Vaillant Ecomax 835 Hot Water Piping hot then luke warm

[guitarbonz]

Any tips gratefully received that might help to diagnose my Vaillant 835 ecomax DHW problem.

Symptoms:

Turn on hot tap, boiler fires up, get piping hot water at 60 degrees (as set by the DHW dial).

1. Boiler temp hits 75 degrees (Vaillant default) and boiler appears to scale back its activity
2. burner indicator on LCD display still on
3. burner still going but at what appears to be a reduced level
4. temp of water at tap begins to drop
5. boiler flow and return temps begins to drop...and keeps dropping, boiler doesn't ramp up to previous levels of activity
6. boiler temperature just keeps falling back...to around 40 degrees...with temperature at tap dropping along with it.

At this point I turn the tap off!


Action so far:

a. Measured water flow rate: 14litres per minute.

b. Disabled the warmstart option by turning DHW control to left and then setting DHW temp to 60 degrees. (No 'C' shown on LCD display)

c. Cleaned and descaled Heat Exchanger, which was not too bad...

d. Tried to work out if diverter valve might be to blame....flow pipe near boiler gets warmer but that could just be conductivity of copper....flow pipe 2M from boiler still cold.....so perhaps it's not the diverter.


Other information:

The boiler has been driving an S-plan system for CH and HW, the DHW from the boiler has not been used since boiler installed 6 years ago, i want to feed HW tap in new utility room from boiler HW rather than cylinder HW so this is first time it has been used; so it could be a sticking under-used and neglected diverter valve.....or it could be something else....i am hoping that you helpful types might be able to point me in the right direction.


Many thanks in anticipation...

Phil G.

 

[guitarbonz]

Slightly baffled after conversation with Vaillant tech support, i checked out the following.

When tap initially turned on boiler goes to full burn, flow pipe very hot to the touch (cannot touch!), water from tap very hot (60degrees) then boiler switches to slow burn, flow pipe gradually cools (presumably as result of cooler water returning from DHW heat exchanger), boiler continues on slow burn doesn't fire back up to full burn. (perhaps it is not supposed to?)

Would this indicate that thermistor (NTC?) is not detecting drop in temperature of DHW as it leaves the heat exchanger (HEX) and heads for the tap?

or is it as Vaillant tech support insisted today that the HEX has had it and needs to be replaced.

I'll check thermistor values; Vaillant say that they should be 3k at 20C and 250ohms at 90C....if outside this range an f1, f2 or f10 fault would be reported...but maybe not...
(Vaillant technical expected thermistor values at different temps: 20C = 3K, 30C = 2.2K, 50C = 1K, 90C = 250ohm)
(I tested thermistors by unscrewing from boiler, and placing the threaded part into a bath of water, using thermometer i checked the temp. and used a multimeter to measure resistance.)

Vaillant tech support guy was very knowledgeable, very patient, advised that he receives calls about this subject between 6-10 times per day...and "it's always the secondary heat exchanger", he is probably right....2000 calls per year does tend to support his argument

 

[cross thread]

sorted a few with same problem,cleaned main heat ex and secondry also repla ed black hose,cleaned using a sonic cleaner ,

 

[guitarbonz]

I have just realised that I have broken my own rules and used a TLA (Three Letter Acronym) that i don't know the meaning of in my posts.

Could anybody explain what NTC stands for? Wikipedia has a number of stabs at it, but none of them relate to boilers.

i found the answer to my own question on wikipedia:
Thermistors can be classified into two types. If the resistance increases with increasing temperature the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If the resistance decreases with increasing temperature the device is called a negative temperature coefficient (NTC) thermistor

 

[guitarbonz]

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

 

[guitarbonz]

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 to temp (75dg), v hot (60dg) water flows out of tap, boiler drops back to low burn, flow temp drops....and keeps dropping...consequently DHW temp drops.

The burner does not appear to be turning up again to reheat the boiler water, my assumption is that as flow temp drops below DHW temp that the boiler should pump more heat into the system (ie burn more gas) so that the flow temp rises and the DHW can be heated to 60dg. Is this assumption correct...it is based on schoolboy physics, ie heat in = heat out + any losses.

Flow and return DTCs are in range...

Please can anyone help me to shed any light on this before i shell out £150 to replace the heat exchanger (as advised by Vaillant).

Many thanks
Phil G

 

[twgas]

How did you clean the h/ex etc, just washing it through is not usually good enough.

 

[guitarbonz]

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 by boiler diagnostics so why would little used Hex gunge up?. (our DHW is normally supplied from unvented cylinder, that is heated off the CH circuit using S-plan..

Struggling to understand how putting water (at 50dg) into a h/ex can possibly heat fast flowing mains temp cold water to 60dg, surely the boiler should be sending water down the flow pipe at 60dg minimum? (Or is my lack of relevant knowledge a dangerous thing?

 

[gas4you]

You need to see what d40 & d41 read whilst it is faulting.

 

[Agile]

You need an acid to clean the plate which will actually dissolde the dirt.

One wonders how you think you cleaned the main heat exchanger!

Tony

 

[guitarbonz]

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 tap/pressing buttons etc)

Output temp of water at tap 58dg

boiler drops to low burn..

and gradually the temps hit a low point..

temp at tap reduces to 32dg
d40 flow temp 42dg
d41 return temp 31dg

(so a 14dg rise at tap with a 11dg drop on boiler - Newton was wrong! or my measurements are slightly out.)

Meter readings (last 3 digits metric meter) taken at 30 second intervals during slow burn phase:
start: 985
after 30 secs: 995

= 020 per minute on gas meter

Reducing HW flow rate to trickle by turning HW tap down gradually increases temp of HW at the tap

 

[guitarbonz]

You need an acid to clean the plate which will actually dissolde the dirt.

One wonders how you think you cleaned the main heat exchanger!

Tony

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 suggest, by applying some serious chemicals and listening for fizzing noises.

A long soak in X800 and then a flush with the garden hose was probably optimistic of me, but from my point of view it was worth a try.

 

[Agile]

Also expensive at about £17 for X800.

Whereas HCl is under £2.50 on the secondary shopping areas.

Tony

 

[guitarbonz]

Also expensive at about £17 for X800.

Whereas HCl is under £2.50 on the secondary shopping areas.

Tony

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 smaller than a 25 Litre drum with google search.)

 

[gas4you]

With those readings it certainly points to a blockage.

When on full power, is the boiler rumbling or groaning?