Boiler bypass pipework – correct design?

[StephenL]

A plumber recently told me my bypass pipework is poorly designed as it’s offering a low path of resistance v the motorised valve T junctions, and therefore is passing water constantly. I have noticed that, some minutes after the CH starts up, this bypass pipe gets hot along its entire length up to the point at which it meets the Return pipe to the boiler which is cooler.

The rads & DHW cylinder work well (they get hot quickly and there is no boiler cycling).
The boiler, DHW cylinder and MVs are all located in the basement.

The attached picture shows 3 MVs ['CH top 3 floors' (28mm In/Out), DHW cylinder (22mm In/Out) and 'CH Basement' (22mm In/15mm Out)], then the bypass which begins at the bottom of this short run. Below this you can see the gas pipe to the boiler and the filling loop with pressure gauge.

The bypass has a fixed valve set slightly open + an ABV set to 0.5.

The plumber thought the bypass should T off between the top & middle MVs (ie, 'CH top 3 floors' MV and DHW MV), but there is not enough pipe length to do this.

Is he right about the poor design?
What would be a better design?
If this is complex to achieve, is it worth it V the potential efficiency improvements?

 

[StephenL]

keep turning the bypass up until it doesn't do that. Then once you've got it so its not red hot the other side of the bypass, turn off your demand and let pump overrun work, it should now get hot very quickly.

If it does that I wouldn't look much more into it. If that doesn't work then replace the bypass and re-configure the pipe layout. No point wasting time refilling and draining for the price of a new bypass, just do 2 birds one stone.

Thanks for the guidance. The ABV is new, but might be faulty. I have set it to its absolute maximum (0.6), but the bypass pipe still gets hot on the output side of the ABV, with all TRVs set to maximum.
You mention re-configuring the pipe layout - what should the new layout be?
(I note that a bypass pipe should T off between the pump and the first Motorised Valve, whereas in the current configuration my bypass is located after the pump and the 3 MVs.)

 

[ShaunCorbs]

What speed is your pump on ?

 

[StephenL]

What speed is your pump on ?

What speed is your pump on ?

Originally Constant Pressure 2, but after feedback from Chuck regarding incompatibility with an ABV, I set it to Constant Speed 2.

 

[ShaunCorbs]

Try knocking it down to cp1 and see if your system works as normal eg all rads heat up etc

 

[StephenL]

Try knocking it down to cp1 and see if your system works as normal eg all rads heat up etc

Sorry, my mistake. The pump was originally set to CP1 at the beginning of this thread, then changed to Constant Speed 2. The problem is the same with both settings.

 

[Chuck]

(I note that a bypass pipe should T off between the pump and the first Motorised Valve, whereas in the current configuration my bypass is located after the pump and the 3 MVs.)

If your description is accurate, the current configuration needs fixing so that the bypass allows circulation when the zone valves are closed and the pump is operating for the 'overrun' period.

 

[Chuck]

Originally Constant Pressure 2, but after feedback from Chuck regarding incompatibility with an ABV, I set it to Constant Speed 2.

I wouldn't use constant speed on a Y-plan system like yours. CP works better, you just need to set the ABV carefully, by trial and error, rather than guessing a setting.

Some of what you say makes me suspect your ABV is never closing, i.e. faulty.

 

[Chuck]

... on a Y-plan system like yours.

I should have said "S-plan plus", of course!