How I Sort Of Converted My Single Impeller Shower Booster Pump To A Pretend Double Impeller Pump.

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It’s interesting how one job leads to another. And then another. I’m perfectly happy with a hot bath, but the missus really wanted a mixer tap and shower head. Takes all sorts, I suppose. Rather naively, I thought it would be a piece of cake to fit such a thing to out bath. But no, what we ended up with was a trickle of cold water. Research indicated that the cause was that the rather pathetic pressure afforded by the header tank in the loft was no match for the mains pressure of the cold water. The cold water was actually pushing the hot water back into the storage tank, rather than playing nicely, and mixing with it.

What’s needed, I thought, was some sort of booster pump that automatically detected when the water was flowing. It seemed natural to me that someone had invented such a thing, and sure enough, there were several models for sale. The main choice was whether to get a single or double impeller. Since I saw no need to pump the cold water, I plumped for the single version. I chose a Grundfos Nile, mainly because our central heating pump is a Grundfos, and it seems to have been running forever with no problems. To save time, I put it in the airing cupboard for now, although it was always my intention to stick it under the bath eventually. I soon found out that having all the hot water in the house being pumped is fairly annoying. Not least because of the noise.

Of course, plumbing experts will know what happens next. The mixer produces nice hot water, but you have to remember not to open it when it is pushed all the way to the cold side. The hot water never gets the chance to flow against the cold water, so it is still being pushed back, and the pump never gets switched on. Drat. Perhaps I should have got the twin impeller version after all. That way, the pump would come on when the cold water flowed, too.

But am I going to go out and buy a new one? And waste the money I’ve already spent? Not a chance. What I really want to do is to have the existing pump come on when the cold water is turned on, thus giving the hot water a fighting chance. IS there any way, I wondered, that I could get a flow switch into the cold water pipe, and wire that into the pump in parallel with its existing flow switch? Happily, the answer is yes.

It was easy enough to find a cheap 15mm flow switch on eBay. I was less sure about the actual wiring bit. Anyway, on the day I decided to move the pump under the bath, I thought I’d take the wiring cover off. What I found was a very pleasant surprise. They’d obviously anticipated my wanting to add a flow switch, since there were spare connectors just waiting for it. Presumably, what I’m attempting to do is fairly common.

Below is a picture of the wiring of the Grundfos. My extra connectors (they even provided these, already waiting on the extra posts under the cover) from the flow switch are the red ones on the left.

Grundfos Extra Flow Switch Wiring

What really took most of the time was wiring in an FCU switched spur on the outside landing to provide the power for it. Fortunately, the partition was made out of that Paramount board stuff, so it was easy to push a hole through and along it.

Anyway, it’s in now, the hot water is pumped as soon as either tap is switched on, and the whole thing works a treat. If anyone else has the same problem we had, and is thinking of trying this, then I hope this is a source of encouragement.





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