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combi boiler questions



 
 
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  #1  
Old April 2nd 06, 05:13 PM posted to uk.d-i-y
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Default combi boiler questions

hello everyone

I am new member here, so please excuse me if some of my questions were
answered before or just stupid. 8) reading the archive on internet and
googling did not help much.

I have a combi bolier (modena 102) that keeps giving me grief. it is
getting worse - just a couple years ago it worked fine. the main problem
is that it keeps cycling, and it does not seem to do it right - water
temperature of the shower keeps fluctuating from really cold to very
hot. I tried to eliminate this problem by installing a new shower head
with better throughput, but it did not help. I don't think the flow is
to blame. the same is happening with hot taps everywhere in the house.

the service manual for the boiler states that it should modulate the
flame and I don't see that happening at all. I inspected the flow meter
visually and cleaned the inlet filter. the output HW temperature sensor
seems okay as well (~4KOhm). I may be wrong, but it seems like the HW
running is just too hot and that boiler's HW temperature regulator does
not work. perhaps the limit thermostat is cutting everything off to
protect the boiler.

is it possible that the gas pressure is to blame ? I can't check the
flow meter, since I have no oscilloscope.

any ideas that I can try before calling for professional to rip me off ?
I am pretty confident he'll say that I need a new pump. 8)

thanks very much in advance.

/max
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  #2  
Old April 2nd 06, 07:57 PM posted to uk.d-i-y
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Default combi boiler questions

Try an easy thing and renew the hot water sensor. It will not cost much and
is easy to do. The sensor is a common fault on these boilers

"maxim naumov" wrote in message
...
hello everyone

I am new member here, so please excuse me if some of my questions were
answered before or just stupid. 8) reading the archive on internet and
googling did not help much.

I have a combi bolier (modena 102) that keeps giving me grief. it is
getting worse - just a couple years ago it worked fine. the main problem
is that it keeps cycling, and it does not seem to do it right - water
temperature of the shower keeps fluctuating from really cold to very hot.
I tried to eliminate this problem by installing a new shower head with
better throughput, but it did not help. I don't think the flow is to
blame. the same is happening with hot taps everywhere in the house.

the service manual for the boiler states that it should modulate the flame
and I don't see that happening at all. I inspected the flow meter visually
and cleaned the inlet filter. the output HW temperature sensor seems okay
as well (~4KOhm). I may be wrong, but it seems like the HW running is just
too hot and that boiler's HW temperature regulator does not work. perhaps
the limit thermostat is cutting everything off to protect the boiler.

is it possible that the gas pressure is to blame ? I can't check the flow
meter, since I have no oscilloscope.

any ideas that I can try before calling for professional to rip me off ? I
am pretty confident he'll say that I need a new pump. 8)

thanks very much in advance.

/max



  #3  
Old April 2nd 06, 11:20 PM posted to uk.d-i-y
external usenet poster
 
Posts: n/a
Default combi boiler questions

On Sun, 02 Apr 2006 15:13:57 +0000, maxim naumov wrote:

hello everyone

I am new member here, so please excuse me if some of my questions were
answered before or just stupid. 8) reading the archive on internet and
googling did not help much.

I have a combi bolier (modena 102) that keeps giving me grief. it is
getting worse - just a couple years ago it worked fine. the main problem
is that it keeps cycling, and it does not seem to do it right - water
temperature of the shower keeps fluctuating from really cold to very
hot. I tried to eliminate this problem by installing a new shower head
with better throughput, but it did not help. I don't think the flow is
to blame. the same is happening with hot taps everywhere in the house.

the service manual for the boiler states that it should modulate the
flame and I don't see that happening at all. I inspected the flow meter
visually and cleaned the inlet filter. the output HW temperature sensor
seems okay as well (~4KOhm). I may be wrong, but it seems like the HW
running is just too hot and that boiler's HW temperature regulator does
not work. perhaps the limit thermostat is cutting everything off to
protect the boiler.

is it possible that the gas pressure is to blame ? I can't check the
flow meter, since I have no oscilloscope.

any ideas that I can try before calling for professional to rip me off ?
I am pretty confident he'll say that I need a new pump. 8)

thanks very much in advance.


It could be a combination of scaling in the DHW heat echanger and/or the
modulation failing.
It can be quite hard to see the modulation on the gas flames the real
test is to measure the burner pressure and see it varies within the limits
specified in the book.

To be sure measuring the flow rate and temperture rise.
Check the gas rate (see the Gas Fitting FAQ) all these test don't require
very expensive tools.

Why do you think that the fitter will recommend a pump - especially as the
pump has zero to do with the DHW on a Ferroli Moderna?

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html


  #4  
Old April 3rd 06, 07:46 PM posted to uk.d-i-y
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Posts: n/a
Default combi boiler questions

Ed,

thank you very much for your reply.

Ed Sirett wrote:

It could be a combination of scaling in the DHW heat echanger and/or the
modulation failing.


how can scaling cause this ?
I saw some "descaler" products being sold -- do they really work ?

It can be quite hard to see the modulation on the gas flames the real
test is to measure the burner pressure and see it varies within the limits
specified in the book.


afaiu, I need a way of measuring the actual burner pressure.

Check the gas rate (see the Gas Fitting FAQ) all these test don't require
very expensive tools.


thank you, I'll read up on this.
is it common for gas valves to fail to modulate properly ?

I checked overheating thermostat -- it does not seem to be causing the
problem. the same for air pressure switch. voltage drop on the DHW
sensor seems to vary according to temperature.

I am completely lost now. I ordered the sensor though.

Why do you think that the fitter will recommend a pump - especially as the
pump has zero to do with the DHW on a Ferroli Moderna?


perhaps because he happened to have one in his boot ? 8) replacing a £5
thermistor is not that attractive as replacing a £200 pump, especially
if he got it from another unsuspecting customer. 8)

of course pump has nothing to do with DHW circuit, I was just trying to
make an example with something completely irrelevant.

/max
  #5  
Old April 4th 06, 01:25 AM posted to uk.d-i-y
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Default combi boiler questions

On Mon, 03 Apr 2006 17:46:40 +0000, maxim naumov wrote:

Ed,

thank you very much for your reply.

Ed Sirett wrote:

It could be a combination of scaling in the DHW heat echanger and/or the
modulation failing.


how can scaling cause this ?
I saw some "descaler" products being sold -- do they really work ?


The heat transfer is so restricted by the chalk that the boiler cylces
between minimum and off gas rates.
The problem is to get the descaler into the pipework.


It can be quite hard to see the modulation on the gas flames the real
test is to measure the burner pressure and see it varies within the limits
specified in the book.


afaiu, I need a way of measuring the actual burner pressure.


You will need to make or buy a manometer & tubing, and leaks detector
spray to check the nipple is closed afterwards.


Check the gas rate (see the Gas Fitting FAQ) all these test don't require
very expensive tools.


thank you, I'll read up on this.
is it common for gas valves to fail to modulate properly ?

IME this fault is not common and would be at least as likely to be PCB
driving it as the gas valve itself.


I checked overheating thermostat -- it does not seem to be causing the
problem. the same for air pressure switch. voltage drop on the DHW
sensor seems to vary according to temperature.

I am completely lost now.

That's why you need help.

I ordered the sensor though.
Why?


Why do you think that the fitter will recommend a pump - especially as the
pump has zero to do with the DHW on a Ferroli Moderna?


perhaps because he happened to have one in his boot ? 8) replacing a £5
thermistor is not that attractive as replacing a £200 pump, especially
if he got it from another unsuspecting customer. 8)

In this type of job you are paying mostly for someone to make time to see
you. Then you are paying for the time they are there and finally you pay
for the parts. Selling parts that are not required at prices that are
highly inflated is not a good strategy for building up a base of loyal
customers.


of course pump has nothing to do with DHW circuit,

....on a Ferroli but on most other makes it has a lot to do with it.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html


  #6  
Old April 5th 06, 12:16 AM posted to uk.d-i-y
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Default combi boiler questions

Ed Sirett wrote:

The heat transfer is so restricted by the chalk that the boiler cylces
between minimum and off gas rates.


don't understand. surely this means that the heat is just lost into the
flue and that water temperature in the exchanger would be lower ? I see
something quite different -- the water is quite hot.

The problem is to get the descaler into the pipework.


how is this generally done ?

IME this fault is not common and would be at least as likely to be PCB
driving it as the gas valve itself.


yes, I'd think so. I'll try to check the driving signal.

That's why you need help.


yeah, maybe it is time to call for professional. 8)

Why?


the old sensors looked dodgy. the DHW one is fit loosely in it's casing
and I suspect the internal contact may be bad. I bent the pins and
rotated the internal plastic part -- it helped. it does not cycle that
often anymore.

for the parts. Selling parts that are not required at prices that are
highly inflated is not a good strategy for building up a base of loyal
customers.


of course you are right. perhaps I watch TV too much. 8)

...on a Ferroli but on most other makes it has a lot to do with it.


to maintain DHW's constant flow ? seems unnecessary for common water
mains supply, where pressure is always quite high.

thanks for your explanations, Ed.

/max
  #7  
Old April 5th 06, 09:09 PM posted to uk.d-i-y
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Posts: n/a
Default combi boiler questions

On Tue, 04 Apr 2006 22:16:14 +0000, maxim naumov wrote:

Ed Sirett wrote:

The heat transfer is so restricted by the chalk that the boiler cylces
between minimum and off gas rates.


don't understand. surely this means that the heat is just lost into the
flue and that water temperature in the exchanger would be lower ? I see
something quite different -- the water is quite hot.


OK you may simply not have the required flow rate of DHW. since you
haven't reported back the flow rates. I haven't got a clue.


IME this fault is not common and would be at least as likely to be PCB
driving it as the gas valve itself.


yes, I'd think so. I'll try to check the driving signal.


Typically the modulator is driven with 0-9Vdc.


That's why you need help.


yeah, maybe it is time to call for professional. 8)


the old sensors looked dodgy. the DHW one is fit loosely in it's casing
and I suspect the internal contact may be bad. I bent the pins and
rotated the internal plastic part -- it helped. it does not cycle that
often anymore.

OK.

...on a Ferroli but on most other makes it has a lot to do with it.


to maintain DHW's constant flow ? seems unnecessary for common water
mains supply, where pressure is always quite high.

No on most combis the pump move the primary water between the primary and
secondary heat exchangers. ON a Ferroli there are two primary heat
exchangers.

In neither case does the pump move the DHW which would be unnecessary,
bad for the pump and illegal.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html


  #8  
Old April 13th 06, 01:00 AM posted to uk.d-i-y
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Posts: n/a
Default combi boiler questions

Ed,

replacing the DHW temperature sensor helped !

the original sensor still had variable voltage drop on it, but I think
the problem was as follows: the sensor consists of several parts. one is
the external housing, made of metal. inside there is the actual sensor
and leads, kept in place by some sort of plastic or epoxy.

___-__
+----+\_____
====|XXXX! _____| this side is immersed in water
+----+/
---_--

I noticed that with my old sensor, the internal part was a bit loose in
the housing (epoxy shrunk ?), which was perhaps causing bad heat
transfer to the sensor. maybe the semiconductor has degraded with age, I
don't know. the sensor was testing ok with an ohmmeter.

now after replacing it I can enjoy shower without boiler cycling, given
that the mains pressure is sufficient. most of the time it is. when it
is not, I just feel that the flow is weak.

thanks for your replies !

/max
 




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