How thermostats work?

[rkinsella]

Hi all,

Quick question.

I have an oil burner, a feed and return radiator system, two motorised isolation valves on the system, one upstairs and one downstairs.

I currently have no thermostats in my house, I mean none. So i switch the heating on, the burner kicks in, the house gets hot. In my current setup, how does my oil burner know how to stop burning? Is it when the water in the return is the same temperature as the feed?

If I were to put thermostats upstairs and downstairs, how would these control the burner. Would they be wired directly to the burner (tell its to switch on and off?), or would they be wired to the motorised isolations valves (controlling the burner by stoping the flow of water)?

Thanks

Ray Kinsella

 

[Hugh Jaleak]

The boiler will stop firing when its internal stat tells it the water inside the boiler has reached set temperature. External stats will keep calling for heat unless they are satisfied, i.e. room or cylinder up to temp. Boiler may continue to cycle until external stats are satisfied if the system is slow to disperse heat, slow circulation for example.

Yours will fire until boiler stat is satisfied, boiler will stop firing but the pump will continue to circulate the water. As the heat is given up around the system, the returning water into the boiler is cooler, the internal water temp in the boiler drops, and the stat tells it to fire again. This will continue for as long as the programmer is set to 'on'. Whilst simple, it is rather inefficient, and there is little control over room or hot water temp, other than turning boiler stat down.

Stats and motorised valves are wired as part of a "Honeywell Sundial" wiring plan. Common types are 'S', 'Y' and 'C' plans, which include the wiring connections for the programmer, pump and boiler. Also depends on type of boiler, some require the pump to be wired from the boiler. The boiler then runs the pump for a while after firing to allow the pump to circulate the water and dissipate the residual heat in the boiler.

Basic operation (as I understand it!), programmer energises system during 'on' periods' This supplies power to stats. If stats sense requirement for heat they will then supply power to motorised valve to open appropriate circuit. Valve opens, activating microswitch which then gives power to boiler to fire and pump to operate thus heating required circuit.