Boiler cycling

[Matt0029]

Any ideas what could cause a boiler to repeatedly cycle on and off on both hot water and central heating. Until the boiler gets to temperature then settles down and stays on for both.

 

[ShaunCorbs]

Lack of flow / not enough load eg boiler too big

 

[Matt0029]

Its a 24kw obviously heating load be nowhere near that. Lack of flow possibly low performance of the pump?

 

[Matt0029]

Just at abit of a lose why it settles when the boiler gets to temperature

 

[Chuck]

Just at abit of a lose why it settles when the boiler gets to temperature

Assuming it's a gas boiler, I have two guesses:

(a) when the system is cold the boiler tries to run at 24kW and cycles due to lack of circulation but once everything is warm it modulates down to a level that the circulation rate copes with.

(b) when the system is cold the pump is particularly weak but warming it frees it up a bit.

If it's got a few years on the clock, I'd give it a 50% chance the capacitor in the pump is on the way out.

 

[Matt0029]

Assuming it's a gas boiler, I have two guesses:

(a) when the system is cold the boiler tries to run at 24kW and cycles due to lack of circulation but once everything is warm it modulates down to a level that the circulation rate copes with.

(b) when the system is cold the pump is particularly weak but warming it frees it up a bit.

If it's got a few years on the clock, I'd give it a 50% chance the capacitor in the pump is on the way out.

Thanks. With regards to point A. Once hot it doesn't cut out on hot water. I'm correct in saying the boiler doesn't modulate down in hot water mode is always on max? But very good points

 

[Matt0029]

It does seem a good explanation for it.

 

[Chuck]

I'm correct in saying the boiler doesn't modulate down in hot water mode is always on max?

Not sure whether we're discussing a combi or cylinder, etc. but not necessarily.

For a combi it will depend on the draw rate, incoming supply temp and DHW set point. The boiler maximum should be sized to run a bath or have a decent shower in winter so running a kitchen or basin tap should need less. For a cylinder it depends on the coil and the stored DHW temperature. A fully cold tank and a high spec coil might transfer 24kW but as the tank warms up this will drop.

But you're right, in normal circumstances I'd expect heating the DHW to require quite a lot of power so perhaps (b) is a bit more likely than (a).

 

[John.g]

Just at abit of a lose why it settles when the boiler gets to temperature

What is the make & model of the boiler?

 

[John.g]

I see from another post that its a Alpha CB24 which has a minimum output of 9.3kw and a max of 23.3kw.
All gas boilers fire up at ~ 60% of max output, in your case, say 14kw, the boiler should then modulate to maintain its SP temperature.
Boiler circulation flowrates are normally based on a flow/return of 20C, a 23.3kw boiler might have a flowrate of 16.7LPM, if so then the flow temperature on fire up (at 14kw) must be 48C to avoid it exceeding its SP of 60C, otherwise the boiler will trip at its SP+5C, 65C in this case, it will then run for the anticycle time with the circ pump still running and refire again. Some boilers have a adjustable anticycle time, others don't and might be ~ 3minutes or so.
So possibly a circ problem, pump etc or fouled boiler HEX, can you check the boiler flow temperature just as it refires and when of if the burner trips out, that will give you a good idea of the flowrate.
What is the boiler flowtemp SP and is there a ABV (automatic bypass valve) installed?.

 

[Matt0029]

Thanks so I rang technical they told me to do a test on the divertor valve. I thought this would only possibly cause a problem with the hot water. But they said it can cause both to cycle. Unsure why it settles when hot though. Will try this and see how get on.

 

[John.g]

Came across a rather strange statement in the MIs, where it states "When the set temperature is reached, the burner is turned off" this can't be correct as the boiler would just keep firing on and off all the time, most boilers will not shut down the burner until the temperature is at SP+5C, it will then refire when the anticycle time has elapsed, providing the temperature is < SP-5C.