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Ed Sirett
 
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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