Hot water cylinder heating issue

[fixitflav]

I wouldn't expect it to satisfy the demand in a single run, due to mixing and thermal currents in the water of the cylinder. It will also cycle more, if the boiler cannot effectively modulate, to a low output.

My boiler is on/off. Takes 3-4 cycles to heat the water. I leave mine on 24/7/365 CH and HW. When the weather gets cold CH comes on!

 

[Harry Bloomfield]

I leave mine on 24/7/365 CH and HW. When the weather gets cold CH comes on!

Same here for the CH 14C night, 18C in the day. It never gets anywhere near the 14C during the night, and so never fires. The 18C has fired maybe once, since the summer. HW is on from 15:00 through 20:00, which takes care of baths, and washing up - that avoids wear and tear on the boiler, firing for just a few minutes, to top the cylinder up.

 

[Johntheo5]

My 150L twin coil cylinder has the top (oil fired coil) heating the top 100L, solar coil in the bottom. The top 0.62m2 coil outlet is exactly "100L" from the top and the top coil inlet "50L" from the top, with a coil height of 0.5m, the "stat" PT1000 probe is located almost exactly midway between the coil inlet&outlet so ~ "75L" from the top, I use my solar controller and this probe to monitor and control the motorized valve to 60C, with the hysteresis set to 5C, what I find a bit strange is that the HW outlet temp measured with a digital thermometer is almost exactly the same as the PT1000 probe, I measured the temperatures at 40C, 50C and 60C, the coil dT is 10C with a cold cylinder and 3.5C when the cylinder temperature is almost up to temp at 60C.

 

[Mark Batcheldor]

Just a bit of an update on this drama. It appears the cylinder stat itself if being affected by ambient temperature effect. Was clicking in and out quite regularly and I was wondering where the energy was being wasted. I put some insulating tape around where the very small gap is around the cut out of the foam cylinder effectively sealing up the stat around its edges and now it seems to have settled down. I didn't think this would affect the cylinder stat operation but it does seem to have. The cylinder is probably very old so the foam insulation probably not as thick as more modern cylinders etc. I am still using my old Honeywell stat as the new one appeared to have zero hysteresis so would click in and out at the slightest variation in cylinder temperature. This forum has been really helpful and a lot of getting to this stage has involved trial and error over a few days.

 

[DP]

This is my concern I think that it's full of scale so it will take ages for the heat to transmit through the cylinder wall to the stat. Where i have messed around with the stat setting its maybe thrown it out of calibration. In an ideal world i probably need a new cylinder but this is just what I have always been used to even tho not right. Thank you for replying as it stresses me out all this stuff.

The hysteresis on the new stat might be tighter. Old stat less so.
A modern cylinder that is factory lagged will reach 75% heat capacity in quite a short time and factory applied lagging will reduce heat loss from the cylinder.
It may be more prudent to split heating and hot water programming on controller to reflect usage rather than all day. Example of this would be to heat water early hours of the morning when heating is off so you have a fully heated cylinder when you are in need of hot water. In the evening run a short duration HW heat up schedule.

 

[Mark Batcheldor]

Thank you I may try this I was worried about heating starving the HW and visa versa but your suggestion of putting the HW on a timer (early hours then maybe in the afternoon before the heating comes on) is probably a more efficient way of running things.

 

[Harry Bloomfield]

Just a bit of an update on this drama. It appears the cylinder stat itself if being affected by ambient temperature effect. Was clicking in and out quite regularly and I was wondering where the energy was being wasted. I put some insulating tape around where the very small gap is around the cut out of the foam cylinder effectively sealing up the stat around its edges and now it seems to have settled down. I didn't think this would affect the cylinder stat operation but it does seem to have.

Cylinder stats, take their temperature sensing, through the metal back panel. so long as that rear panel is in good, tight contact with the cylinder, that should be all that is needed. Hysterisis, is usually around 6C, between switch on, and switch off.