Low hot water outlet pressure fron unvented cylinder

[Tam345]

Having problems resolving a low hot water outlet flow rate from an unvenetd water heater system at all the hot water outlets throughout the property.

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 does not appear to resolve this issue.

I have also fully drained the system and subsequently refilled it which does appear to resolve the low flow rate. Unfortunately after the water is heated the flow rate drops off yet again.

If I drain the system and refill it to restore the pressure then subsequently ensure both the indirect heating and direct immersion heater are switched off then pressure can be maintained. For all locations in the property the hot water outlet flow rate seemed comparable to the cold water outlet flow rate after a cold refill. I have run an outlet tap for about 20 minutes in this configuration and there was no apparent drop in flow rate at the outlet. This suggests that the cold water inlet to the cylinder was sufficient and there is no issue with the pressure reducing inlet valve. I have also removed this inlet valve and cleaned the strainer to ensure no blockages.

Even after a very short time of heating then the drop in flow rate at the hot water outlets is very apparent. There is no apparent discharge at the tundish (neither visually or no sounds) from the Temperature and Pressure relief valve during this short heating period suggesting this valve is also okay.

Thinking this is likely to indicate a definite problem with the cylinder itself. Any advice would be much appreciated.

Thanks.

 

[diamondgas]

Hi Tam345 and :welcome: to the form 🙂

Sounds like a strange scenario you're describing ... flow okay when cold but not when hot! Is there any kind of temp controlled mixer valve fitted to this system?

 

[Tam345]

Hi diamondgas, thanks for the response.

Think there is only one, which is a thermostatic mixer for a shower on the hot water outlet.

 

[diamondgas]

How many outlets we talking just to give a picture of the system?

Like yourself I'd be thinking something going on within the cylinder that doesn't like heat ... I'm not familiar with the heatrae you mention so will have to check out the mi's ....... unventeds tend to be straight in and out regards the hot water supply though yeh?

 

[Tam345]

6 outlets in total and all would appear to have been fine previously. Periodic recharging of the air volume sometimes required and resolved simillar problems.

One cold feed and one hot outlet with a couple of safety valves for pressure and temperature relief in the system.

Single flow and return also for the indirect heating.

Just thought I might try heating by the immersion only instead of the indirect boiler on the off chance that it may be something in the indirect heating loop.

Thanks again.