Pump overrun and Satisfied / Call crossed

[Rob69]

Can somebody firstly tell me what the reason is for the pump overrun on combi boilers?

Secondly what would the fault be if the Satisfied and Call connections were crossed from the cylinder thermostat?

Many thanks

Rob

 

[kme]

Pump overrun: keeps moving the heat around the pipes, after the boiler has switched off - otherwise the heat the boiler created just before shutdown sits in the pipe near the boiler, and cools uselessly - i.e. waste.

Call / sat reversal: If the water is cold, it`ll be "satisfied"; so the boiler won`t come on.

If the water is above the stat. setting, it`ll "demand", and start pumping MORE heat into, until one of the following occurs:

1. the overheat stat trips on the cylinder, and shuts things down (depends how its wired).

2. the overheat stat trips in the boiler (usually non-resettable).

3. the cylinder overheats, and boils the header tank (if vented), blows outh the TPR valve (if unvented), or explodes (unvented with faulty TPR).

KME

 

[Rob69]

Thanks KME! So to test the satisfied / call circuit.........if i just turn cylinder stat up to say 55 the boiler will kick in.......turn it down and boiler will turn off(after a period).

So what is the point of the satisfied (to 3 port valve)connection , surely all you need is the above?

Thanks Rob

 

[steptoe]

If you have hot water and heating both calling at the same time,

hot water will usually have priority, so the valve needs to know what position to be in.

 

[Rob69]

Thanks All

 

[kme]

Scooby snacks then????? C`mon - you KNOW you want to.............................

 

[PC Electrics]

Many thin aluminium heat exchangers (eg Worcester Bosch) would warp and split due to the heat soak if the water flow were cut off at the same time as the flame. This is also why these boilers require the pump to be on max speed.

 

[wozz]

Many thin aluminium heat exchangers (eg Worcester Bosch) would warp and split due to the heat soak if the water flow were cut off at the same time as the flame. This is also why these boilers require the pump to be on max speed.

As ive always thought to get the heat out the boiler or it will cook it. Didnt know about the pump speed always been for the plumber to decide.

 

[kme]

On coal systems; slower speed was always the norm, as the heat was more gradual / permanent - with an "instant heat / instant cold" gas boiler, you need to move the hot water to the rads as fast as you can (that, at least, is what I was told.................but that WAS by a plumbit....)

 

[DarkInferno]

Most systems don't do priority anymore, it needs the Satisfied because the 3 port valve is a mid position valve. Which is sprung return to Hot Water only.

Hot Water Call, - Direct to boiler

Heating and Hot water Call - Opens the valve to midway (hits a microswitch that injects DC, stalling the motor)

Heating and Hot water Sat - Opens the valve fully (the Sat also provides the power for the second microswitch to fire the boiler.)

Quiet a few boilers would self destruct in short order as mentioned due to the amount of heat stored in the heat exchanger.

 

[Whiskey]

Without pump overrun the water in the boiler will continue to be heated for a few minutes after the pump is switched off. This remaining heat in the boiler can raise the temperature, of the small amount of water it contains, and cause the over-temperature cutout to operate.

Also transfers the remaining little bit of heat to the water making it more efficient.

 

[batty]

unusual to have a cylinder on a combi boiler thought the idea of combis was to do away with cylinders so no waste. I have just wired an underfloor heating circuit into a combi and have done so in past also.

 

[Robin Spark]

Either 2 different jobs or perhaps he means system boiler?