Short cycling Rayburn boiler

[Nozzle]

Do you know if there is a way to identify what power output a burner is set to? i.e. is it stamped on a nozzle or housing or something? Either directly or a part number that the man on the street can identify without needing to get internal.

Nozzle

 

[D_Hailsham]

Its is the newer ups2 pump but basicaly the same as a 15-50. Speed is set to max.

Is the green light on the pump steady or flashing?

If it is flashing the pump is running in proportional mode. It needs setting to fixed speed mode

Press the button three times. It will go to II (flashing) - I (flashing) - III (steady).

Now check what happens.

 

[Terrywookfit]

Do you know if there is a way to identify what power output a burner is set to? i.e. is it stamped on a nozzle or housing or something? Either directly or a part number that the man on the street can identify without needing to get internal.

Nozzle

Gas rate it !!

 

[jtboldon]

Think nozzle is asking about the boiler not the pump. Have you downloaded the installation manual? There it describes setting the boiler output by tweaking the gas input valve and measuring gas pressure. In any case there is not a great deal of flexibility and it will be between 17.6 and 23.4kw. Btw have you the chance to measure your flow and return temperatures?

 

[jtboldon]

Actually, the manual says that whoever set the output power should have marked the value set on the inside of the boiler door. Must check mine when I get back home.

 

[D_Hailsham]

Think nozzle is asking about the boiler not the pump.

Is that referring to my question about the pump? If so, it was not connected to Nozzle's question about the boiler.

The reason I asked what setting the UPS2 pump is on is that proportional mode could cause the problem you are having.

 

[jtboldon]

Sorry, I should read the quoted text before replying
The pump is in standard mode on the highest speed. I did try proportional mode just as an experiment, but the pump slowed right down and I had the impression the boiler was starting to kettle.

 

[D_Hailsham]

That removes one possible cause.

In your first post you said:

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.

1. How did you measure the temperatures of 75°C and 55°C?

2. How did you measure the 20-25°C differential?

 

[jtboldon]

That removes one possible cause.

In your first post you said:


1. How did you measure the temperatures of 75°C and 55°C?

2. How did you measure the 20-25°C differential?

After spending quite a bit on plumbers who did not help, I decided to invest 40 quid in a dual input K-type bead thermocouple digital thermometer. The Rayburn has the flow and return pipes exiting the range on the left hand side. I therefore clamped the two bead thermocouples there. Since I am simultaneously now measuring flow and return I can subtract one from the other. In addition the meter also gives a direct T1 - T2 readout. I have been away for a few days and posting from my mobile, which was not ideal. I measured again on returning home this evening and saw a differential of about 22°C from 70°C flow to 48°C return. So it might vary from time to time but bottom line is that the differential falls in that range of 20-25°C. This differential is really only noticeable before the boiler starts short cycling. I tried to make a sketch of how it looks. Red is flow and blue is return.

 

[D_Hailsham]

Thanks for the explanation. The short cycling is due to the boiler output being more than the rads can absorb. (Do you know the total kW of the rads?)

Your boiler has pump overrun (the pump keeps going for a few minutes after the boiler switches off) so, as you have an S plan, you must have a bypass connecting flow to return. This should be connected to the flow after the pump and before the motorised valves.

1. Do you have a bypass? If so what type (gate valve or automatic) and what setting is it on?

2. Does the pump overrun work? (Pump light stays on for a short time after boiler goes out)

 

[jtboldon]

Thank you, but I'm afraid I have to differ. As you can see from the graph, I have 20°C difference between flow and return as the system warms up from cold. Since Rayburn specifies 10-14°C and plumbers tell me that 11°C is the magic number, the only interpretation is that I have insufficient flow. The reason that the boiler is short cycling is that the deltaT that I have is bigger than the hysteresis range of the boiler thermostat.
Pump overrun is nothing to do with it. The boiler is on continually during the period of the sketch graph I made. The sawtooth pattern of short cycling is caused by the boiler thermostat turning the burner on and off.
But yes I do have pump overrun and an automatic bypass set at 0.4

 

[MrTherm]

Hi jtbolton,

Your auto bypass valve appears to be ineffective to requirements.

They often are diagnosed as 'stuck' and need to be cleaned of deposits and re-adjusted, many are set by pure guesswork. The 'screwdriver-to-the-ear' test always works best for me:-

a: No flow sound = shut
b: High flow sound = open
c: Low swish sound = partially open

By listening to the flow sound while gradually turning the adjuster control from fully open to fully closed will give an insight to the sound range prior to setting of any ABV.

Good luck

 

[D_Hailsham]

Thank you, but I'm afraid I have to differ.

No problem.

As you can see from the graph, I have 20°C difference between flow and return as the system warms up from cold. Since Rayburn specifies 10-14°C and plumbers tell me that 11°C is the magic number, the only interpretation is that I have insufficient flow.

There's no magic in 11°C, it's 20°F converted to Celsius. 20°F was supposed to be the ideal drop across a non-condensing cast iron boiler.

Yes, a low flow rate will produce a high differential. This will be caused by a circuit resistance higher than the pump can handle at the required flow rate. This could be due to a partially blocked heat exchanger.

The reason that the boiler is short cycling is that the deltaT that I have is bigger than the hysteresis range of the boiler thermostat.

Sorry, but I don't agree with that.

The boiler is turned off by its internal stat because the flow temperature is rising above the set value, which will happen if the boiler is producing more heat than the rads can emit.The hysteresis just determines the temperature at which the boiler turns back on. This is independent of the flow/return differential.

 

[jtboldon]

Up to a point. With a fixed output boiler where you cannot turn down the heat, the only way it can work is to turn the burner on and off. This will always happen as the system and house get up to temperature. But there is an enormous difference between running for five minutes and off for 5 minutes, which is what I would call normal behaviour, and on for 30 secs and off for 30 secs, which is the short cycling I am seeing . I have expanded my sketch so please take look. My boiler is behaving as in the upper sketch, on and off every 30secs, and I want it to behave as per the lower sketch.

I am also inclined to think my problem is a partially blocked heat exchanger and will be looking to see if a power flush helps.

 

[D_Hailsham]

I didn't appreciate that the boiler was cycling so rapidly as there is no time scale shown on the x axis.

The temperature drop when the boiler is off will depend on how pump overrun is controlled. Some boilers have a timer which runs the pimp fro a fixed time; others are thermostat controlled - the pump runs until the flow temp has reached a set temperature. The manual does not say which type is used, but I suspect, from the wiring, it is temperature controlled.