Short cycling Rayburn boiler

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I have a problem with my boiler, a Rayburn 480ag (gas fired), which is short cycling. From cold it will burn continuously for 10 to 15 minutes until the boiler thermostat turns it off. It is off for just 30 seconds before firing up again, running for 30 secs, turning off again, etc, etc.
It may well have been doing this since we moved into the house in the spring, but is now really noticeable because the heating is on. The system is pressurised (no header tank) and is a Honeywell S plan arrangement, one motorised valve for the hot water tank, and one for the radiators.
I have measured the flow and return temperatures to the boiler and found that with the boiler thermostat in the mid position, the boiler turns off at about 75°C and back on again at about 55°C which is probably what I would expect. However, while the Rayburn manual says that the difference between flow and return should be 10-14°C, I am measuring 20-25°C. That implies that the pump is not taking the heat away quickly enough, i.e., insufficient flow.
The plumber who came out suggested the pump was duff and changed it. However, though the new pump is much quieter than the old one, it has made no difference otherwise. I also took a look at the required flow rate for 20KW heat input and 10°C rise and came out with about 1.8m3/hour which is within the capabilities of the pump. So eliminate the pump as being the cause of insufficient flow. Now looks like a restriction or blockage??
The boiler short cycles regardless of whether I have HW only, CH only, or both. This would imply that the restriction is not in either zone but in the common path, i.e., from boiler to pump, pump to branching point to the two zone valves, or in the common return to the boiler.
I have gone round bleeding radiators. Most of the flow and return pipes are 28mm, only reducing to 22mm to feed the zone valves. The system is about 10 years old installed by the previous owners, but the Rayburn dates from 2002.
I am now thinking about getting Rayburn out to take a look at the heat exchanger as that seems to be the only place left for a flow restriction.
I am tearing my hair out trying to solve this problem and the couple of plumbers I have tried have not helped.
I am just a normal guy with no plumbing expertise trying to get my central heating fixed. Have I made any false assumptions in the tale above? Is there something else I should be doing to narrow it down? What is the probability that it is indeed a clogged up heat exchanger???
Thanks in advance for any help.
 

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That implies that the pump is not taking the heat away quickly enough, i.e., insufficient flow
The main problem is likely to be that the radiators cannot disipate enough heat to bring the return temperature low enough for the boiler
 
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Not so much help but... I have a very similar problem. In fact so similar I thought your post was actually mine but lifted from another forum by a spamming software bot! Alas, I cannot offer a solution to you, though I do have someone lined up who is coming over to inspect the system and I'll let you know the findings when I know... Send me a PM if you want the details of "the other place" - I'd probably get spanked by the admin/moderators if I posted the link publically.

Nozzle
 

DP

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The main problem is likely to be that the radiators cannot disipate enough heat to bring the return temperature low enough for the boiler

No Bernard. The radiators are not getting enough water. That is why return temperature Around 20 less
 
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No Bernard. The radiators are not getting enough water. That is why return temperature Around 20 less

In that case then the bypass valve or some other route is allowing the hot water to bypass the radiators and return to the boiler without having lost much heat.

If the radiators do not get "enough" water to satisfy the radiators demand ( the amount of heat they disipate ) then the water that does pass through them will be even cooler when it leaves the radiator
 
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More likely the pump is not powerful enough to maintain the flow rate against the system resistance, the op has worked out the flow rate but against what resistance ? index circuit anyone ?

Just had a look at the head loss on the heat exchanger for the ops boiler, makes interesting reading.
 
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More likely the pump is not powerful enough to maintain the flow rate
If it couldn't maintain the flow then the water would be passing slowly through the radiators and losing most of it's heat and thus coming out much cooler.
 
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I thought the OP had calculated or cross-referenced (from a chart) the flow rate, rather than actually measured it with the system in service.

Nozzle
 

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