Gas boiler: correct layout of bypass circuit

[downthetube]

Folks, I'm new to the forum and a DIY chap. This is by far the best source of help I've found on the Web.

System:
10-year-old installation
Fully pumped S-plan
2 x 2-port zone valves (HW and CH)
Potterton Suprima 80
Honeywell 2-zone programmer
Manual room thermostat

I am moving the HW vessel and all the control gear to a different part of the house to make way for an alteration. This involves a fairly major rework of the primary circulation, much of which is buried in concrete floors. At the same time I'm fitting an ABV (Honeywell DU144) in place of the bypass gate valve. Halfway through the installation I checked the boiler installation instructions and realised I may have made a mistake with the bypass circuit.

In their new position, the pump and valves are all mounted close to the boiler. I've fitted the ABV between the output of the pump and the adjacent return. However, the total pipe run of the bypass, from the boiler unions, is about 2.5m. The installation guide is unclear (to me). It says:

"Where a pair of two port valves are used, a by-pass is
necessary. The total length of the by-pass circuit taken from the
boiler connections should be greater than 4 metres of 22mm
pipe. It should be fitted with a lockshield valve and be adjusted
to maintain a minimum flow through the boiler of 4.5litres/min (1
gal/min).

Systems fitted with controls which allow the boiler to operate
when both the hot water and central heating circuits are
closed i.e. mechanically operated thermostatic control
valves, must be fitted with a by-pass circuit of:-
1. A minimum length of bypass of 2m of 22mm dia. Pipe.
2. Maintaining a minimum water flow rate through the
boiler of 9 litres/min (2 gal/min)."

Now, it's pretty clear that my system is in the first category, which would make my bypass circuit too short. But why the difference between the two, and why wouldn't my circuit be adequate, given that the second option allows for a 2m circuit?

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[diamondgas]

:welcome: to the forum downthetube ... Personally, you have a suprima, nevermind! Why not change it?

Regards the bypass .... Manufacturers recomended a certain distance because in theory that would allow a certain loss of heat to be accomplished and allow the heatexchanger to dissipate its heat! My theory anyhoo's

 

[AWheating]

if possible you need to keep the bypass away from the boiler to allow the boiler to remove its heat. But if its not possible then you have to fit it where you can and atleast its protecting the pump and allowing the min flow rate.

 

[downthetube]

Thanks, AWheating and diamondgas. I'll have to rethink the layout - it's all a bit tight in the new position. It's a corner of the bathroom that's going to be the airing cupboard, so I guess I can use the bathroom rad as a bypass. Shame about the manifold I've made up for the valves and ABV - scrap, now. :annoyed: