combi boiler questions

[maxim naumov]

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

[IAN CAPEL]

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

[Ed Sirett]

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?

[maxim naumov]

Ed,

thank you very much for your reply.

Ed Sirett wrote:

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

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

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.

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

[Ed Sirett]

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.

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

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

That's why you need help.

I ordered the sensor though. Why?

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.

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

[maxim naumov]

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.

how is this generally done ?

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

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.

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

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

[Ed Sirett]

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.

Typically the modulator is driven with 0-9Vdc.

OK.

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.

[maxim naumov]

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