Ferroli Modena 80 fluctuating DHW

[HB79]

Hi, I have searched and read the many posts about similar problems with the Modena, however I believe my case is slightly different:

CH is OK, works fine.
DHW downstairs in the kitchen is OK, full time hot water.
DHW upstairs in the bathroom (bath) fluctuates hot & cold.

When the bath hot water is fluctuating, the boiler is not resetting, the overheat and max temp thermistors are OK.

I have had the boiler serviced by a heating engineer (2 visits) and between us (I'm an electrical engineer) we have serviced it, checked the flue and adapter plate, removed & checked the fan and venturi, checked the heat exchanger. We have tested the pressure switch, checked gas pressure. The flame detect electrode wasn't so good so we replaced that as a precaution.

Interestingly, the boiler runs fine with the front cover removed. I haven't replaced the DHW temp sensor yet (have one on order), but I'm not convinced this is the problem as the hot water downstairs works OK and the flame modulates.

So, as you can see, its been looked at by a pro (twice), I've changed a few cheap bits but thought I'd post on here for any last ditch suggestions before I look into getting a new one! Please help as the new ones don't sound much more reliable

Thanks in advance!

 

[BingoBongo]

is it only the bath tap?

run a sink tap for a good 10mins if its ok run the bath SLOWER as its the same hw

is the flame cutting out when its going cold?
has the engineer checked the flow switch?

with regards to the case it points to venturi or Modena`s are common for white dusty crap to cling to fan blades.Small paint brush or 1 of those for eggwash i had to use recently in a womans house.

The modena is a decent boiler and 1 of the easiest to work on so id keep it

 

[picasso]

dont think your boiler is faulty, it sounds like a non return valve is passing on your shower (I hope you have got a shower) .

 

[BingoBongo]

just re read your post

run the bath slower/a lot slower if needbe

and you shouldnt be opening the casing any way

badman

 

[gaspro1981]

something in your post doesn't quite make sense

what do you mean about the boiler operating ok with the cover off

 

[HB79]

Hi Guys, thanks for the replies so far. Firstly and foremost I am not touching any gas/combustion parts of the boiler - my heating engineer is doing the work, I am just interested in helping him to get it working. I am a electronics engineer so I don't mind changing a couple of temperature sensors...

The front cover was removed when he replaced the flame electrode, he tested it before putting it back together and the boiler ran fine during this test (this was also to see if there is a possible air starvation problem from the flue intake).

The shower is brand new mixer which is spec'd for combis (although I haven't been able to use it yet due to the fluctuating HW!) so am relying on baths as the mean temperature is OK.

I have tried running the bath at lower flow rates but it didn't seem to help - I am going to do a proper flow test tonight upstairs vs. downstairs.

The flame does not go out when it goes cold. He didn't check the flow switch (suggestions how to test?). We saw the white dusty stuff you mention and hoovered it all out - yes I'd like to fix & keep it!

 

[HB79]

Correction to my last post, the flame is going out when the water is going cold - I was getting duff info from my wife downstairs....

 

[numpty1]

remove one of the rubber bungs on the combustion chamber lhs one is best and try it.run the bath and see if the problem persists.
if it does replace venturi and the lower connection.are you sure the min gas pressure is correctly set?

 

[bellairaphon]

i,d be changing the venturi. even tho it looks ok i would buy a replacement then remove the fan and give it a little squeeze me reckons it will crumble into little bits

 

[HB79]

Hi numpty1, thanks. I just removed the lhs (flue) bung, the problem still persists and I would say its worse with the bung removed (it shuts down quicker/does run as long).

I have done some tests on the DHW & CH sensors and the resistance vs. temp profile matches the spec in the manual. I swapped the wires over to double check.

I'm coming round to thinking its something to do with the venturi/flue as you are suggesting. Once the dhw flow sensor clicks & tap LED comes on you can hear the fan start up but it runs for a long time (say 15-20 secs) before the burner ignites and the boiler fires.

I also suspect the restrictor plate in the flue as originally the boiler was fitted with an up & rearwards flue which at some point was changed to an upright flue directly through the flat roof above (both concentric). The upright flue is ~1m long at which point the manual recommends removing the restrictor. I will get my heating engineer back to check this and the min pressure. What do you mean by 'lower connection'? I'd just like to try and home in on the problem as far as possible before handing over to the engineer! Thanks again.

 

[numpty1]

replace the bung and remove the restrictor,its sole purpose was to lower emmissions for the uk market.the venturi has two connections one red or grey upper and a lower one white or red.

 

[Agile]

I'd just like to try and home in on the problem as far as possible before handing over to the engineer!

Thanks again.

It sounds as if the engineer is not up to scratch!

Its you who choose who to call !

Tony