Boiler Control

[ChrisinShropshire]

Once the demand for heating has gone (I.e. wall thermostat reaches temp), does the time for the boiler to go out vary?
Only asking because today the boiler ignition light stayed on for about 1 minute and this seems a bit long, as I thought it should be almost instantaneous

Thanks
Chris

 

[ianmcd]

you might want to add more information, boiler make and model, thermostat make and model system set up, if your thermostat is a wireless RF one this is normal

 

[ChrisinShropshire]

Worcester Bosch Greenstar 30CDi
Honeywell room stat T4360
Grundfos pump UPS2 15 50/60

 

[ianmcd]

Worcester Bosch Greenstar 30CDi
Honeywell room stat T4360
Grundfos pump UPS2 15 50/60

Honeywell T4360 is a frost protection thermostat, not a room thermostat , post some pics of your set up and I will see if I can help you

 

[ChrisinShropshire]

Starting conditions

1. Garage air temp (according to digital thermometer) 7.8°C

2. Water temperature in Boiler approx. 55°C

3. Garage Honeywell T4360 Froststat set to 5°C

4. Garage Honeywell L641B1004 pipestat, set to 20°C

5. System controller set to heating on continuous (so demand controlled by hall thermostat)

6. The Froststat was replaced by Worcester Bosch this month.



The Thermostat in the hall turned down to below 15°C (NO heating demand) and the Water demand set to OFF at the system controller in the Utility room. The pump was running!

I turned the dial on the froststat down. At 3°, the froststat ‘clicked’ and the boiler ignited for approx. 30 seconds. The water temperature rose by about 7°C (55°C to 62°C), according to the boiler.

Question: This appears to be the boiler operating Frost Protection. But why does it happen when the Pipestat, which is wired in series with the Froststat, should be open, as the water temperature is above the 20°C setpoint?

Question: Is there another protection device in the boiler that initiates the Frost Protection cycle, even though the external froststat and pipestat have not demanded it and the temperature dial on the boiler is NOT set to *?

Question: Should the froststat have wires connected to terminals 1, 2 and 3? This is not in accordance with the circuit diagram in the T4360 Reference guide where terminal 2 is not used? If not, could this cause our problems?

Question: Could the terminals of the pipestat have welded closed so that frost protection demand is controlled only by the froststat (this would explain why the Frost Protection operates even when the temperature of the water in the boiler is above the 20°C set point). I have bought a replacement pipestat.

Generally, in winter, our garage is cold and the central heating pump runs all the time, irrespective of demand. In summer, it runs only when there is a demand for heating or water. (except if the garage gets really warm in the winter or cold in the summer). In the warmer weather, we have no issues.

Question: Does the boiler control the operation of the pump?

Question: Why does the pump run after the 3 minute boiler overrun time when there is no heat or water demand?

 

[ianmcd]

edit terminal 2 is Neutral and the frost stat switches between 1 & 3 so the wiring is correct

 

[ChrisinShropshire]

I have now replaced the pipe stat. It was wired between Common and terminal 2. The Honeywell manual (see diagram above) says Common and 1. I have wired the new Pipe stat in line with the instructions.
The Frost protection on the boiler is now happening much less frequently and the pump no longer runs all the time. However, it still happens every 30 mins, even though the boiler indicates that the water temp is 48°C and the garage temp is 8°C (frost stat set at 5°).
Any ideas please?

 

[stem]

Are you are looking at the temperature of the water inside the boiler (48°C). The pipe stat is usually located on the flow pipe back to the boiler after the water has passed though the radiator circuit, so could possibly still be under the set 20°C.

 

[ChrisinShropshire]

Stem,
Yes, the pipe stat is on the return between the magnetic filter and the boiler.
For water temperature, I am using the display on the front of the boiler. I understood that in normal operating conditions you would look for a 20° drop in water temp between flow and return. I have just touched the return pipe, it's warm, not hot.

 

[stem]

It doesn't really explain why the frost stat is still calling for heat at 8°C when it's set at 5°C though. Although I suppose there could be a degree or two of error between the two devices.

The boiler will look after the entire operating sequence, pump run times etc, if it has been installed properly. The supply to the frost stat terminal '1' should be connected to Fs (Frost stat supply) at the boiler and output from the pipe stat terminal 'C' should be connected to Fr (Frost stat return) at the boiler, and the two stats are wired in series as per the diagram (Frost stat 3 to pipestat 1). I have found installations with them wired in parallel on more than one occasion.

The 20°C difference is a typical figure and will vary from installation to installation. When first starting from cold it can take a while for any of the heat to get back to the return. The pipe stat is also measuring the temperature via conduction of the metal and if the stat is in a much cooler area so heat is lost from it that might also have an effect.

 

[ChrisinShropshire]

Thanks Stem,
I have always assumed that there would be a 2-3° difference in opening and closing temps on the switches to keep the system stable so it's not continually going off and on.
Thanks for confirming the frost stat connections. I could see no reference to them in the 'standard's control wiring diagram.