central heating boiler (oil) control cycle

[jim]

What is the correct heating cycle for an oil boiler?

1. starting from a point when all calls for heat are OFF, and the flow and return pipes have cooled:

1. A call for heat comes ON resulting in the flow pipe temperature ramping up to the boiler limit, which turns off the boiler firing.

2. Now what should happen? SFAIUI there are 2 possibilities

- either the flow pipe drops to the lower boiler limit at which point the boiler refires & the heating cycle resumes.

- or the ch control system senses the boiler is OFF; waits a short time, then forces the boiler to refire, giving much more of saw tooth pattern.

Apols if that is not very clear. Alternately can anyone please point to a typical timing diagram? Being able to work out the whys and wherefors of a diagram would probably clear up my queries.

TIA

[Dieseldes]

Im can only answer for my oil system which is a pressure jet Stanley, the call for heat is controlled by the temperature of the outlet of the boiler, Call for heat burner fires up until correct water temp. is reached at outlet of boiler, burner is then switched off until temp. drops, if it is heating HW only, the length of time between firings is longer than if the CH is running (obviously), There is also a CH pump cut out if the temperature falls below 40deg IIRC to prevent condensation in the boiler. I have installed a solid fuel converted to oil fired Stanley and now have installed a proper oil fired Stanley, so have been intimate with the operation of both (and next door neighbours oil fired combi!!)

I will try and find the installation manual for the Stanley, I have it on pdf somewhere in the depths of this PC.

Des

[Calvin]

I agree with Des on this. If your boiler is straightforward you will have two thermostats. One will be a high-limit safety stat designed to protect from other control failures, probably fixed at 90 degrees. The other will be the operating stat and you will probably have a control on the front panel to set its operating point. The boiler will come on and off depending on whether the *return* temperature is above or below your set point allowing for the hysterisis of the sensor. There will possibly be a small time delay to prevent it refiring too soon after switching off and there will almost certainly be a purge cycle at the beginning of the heat cycle to prevent it booming as gasses in the boiler ignite in an uncontrolled manner.

[Michael Chare]

My boiler(s) fire if

a) The programmer supplies power. b) The boiler temperature is below the adjustable limit, set using the control knob.

There is no control system to sense that the boiler has been off and cause a refire.

On the boilers that I have had it is the boiler temperature(s) rather that the return temperature that is monitored. (Though clearly the return temperaure is a factor in this.)

Most of the on/off cycling of my boiler is triggered by my programmable thermostat.

The jet size is chosen to match the boiler output to the ability of the radiators to give off heat.