Can a second pump be added to Gravity HW system?

[Clive M]

I have a central heating system as per the diagram on page 4 of

with the exception that the boiler only has two pipes and the HW circuit comes off the boiler just before the CH pump.

Is it possible to convert this layout to fully pumped without serious replumbing work? I did think about adding a second pump on the gravity circuit, but reckon that this will overpump into the header tank - Is semi gravity the way to remain?

[Christian McArdle]

It should be possible. However, the exact layout of the vent/feed arrangements is critical to avoid pumping over.

Christian.

[Set Square]

It may be possible, depending on the exact pipe layout.

In essence, you move the pump into the common bit of pipe before the circuits split, and put a 3-port mid-position valve in place of the T-piece in the flow pipes. You then end up with a Y-plan system [See

]

HOWEVER, you must have a clear path from the boiler to the vent pipe - with no valves in the way - so a lot depends on where the vent pipe is currently connected.

If the vent and fill pipes are in the wrong place, it might be easier to go for as S-Plan (which many people think is better anyway) with two 2-port valves rather than one 3-port valve. You can put the HW valve right next to the cylinder (as in the diagram in the danfos document which you site,

*after* the vent pipe connection) and put the CH valve where the pump is currently fitted.

Yet another alternative to keep a semi-gravity system, but convert it to a C-Plan which will turn the boiler off when it is not required. Conventional gravity hot water systems suffer from two basic problems:

• hot water recovery is slower than pumped because of the slower gravity circulation

• there is no boiler interlock or control of hot water temperature - so the boiler cycles on its own stat - thus wasting energy - when not really required, and the hot water gets hotter and hotter (and also gets hot when the CH is on, even if not required) If you have all these problems, you need a fully pumped system. If your hot water recovery rate is acceptable, going for a C-Plan is a lot less trouble - and will fix all the problems described in the second bullet point.

[Alec]

I deliberated over my similar system for ages, then last weekend converted to a Y plan sealed system. It has worked out really well. No header tank in the loft and no over-pumping worries.As a bonus it has held pressure all week...that was the biggest surprise! This newsgroup's FAQ was a great starting point. Go for it.

Rgds Alec