Hello all... We have a three year old condensing boiler but the hot water now is unreliable, it may (or may not!) run hot for a few minutes and then cold, more often cold than hot and then luke warm water followed by a couple of minutes of hot again! It makes trying to have a bath a nightmare!! Can anyone offer me advice or suggest a fix? Thanks ... Richard.
Replace with a proper water heating system?
:-)
Even if the oil system heater totally fails, I can crawl into the attic and switch the immersion heater on..
My immediate _suspicion_ will be that the DHW heat exchanger has become scaled up (on either side, the secondary most likely, but the primary also if the system was not flushed out).
What happens is that the heat transfer gets poorer and poorer until eventually the boiler can maintain its lowest burner setting when doing DHW without over heating. Therefore the boiler flame has to cut out for a while and this produces the fluctuations.
Any further diagnosis will require specific temps, flowrates and if possible burner gas pressures.
In my (limited compared to Ed's) experience scaling-up on the secondary side tends to produce a feeble stream of scalding hot water. I've just had one blocked on the primary (flakes of rust in it when I attempt to flush it out). The symptom was that the diverter valve moved over OK and the boiler would fire up on HW demand but quickly shut down again with the flow pipe & port on the PHE roasting hot but the DHW out of the PHE cold.
If it's scaled on the secondary side it's relatively easy to pump something like Kilrock K through it to open it up again. With a bit of ingenuity and care this can be done in situ, without dismantling the boiler. Blocked primary is probably a new PHE job. I put a magnetic separator (like a Magnaflow but Pulsar's own-brand) on the return before replacing the one I did.
That seems to be the more severe cases.
The progression as I see it is something like.
1) Boiler works as intended but can't break the laws of physics. 2) Boiler produces less and less DHW. 3) Boiler starts to produces fluctuating DHW temperatures (when the effective rate is reduce below the minimum modulation setting). 4) Boiler works like a 'Still' boiling water machine that were once very popular in caffs. This is caused and assisted by the user turning the maximum DHW temp setting to the max, in order to avoid, in vain, problems in (2) and (3).How quickly the boiler moves down the list seems to vary from weeks to years depending on more factors than I am able to pick over.
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