Boiler over-run, hot radiators?

[RubberJ]

Hi,

I've got a two a zone system that was installed a year or so ago. One thermostat upstairs that controls the radiators up there, one downstairs that controls the underfloor heating (one large zone - open plan).
I've noticed that the upstairs radiators get hot when the underfloor heating is on downstairs, but the upstairs thermostat hasn't called for it. I've just checked the motorised switches, and they work correctly, opening the called for zones independently as they should. But I think the radiators upstairs are being heated when the downstairs thermostat switches off and the pump over-run is dissipating the heat through them.

My question is: should this go through ALL the radiators upstairs? I thought if there was the need for a bypass circuit to help dissipate the heat on the over-run it would just go through, say, the towel rail. Oh, and that's another question: should my towel rail have a trv on it (which it currently does, as all the other radiators do)? Shouldn't it be always on to allow for a bypass loop?

You'll have to excuse me if I'm getting confused about old v new boilers, etc., I'm just picking up what info I can before I get on to the plumber. His answer when I asked this was that it was all normal, and I just want to check.

And yes, I know I can control each radiator separately via each trv, but although they're set, they're still coming on when there's no-one up there, which is what I was trying to avoid.

The boiler is a Worcester Bosch Greenstar 37 CDi combi.

Thanks for any pointers or suggestions.

Cheers.

 

[D_Hailsham]

Check which of the two pipes connected to each radiator is the hotter, (a) when the upstairs rads are on, (b) when the fault occurs.

If the hotter pipe stays the same pipe in both cases, water must be getting past the motorized valve for the ground floor circuit.

If the hotter pipe swaps over, you have reverse circulation caused by incorrect connection of the return pipes.