ecotec 418 bypass loop is after external pump

[easterlywinds]

Hi everyone, I've been trying to understand this heating loop for a while and wanted to check if my understanding is missing anything.

Context: an Ecotec 418 set up in heating mode in attic. A single pipe runs down to the flat, where it splits into two (Y configuration), with one pipe charging an unvented cylinder, and one pipe going to radiators. The pipes join up again and travel back to the attic to the boiler. Each of the two branches at the Y-split has it's own zone valve controlled by the heating logic.

There is an external pump on the return pipe in the attic, controlled by the boiler. There is also a bypass valve + pipe, but this forms a bypass right between in the inlet and outlet of the boiler itself, and the "bypass circuit" does not include this external pump.

My understanding is thus that if both of the Y-split zone valves are closed, then no matter how hard the pump spins, there will be no water flowing around the bypass loop because the pump is outside this loop. Thus the boiler overheats and shuts down, and this is indeed what we were experiencing. We implemented a quick fix a while back - for the cylinder charging zone valve to always be open, so that the boiler can gracefully turn off instead with proper pump overrun through the cylinder if there is no radiator heat demand.

However I suspect this is causing a lot of inefficiency as it means water is constantly circulating all the way down from the attic whenever the boiler is checking for heat demand, even if the cylinder is hot enough. Likewise, the boiler is always "drip feeding" heat into the cylinder and thus endlessly cycling.

Mainly two questions:
1) Is it correct that the Ecotec 418 does not have an internal pump, and thus a bypass loop around the boiler itself is useless/likely incorrectly installed)
2) Assuming (1), is the only option to fix this to reroute the bypass to include the pump?

 

[fixitflav]

My understanding is thus that if both of the Y-split zone valves are closed, then no matter how hard the pump spins,

If it's wired up correctly the boiler cannot fire when both valves are closed. The boiler and pump are powered from a microswitch in the valve, which only closes when the valve is fully open.
Having said that, the bypass doesn't sound right, it should be between pump outlet and boiler return. But do you need a bypass? If you have at least one rad without a TRV there isn't a no-flow situation.

 

[Johntheo5]

All series "4" (heat only boilers) vaillants dont have a internal pump, the "6" (system boilers) do as well as the "8" (combi boilers) series, the pump above should have been installed on the boiler flow with the bypass then installed between that and the return, the "4" doesn't have a internal bypass but the others do, but the 418 ecotec plus does show a adjustable time pump overrun parameter, d.1 but don't know if this provides a volt free contact or whatever to send a signal to the external pump.

 

[fixitflav]

1) Is it correct that the Ecotec 418 does not have an internal pump,

You said there is an external pump, and it's very unlikely there is an internal one as well.

2) Assuming (1), is the only option to fix this to reroute the bypass to include the pump?

Do you need a bypass? See my #2. You haven't got an effective one at the moment, and you haven't said it's a problem.

 

[easterlywinds]

Thanks both for the replies!

If it's wired up correctly the boiler cannot fire when both valves are closed. The boiler and pump are powered from a microswitch in the valve, which only closes when the valve is fully open.

The cylinder/rads and boiler are in completely different locations (building attic and flat), and I'm almost completely certain they have no electrical connection to each other. The boiler definitely still fires when both valves are closed, and immediately proceeds to overheat and error out due to no flow. This is the state we received the system in, and our temporary fix was to keep the valve to the cylinder always open.

Having said that, the bypass doesn't sound right, it should be between pump outlet and boiler return. But do you need a bypass? If you have at least one rad without a TRV there isn't a no-flow situation.

Indeed, the pump itself is on the return flow pipe going to the boiler, and outside the bypass loop. So as there is no internal pump, it seems the bypass loop does nothing at the moment (as I suspected).

I don't strictly need a bypass, but I suspect that the current solution of keeping the valve to the cylinder in the flat open, to ensure no-flow is causing a large amount of wasted heating, as every 10 minutes or so the boiler will circulate water all the way down from the attic to the cylinder and back up (a lot of floors), when instead both valves could be just be closed when there's no heat demand, and the boiler would flow some water through the bypass and shut off.

 

[fixitflav]

The cylinder/rads and boiler are in completely different locations (building attic and flat), and I'm almost completely certain they have no electrical connection to each other. The boiler definitely still fires when both valves are closed, and immediately proceeds to overheat and error out due to no flow. This is the state we received the system in, and our temporary fix was to keep the valve to the cylinder always open.

Sounds like the wiring needs correcting. If the boiler isn't powered from the valves, how is it? Just from a timer, or a manual switch?

I don't strictly need a bypass, but I suspect that the current solution of keeping the valve to the cylinder in the flat open, to ensure no-flow is causing a large amount of wasted heating, as every 10 minutes or so the boiler will circulate water all the way down from the attic to the cylinder and back up (a lot of floors), when instead both valves could be just be closed when there's no heat demand, and the boiler would flow some water through the bypass and shut off.

You could re-pipe the bypass, and if it's close to the boiler it would then likely waste less heat than the water going all round the house. But re-doing the wiring would better (for wasted heat) and might be easier to do, no draindown needed.