combi-boiler behaviour

hello everyone

I have been struggling with this combi since I moved into the flat. first, the shower would periodically go cold due to boiler cycling. the lower the mains pressure, the more often/likely it will go off and then on. very annoying. I "fixed" the problem by buying a shower head with a better throughput. with the new head it would only cycle if the shower was set too weak/cold or the mains pressure was too low.

recently I started renovating the bathroom and installed a thermostatic valve with divertor (hudson reed). with the valve in place, boiler keeps cycling and although the thermostatic valve does its best (?), the temperature fluctuates a bit, around 1.5C colder shortly after boiler has cut off.

today I did some experiments on the whole setup. with the kitchen tap fully open on hot -- no cycling. temperature ~60C. with shower AND kitchen tap running -- no cycling. cycling only starts when I consume less hot water. I made some measurements and from the shower hose, with thermostatic valve set to 37C, an 11-litre bucket gets filled with 64 seconds, no cycling. with a good shower head, in 78 seconds, cycling.

AFAICS, with figures like that I cannot be suffering from a low mains pressure. it looks like my boiler needs good flow of hot water in order not to cycle.

the boiler spec says that the domestic water production at delta 30C is

14.3liters per minute and 12.3 liters per minute for delta 35C. it doesn't specify the minimum flow required. what is also interesing, is that the boiler's panel has a knob for adjusting the DHW temperature. when I turn it to minimum, the hot DHW output from kitchen tap is 62C, on maximum it is 66C (both for high flow). not that much of a difference. also, I don't see flame modulating at all (through a window). I measured hot only from a kitchen tap and got the following data: 11l bucket in 1:47 (no cycling), the same bucket with lower flow in 2:50, no cycling but the temperature was 78C (!!!).

so now I am completely puzzled, whether the boiler is operating incorrectly, or my installation is incorrect. but recalling that I always had problems with DHW temperature, I am not sure about the latter. to get sensible temperature from a kitchen tap I have to open it to full, which is very difficult to use, water splashes all over the place. if I reduce the flow, boiler starts cycling.

I looked at the valve, and despite 1/2" and 3/4" sizes used everywhere, on the divertor I can see that the holes are ~5mm in diameter. surely this restricts the flow.

could it be that my mains pressure is too high and I need to fit a pressure reducing valve?

my setup is as follows:

boiler: ferroli modena 102, 30KW pipework: 15mm copper, thermostatic valve ~5m from the stopcock, the same distance to the boiler.

I would be extremely helpful for any advice or analysis. I'd like to diagnose the problem myself and only then call for a boiler specialist once the boiler is identified as a culprit. maybe I could take some additional steps to increase the flow through the hot circuit? or reduce the cold pressure?

my suspicion is that the boiler is running too hot, doesn't modulate and the lower the flow, the higher the temperature. the shower valve might be reducing the hot flow to get the temperature right, therefore boiler cuts out.

thanks very much for reading.

/max

Reply to
maxim naumov
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a follow-up with an important piece of data:

the manual says that: "maximum temperature for domestic hot water: 55C, adjustable between 40C and 55C", however, reading the same manual, on the PCB there is a jumper to set the maximum to 62C. still, nowhere close to 78C that I just got. also, around a year ago a PCB and temperature sensors were replaced.

/max

Reply to
maxim naumov

If you require hot water at a lower flow rate than the boiler can provide at max output then *either* the flame has to modulate down to a lower heat setting *or* it has to cycle on and off - the second option producing fluctuations in water temperature. Simple laws of Physics!

Is your boiler *designed* to modulate?

Reply to
Roger Mills

thanks for reading my posting carefully: %)

well, the boiler is pretty modern and when I look through the service manual, I find word "modulate" here and there, for instance: "the burner will light at ignition burner pressure, automatically range rating itself up to the heating load, then *modulating* down when the boiler reaches the desired flow temperature". also, troubleshooting charts mention checking modulation. the introduction says that one of the main components is "combination safety gas valve with *modulator*, complete with pressure stabiliser". I also see in the manual that there are four wires going to the valve and know that the solenoid is controlled with PWM.

does all this look like the boiler is designed to modulate, but doesn't do it for some reason? I still don't understand what the range of modulation is supposed to be in modern combi boilers.

thanks!

/max

Reply to
maxim naumov

In that case, the boiler is clearly intended to modulate. Even so, it only does it over a finite range. The manual should give the max and min kW outputs when modulating. This may represent something like 60% - 100% of rated output rather than 0 - 100%. So if you're actually demanding less than the lower figure, it will still need to cycle on and off.

Reply to
Roger Mills

does it look like a bucket of water of 37C is less than the lower figure? the manual doesn't say anything about the lower figure. it only mentions 2.5L minimum flow to initiate DHW production. I am certainly above that. in regards to wattage, it does have two figures for heat output (30kW and 12.7kW), but these are not annotated. just two numbers.

the reason I was asking all that here is because I was hoping that people who dealt with this before (this can't be an unique setup) would say whether this is normal or not. after all, boilers should match some general requirements. it doesn't seem to be normal for me if the kitchen tap is unusable because of cycling.

again, am I supposed to see modulation by looking at the flame through the window? why am I getting water of 78C if the manual clearly states that the maximum is 62C.

thanks for your reply.

/max

Reply to
maxim naumov

DHW at 70 degC definitely sounds wrong - and is actually dangerous! That would be ok for CH flow, but it should control the DHW down to 60 or so. Sounds like you need to get it looked at.

Does it still cycle if you turn the kitchen tap on full? If not, that indicates that, at lower flows, you are operating below the minimum output of the boiler. If it does, it suggests that modulation isn't working properly.

Does it *matter* if the kitchen tap temperature fluctuates a bit? When filling a bowl for washing up etc. isn't it only the *average* temperature which matters?

Reply to
Roger Mills

thanks, I should have done that long ago.

when the tap is on full, it doesn't cycle. when the flow/temperature are reasonable, it cycles.

it depends on how you use the kitchen tap. in most cases I don't need to fill the bowl to wash several plates/cups and use flowing water. and I am used to washing dishes in running water anyway.

with shower, it is even more important. I know that with an average shower head it would not work. it will keep cycling regardless. only with a select shower head with lots of larger holes it would not cycle. it did not, anyway, before I fitted the thermostatic valve. it seems to have restricted the flow a bit. but still, the amount of water coming through the shower head is, I would say, is above average. how do other people manage to get constant shower temperature with combis? wouldn't that mean that the boilers should work in that region of flows/temperatures?

thank you for your replies, Roger.

/max

Reply to
maxim naumov

Showers are different in that they usually mix hot and cold in whatever proportions are required to provide the desired temperature. I personally use stored hot water, and have a power shower with a manual mixing valve. Once I have adjusted the temperature, it stays pretty constant.

My only experience of a combi-powered shower was in a recent holiday let. That had a thermostatic mixing valve - which I reckon is pretty much essential with a combi so that it can adjsut the hot/cold ratio in real time to compensate for fluctuations in the combi's output temperature. That shower had one particular quirk which I didn't like, but was otherwise ok. If you turned up the temperature a bit, it suddenly got a *lot* hotter - and then settled down a bit lower once the thermostatic valve had stabilised. I don't know whether this is normal or not.

Reply to
Roger Mills

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