Motorised valve passing?

27 Jul 2008
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United Kingdom
Would the following symptoms suggest a passing motorised valve - heating side?

HW on: rads heat up from return side. Path is via bypass (gate valve). Being summer (!) we shut off all the TRVs to save this unecessery heat and waste of fuel. When we do this, return is cold but the flow pipe beyond the CH 2 port valve does get hot but only for about 30cm or so.
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Sounds like an Installation Fault, with the Return connections, It is very important that the nearest Return to the boiler is from the Cylinder, if any Heating Returns are fitted between the boiler & Cylinder, then reverse circulation will occur, You will need to find the fault and re-route the pipework correctly, could be easy, but also a Pig i'm afraid

Good news though - your M/Valve sounds fine! ;)
Should there be any heat getting beyond a closed M/valve?

No, but sometimes not 100% positive shut off. That is not cause of your problem in this case.
Follow Boilermans advise.
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Yes, but problem pre-dates this.

Regarding return connections. Return from cylinder tees into main return immediately before it enters the boiler.
But do all the heating returns join together before the join the common return.

If you could sketch the pipework, we could tell straight away.

Image attached (not fantastic quality). For info, shutting the bypass (gate valve) does stop the radiators getting hot.

As for common return. The only thing that is common about it is the final few inches. It is the main reurn pipe from the heating circuit that enters the boiler and the return from the HW tees in just (i.e a few inches) before it enters.

Have had professionals round 4 times. 1 said "must be pipe in wrong place" but we never heard from him again, 2 had no idea why the problems occur and the most recent thinks its the m/valve letting by hence the title of this post.
As I said a classic case of reverse circulation, it cannot be anything else.

Unfortunately your drawing is missing all the important pipes.

Forget the cylinder for now, we need the htg flow and return pipes.

In the meantime shut that by-pass valve off and see if it makes any difference.
OK, The return pipe from the rear of the house is the wrong side of the by-pass valve.

Which opens the loop from the other circuit.
I cannot see why the inaccessible parts of the system are anything other than the flow pipe with microbore feeds to radiators, coming back as the return with the microbore returns from the radiators. What other configuration would lead to the problems? The cylinder is directly above the boiler at the rear of the house, well away from this part of the heating system. For info I have a cellar so the bits that I have drawn are totally visible.

The plumber we had in suggested closing the bypass which he did. This does stop the radiators heating on “HW only” but the boiler cut out. All was cold and the pilot was out. Boiler has pump overrun.

[OK, The return pipe from the rear of the house is the wrong side of the by-pass valve.]
The radiators at the rear of the house are not affected. Would you expect this? (NB: I’m a total layman so sorry if asking obvious questions!) Can you explain to a layman why in reverse circulation there is flow against a closed m/valve?
The plumber we had in suggested closing the bypass which he did. This does stop the radiators heating on “HW only”

There is your answer.

You would need to move the bypass connection (on the return) to right before the boiler as the reverse circulation is coming from there. The microbore runs at the front of the house from the manifold may be shorter.
Your boiler is about 20 years old (and is getting near the end of its life although it is still working) and may have a bit of a blockage inside which is worsening the problem. You could maybe do with giving the system a good clean out or even better think about replacing it. You wouldn't have kept a Morris Marina ((out around the same time as this boiler) just because it was still working.
Would an automatic bypass be a solution/compromise. Boiler couldn't cope with manual bypass closed with just HW on.
Gawd blimey, I tell ya what the problem is and you don't believe me :rolleyes:

The return from the rear heating is the wrong side of the by-pass

Either move the return or the by-pass connection, or just swap them over if it's easier.

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