Vaillant Ecomax 835 Hot Water Piping hot then luke warm

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 comparitively quiet once the initial burn is complete.

Should this boiler just have two burner settings....ie full on 37kw and 1/3rd? 12.5kw (which is approx the energy input on a 'low burn'?

Just off to see if the same happens when there is demand for CH...however during last cold spell CH was getting up to temp and staying there.
 
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:( i sent Vaillant technical an email with all of the information posted so far on this topic...

received a 'phone call from Vaillant :eek: 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 time i drained the system and fitted a Fernox TF1. (I have also cleaned the black hose, the filter, the diverter valve, the bypass valve, pump, verified that pump is rotating and soaked primary heat exchanger in X800 and flushed with mains pressure water (both ways), checked/changed NTC thermistors)

After fitting the new heat exchanger there has been a slight change in symptoms...the old symptom is still there: HW at full flow (14L/minute) comes out as before piping hot, then boiler refuses to modulate so HW from tap cools gradually, because the cold water going in cools the boiler water which isn't heated again.

I changed HW flow rate to 8L/minute and get initial v hot water AND the boiler modulated....burning more gas to keep water up to temp...but eventually it stopped modulating and HW from tap cools gradually again.

Interestingly i ran tap at half flow (say 7L/minute) and when i whack it up to full flow the boiler goes to full burn.....but eventually we get the old familiar symptom. :(

(Flow rates measured with a bucket and stopwatch)

The central heating works fine, boiler goes to full burn as required and then modulates down to low burn, with occasional boosts to keep things up to temp.

At full burn the flow to 2ndary h/ex gets v.hot so that i can't touch it, the return from the h/ex is also red hot, then burner drops and at any DHW flow rate all temperatures start to fall.

I tried calling Vaillant, but as it is saturday afternoon the hot line was cold and i gave up after 25 minutes.

If anyone out there has any bright ideas i would be most grateful to hear them. :cry: :confused: :cry:

Many thanks.

Phil G.
 
you need to get the solution completley in ,the circular design of the heat ex is good at stopping the solution
getting round .try putting the solution in under pressure so you know it is completley cleaning the inside
 
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 (imediately above boiler in bathroom) on and all other rads off. I guess the boiler pump doesn't provide enough pressure/agitation.

Last resort i guess is to borrow or hire a power flush and try that...

Thank you for your continued interest & advice Gordongas :) much appreciated.

Just out of interest what stops the boiler from modulating? I am really struggling to understand why the boiler won't burn more gas when the flow temp. drops? Or once it is up to temp. is it engineered to be capable of maintaining full HW temp at low burn?
 
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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 the nearest rad (towel rail in bathroom)

First clean of the Fernox TF1 filter on Sunday morning yielded lots of wonderful black gunge, throughout sunday and monday it produced plenty more and then out of nowhere the boiler started to modulate on HW demand......so it seems that the blockage has begun to clear....

My plan now is to continue with the hot X800 treatment for a few more days (and to flush other rads) to see if the last remaining symptom will disappear:.

Boiler will now modulate nicely if HW tap is turned on half way and then is gradually opened until it is at full flow. If HW tap is turned full on to start with then the old symptom re-appears. Any final ideas or should i continue with my slow hot clean and hope that the problem gradually fades away?


Incidentally i am very impressed with the Fernox TF1, it was easy to fit, came with valves etc, instructions advised how much pipe to cut out..(210mm for 22mm pipe), very easy to drain and flush out and ideal as a dosing point for cleaner/inhibitor. But beware the magnet which collects spanners like they are going out of fashion! :)
 
HI, I AM GOING THRO SAME EXPERIENCE, COULD YOU LET ME KNOW WHAT IF ANYTHING YOU HAVE FOUND TO DATE. I HAVE JUST CHECKED AND AM GOING TO CHANGE THE HEX SEAL ON THE MAIN BURNER. INSPECTED THIS AS SUSPECTED AIR LOCK AND ITS SHOT COMPLETELY ALL AFTER NOTICING AIR SUCTION INTAKE HAD MELTED AT ONE SIDE OF HEAT EX.
 
I KNOW, STARTED OFF WITH HOT WATER RUNNING COLD THEN HOT ALTERNATE, SO CHANGED CHEAPEST FIRST, NTC'S, TO NO EFFECT, THEN SUSPECTED DIVERTER VALVE, SAME THING NO DIFFERENCE, AT THIS TIME NOTICED NUTS WERE LOOSE ON HEAT EX BURNER, AND WHEN I DROPPED THE AIR SUCTION DOWN IT HAD BEEN MELTED WHERE THE HEAT FROM THE BURNER HAD ESCAPED COS THE BURNER SEAL HAS PERRISHED..GOT TO RESEAT THE SEAL, AND RECOMMISION, AND SEE WHAT HAPPENS. BUT IN NEED OF A SHOWER..LOL
 
Hi. 835's are prone to a large temperature difference with a small amount of system debris in the boiler. In order of probability I would look at:

Filter in flow pipe blocked, domestic plate blocked (primary side), pump impellar blocked/pump running slow, main exchanger blocked.

As previously mentioned you need something stronger than a splash of X800! Problem being the boiler needs to be run in dhw mode to pass this through the plate and filter effectively.

If you press the i button during dhw and display shows S.53 or S.54 the temperature difference is too great and the boiler will enter 'get me home' mode for 10mins and just operate on low output to prevent any damage.

In ref to the seal problems above, listen to Bunnyman, you must get this replaced by a Gas Safe engineer who has the correct equipment to check and adjust the air/gas ratio correctly.

Bunny
 
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 main H/ex. You need to undo the pipe coming in from the top, the filter is within the brass body and on my boiler was tightly fitted, had to get little finger inside to pull it out...risky because you can get your finger stuck!

2. My excellent GasSafe bloke told me that 835's are prone to leaks around the seal. Original models had rubber seal which hardens and cracks, he has seen instances where the flame gets past the seal. Get it fixed by a pro, he'll do all the proper checks and tests if he is any good!

Incidentally Vaillant issued a notice to installers advising seal replacement, new seal is 'graphite with fibres in it'.....of course my installer didn't come back to me and advise seal replacement.....in fact since the install he has been very difficult to get hold of, last seen drinking in a Brazilian bar with Lord Lucan. :) (This is one of the reasons that many of us are tempted to DIY...some of the so called professionals are anything but....whereas if you find a good one they are worth paying for and keeping in touch with.


Finally I thought i ought to close this thread off with summary of solutions.

1. Plate Heat Exchanger changed

2. Fitted Fernox TF1 filter

3. Flushed system with X800 (original installer didn't flush system after install) using mains pressure, and CH pump with Fernox filter. (Mains flush is detailed elsewhere by Chris on this forum in the FAQs section.)

4. Still slight problem of filling the bath (22mm Taps off a 15mm pipe - which can cause problem apparently), can only open taps 1/2 way or boiler doesn't modulate and water eventually comes out luke warm. (When i get time i plan to check out the Aquaflow because is suspect that this might be sticking at high flow rate.). This isn't really a problem anymore because unvented cylinder and S-System is being refitted next week after boiler was moved to its new location last week. (And all done properly by a GasSafe registered fitter.) We shall run the utility room taps and washing machine off the Boiler HW supply.

My thanks to all those Forum contributors who responded with sound helpful advice.

Can we consider this thread closed now?

Cheers
PG
 
you can close it if you want mate but this boiler has a known prob with the pump getting stuck on low speed causing the probs you indicated.. it is a variable speed pump only on this model,,this would cause your bath problem you are still getting..vaillant technical would probably not know this as they have no field experience
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