Return issues???

[Gasmouth]

Hi all.
ive been called to a job and they've had an extension around a year ago and upgraded from a combi to an unvented system.
Theyve stumbled across an issue now as they've only just turned their heating off.
when the heatings on all is ok.
heating and hot water ok.
when just hot water the return pipes on the radiators in the extension get hot first before heating the radiators (just in the extension).
when the heating is on the flows heat up first.
what could be causing this backfill?
i know it must be piped up wrong but how and why is this happening?
its probably very simple but something I've not come across before.
look forward to hearing some replies.

 

[Mike Jackson]

Google the three t rule. Hot water return is probably piped up wrong.

 

[Gasmouth]

That must be it. Bypass, cylinder, heating.
I'll be honest I was never taught that as an apprentice. Good to know!

 

[gassafe]

Sounds like the radiator return pipework from the extension have not been teed in correctly...

Iv seen similar problems before when rads are added & teed into the cylinder return, it causes reverse circulation ( hence different sides of the rad heating up )

 

[Leo21]

It happens a lot on loft conversions, the t is probably piped up wrong. To close the cylinder return, a non return valve is an answer sometimes.

 

[bright spark]

Yeh I'd probably go with a non return valve on d extension return circuit. That should do it. (Famous last words!)

 

[Best]

Yeh I'd probably go with a non return valve on d extension return circuit. That should do it. (Famous last words!)

But would that work? The return is still a return, - it is just that it is joined into the "live" part of the return circuit. A non return valve would still have to be allowing direction of return flow,- so no good.
I have came across this done where the plumbers teed off the return first to some rads in new builds and not a thing I could do with it where I couldn't get at pipe work below oak or solid floors

 

[Leo21]

But would that work? The return is still a return, - it is just that it is joined into the "live" part of the return circuit. A non return valve would still have to be allowing direction of return flow,- so no good.
I have came across this done where the plumbers teed off the return first to some rads in new builds and not a thing I could do with it where I couldn't get at pipe work below oak or solid floors

You'll find it's going the other way, flowing down the return so to speak.

 

[Best]

It's been a long time since I have thought about or came across such issues. I will think on it when I get my brain working. Think you are correct, - the return coming back creates a loop through some or all rads if there is more than one tee on the return. But would that not mean that if it can't use one of the return connections as an entry - it surely will go through the other one? Or is my mind too muddled?
I do understand it, but just gets so confusing when I go back to it.

 

[gassafe]

I fit a check valve to remedy it once but the lady phoned up & said there was a rattling noise when the heating was on plus it acts as a restriction on that circuit!

I fit a swing valve in the end, same as a non return valve but operates on a swing arm hence no noise it fixed the reverse circulation & never been back since!

 

[paulusgg]

I've tried to solve a reverse circulation problem twice (on two different jobs) but using a non-return valve and it works brilliantly for a while.
Then a couple of months later you get the call complaining of strange noises coming from the central heating and you try all sorts of things to get rid of the noise, changing pumps, swapping servos in the two port valves etc.
The only remedy that's worked is to repipe correctly following the 3T rule and ensuring the return is far enough way from the cylinder return

 

[Best]

The only remedy that's worked is to repipe correctly following the 3T rule and ensuring the return is far enough way from the cylinder return

I don't think it should really matter where you tee off the return to the rads. One thing to note is you can get a slight gravity flow from that connection if you have the pipe rising or even if it is level and nearest rads are close to it.

 

[leerdonald]

If the CH return lies before and too near the HW return you can't help but get some reverse circulation, hence the nearest rads warming when only HW is on.