Vaillant EcoTec937 hot water cool after 10min...design flaw?

[rahbar]

I have a 2 year old EcoTec937. If you run the shower all is well for the first 10 mins or so, then it seems as if the water in the charging tank seems to run out and the boiler does not seems to be able to keep up with recharging it quick enough so the water goes cold (not stone cold, but cool enough to be unpleasant for a morning shower!). The tank does seem to recharge quickly if you switch off the water for a few minutes - it just cannot keep up with any reasonable sustained flow of hot water.

Given the whole point of this boiler was the high water flow rate, but the apparent ease of draining the hot water with a single shower, let alone other taps, showers etc...is there a problem here? If not, what on Earth is the point of the storage tank?

 

[Agile]

Either the boiler has a fault or the shower consumption is greater than the boiler can provide. Probably the latter!

You should measure the litres per minute flow the shower is taking.

I always used to ask those who used to install them how long they would keep up a good flow for but I never had any replies.

Tony

 

[ALEC1]

what you need on this boiler is a mixing valve....so you store it at a high temperature andmix it down to a usable temperature...

it's not a design fault although it doesn't say it in the book either...

if you do that it will be fine...I pretty well guarantee it!

 

[Biggles..]

It should be fine anyway, 220 litres in 10moons and then goes back to standard 837 performance of 16 lpm whilst recharging.

Could possibly be a fault in the store,not uncommon for failed store pumps and sensors to give poor performance.

Never fitted a blender and never had bad performance.

 

[ALEC1]

is that 220 litersin the first 10 minutes?

the blender with a high storage temp makes this a phenomenal combi boiler IMHO, worth a try!

 

[Agile]

When the store is depleted it reduces to the 16 li/min but will not be able to recharge until less flow is taken.

The OP needs to give us flow information.

Tony

 

[rahbar]

Thanks for all the replies folks.
Difficult to give precise flow info on an existing shower head (it's a normal shower head, about 3 inch diameter, good flow) - I could measure the water out per minute into a bucket I guess to get a feel for the throughput, but then only a % of that will be from hot water side.
After 10 mins the water is stone cold - I have the shower full over to the hot water only by this point and the water is cold. Its as if it's not recharging properly whilst the water is running, only when the water switches off - is this possible?

@Alec1 - I don't understand; I thought the 937 had a built in blending system for the storage tank? Regardless, the shower mixer will do this job?
It should be phenomental...but in practice atm its pretty useless and worse than a cheap combi you can buy for a few hundred from screwfix.
@lcgs - My thoughts as well - I would never expect a boiler to run cold at any time, even when the tank is empty (because thats kinda pointless).

Some further background on this: We've had engineer(s) out from Vaillant for this but I'm tearing my hair out as they cannot seem to resolve it. It turns out that the water from the boiler runs through the tank even if the charging tank is turned off so the hot water gets cooled by that big slug of cold water (bizarre). Essentially the charging tank has to be left on no matter what.
We've had endless faults on this boiler (including current F75 faults every morning) so would not be surprised to see another one...

Are we still thinking my expectations are out of whack on flow or fault? Will try to get some idea of flow in the meantime!

 

[Agile]

Turn the shower control to full hot and measure into bucket of known volume! We must have that info to enable us to give any useful advice.

Also turn a hot tap on and see if the boiler fires up, keep watching for a few minutes.

The F75 fault is a common one and easily dealt with by any competent engineer.

Tony

 

[rahbar]

Ok, I get roughly 9l per minute turned to fully hot.
Boiler fires up when water is called for as expected and water temp gradually 'fades' over time. Looking at the boiler and feeling the pipe coming out I can see that the temp is bang on 63C but when coming out from the output of the boiler's storage tank it has lost quite a bit of temperature.
I can only conclude that as before the 975's storage tank is 'draining' the heat from the water as it passes through.

 

[JoeMar]

hi

...as the water temp starts going down, did you notice if the boiler was still lit up?

switch off tank charging overnight and see if you get hotwater in the morning.

 

[rahbar]

JoeMar, yes as water temp drops boiler is very much lit and actually appears to be pumping out hot water at the correct temp. The problem appears to be that the water is getting cooled by the charging tank on the way to the tap, which seems like a design flaw to me.

The charging tank used to be switched off, which made the problem worse (i.e. even less time before it ran out of steam). Vaillant engineer switched on the charging tank as the 'solution'. Clearly though, that isn't the solution...

 

[David937]

I always used to ask those who used to install them how long they would keep up a good flow for but I never had any replies.

Tony

Not that I install them..but ours does give a very high flow for the 10 minutes as specified and then it goes down a little. Temperature seems unaffected and flow is not bad at all too. We have the tanks set as low as is possible i.e. 50º and it does the disinfection run every week or so.

 

[Hufty]

The charging tank used to be switched off, which made the problem worse (i.e. even less time before it ran out of steam). Vaillant engineer switched on the charging tank as the 'solution'. Clearly though, that isn't the solution...

Do you know what the engineer actually did?
Also what timer/controller do you have for the boiler?

 

[rahbar]

David937 - that sounds exactly like I'd expect it to be Ours has the high flow rate at the start but then does not maintain it after the tank is exhausted. At least I have an idea of what 'normal' is then.

Hufty - The engineer turned on storage tank charging via the front panel (so the 'C' is lit on the main panel all the time). The time is a Vaillant digital 'timeSWITCH 140'.

 

[Agile]

If you are only using 9 li/min then the boiler can easily provide that constantly!

Does it also do the same thing at a bath tap set to 9 li/min?

Either your boiler has a fault or your shower!

Tony