DIY Doctor UK DIY Forums

I have currently got a floor standing cast iron boiler (Myson Marathon 1000B) that I wish to upgrade to a more efficient condensing type. ( I was looking at the Glow worm flexicom range). The current system is gravity DHW and pumped CH.
There are 4 pipes coming down to the boiler. 2x28mm flow & return for the DHW and 2x22mm flow & return for the CH. The 28mm pipes go up to the attic where the indirect DHW cylinder is located. The 28mm flow carries on past the cylinder up to the F&E tank. The cold water makeup from the F&E joins into the 28mm flow adjacent to the DHW cylinder.

I wish to convert this to a fully pumped ‘S’ plan system. In order to maintain an unrestricted path from the boiler to the F&E vent means I would need to place the 2 port motorised valve in the return 28mm pipe from the DHW cylinder. Similarly it would mean placing the circulating pump in the return leg before the boiler. Commoning up the CH & DHW flow pipes at the boiler outlet.

All the diagrams I have seen show the pump and the motorised valves in the flow side, after the boiler. Is there a problem in putting the valves and pump in the return side before the boiler? After all, the pump is just there to circulate the water around the system. The static pressure comes from the height of the F&E tank? So it shouldn’t overflow the vent into the F&E? Or am I missing a trick here?
I realise that I will need a bypass circuit for pump ‘run on’ when both valves are shut but I can accommodate that.

I would have thought this would be a common upgrade / problem but can’t find any references in the forums. Any hints / tips would be much appreciated.
Thanks
Matt

Specifically there is no reason why a pump or valves cannot be fitted in the return. In fact, originally, pumps were always fitted there.

As for the F&E pipes, what is important, is not to have them too far apart. The further away they are from each other, the more likely there is to be a pressure difference resulting in the problem you mention (and don't forget the opposite can apply too, where air is drawn down the expansion into the system). Just make sure there is always a direct route for steam and boiling water to escape into the F&E tank should there be a boiler fault. The trick to a successful installation is to choose the highest point on the system for the expansion, and a point slightly lower for the feed so when you are filling the system, air can easily escape and the feed is unhindered.

If the system is correctly wired, and the boiler does not require an overrun, strictly speaking, a bypass is not required because the valves closing should automatically shut down the pump.

Thanks for that Plumbbob.
I thought that it would be OK. Its just that when you see every diagram showing the pump in the flow, you begin to have doubts.

I don’t think I will have a pressure difference problem with the feed and makeup as they are within a few inches of each other.

Thanks again for your reply
Matt