Minimising boiler short cycling

[Hereward]

WB 40CDi conventional gas boiler heating two HW tanks, both with cylinder stats set to 60 degC, using Honeywell controls:

Is there anyway to minimise boiler cycling? My boiler constantly cycles when the HW timer period is ON. Is there an add-on product available to override whatever is calling for heat?

[andy43]

You can use two stats on one cylinder with a hold-off relay, lower stat at a higher setting than the top stat - so boiler only fires when both stats call for heat, then keeps going until both stats are satisfied. Two cylinders - I'm stumped.

[numberjack 4]

can you not just stretch out the hysteresis on the main stat?

[Hereward]

numberjack 4 said:

can you not just stretch out the hysteresis on the main stat?

I can do that on the CH side of the system via the roomstat, but I couldn't see any reference to it on the HW programmer (Honeywell ST9400C)...

[andy43]

Actually, do you mean that the stats cycle on and off, on and off, or that the boiler cycles?
If it's the boiler, try slowing the pump speed down - if it's on speed 3 the boiler could possibly cycle if the return temp is too high due to the high speed - slowing the flow through the cylinder coil might transfer the heat better. Boiler is a big one! Some can be range-rated down, which might help.

[bdx]

Use an on delay timer, set at 3 or whatever minutes.

[Deva Link]

Unless the tanks are uninsulated, or someone is using hot water, then the boiler shouldn't cycle very frequently at all. Unless the 'stats have hardly any hysteresis.

Are you sure the control circuit is wired properly - sounds like the 'stats are not controlling the boiler properly?

I work from home with the HW on all day and the boiler fires up once or twice during the day.

[Hereward]

It's the boiler that cycles, for seemingly random periods, typically off for between 1 to 3 mins then on for 5-30 seconds. It heats the water from cold effectively, the cycling only happens when the hot water tanks are at target temperature.

It's a big pump, Grundfoss 25-55, but already set on "2". The cycling means it's running the whole time, on overrun.

[andy43]

You need Ferg to answer, but I'm guessing the big boiler and big pump are putting too much energy into the cylinder coil - it can't transfer the heat effectively, so the return to the boiler keeps getting hot enough to make the boiler cycle on and off.

[Ricky_M]

CDIs have fairly advanced software for a boiler, there are a few parameters that can be adjusted. Give Worcester Technical a ring and discuss it with them, they may be able to offer a solution.

[Deva Link]

Hereward said:

It's the boiler that cycles, for seemingly random periods, typically off for between 1 to 3 mins then on for 5-30 seconds. It heats the water from cold effectively, the cycling only happens when the hot water tanks are at target temperature.

It's a big pump, Grundfoss 25-55, but already set on "2". The cycling means it's running the whole time, on overrun.

Whereabouts (how high up) the tanks are the tank stats positioned?

It sounds like the tanks are not quite getting up to temp and so the boiler is cycling at (probably) just under the 60C set point. What setting s the bolier stat on? Does the situation improve if you reduce the tank stats a few degrees.

The other thing that could be happening, although this strikes me as unlikely, is the the pump overun is cooling the boiler rapidly and too much heat is being lost in the whole water circuit, thus dropping the tank temps. Normally (I think - perhaps Ferg would advise) the heating coil in the hot water tank is isolated by a motorised valve when the tank it at set temp, so isn't cooled by water continuing to circulate.

[F i F]

Is there a stat on the boiler and what is it set to?

[Hereward]

F i F said:

Is there a stat on the boiler and what is it set to?

There is no stat on the boiler to control the HW side, just one for CH.

[Deva Link]

Hereward said:

F i F said:

Is there a stat on the boiler and what is it set to?

There is no stat on the boiler to control the HW side, just one for CH.

If it's a conventional boiler (as you said in your OP) then all the boiler does is heat water. The rest of the system steers it towards the heating and/or the hot water. So the boiler stat controls the temp of both.
Combi boilers have separate heating and hot water controls.

[F i F]

Deva Link said:

Hereward said:

F i F said:

Is there a stat on the boiler and what is it set to?

There is no stat on the boiler to control the HW side, just one for CH.

If it's a conventional boiler (as you said in your OP) then all the boiler does is heat water. The rest of the system steers it towards the heating and/or the hot water. So the boiler stat controls the temp of both.
Combi boilers have separate heating and hot water controls.

I'd be tempted to wind the boiler stat up a bit and see what happens. Wonder if the cycling is on the boiler stat?

[Hereward]

Thanks, I see where you're coming from, the boiler stat could be kicking in before the tank stats are satisfied. However, I am sure the boiler stat is just for the CH side of the system. Anyway, I'll turn it up tonight and see what happens. Cheers.