads.
If the power flusher is connected across a Rad as appears to be the more usual practice, due to the combi boiler design and its internal diverter valve, this is not going to flush the DHW FPHE satisfactorily.
If the DHW FPHE has to come out for cleaning, then I will need to find a Gas Safe registered engineer as well.
Failing that, the nuclear and the most expensive option is to rip out the 7 year old combi boiler before its natural end of life, power flush the radiators and then fit a new combi boiler which would solve the problem once and for all.
I then could clean out the DHW FPHE on the old Glow worm and flog the boiler onto Ebay perhaps to recoup some of the cost.
Comment welcome on how effective a power flusher is at actually cleaning the DHW FPHE in combi boilers.....
Regards,
Stephen.
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the power flush company I used removed our pump and put their own filtering system and pump it its place. This cleaned out the boiler as well as the radiators.
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On Tue, 18 Dec 2012 20:34:04 +0000, Stephen H wrote:

Because the PHE has multiple parallel channels it's difficult to thoroughly clean it by flushing using any technology - powerflushing machine or mains water blasted through. Paying an engineer £150 a pop to imperfectly flush out a PHE rather than sticking a new one doesn't seems good value for money in the long term.
A new PHE and a decent magnetic/cyclonic filter should do the trick.
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John Stumbles -- http://yaph.co.uk

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You can buy a filter that has large magnets that gathers most of the sludge up as water passes through it. This needs to be installed on the heating pipe immediately before the boiler. The filter has to be periodically cleaned out. http://www.britishgas.co.uk/products-and-services/boilers-and-central-heating/heating-accessories/magnetic-system-filters.html
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harry wrote:

http://www.britishgas.co.uk/products-and-services/boilers-and-central-heating/heating-accessories/magnetic-system-filters.html
Does he install this on the pipework before his magnaclean or after it?
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Oh, I missed that bit!
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On 19/12/2012 14:33, Phil L wrote:

My Magnaclean is installed on the radiator return pipe to my Combi boiler.
As mentioned, the magnaclean is the same principle as in the weblink provided.
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On 19/12/2012 08:45, harry wrote:

http://www.britishgas.co.uk/products-and-services/boilers-and-central-heating/heating-accessories/magnetic-system-filters.html
Already been there and done that.... The Magnaclean I have does the same thing as the device you provided the web link to.....
And the problem has reoccurred since.....
Thank you for your comment though....
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