Short Cycling, any advice?

[Johnnyt]

During the recent cold spell my 25kW Viessman w200 system boiler returned 5 firings over a burner run time of 62 hours modulating down to 8.7%. Mind you it was minus 8.5C outside. I wish I could repeat as a general rule.No weather comp just setting flow temp of 43C and ufh 5 zone with room stats at 22C and 4 rads on 1st floor.

[Super_Paulie]

9 minutes ago, John Carroll said:

So, what is the cycle time?, ie, burner ON time & burner Off time.

 

Can you post a link to your Boiler MIs.

 

I don't have that exact figure at the moment, but setting the flow at 60, and the mixer at 45 it took an age burning while the buffer gets to temp, at least 30 minutes. At that point it goes over the target temp +5 then the flame out (anti cycle?) kicks in for exactly 3 minutes. In that time the flow temp has dropped to match the buffer return temp then it takes another 30 minutes to get it back up again before repeating. So it's pretty much always firing, except for the 3 minutes it isn't.

 

I have the manual if that's what you mean? The figures start from page 14 and I can't find anything on there or the internet about being able to adjust the cycle timer.

Baxi 800 Combi Installation and Service Manual.pdf

[marshian]

Thinking about it that's a heck of a circuit temp drop in 3 mins

 

65 deg down to 50 (ish)

 

Have you got an ABV in the circuit?

[Super_Paulie]

10 minutes ago, marshian said:

Thinking about it that's a heck of a circuit temp drop in 3 mins

 

65 deg down to 50 (ish)

 

Have you got an ABV in the circuit?

I've got a rad in the bathroom, not sure if it's better to have that thing fully open or throttled back.

 

As soon as you get flame out the pump is still running so I guess it cools quickly to the temperature of the return from the buffer.

[John Carroll]

29 minutes ago, Super_Paulie said:

 

I don't have that exact figure at the moment, but setting the flow at 60, and the mixer at 45 it took an age burning while the buffer gets to temp, at least 30 minutes. At that point it goes over the target temp +5 then the flame out (anti cycle?) kicks in for exactly 3 minutes. In that time the flow temp has dropped to match the buffer return temp then it takes another 30 minutes to get it back up again before repeating. So it's pretty much always firing, except for the 3 minutes it isn't.

 

I have the manual if that's what you mean? The figures start from page 14 and I can't find anything on there or the internet about being able to adjust the cycle timer.

Baxi 800 Combi Installation and Service Manual.pdf 3.09 MB · 0 downloads

(say) 30 minutes on & 3 minutes Off is a cycle time of 33 minutes, not even 2 cycles/hour, nothing to complain about now?, and assuming 6kw min output means a UFH demand of 6*30/33, 5.45kw?.

[John Carroll]

9 hours ago, Super_Paulie said:

 

I've had to call it a night up there, but before I did I set the boiler at 60, the mixer at 45. This takes a while to fill the buffer and the boiler creeps to 65.1 then flames out for 3 minutes, the buffer is at 51 at this point. After the 3 minutes the boiler is now at 54 and it starts up again. Runs back up to 65.1 in around 30 minutes. 

 

Seems to be a lot of bypassing , if the UFH return temp is say 38C then ~ 3.5LPM at ~ 38C must be mixing with ~ 5.0LPM at 60C of bypass (assuming no temp drop through the rad), to give a buffer temperature of 51C, the manifold return should be well down to its return temperature after 30 minutes, your UFH system with 400M of 16MM (12mm ID) piping should contain around a total of 95L, (45L in the pipework + the 50L buffer) will/should have circulated through the system after ~ 95/3.5, 27 minutes, check those readings again sometime after say 3 cycles. Its no wonder the boiler was short cycling with (If) that sort of bypassing.

[JohnMo]

8 hours ago, John Carroll said:

What make/ output Boiler do you have

Atag A325ECX, rated modulation is about 7 to 31kW, at lower flow temperatures.