Short cycling Rayburn boiler

DP

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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

That is a kind of thing a heating engineer would look at and then attempt to repair instead of speculation.
 
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with the boiler thermostat in the mid position, the boiler turns off at about 75°C and back on again at about 55°C
How can 75 be the return temperature?

It could be if the flow temperature was more than 75 and there was no loss of heat ( temperature ) in the pipe work from boiler flow to boiler return. Automatic bypass valve close to the boiler and fully open all the time would create that situation. What temperature should the flow be controlled at when the boiler thermostat is mid position ?
 

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At mid position, 75 appears a bit high.. May be residual heat pushing the temperature to 75

Bernard, automatic bypass would not be open during a demand. If it is open, then it is a bypass, but not automatic. No point in having unwanted unrestricted short circuit to water flow
Return temperature very unlikely to be 75 with boiler stat set at mid position.
 
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automatic bypass would not be open during a demand

It could be a defective automatic that has decided for some reason to remain open. There is obviously something wrong and not operating as it should be. Consider every item as suspect and then work out which is the culprit component using logic and not by changing items one by one until the fault seems to dis-appear.
 
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automatic bypass would not be open during a demand. If it is open, then it is a bypass, but not automatic.

What if the bypass is faulty? (Stuck open or opens too lightly). It seems plausible and it's not always possible to know if a {copper} pipe is hot or warm because of flow through it, or conduction along the copper.

Nozzle
 
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If the bypass was stuck open the return temp would be higher , the op said the primarys are 28mm so how many rads are on the system ?
 

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Clearly several replies typed out without brain in gear.
Fault that OP has, first thing I would do, without reaching for ANY tools, is to FEEL with my HANDS the temperature difference on flow and return.
How difficult is that. No speculation that auto bypass is stuck, badly set or whatever
This right away tells me if the heated water is coming right back to the boiler ( in which case look for reason why that is happening), or lack of flow indicated by return that is unaccountably cooler.
 
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Think picasso could be on to something with system resistance. Maybe a Rayburn system requires a high head pump to overcome the heat exchanger resistance. Resistance curve for Rayburn on dara sheet looks a bit steep.
Want to reply more on this but cant till weekend. Away in hotel with 1kbps internet
 
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Just to clarify some points. When talking about 75degC I meant flow temp. From cold when both flow and return are 20degC, turning on the boiler will cause flow to rise rapidly to 45degC in about 30 seconds. Then will rise slowly to 75degC over 15 minutes. Return rises to 50degC over same period. 1.8m3/hour comes purely from the flow required to shift 10degC rise with 20kw heat input regardless of what resistance has to be pumped against.
 
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Is the boiler actually set to 20kW (min =17.6kW, max = 23.4)?

What pump do you have and what speed is it on? The boiler heat exchanger has a very high resistance - over 2m at 1.5m³/hr. That doesn't leave much for the resistance of the heating circuit if you have a Grundfos 15-50.

Setting the output to match your heat loss and balancing for a 14C drop will reduce the required flow rate. The resistance of the boiler and heating circuit will be reduced, probably bringing it within the capabilities of the pump.
 
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Its is the newer ups2 pump but basicaly the same as a 15-50. Speed is set to max. To be honest I was asuming 20kw for the moment. Think I wil get someone in to adjust it down to 17.6kw. However with my current 20 to 25 acros the boiler it is increasingly looking like a bunged up heat exchanger.
 

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