Rads coming on when hot water is on

[jaydebruyne]

ive just changed the 2 port on the heating side as I thought it was letting by as all rads started to get hot when only water on demand.

Fired up boiler on demand for water once changed 2 port, flow pipe to rads underneath 2 port stone cold but rads still warming up at a slow rate.

Checked bypass and it's fine. I put 10 replacement rads a couple months ago and when heating on demand all heat up nicely.

Heat only boiler, unvented cylinder.

**update**
I've just noticed it's the return pipes from rads heating up when hot water is on

Any suggestions? Blockage somewhere?

 

[Stigster]

Is the pipework following the three tee rule?

 

[jaydebruyne]

Is the pipework following the three tee rule?

Not sure as return paths are concealed. "Apparently" this only started happening when I changed the rads! Pffft

 

[Stigster]

I wonder if the old rads had unidirectional TRV's (if they had TRV's) which prevented this reverse circulation? Now you have new rads in with bi-directional valves the problem has been able to show itself?

 

[jaydebruyne]

I wonder if the old rads had unidirectional TRV's (if they had TRV's) which prevented this reverse circulation? Now you have new rads in with bi-directional valves the problem has been able to show itself?

Good point!! I'm gonna try pop back today so I'll see if I can get a butchers at the return path and order of the heating, water and bypass.

 

[jaydebruyne]

Good point!! I'm gonna try pop back today so I'll see if I can get a butchers at the return path and order of the heating, water and bypass.

So the order after the pump is:
DHW, Heating, Bypass
Should be:
Bypass, DHW, Heating. Same order on return. But return pipework is concealed.
That right?

 

[KingDoey]

DHW should be the final connection before the boiler

 

[Stigster]

Yes, KingDoey has it right. The original installer didn't follow the 3 tee rule. DHW last connection before boiler. This diagram shows you what happens when you tee in the DHW return before the Heating circuit. I reckon that's what you've got going on and it has only shown as a problem with your new bi-directional TRV's.
http://www.hertsboilers.co.uk/tech/backflow.jpg

 

[GH77]

Put a check valve on CH return