differential bypass valves

[Sam]

Hi,

Thanks to the advice I have received from this group I am undertaking upgrading the CH to fully pumped. I am going to use 2-port valves, which as you pointed out to me, means that if the valves are off, there will be no flow path (other than up the vent). I understand this could damage the pump. I was thinking about fitting a differential bypass valve.

Screwfix sell one (67854) for £15, so for that price I figure better safe than sorry. Are these things pre-set or do I have to set the bypass pressure; if so, what do I set it to?

Do I just connect it to "short circuit" to the other side of the boiler?

Thanks.

[Ed Sirett]

Depending on the type of boiler you have you may not need one. Unless your boiler controls the pump (so that it does pump-overrun) you won't see any benefit anyway.

Even so adding one is probably a good idea. You will need to adjust it so that it lets by when all zons are shut and the pump is still running. Feeling the pipe is probably a way but you have to allow for how much the pipes will heat up by conduction (quite a bit) even when no water flows through them.

If the valve goes from 1-6 that's roughly 0.1 to 0.6 bar. A flow stalled pump will be at various pressure depending on what setting it's at and whether its a nominal 0.6 bar (at zero flow) or 0.5 bar (at zero flow) unit.

[Sam]

Thanks, the cheap one at SF comes with no useful instructions. I'm not sure whether to take it back and pay extra for one that does! There is no overrun and I have a couple of rads without TRVs so I'm wondering whether to save some money and not bother. Of course, if I don't and the pump fails, I'll wish I had!

[Pete C]

You can test the valve it with a hosepipe if you wish, 0.6 bar is 20ft vertical 'head' of water.

cheers, Pete.

[Sam]

The £15.99 one from SF has no instructions. Are they preset to open at

0.6 bar or do I have to set that myself? If so, how?

Thanks,

[Pete C]

HI,

Looking at the paper catalogue it says the valve is made by Comap, and a google leads to:

So it looks like it's preset to 0.2bar and the red needle indicates the opening pressure. But can be adjusted to different opening pressures by twisting the black cap.

May be worth trying on the factory setting first to see how it goes.

cheers, Pete.