Cyl Stat wanted with big lag

[JohnD]

I am troubled by my boiler repeatedly firing up for very short periods od less than a minute when I have used less than a sink of water, and doing this 8 times in 25 minutes even though I had not used any more

I am thinking that the cyl stat detects the drop in temp near the bottom of the cyl (stat is mounted 300 mm up from the bottom, coil tappings are at about 100mm and 450mm up, was fitted Sept 2002 and is modern blue/green foamed)

I am considering the idea that if I had a cyl stat with a big gap between "off" and "on" temps, it would not start the boiler up until the bottom of the boiler had gone cold, so it would not fire up so often. the cylinder is plenty big enough.

The boiler is a Vitodens with the coiled stainless tube HEX, so it does not have much mass.

I have not got pipe thermometers but I have tested the temp of the tap water and it is coming out at the temp set by the cyl stat

Any comments or suggestions?

the cyl stat is a (new) Honeywell L641

Working parts of installation are new
Boiler is Vitodens 100 Compact (modulating)
Open-vented
Pump is Grundfos Alpha + with 28mm gate valves (self-modulating)
Pump runs for 10mins after final call for heat ends.
Boiler stat is "normal" 60C
Cyl stat is set to 50C (I have been turned it down a bit to see if it helps, but the short-cycling still occurs when the cylinder is almost, but not completely, full of hot)

This is a different approach to solving the problem I reported earlier.

 

[D_Hailsham]

Ah! I see you have still not solved this problem.

the cyl stat is a (new) Honeywell L641

The L641 has a temperature differential of 10°C; so I doubt if that is the cause of your problem.

I am considering the idea that if I had a cyl stat with a big gap between "off" and "on" temps, it would not start the boiler up until the bottom of the boiler had gone cold, so it would not fire up so often. the cylinder is plenty big enough.

Don't forget that the cold water goes into the cylinder at the bottom and the hot comes out at the top. So the temperature at the bottom will get colder when hot water is drawn.

With your cylinder stat set to 50°C, the boiler will come on when the temperature drops to 40°C. A basin full of hot water water will get replaced, in the bottom of the cylinder, by cold water at 15°C. This may be enough to cool the stat below 40°C, so the boiler will light. If you turn the stat up to, say 60°C, the boiler will not light until the temperature has dropped to 50°C. You would then have to turn your boiler temp up to say 75°C, but I don't think that is a bad thing as the boiler will automatically modulate to give the correct temperature. Setting the boiler temp to 60°C just restricts the range within which the boiler can modulate. The cycling could be due to the boiler reaching 60°C, turning off, cooling, turning on and so on.