This will be quite a long post, so make yourself comfortable... 
I have a Vokera Maxin 28e combi installed. It is about 10 years old and just recently it has been only delivering intermittent hot water. This mostly showed itself in the shower when it would be hot for 30 seconds - 1 minute then cold for a couple of minutes. The same pattern would be seen at the hot tap.
The first thing I did was to check the plate heat exchanger. I smacked it with a bit of wood a few times and flushed water through it and a few bits of stuff came out but nothing substantial. I put it back in place and the symptoms remained as they were.
The second thing I did was to clean out the fan. There was some loose sand-like substance on the fan blades and in the housing, and that just brushed off. It's not like it was clogged with the stuff, and I'd be surprised if there was enough to make any difference. But I cleaned it anyway.
At the same time I checked that the tubes to the air pressure switch were clear, and blowing gently through the tubes caused the switch to sound like it was opening and closing. I didn't check this electrically.
Also I cleaned the combustion chamber because there was a thin carpet of ash in there. I suspect this was the final resting place of many insects, since there's nothing to stop them flying in if the fan isn't running and the boiler is off.
This all seemed to improve things a bit, in that the hot water would stay on more reliably but it was still not right. While the shower was running, and the gas was turning on and off, I tried adjusting the minimum flow rate at the modulator coil. This might have helped, but it's hard to say. In any case, the boiler does modulate the gas supply - there is a clear difference between high and low flames.
Last night I checked the two diaphragms, and they are fine. The inlet and outlet pipes to the bigger diaphragm housing were blocked (probably 'very nearly blocked' would be more accurate') by gritty sludge, so I cleaned them out. As a result the burner microswitch now turns on much sooner than before because the push rod comes out much quicker, which is good.
But I'm still left with the DHW going cold for a time before the burner kicks in again.
When the water does run cold it is, and this sounds obvious, because the burner has been off for a while. The temperature gauge on the boiler shows the water is cold. If I'm standing at the boiler at this point I can hear clicks and such like coming from the area of the gas valve behind the modulator and the ignition control box, but the burner doesn't light at this point. It will light again at some point after, so I'm wondering if maybe the ignition circuit is a bit dodgy. (Presumably if gas is turned on but ignition doesn't happen, i.e. the temperature doesn't rise, the gas is turned off again to give the combustion chamber time to clear before trying again?)
So I'm looking for any clues as to what to look at next. All suggestions gratefully accepted.
I have a Vokera Maxin 28e combi installed. It is about 10 years old and just recently it has been only delivering intermittent hot water. This mostly showed itself in the shower when it would be hot for 30 seconds - 1 minute then cold for a couple of minutes. The same pattern would be seen at the hot tap.
The first thing I did was to check the plate heat exchanger. I smacked it with a bit of wood a few times and flushed water through it and a few bits of stuff came out but nothing substantial. I put it back in place and the symptoms remained as they were.
The second thing I did was to clean out the fan. There was some loose sand-like substance on the fan blades and in the housing, and that just brushed off. It's not like it was clogged with the stuff, and I'd be surprised if there was enough to make any difference. But I cleaned it anyway.
At the same time I checked that the tubes to the air pressure switch were clear, and blowing gently through the tubes caused the switch to sound like it was opening and closing. I didn't check this electrically.
Also I cleaned the combustion chamber because there was a thin carpet of ash in there. I suspect this was the final resting place of many insects, since there's nothing to stop them flying in if the fan isn't running and the boiler is off.
This all seemed to improve things a bit, in that the hot water would stay on more reliably but it was still not right. While the shower was running, and the gas was turning on and off, I tried adjusting the minimum flow rate at the modulator coil. This might have helped, but it's hard to say. In any case, the boiler does modulate the gas supply - there is a clear difference between high and low flames.
Last night I checked the two diaphragms, and they are fine. The inlet and outlet pipes to the bigger diaphragm housing were blocked (probably 'very nearly blocked' would be more accurate') by gritty sludge, so I cleaned them out. As a result the burner microswitch now turns on much sooner than before because the push rod comes out much quicker, which is good.
But I'm still left with the DHW going cold for a time before the burner kicks in again.
When the water does run cold it is, and this sounds obvious, because the burner has been off for a while. The temperature gauge on the boiler shows the water is cold. If I'm standing at the boiler at this point I can hear clicks and such like coming from the area of the gas valve behind the modulator and the ignition control box, but the burner doesn't light at this point. It will light again at some point after, so I'm wondering if maybe the ignition circuit is a bit dodgy. (Presumably if gas is turned on but ignition doesn't happen, i.e. the temperature doesn't rise, the gas is turned off again to give the combustion chamber time to clear before trying again?)
So I'm looking for any clues as to what to look at next. All suggestions gratefully accepted.
