w/bosch 280 RSF hot water over heating

[podpod]

This combi keeps popping the overheat cutout when running hot water. I suspect the modureg valve may be faulty. The voltage across it drops from 20v to 3v, the resistance is 120 ohms but the gas flame does not seem to reduce and water begins to boil in the heat exchanger. Are parts still available for this combi and is the modureg (part no ZAGAS215)available separately from the gas valve?
Any help would be appreciated
Thanks
John

 

[fitz1]

dhw h x is common for scaling up on these.how old is it.

 

[ChrisR]

Parts are available.
You could check the gas rate at low modulation but it would be unusual for the burner output to stay too high..
To help diagnose the problem I'd measure a number of temperatures round the boiler/heating, gas pressures and the pump speed.
The modureg section is not separately changeable.
Should the gas valve need changing, it's a corgi job.

 

[podpod]

combi is approx 9 years old. Previously leading upto this problem when say filling a bath the burner would run for approx 2 mins then would cut off for 1 minute then would relight for another 2 mins and so on. At no time did the temprature of the water seem excessivly hot.

 

[ollski]

I would suspect the dhw hx aswell, but they also block in the primary circuit between the pump and main hx. voltages at the modureg don't tell you a great deal if you cannot correlate them with the burner pressure. The honeywell numbers for the moduregs are usually v7335A followed by a 4 digit model number.

 

[Agile]

Its odd that all the others seem to have overlooked the most likely problem.

The gas valve has two adjustments, one for the minimum gas pressure with no voltage on the mod coil and another for maximum pressure with about 20 volts on the coil.

It seems pretty obvious to me that the valve is not set up correctly. Unless you are CORGI registered then I cannot tell you how to set up the valve ( although it might be in your book!!! ).

Its amazing to me that over 50% of CORGI registered people do not know how to set up a gas valve properly! But its not part of the training or assessment even though its important when fixing boilers.

Just had a call from a friend who asked be for advice on a similar fault yesterday. He has confirmed that I was right although as he is in Ireland he will give me the full details on Monday.

Tony Glazier

 

[fitz1]

surely if it was valve set up it would have been from day one and not an occurance.faulty valve possibly, would still go for the dhx on these boilers.

 

[BOB.DOLE]

This combi keeps popping the overheat cutout when running hot water. I suspect the modureg valve may be faulty. The voltage across it drops from 20v to 3v, the resistance is 120 ohms but the gas flame does not seem to reduce and water begins to boil in the heat exchanger. Are parts still available for this combi and is the modureg (part no ZAGAS215)available separately from the gas valve?
Any help would be appreciated
Thanks
John

TO CHECK THE MOD VALVE turn stat down slowly and watch the flame picture if it stays the same its not working.

 

[ChrisR]

Tony's gas pressure had already been covered.
Podpod you can check the modulation, as previously stated, by the gas usage rates, measurable at the meter. It wouldn't be practical to try to set it that way, though.

Your hot/cold/hot experience is evidence, as has been suggested, of a partially blocked secondary heat exchanger. If the same sort of blockage got to the main heat exchanger, it would very likely "boil".