Cold Rads Downstairs when hot water on.

[richardhill84]

Hi,

My CH system works fine (i.e. all rads up and downstairs are hot) most of the time.
However, when i run a bath, the rads go cold downstairs.
The rads downstairs only heat back up again until when the HW cylinder is back at the correct temperature.

I understand that when the temperature drops in the HW cylinder that the 3 way MV value will open (HW side). It's strange. It's almost like theres not enough pressure (i.e. to much resistance) to heat the rads upstairs, downstairs and the HW at the same time.

Note that the MV and pump were new last year.
The pump is on max speed.
I have had fernox system restorer/cleaner around the system twice.
I have flushed out every RAD downstairs and flushed the flow and return pipes out the ran clear.
Turned all the locksheilds on the upstairs rads to half turn then quarter turn.

Any ideas???

Any ideas?

 

[kevplumb]

is there a gate valve on the return from the cylinder

 

[richardhill84]

i'm not sure to be honest.

I take it that will make a difference? Are you suggesting it may be open to much????

 

[kevplumb]

yup

 

[richardhill84]

What does this value actually do/control.

 

[kevplumb]

the valve controls the flow of water through the cylinder

if the water is passing straight through it's the path of least resistance to flow

so it goes through the cylinder coil and back to the boiler not the rads

 

[richardhill84]

I see. I thought it might be the the stop cock on the supply side of the HW cylinder (i.e HW side of the MV). I have tried throttling that back but it didn't seem to help.

I'll look to see if there is a stop cock on the return.

Thanks for your help.