The last tee

[Mike Jackson]

But when the heating is off there is no water flowing towards the boiler in that pipe and you get reverse circulation. Reverse circulation isn't an issue when the heating is on.

 

[Gunner-rob62]

To get reverse circulation when heating is off the water would have to flow down the heating return the wrong way. It would then have to circulate through 1 or more rads the "wrong" way before getting into the flow pipe. Assuming that the zone valve is closed correctly the only way to complete the circuit would be to flow the correct way through 1 or more rads and back down the same return to the boiler. For this to happen would involve water flowing through the same section of pipework (ie between last heating return tee and cyl return tee) in opposite directions at the same time.

 

[brightcrow]

To get reverse circulation when heating is off the water would have to flow down the heating return the wrong way. It would then have to circulate through 1 or more rads the "wrong" way before getting into the flow pipe. Assuming that the zone valve is closed correctly the only way to complete the circuit would be to flow the correct way through 1 or more rads and back down the same return to the boiler. For this to happen would involve water flowing through the same section of pipework (ie between last heating return tee and cyl return tee) in opposite directions at the same time.

Not necessarily, as have seen this happen also. Normally caused by something similar to the 'Venturi' effect, where water travels at speed accross top of tee.

 

[Gunner-rob62]

Bear in mind to get reverse circulation you need to complete a "circuit".
We are not talking about hydraulic compression or thermal conduction.
A venturi can only draw water in to an already flowing body of water, still only flowing in the same direction. To make a circuit it needs to get back again. Check out tamz diagrams earlier in thread.

 

[brightcrow]

I see what you are getting at and generally i would agree had i not seen it for myself.

If you had a complete circuit then all the rads would get hot by that principal. Not partially and those closest to cylinder.

My understanding of venturi effect is that if it was across the top of a bottle for instance then would a slight flow in on one side of opening and a slight flow out on other side.

The other theory ventured in this particular situation was that the smaller inner workings of heat exchanger restricted the flow from pump and the excess pressure has to go somewhere. Who knows, but all i can say is that it doesnt seem to occur when you tee the cylinder into the return pipe on the flow up to the boiler and regulate the flow with L/sheild Gate valve etc

 

[Mike Jackson]

To get reverse circulation when heating is off the water would have to flow down the heating return the wrong way. It would then have to circulate through 1 or more rads the "wrong" way before getting into the flow pipe. Assuming that the zone valve is closed correctly the only way to complete the circuit would be to flow the correct way through 1 or more rads and back down the same return to the boiler. For this to happen would involve water flowing through the same section of pipework (ie between last heating return tee and cyl return tee) in opposite directions at the same time.

You don't need complete reverse circulation for warming to take effect. If hot water is pushed up the pipe far enough to flow below a radiator the radiator will heat up by gravity. This will obviously only effect the radiators on the same level or higher than the return connection but I've seen it plenty of times. It tends to be worse when an old boiler is changed for a modern one as the newer heat exchangers are much more restrictive leading to an increase in back pressure.

You may not have seen it happen but I've seen it a lot and altering the return configuration always solves the problem. You've obviously not come across as many poorly installed systems as I have.