CWS Tank Not Draining

[GJS]

I have a conventional CWS tank in the loft which feeds a hot water tank in the airing cupboard. The CWS tank has three outlets, 28mm to the hot tank, 22mm to the rest of house cold and 22mm dedicated feed to a shower pump.

When I try to drain the tank by shutting off the supply to the ball cock and turning on a cold bath tap, the tank drains to about one third full then stops. I can finish draining it using a hot tap or the supply to the shower pump but why wont the rest of house cold outlet drain the tank???

I've checked that there is absolutely no pipework above the bottom of the tank, all the pipes from the tank have a slight fall on them. This is really weird! Any ideas what's happening?

Thanks.

 

[Blasphemous]

The cold water outlet is at the bottom of the tank isn't it?

 

[GJS]

Yes, they're all at the bottom of the tank, hot outlet is slightly higher which I believe is correct (avoids scalding under mixers if tank runs dry).

 

[larrytheleak]

are there any gate valves--stopcocks--or isolation valves on downservice pipework