Limited Hot water in cold weather

[John.g]

Just read your thread "water not hot after changing room thermostat" when you had the same DHW problem, you said in one post that when reheating the store without a CH or DHW demand that the boiler was cutting in/out every 3 to 4 minutes, your boiler stat was set to 77C which means the burner was tripping at 82C with the green circ pump continuing to run (correctly). this cycling while the store is heating up doesn't seem quite right IMO, if the store stat is set to say 60C then the boiler should run continuously IMO until this temperature is reached which means that the flowrate should be sufficient to maintain a dT of 17C to allow continuous boiler firing up to the store stat setting of 60C, so a 30kw boiler will require a flowrate of 25.3 LPM to allow the boiler to fire at 100% output, if the flowrate is half this at say 12LPM then the boiler will just modulate to 50% power to maintain 77C flow temperature. I don't know what the rated output & the minimum output of your boiler is but say its 7kw minimum then that can only mean IMO that the flow rate is far too low (theoretically, only ~ 5LPM at 7kw ) to prevent the boiler tripping at 82C and probably well before the cylinder stat is satisfied

 

[Rincewindwiz]

one pushes hot water round the CH system and the other, I presume, to the taps

Thermal store john

Th

A thermal store so maybe check that the cylinder stat is starting up the boiler once there is either a DHW demand or a CH demand, I think some have a flow switch on the DHW outlet to start the boiler immediately, once the store is used up then the DHW output is probably determined by the boiler output and flow temperature, a 30 kw boiler might give a continuous DHW flow rate of ~ 10LPM at 50C from mains at a now 7C. In cold weather the CH will also have a effect although one might think that the rad temperatures would start falling before affecting the DHW temperature. If the circulating flowrate through the store/boiler is too low for whatever reason then the boiler will still give its target flow temperature but the boiler could be running well below its rated output and the store temperature and DHW temperature will fall depending on the DHW demand so maybe thats a reason for the 5 minute run of DHW?. Possibly a circ pump problem?, the only sure way of checking the boiler output is to take the gas consumption over say 3 minutes, its easy to calculate the boiler output then.

THe thermostat is definitely a thermostat
(https://www.ibhs.co.uk/thermostats-aquastats/honeywell-aquastat-controller/l6188.html) defined in this link a limit thermostat having a tamper proof concealed adjustment.

How do I measure gas consumtion. its an old fashioned external dial meter. Surely its not sensitive enough to allow this sort of measurement?

Sounds like JohnG's comment on the mark. I dug out a copy of the instructions (from google - the previous owners were short on docmentation!) and discover that the boiler thermostat (in the boiler) should be set to 5 or 6 in winter and 3 in summer. I'll wind it up to 6 in the next cold spell, see what happens and report back.
Thanks for your input todate

 

[John.g]

They were quite accurate IMO and might look something like one of these so you might take two photos say exactly 3 minutes apart with a big demand for DHW, open up eveything in sight! for say 5 minutes before taking the first reading, better again if CH is on as well, all we want to see is if the boiler is firing flat out under these conditions, but if the flowrate through the boiler is too low it may well not be firing at 100%, should get a reasonable estimation even from these old gas meters.

Can you also post a photo of the circulating (green) pump while running, also see that the arrow on the pump body is pointing upwards.