Limited Hot water in cold weather

[Rincewindwiz]

I have a 2 pump hot water tank (Flomax 180L fitted in '94) in airing cupboard system with an Ideal classic boiler (downstairs next to an external wall) which for the most part seems to provide excellent hot water. The central heating also works fine - though sometimes I think the radiuators could be hotter, then on other occasions they are uncomofortable to touch.

Normally the whole system works fine but during cold spells (overnight <=0C) I get very limited hot before the water starts to get cooler. What water I do get starts off fine and hot, runs for maybe 5 minutes then starts to cool down.

If I run a bath, I get maybe 6" before it starts to go cold. But the boiler will produce more hot water quite quickly so a soak in the bath is possible with constant topping up

It behaves as if only part of the HW tank is being heated but I cant see why that should have anything to do with temperature
https://community.NoLinkingToThis/styles/default/xenforo/clear.png

Any suggestions as to why this might be?

 

[John.g]

What do the 2 pumps do on the cylinder?

 

[Rincewindwiz]

What do the 2 pumps do on the cylinder?

one pushes hot water round the CH system and the other . . . I dont really know. I presume pumps water through the boiler. One connects to the top of the HW cylinder the other just over half way down

 

[John.g]

One of those pumps (secondary recirc pump) recirculates the water in the cylinder to your hot taps etc, and back to the cylinder, if that's running continuously and is not on a timer or pipestat then it will quickly dissipate heat through the pipe work.

The other one circulates the system (primary) HW from the boiler through a coil in the cylinder to give DHW.
Is there a 3 port mid position valve after this pump to give single zone CH or DHW or both or is there a number of motorised valves after the pump?.

 

[Rincewindwiz]

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

One of those pumps (secondary recirc pump) recirculates the water in the cylinder to your hot taps etc, and back to the cylinder, if that's running continuously and is not on a timer or pipestat then it will quickly dissipate heat through the pipe work.

The other one circulates the system (primary) HW from the boiler through a coil in the cylinder to give DHW.
Is there a 3 port mid position valve after this pump to give single zone CH or DHW or both or is there a number of motorised valves after the pump?.

The boiler pump is clear.
I thought the other pump circulated water thourh the radiators
THe Cylinder is described as Flowmaster Mains pressure System so I presume this (the mains) provided tap pressure
Both pumps are controlled by external sensors. I presume CH is by the main thermostat in the hallway and Boiler via the boiler thermostat (the one with the arrow) Ive attached a picture (Please dont give me hard time about the wiring!!!)

 

[John.g]

Can you see which pump the cylinder thermostat is wired to, when this pump is running feel the return from the cylinder coil?? and see if its hot, cool or cold.

The red (grundfos?) pump seems to be the one circulating through the coil.

 

[Rincewindwiz]

Can you see which pump the cylinder thermostat is wired to, when this pump is running feel the return from the cylinder coil?? and see if its hot, cool or cold.

Cylinder thermostat is not wired to either pump. It goes into the collection of wires on the wall. There it must go either to the hall thermostat or to the boiler. My best guess is the boiler . Ive done a diagram of the wiring which might help

 

[John.g]

What you refer to as a thermostat looks more like a immersion heater to me, a cylinder stat usually has a dial on it that you can set, usually to 60C.

 

[ShaunCorbs]

What you refer to as a thermostat looks more like a immersion heater to me, a cylinder stat usually has a dial on it that you can set, usually to 60C.

Thermal store john

 

[John.g]

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.

 

[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.