The last tee

[Damon Apple]

Help:

I am an apprentice so was dissapointed at college when my tutor belittled me in front of the entire class rather than explain what id done wrong..

I drew a y plan including cylinder flow and returns yet he said i was causing reverse circulation because of the location of my last tee position( grumbled something about HW).....I did not understand and was not explained how to rectify the fault....

Can anybody explain this in simple detail please

 

[quality]

Possibly referring to the last tee in the return line from the indirect cylinder return connection

 

[brightcrow]

yeah no doubt about it, all heating returns need to be commoned (joined) before the last tee where hot water is joined. There is much debate about the orientation of that Tee but path of least resistance works for me.

 

[Damon Apple]

I am conufused.. all i had was flow into then return out of cylinder and all my heating returns were on the pipe return back to the boiler......whats the HW return...surely the HW just goes down the sink...sorry but i am an apprentice...

 

[brightcrow]

No worries. so there are four connections to indirect cylinder (generally!!) Cold water feed from cold water tank, Hot water draw off to taps/outlets, heating circuit flow connection and heating circuit return.

In this instance, the Heating circuit return should join the last tee on the return to the boiler in your drawing.

Hope this helps but some research on indirect cylinders may be more useful to you.

 

[tamz]

Here are a couple of drawings to explain it a bit.

This drawing is piped correctly and the arrows show where the water would flow when on for hot water only


On heating only the water would flow like this

If you put the tee in the wrong place as in this drawing, the water would flow backwards through the nearest rad(s) and they would heat up (back heat).

How far it travels and how many rads would "back heat" depends on the distances and resistance.
The water will always flow in the path of least resistance.

 

[Simon F]

Not forgetting the bypass, this could well be the last tee.

 

[midge]

Damon, its never nice to be shown up like that, but I bet in all the years to come you won't get caught out by installing a system incorrectly. Take it on the chin and learn from it.

I've lost count of how many systems I have attended over the years where the rads get warm in the summer! Drawings of systems on paper bear little resemblance to peoples houses but once you've a picture in your minds eye of what to look out for it gets easier.

 

[Gray0689]

Or it could be inbuilt into boiler

 

[Mike Jackson]

I've lost count of the number of jobs that I've been to that have been reported as a faulty 3 port valve because the rads get hot in the summer. I think I've had a couple in 28 years where it was a faulty valve, most of the time it's an incorrect cylinder return. The favourite one is where the boiler has been relocated into the airing cupboard and the cylinder and heating returns flow into either end of a tee with the return to the boiler in the centre. It just screams reverse circulation to me.

The best one I had was where a system had been re plumbed because the pipes under the concrete floor had burst. The cylinder return was connected to the return furthest from the boiler. This was reported as a faulty 3 port and I had to run a complete new return from the cylinder back to the boiler.

 

[brightcrow]

I've lost count of the number of jobs that I've been to that have been reported as a faulty 3 port valve because the rads get hot in the summer. I think I've had a couple in 28 years where it was a faulty valve, most of the time it's an incorrect cylinder return. The favourite one is where the boiler has been relocated into the airing cupboard and the cylinder and heating returns flow into either end of a tee with the return to the boiler in the centre. It just screams reverse circulation to me.

The best one I had was where a system had been re plumbed because the pipes under the concrete floor had burst. The cylinder return was connected to the return furthest from the boiler. This was reported as a faulty 3 port and I had to run a complete new return from the cylinder back to the boiler.

Hey Mike,

Now was that a w/sure diagnosis by any chance LOL

 

[Damon Apple]

This help has been absolutely spot on!

Thanks a bunch especially for the drawings..chin up from now on eh

 

[Halesowen1]

Unfortunately nowadays, the only time many come up against a Y or S plan etc is when they are ripping them out to fit a combi.
But if you do have to fit one, you will now remember the common Tee.

 

[Mike Jackson]

Hey Mike,

Now was that a w/sure diagnosis by any chance LOL

w/sure diagnosis on a w/sure repipe

 

[Gunner-rob62]

I've lost count of the number of jobs that I've been to that have been reported as a faulty 3 port valve because the rads get hot in the summer. I think I've had a couple in 28 years where it was a faulty valve, most of the time it's an incorrect cylinder return. The favourite one is where the boiler has been relocated into the airing cupboard and the cylinder and heating returns flow into either end of a tee with the return to the boiler in the centre. It just screams reverse circulation to me.

It does not matter which orientation the last tee is installed.
The important thing is to have all heating returns connected before the cyl. return tee (water cannot flow in opposite directions at the same time in the same pipe)

 

[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.