3-tee rule on unvented cylinder

[Jennie]

Hi all,
Just getting my head in a twist regarding the 3-tee rule (last tee on the return pipe should be hot water).
I have a customer with an unvented installation, with two 2-port valves for CH and HW. He's built an extension, and wants to add a new circuit of radiators to the system. The new leg of rad pipes would cut in just under the boiler, with the new rad pipes becoming the last tee on the return (instead of the hot water), and also cutting in after the bypass valve below the boiler. The flow would cut in just after the central heating 2-port valve.
Could breaking the 3-tee rule lead to reverse circulation (when hot water is on, the new radiators get hot)? Surely a closed 2-port valve should stop any circulation, or could the circulation occur up/down within that return pipe? Or if it's a problem, could i put a non-return valve on?
My preferred option is to follow good piping design, and cut the new rad pipes in further down the pipework, before the HW return and away from the bypass. Though the pipework won't be as convenient, and I'd need to drain down the system.
He's particularly keen to put the connections under the boiler, as I'm due to change a faulty zone valve using a pipe freezing kit, (to save draining down a big property). Hence he's keen to fit the new pipes in at the same time, in the frozen section, and get everything up and running quickly.
Advice would be much appreciated.
Thanks
Jennie

 

[Ric2013]

I think the concern in your case is parasitical circulation within a single pipe when in DHW-only mode. To be honest, I think there is a risk of the same happening to the cylinder when a system that complies with the 3 tees rule is in CH-only mode, but it's less of a problem which is probably why we generally ignore it.

I suppose non-return valves would prevent this happening. Perhaps an anti-gravity loop would also work with fewer potential downsides?

 

[Vee]

How far is the cylinder from the boiler, or where do the returns meet? Is it a sealed primary system?

 

[fezster]

The 2 port valve won't stop the reverse circulation (as the water doesnt flow through it when this occurs).

The return water from the HW cylinder may go back up the return pipe for the CH (as it is the last tee). It will then flow in reverse through the radiator flow pipe to the next radiator and then back down the return pipe of the second radiator, before returning to the boiler.

There was a brilliant diagram posted on this forum many years ago, but I cant find a link to it right now.

 

[Jennie]

The 2 port valve won't stop the reverse circulation (as the water doesnt flow through it when this occurs).

The return water from the HW cylinder may go back up the return pipe for the CH (as it is the last tee). It will then flow in reverse through the radiator flow pipe to the next radiator and then back down the return pipe of the second radiator, before returning to the boiler.

There was a brilliant diagram posted on this forum many years ago, but I cant find a link to it right now.

Thanks so much. It finally makes sense.