Odd setup?

[PaulAH]

Quiz time. What’s wrong with the configuration below? It’s drawn roughly to scale / orientation but only shows the essential bits.

I’m particularly interested in the effect of Rad 4 and what might happen in HW only mode.

Answers gratefully received.

 

[kevplumb]

would think it will heat with gravity

 

[PaulAH]

Thanks, Kev.

Can you or anyone else expand on this? It's not Trivial Pursuits - the problem is very real. Our new boiler installation, where we converted from gravity HW and valve-less CH to a fully pumped 3-zone system, is doing strange things.

We have a theory that the builder who messed about with the system when he built an extension has made some duff connections (he got most other things wrong). Trouble is, much of the pipework is hidden and I'm trying to work out what's wrong without taking a can opener to the house.

The diagram shows what I think he's done. If someone can predict the behaviour of this configuration, and it matches what is actually happening, then I can be pretty sure the theory is right and we can take remedial action without unnecessary vandalism.

Thanks again
Paul

 

[PaulAH]

Kev - forgot to tell you. You're warm, very warm!

 

[AdamW]

From the way you have drawn it, RAD-4 should not behave any differently to any other rads in the system. It is plumbed in parallel to the rest of them.

How is it behaving oddly? Is RAD-4 permanently warm whilst HW only mode is on?

Is this rad in a bathroom perchance, perhaps a towel rail?

 

[oilman]

AdamW, Rad 4 may be in parallel with the others, but when water is flowing pressure drops have to be modelled too.

The circulation problem in radiators when the hot water is running is a common problem, and can be cured in this case (I just hope I'm right) by connecting the return from the cylinder to the heating return as near to the boiler as possible.

You might have to take some of the floor up.

 

[DP]

During CH mode the water will flow into the radiator via the inlet valve and during HW demand with CH off, water will now enter radiator through the outlet (return) connection. Radiator 4 will have to heat in tandem with another (or more) radiator to sustain reverse circulation. If all the rads except 4 were isolated, rad 4 would cease to heat during HW mode.

 

[ChrisR]

Is the problem that rad 4 starts to get hot from the return end when HW only is on?

Or is it the bathroom rad, which is meant to get hot whenever the boiler is on?

 

[MANDATE]

It would not surprise me to find rads 1, 2 and 3 got warm also. The hot water having gone through the cylinder coil comes to the tee junction.
On the one hand the hot water rises up to rads 1,2 and 3 , Rad 4 is lower and nearer the tee junction so the pressure has a greater effect and rad 4 will be hotter than 1,2 and 3.
The solution I think would be to fit a one way valve on the return pipe just above the tee so water can only back to the boiler.

 

[PaulAH]

Thank you all so very much, guys. You have confirmed my suspicions.

What actually happens is that in HW only mode, Rad 4 heats up quickly and all the other upstairs rads follow. Not what we intended!

The first oddity was that Rad 4 was getting its heat from the RETURN (or what would normally be its return). DP has it in one - reverse circulation. It seems that water is being pushed back up Rad 4 because of the close proximity of the return connections from the HW cylinder and Rad 4.

Once the water starts moving, there's no stopping it. It comes back from Rad 4 via its "flow" and then, when it joins the 22mm, acts as a feed to all the other rads, further helped by convection. The circuit is complete.

Oilman, as always, has the fix. If we reconnect the HW return to a point BEFORE it splits into upstairs/downstairs, i.e. closest to the boiler, then balance should be restored. Any backflow will meet with a wall of water with nowhere to go.

Does this theory hold good? There's a lot at stake here. With the help of coathangers, torches and mirrors I think we found where the problem lies. Your observations have confirmed this and I thank you most sincerely. If it's a question of re-routing the HW return to the boiler port, that will be relatively painless.

None of this helps a condition in one of the bedrooms where the rad gets hot as soon as you fire up the HW, with no obvious effects elsewhere. I guess it has, quite simply, been plumbed into the HW circuit - and I ain't going to bark any more knuckles over that!

The builder has gone out of business.

Paul

 

[MANDATE]

I came across a similar situation where the HW return and CH return joined at a tee on the route back to the boiler. When the mid position valve was on HW only the hot water went in both directions at the tee and the radiators still got hot. It was quite convenient to lift a floorboard and fit a spring loaded one way valve to 15 mm copper pipe. (about £4 from Wickes 14 years ago)

 

[PaulAH]

Mandate, I'm sorry to have missed you out of the list of acknowledgements! You must have posted yours while I was writing.

Your observation was spot on - those upstairs rads do indeed get hot, very hot. Something else I failed to mention is that all downstairs rads also warm up eventually, though not as hot as upstairs. It's strange to think the whole house is being heated up by water passing through one poxy rad in a spare room, but this would seem to be the case.

The return valve idea sounds good but the more I think about this, the more bothered I am by what is clearly a fundamental imbalance in the system and would like to fix it, if only for the sake of getting it absolutely right. It's also possible that the hot creep effect will remain even with Rad 4 sorted, though to a lesser extent, because of this lack of symmetry.

Luckily we will be able to reconnect the HW return without too much demolition work. It will be relatively simple to re-route the pipe and cap off the tee, which is just reachable through a hole in the ceiling.

Guys, could I ask you to take one last look at the evidence and theory, and confirm that you think we have nailed the problem. Any other cause will probably call for major surgery, something I want to avoid if at all possible.

Thank you all
Paul

 

[AlanE]

Paul the simplest way to view the situation is firstly assume that the heating returns do not exist only the one from the cylinder to boiler.

Now connect ALL the heating returns together, but seperate from the boiler/cylinder return.

Join this common heating return to the cylinder/boiler return and your problem is solved.

Your system has the common failing, particually when a system is extended, of having multiple, return, connections to the common return. Makes system ripe for suffering from secondary circulation as the hot water return has multiple paths back to the boiler.

Alan

 

[PaulAH]

Cheers, Alan. That makes sense. I guess the Oilman solution amounts to the same thing. At least I hope it does ... we're going to do the surgery tomorrow.

Paul

 

[MANDATE]

I know you think your problem is based on radiator 4 only, but if that radiator is closed off, does it solve the problem?.
I feel the return and flow pipes become totally reversed. I agree changing the pipes as planned will improve matters but I'm not 100% convinced it will cure the problem fully. I think some hot water may still find its way back up the return pipe and your upstairs rads maybe warm.
No doubt you will let us know the result!