wired 2 zone heating problem.

[Stephen H]

Hi all,

I have two heating zones. one upstairs and one downstairs with seven rads each making a total of 14 rads.

There are two Salus zone vales in the flow side with a upstairs flow and downstairs flow pipes and a common return pipe (to keep the system volume down as a combi boiler is being fitted) and to avoid too many joist notches by notching for 3 22mm pipes rather than four.

Now upstairs zone works fine, heating all seven upstairs rads.

Now when I put on the downstairs zone, all 7 downstairs rads heat up.

But two upstairs rads heat up.......

The bathroom and bedroom rads are almost vertically above the toilet and utility rooms with about 2m separating them laterally.

The bathroom and bedroom rads do join together for a short loop of "local" upstairs flow pipe and "local" common upstairs return pipe before rejoining the rest of the system.

The utility and toil rats do share a "local" common downstairs flow and "local" downstairs common return for a vertical 2.4m run before joining the rest of the system.

Upon further investigation, when the upstairs zone is on only, the left hand pipes on the bathroom and bedroom rads is the flow pipe and the right hand pipes is return as measured with two thermocouples (as exactly how I plumbed it).

When the downstairs zone is on, the right hand pipes on the bathroom and bedroom rads becomes the flow pipe and the left hand pipes become the return pipes according to the two thermocouples. So its clear that the water is going through the two upstairs rads the wrong way. There is also a bigger temperature drop across both rads which is consistent with a lower flow rate.

This leads me to believe that thermosyphon flow is occurring via the return from the two downstairs rads to the common return to the upstairs rads and then "flowing the wrong way via the upstairs flow pipe to the upstairs zone valves.

The two zone valves are of the rubber ball type and are brand new. Clearly, they are not good at stopping reverse flow.

Now would a pair of "one way anti-syphon" valves placed immediately after the two zone valves cure this problem? Do such things exist?

I plan to put one of each immediately after the two zone valves so that water *cannot* flow the wrong way past the two zone valves. That hopefully will stop thermosyphon flow occurring between the downstairs and the upstairs zones?

Regards,

Stephen.

[Onetap]

You've built in an unintended alternative circuit route, water goes backwar= ds through an upstairs rad and then backwards through the other rads, or so= mething like that. Sketch out the schematic and look for any alternative ci= rcuit routes. The commonest one is when someone joins the upstairs heating = return to the DHW return to save the length of one of the two vertical retu= rn drops.=20

The rubber balls have been known to swell with some inhibitors, but I doub= t it's that. There is a lot of force holding the ball shut and it's unlikel= y the water could force it open.

[John Rumm]

I have not drawn it all out from your description, but it sounds as if there is a gravity circulation loop of some form getting setup - it may not be back through the valve.

A non return valve, or perhaps even an anti syphon loop (i.e. take the pipe up down and up again for a bit) may be enough.

Assuming that it is getting past the valve, and not some other route... Does the feed pipe to the upstairs rads after the valve get hot when that valve is off and the downstairs is on?

[www.GymRatZ.co.uk]

On 18/03/2012 20:45, Stephen H wrote: his leads me to believe that thermosyphon flow is occurring via the

Without getting lost in all the techno-circuit-design stuff I would suggest it is nothing more complicated than heat by conduction from the interconnected return pipes of both zones.

At some point the return from both zones joins together before entering the boiler, and as water is a brilliant conductor of heat anything that is connected to the higher than ambient temperature return pipes will rapidly heat up to the same average temperature. More so that the upstairs radiators offer an easier and more natural route for the heat to follow.

if the zone valve is shut then even with no-flow the heat will still travel through the water via conduction.

Only way round it would be to mess around trying to mechanically isolate the upstairs return from the downstairs or run the upstairs return to a much closer point to the boiler giving a longer path for the heat to track back through. Seems like a huge waste of time and effort for the sake of a little bit of background heat.

In my opinion... having experienced something similar with our shop fan heaters firing up when zone-off because the return from the flat above was plumbed into the shop return... This was also cured by re-plumbing the return back to the boiler rather than tapping into the shop return.

Pete

--

we have one of those too.