Hot water on - Radiators heat up from return leg

29 Apr 2019
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United Kingdom

I think we have had this issue since we moved into our house about 6 years ago. I've adapted how we heat our hot water (set to short periods and times of day) so it hasn't effected us much but after looking into this further I think there is seriously something wrong with our system.

Basic setup - Gas boiler, Megaflow 210l tank, about 15 rads.

We've noticed since we moved in that the rads seemed to heat up while the heating was off (summer, middle of the night). It was always fairly mild and I could never get anyone to really investigate it properly. More recently (last few years) we have been using solar diversion and cheap night rate electric to heat the tank so it stopped during summer completely because the boiler wasn't heating the tank at all or very rarely.

I've watched a few videos trying to understand why it happens and how to investigate. First thing I can now confirm it 100% will heat the radiators when the central heating is off and only the HW is supposed to be running. It takes a while as the tank needs to get pretty hot before you can really tell the rads are also being heated but it does happen. Hot tank plus 30 mins of extra hot water leads to some radiators being as hot as if the heating was on.

Secondly I've confirmed that when the heating is on one side of the radiators gets hot but when they heat up from the hot water being on its the other side of the radiators that heats up. I believe this is a sign of reverse flow and a sign that the pipework is at fault rather than the more common cause of a faulty valve?

I cannot get my head around the pipework. I've included some pictures but please ask for more if you think you can help.

Some places talk about three way valves and those breaking but I seem to have two separate valves instead.

I'm a bit concerned that I'm getting a bit confused and some of the rads are setup as summer rads. From My understanding for this setup to be true there would need to be a hot feed straight after the pump going to these special rads and there does not appear to be one. So for now I don't believe this is the case. this was the most common explanation from any plumber I've asked. That the radiators I was taking about have been setup to do that. Its all the radiators so I obviously don't believe this to be true.

I would love any help anyone can give me. I'd really like to be able to get someone around to fix it with a good idea myself what I'm needing done and why.


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Sounds like you may have reverse circulation. It takes some getting your head around (or it did me anyway :)) but I found the explanation and graphics here. very useful.
This is video that made me suspicious that reverse circulation might be the issue.

Sounds like you may have reverse circulation. It takes some getting your head around (or it did me anyway :)) but I found the explanation and graphics here. very useful.

So did you have the return from the tank moved in the end to solve the issue?

What I’d really like to do now is identify how the pipes come together and where so I know if the issue is as he describes in that video.
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Sorry it wasn't an issue I had personally, it was just useful for my understanding of what reverse circulation is.
OK I've investigated a bit further and the return from the cylinder is the return for all the radiators but is also before all the radiators in the system. It now makes sense to me that all the radiators can heat up because when the cylinder coil is being heated the return from the coil can reverse feed into the radiators. I assume this is a massive design flaw but if any can check my photos that would be very useful.

Hot water from Boiler from top through pump. There is nothing between the pump and the boiler so no intentional "Summer Heating"

No three way valve. Two on/off valves instead. One for central heating and one for Hot water.

I'm a little confused about why the boiler/pump flow is directly connected to the return leg through the green valve. I assume that is a pressure valve and if both the HW and CH were broken and their valves didn't open the valve would let water back to the boiler?

I think everything on the cylinder is correct and the valves appear to open correctly.

However the return from the cylinder is immediately below the green valve and the first connection to the return. Am I getting this wrong or should the return from the cylinder be at the end of the return after all the radiators have been fed back into the return?

If I get someone in to correct this the solution should be to have the return from the cylinder connected last before everything reaches the boiler?

I'm intending to get a pro in of course but its so much easier if I have an idea what needs to be done. Has anyone got any idea why it has been connected like this?


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