Central Heating Questions and Help

[poolebybirth]

Hi people: i have a few quick questions (probably really easy for you)! about heating that i am not 100% sure about and would like any further help/information on from any top plumbers/heating engineers if i may?:

1) why are air vents (usually on the primary flow to the h/w cylinder, tee'd off just before connection) not allowed to be installed on the return/a return pipe?

2) system boilers/combi's that have aav's built in do not require additional manual/automatic air vents installed on the system do they? apart from rad air vents?

3) room thermostats that do not have an anticipator (which works by the neutral wire): how do they reduce the on/off temp. difference instead?

4) the rotor in a circulating pump is turned by the motor-the impellor is attached to the rotor and drives the water by centrifugal force.

5) what exactly do the bearings do in a circulating pump?

thanks for any help, i appreciate you helping me to learn more. sorry if the ed to check/ask pro's.

 

[powell30]

1. Positive and negative pressure created by pump.......on return it will be negative pressure and 'suck' air in when you open any air bleed valves

2. Usually correct unless you have unusual pipework configurations creating a high spot. This might need an air bleed valve fitting

 

[poolebybirth]

1. Positive and negative pressure created by pump.......on return it will be negative pressure and 'suck' air in when you open any air bleed valves

but if the system is designed correctly, and the neutral point is just before the pump, then surely the rest of the system will all be under positive pressure, so no air would be drawn in?!

 

[seco services]

1. Positive and negative pressure created by pump.......on return it will be negative pressure and 'suck' air in when you open any air bleed valves

but if the system is designed correctly, and the neutral point is just before the pump, then surely the rest of the system will all be under positive pressure, so no air would be drawn in?!

as you say positive pressure then the neutral point (vent/feed pipes) then negative to the pump after pump positive pressure

 

[poolebybirth]

yep that's right - so basically the air vent being on the flow means less chance of air being drawn into the system, through micro leaks, etc?

 

[D_Hailsham]

3) room thermostats that do not have an anticipator (which works by the neutral wire): how do they reduce the on/off temp. difference instead?

Older thermostats uses a bimetalic strip to act as the switch contact. Because of the different expansion coefficients of the two metals, the strip would bend one way when heated and the other way when cooled, thus making contact with either the call or sat terminal. However these switches took a relatively long time to react, so the difference between on and off temperatures could be as much as 8°C. The anticipator was a small resistor, connected between the call terminal and neutral. When the boiler is calling for heat, this resistor warmed up, so the bimetallic strip was heated up quicker. The result was that the temperature differential was reduced to about 2°C.

Modern digital thermostats do not use a bimetallic strip, instead they use a solid state device called a thermistor. This is a device which varies in resistance with temperature. The thermostat then consists of a battery or mains operated micro-computer which measures the resistance of the thermistor, and so the temperature. As a thermistor reacts virtually instantly to temperature changes, an anticipator is no longer required and temperature control with in ±0.5°C is possible.

 

[poolebybirth]

interesting info. thanks d-hailsham. so obviously the modern stats (or digistats) without an anticipator do not need the neutral wire as the thermistor is battery operated or mains operated, ie from the common terminal?

 

[ChrisR]

That's correct now, as far as I know, but there WERE some digital programmable thermostats which used the neutral. While the switched live was live, they'd charge ther batteries in the device.
Now, battery-only units last several years on a set.

4) and 5) - not sure what you're driving at. These days the shaft is in water and supported by bearings which will overheat if you run the pump dry.

 

[namsag]

Also some pumps did not have a shaft connected to the impellor the motor head and impellor part where completly seperate when you split them. Worked by some sort of magnetic effect or whatever

 

[poolebybirth]

i was reading a CH book and it said this:

closing both isolation valves to the pump.

remove the pump vent screw at the front

slowly open one of the isolation valves (the higher one)

if water pours through then the bearings are wron and a new pump is needed

by why would bearings being worn allow water to pour through ??

 

[Softus]

i was reading a CH book

Put the book down, AND BACK AWAY.

Good. Now stop posting nonsense on the forum.