Hot water short cycling boiler
- Thread starter The Woo
- Start date
Sponsored Links
- Joined
- 16 Nov 2018
- Messages
- 112
- Reaction score
- 6
- Country
I have a similar problem on a vented system. I have observed that the boiler flow temperature hits the boiler temp (75 degrees), the boiler modulates back and the 75 is more or less maintained (cylinder temp is set at 65). But quite often the boiler flow temp will overshoot 75 quite rapidly and the boiler will shut off totally. A few minutes later it fires back up and the whole cycle repeats itself.
Prior to adding a new fancy control system to my open vented CH and HW system, where it feedback actual temperatures, rather than an on/off demand, I added some LED's so I could see what my system was doing. The idea was so I could see at a glance what my system was doing, rather than getting a test meter out. I had LED's for HW demand, CH demand, boiler run, pump run etc..
Well, with a UV it won't be stat positioning or grounding on the cylinder then. Might have some furring of the cylinder coil?
But sounds like coil can't dump heat into the cylinder as quick as boiler is dumping heat to the coil. As said, check hysterisis of the stat and put the HW on timer.
Sponsored Links
Think you nailed it dilalio
Very recently serviced boiler with a few new parts = significantly improved efficiency.
CH not coming on at moment thanks to warmer weather = only cylinder now transferring that heat.
Old cylinder not transferring heat as efficiently as boiler heats it = boiler stat being satisfied before cylinder stat is even close (boiler light indicates this - boiler stat satisfied, followed by immediate call for heat from cylinder).
Testing the theory - upped boiler stat a little yesterday and lowered cylinder stat a tad this evening - meet in the middle?
Think it will still short cycle, but hoping only a few times before cylinder stat is happy.
What about leaving HW on - theory being water in cylinder won't have cooled as much as turning it off completely overnight = less effort to re-heat = reduce short cycling even more?
Very recently serviced boiler with a few new parts = significantly improved efficiency.
CH not coming on at moment thanks to warmer weather = only cylinder now transferring that heat.
Old cylinder not transferring heat as efficiently as boiler heats it = boiler stat being satisfied before cylinder stat is even close (boiler light indicates this - boiler stat satisfied, followed by immediate call for heat from cylinder).
Testing the theory - upped boiler stat a little yesterday and lowered cylinder stat a tad this evening - meet in the middle?
Think it will still short cycle, but hoping only a few times before cylinder stat is happy.
What about leaving HW on - theory being water in cylinder won't have cooled as much as turning it off completely overnight = less effort to re-heat = reduce short cycling even more?
That will increase the opportunities to short cycle, every time the temperature falls in the cylinder. I have my system set to boost to 60C, once per day at around tea time. Shortly before it comes on, when I have checked the temperature it shows 50C (when none had been used), so 10C is lost in the 24 hour period. Had I had the HW enabled for the entire time, no doubt it would have fired up several times - there are not just cylinder heat losses through the insulation, but some through convection losses, plus some losses to the cold water in the bottom of the cylinder too.
Not really. Already been shown that boiler will cycle whilst there is a demand for heat from the cylinder stat, so the only way (without further system alterations) to prevent this, is to restrict the demand via the timer so that the conflict between cylinder and boiler is "cut off at the pass" so to speak.
Cheers guys.
Just been chatting to heating bloke who still thinks it's a zone valve issue. Valve not fully opening causing restricted flow on the circuit, which in turn causes the boiler to "surge".
Not convinced at the moment, but I'm not a professional.
Just been chatting to heating bloke who still thinks it's a zone valve issue. Valve not fully opening causing restricted flow on the circuit, which in turn causes the boiler to "surge".
Not convinced at the moment, but I'm not a professional.
Noted this morning that CH is doing the same thing...although not as much.
Burner clicked on - ran for 10minutes (all rads got hot) - clicked off and cycled down as boiler stat satisfied (flashed green for the alloted one min) - clicked back on due to external call for heat (from room stat) - ran for 90secs and then cycled down.
It did this 5 times before room stat was satisfied and everything turned off as expected.
I think this is pointing to a boiler issue - PCB or Thermistor not modulating the boiler properly.
Boiler just ramps up to full heat and stays their until it satisfies its own stat - turns off - just to be met by call for heat from external stat (room or CYL) where it ramps back up to full heat again. It does this until the external stat calling for heat is satisfied.
Spoke to my heat guy who is now recommending we change out the thermistor.
Thoughts?
Burner clicked on - ran for 10minutes (all rads got hot) - clicked off and cycled down as boiler stat satisfied (flashed green for the alloted one min) - clicked back on due to external call for heat (from room stat) - ran for 90secs and then cycled down.
It did this 5 times before room stat was satisfied and everything turned off as expected.
I think this is pointing to a boiler issue - PCB or Thermistor not modulating the boiler properly.
Boiler just ramps up to full heat and stays their until it satisfies its own stat - turns off - just to be met by call for heat from external stat (room or CYL) where it ramps back up to full heat again. It does this until the external stat calling for heat is satisfied.
Spoke to my heat guy who is now recommending we change out the thermistor.
Thoughts?
- Joined
- 16 Nov 2018
- Messages
- 112
- Reaction score
- 6
- Country
Did you get this resolved?
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links