Low hot water outlet pressure fron unvented cylinder

[Tam345]

I have low pressure hot water outlet issue with my unvenetd water heater system which is now almost five years old. This has only recently been an issue and the system has operated okay up until now.

The system incorporates a Megaflo boiler unit (Model CL170 from Heatrae Sadia) which is configured for indirect heating via a gas fired boiler.

Recharging the air volume within the Megaflo unit periodically (as per manufacturers instructions) used to resolve this issue, although now does not appear to resolve it which suggests there is now a fault.

I have since drained the system and subsequently refilled the system which does appear to resolve the low pressure issue. Unfortunately after the water is heated the pressure drops off yet again.

If I drain the system and refill the system to restore the pressure then subsequently switch off the indirect heating and direct immersion heater then pressure can be maintained. I have run an outlet tap for about 20 minutes in this configuration with there was no apparent drop in flow rate/pressure at the outlet. This suggests that there is no issue with the pressure reducing inlet valve. I have also had this inlet valve removed and cleaned to ensure no blockages.

Even after a very short time of heating then the drop in flow rate at the hot water outlets is ver apparent. There is no apparent discharge (either visual or sound) from the Temperature and Pressure relief valve during this short heating period.

Does this indicate a definite problem with the cylinder (i.e. cannot maintain the increase in pressure when heating is applied) or could there be issues with indirect heating and central heating configuration that is causing this reduction in flow rate.

Any advice would be much appreciated.

 

[Agile]

How are you measuring the pressure at the cylinder outlet?

Have you measured the pressure at the inlet?

An unvented should be serviced anually. Have you ever had yours serviced?

Tony

 

[Tam345]

Thanks for the prompt reply Tony.

I have not measured the output pressure but it is visually apparent that the flow rate is significantly reduced after the water has heated.

After the system was drained and before any reheating was applied I compared the flow rate (only visually) between the hot and cold taps at all of the sinks in my property. For all locations the hot water outlet flow rate seemed comparable to the cold water outlet flow rate at each of the locations. The flow rate also appeared to be maintained for about 20 minutes as stated before without any problems while the water was cold suggesting the inlet to the cylinder was sufficient.

I have not had the cylinder itself proffesionally serviced. I have the Installation Manual which indicates anually checking the operation of the safety valves (Temp & Pressure Relief Valve and Expansion Relief Valve which is part of the Cold Water Combination Valve) and both these valves operate correctly as defined in the manual.