Heating system pressure loss

[Tricky1963]

I know this subject has been discussed on a number of occasions but I wondered if anybody can help with the pressure loss in my heating system. I have a Potterton Promax System 24HE boiler and I am finding that I lose system pressure slowly over a period of time, around 0.5 bar in a month during the heating season. Consequently I need to check the pressure on a regular basis and top-up the pressure accordingly. I have checked the system for leaks (when cold) on a few occasions and can't see anything obvious. My theory is that there is air getting into the system through a hole small enough that does not let water out. I have noticed from the schematic for the internal components of the boiler that the connection for the expansion vessel (not too sure if this represents the neutral point in a sealed system) is after the pump which suggests to me that the distribution system will be under negative pressure and hence sucking in air. Has anybody experienced the same problem or is my theory flawed?

 

[bengasman]

Your theory is flawed. You probably have a very small leak that just keeps dripping when the pressure is relatively high during on time. During off time, the pressure gets lower and air enters the system. Typical for a compression joint that is letting by. quantities are so small that it evaporates so you never see anything. Look for limescale traces on compression joints.

 

[chrishutt]

My theory is that there is air getting into the system through a hole small enough that does not let water out. I have noticed from the schematic for the internal components of the boiler that the connection for the expansion vessel is after the pump which suggests to me that the distribution system will be under negative pressure and hence sucking in air.

Air will only be sucked in if the pressure within the system is less than atmospheric pressure (0.0 bar). This tends to occur at high points in a system and/or towards the inlet to the pump but only if static pressure itself is very low.

For example if system pressure is maintained at 1.0 bar at the boiler and the highest point in the system is say 3 metres higher then the static pressure at that high point would be 0.6 bar. A pump would have to create a negative head of more than -0.6 bar to take the pressure below atmospheric, which is just beyond the capacity of normal domestic heating pumps.

If there is nevertheless some suspicion that sub-atmospheric pressures might occur, it is only necessary to increase the static pressure to eliminate that possibility.