central heating pressure keeps dropping

[davygravy]

I keep losing pressure in my system, one engineer said at it and said it was the expansion vessel, but never came back to sort it, next engineer said it wasn't but pumped it up anyway, when the heating is on and the hot tap is turned on the pressure jumps up, to which the engineer said he has never seen that it normal drops.
Can anyone shed some light on the situation?

 

[davygravy]

THANK YOU.

 

[ianmcd]

Make and model of boiler would help

 

[Boiler medics]

Hi the pressure gauge will increase slightly by .25bar if the hot water tap is turned on during a heating cycle because your boilers diverter valve closes off the flow port to the heating circuit and concentrates sending this heating water through the boiler main primary exchanger and hot water platent exchanger ,to produce instantaneous hot water .This change of water circulation provides less resistance to the pump which is capable of 5 to 6 meter pump lift at low static head and low flowrates therefore because of the small amount of water volume reqd to circulate through these two exchangers the pump pressure characteristics naturally increases the internal pressure in the boilers waterways mainly because the pump speed do not reduce accordingly when this changeover takes place .
With regards to your pressure loss check the overflow pipe isn't leaking on the outside of the property this will be a 15mm copper pipe usually a meter or so below the flue terminal ,and see what pressure the gauge goes up to when the heating gets to 82deg c if it rises to 3bar and above I would definetly be looking at recharging the expansion vessel , let me know if I can be of anymore help , Boiler Medics

 

[ianmcd]

Hi the pressure gauge will increase slightly by .25bar if the hot water tap is turned on during a heating cycle because your boilers diverter valve closes off the flow port to the heating circuit and concentrates sending this heating water through the boiler main primary exchanger and hot water platent exchanger ,to produce instantaneous hot water .This change of water circulation provides less resistance to the pump which is capable of 5 to 6 meter pump lift at low static head and low flowrates therefore because of the small amount of water volume reqd to circulate through these two exchangers the pump pressure characteristics naturally increases the internal pressure in the boilers waterways mainly because the pump speed do not reduce accordingly when this changeover takes place .
With regards to your pressure loss check the overflow pipe isn't leaking on the outside of the property this will be a 15mm copper pipe usually a meter or so below the flue terminal ,and see what pressure the gauge goes up to when the heating gets to 82deg c if it rises to 3bar and above I would definetly be looking at recharging the expansion vessel , let me know if I can be of anymore help , Boiler Medics

And as you dont know the boiler make or model I guess you are assuming it is not a Bi-Thermal heat exchanger

 

[Boiler medics]

Your right Ian I am assuming it is a bi-thermal exchanger boiler he has installed as with the majority of manufacturers boilers incorporating a single heat exchanger design the pump is not required to run for hot water production. it would be helpful to know the manufacturer and model of his boiler.

 

[davygravy]

Thanks for the replies, boiler is a potterton performa 30he,expansion vessel has been recharged but still rises to over 3,cheers

 

[flameport]

,expansion vessel has been recharged

Exactly how was this done, and was the air pressure in the vessel measured?

It should have been
a. remove all pressure from system by opening a suitable drain valve and keeping it open
b. check pressure in vessel and add more air if required.
c. close drain valve, add water until correct system pressure is achieved.

 

[davygravy]

Yes this was done, cheers

 

[Boiler medics]

do you know if the expansion vessel is connected with a silver braided hose ? if so it may be kinked or blocked stopping the system expanding into the vessel , this hose can get blocked solid with sludge , also check your shraeder valve on the top of the vessel is not leaking after you removed your foot pump hose .if the problem persists after you put on the heating it can only be a passing filling loop or a defective membrane within the expansion vessel . wait your reply ( boiler medics )