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Point taken Tony.....i won't be doing anything with acids, even though they used to let us play with Conc. Nitric, HCL, Sulphuric and all the nasty caustic stuff in the school labs many years ago. Acid burns in the lab coat were almost a badge of honour. As for whether Dad has seen the TV...

[The much earlier posts have demonstrated the usual DIY misunderstanding of just what X800 does not do. To dissolve dirt in an HE, an acid is needed. This can be citric or HCl but as always acids need special care and experience of handling and use. Tony If you want to cast...

I wish you had seen it earlier too! :) A pump doesn't cost as much as a heat exchanger :( Vaillant technical were helpful but not particularly useful...i made a number of phone calls and sent emails.....the best one was the call i had advising me to change the plate heat exchanger....this...

Thanks Bunnyman, interesting, where is the problem normally, is it with the pump or the electronic control (the dreaded pcb!). I had the pump set to 100% until my GasSafe bloke came in last week, he reset it to Auto... The pump does seem to be running ok....do you have any suggestions...

To answer the question from Dannibob 1. The filter is in a T shaped brass junction that is in the lower third of the boiler in the middle of the boiler towards the back, just to the left of the Diverter valve as you look at it head on. With pipes to DHW Plate Heat Exchanger and i think the...

Having flushed, backflushed, reflushed the system and the boiler using mains pressure water (see FAQ) before fitting the new heat exchanger and seeing nothing but clean water going down the drain I dosed the system again with X800 and then left the boiler running overnight on saturday heating...

Thank you for the cleaning under pressure tip...i'm thinking that the primary heat exchanger has had it. I have tried mains pressure backflush, forward flush (as described in FAQ), X800 at high concentration, system running all day at full blast, max pump speed selected and the nearest rad...

:( i sent Vaillant technical an email with all of the information posted so far on this topic... received a 'phone call from Vaillant :o and was assured that it was 'a blockage' and that the secondary heat exchanger was at fault. Ordered new heat exchanger, fitted it today and at the same...

No need to apologise Oldun; your helpful comments and interest in my problem is much appreciated, as are those of the other respondents. Many thanks Phil G.

SE is suggesting underpinning.....or leaving the slab in place and advising my missus that UFH isn't an option....which I am working up to! :wink: I haven't asked the SE the question about max bay width because I had hoped that a helpful builder might have an opinion, given that SE's do the...

Hi Oldun, thank you for your reply and for your interesting comments, i have already made two inspection holes and fortunately? (or not) what you describe is not the case. There are no foundations under the internal walls, they are built straight off the 4" concrete slab which is resting on...

Beware screed applied directly to foil faced celotex/kingspan; two independent stories of screed attacking the foil and insulation turning to mush as a result. My UFH installer/poured floor screeder insists on installing a membrane over the insulation before fixing pipes and then pouring screed...

To achieve the necessary depth for underfloor heating & insulation we have to remove a 4 inch concrete slab and replace it.......sounds simple until you discover as we have that all internal walls of this late '60s house are built straight off the slab. Structural engineer describes this as...

Dave thank you for your reply; no there is no grumbling or groaning on full power.... What would you expect flow and return temps to be? on full power the flow temp rises to 75dg very quickly (boiler flow temp max default setting is 75dg). Pump and fans v. obviously kick in....then it goes...

Local merchants had special offer of £20 for 1ltr X800 and 1ltr X100, so not too bad...but not too much muck came out so I'll give the old HCL a try, nothing to lose but £2.50 + delivery. Thanks for the tip :) (p.s. any chance of a link to a suitable HCL supplier; struggling to find anything...

Thank you for the tip Tony. I too wonder whether i have been successful in cleaning either of the heat exchangers! If one or both of them is blocked (which i accept is highly likely) there is apparently no way of really knowing short of cutting them open and having a look or, as you kindly...

constants: Cold water in at 18dg (such a lovely warm day!) Flow rate (approx 14 or 15 litres per minute) (boiler rated at 14.5 l/m) As tap opened.. d40 flow temp rises to 75dg d41 return temp didn't seem to rise above 46dg (difficult to catch this when running between boiler and...

Thanks for the reply Mick, X800 at 10% solution (ie 10ml X800 per 100ml boiling water), left to soak for 4 hours...then flushed with hose pipe maybe something a little stronger will do the trick...? :) (what is so odd is that the HW system has only run for 6 hours in total as revealed...

Took all the water system apart, cleaned primary and secondary heat exchanger, black hose, bypass valve, diverter valve, pump...reassembled with new fibre washers and o rings. Changed DHW NTC Thermistor because its range was slightly out. Symptoms remain...Boiler fires on DHW demand, gets...

Thank you gordongas, where did you find an ultrasonic cleaner big enough for the job?