Is this a doable booster pump bodge?

[Max Quad]

We've a booster pump near the hot water tank that boosts the hot water pressure on demand. It works well as long as you realise that you have to turn the mixer to hot first (because if you try to turn it on in the cold position the hot water flow never overcomes the cold one). No great deal, but I'd like to be able to do something about it.

I'm going to have to replace the bath soon (my son made a hole in it), and I'm thinking of moving the pump under it, since I think that is a better place for it (so that it is pumped only when the bath mixer wants it, as opposed to any other tap in the house). I know that the correct thing to do to fix the hot water flow problem is to replace the single pump with a twin, but it's in good condition, and I'm a bit mean. I was wondering if it's possible to get a flow switch for the cold water, and use that to switch on the pump too, instead of just having the hot water flow do it.

Has anyone ever tried that? Of course, it depends on my being able to open up the pump and find where the existing flow switch is, and wire the additional one in parallel. Do pump manufacturers (it's a Grundfos) make it easy for you to do that?

[Brian Gaff]

Ah thats what I need on my shower, but due to both being effectively gravity fed I'd need two pumps I suspect. Brian

[Andrew May]

I have a single pump that pumps both hot and cold. At least I assume it is a single pump, I've never opened it up and it has been installed for ten years or so and still going strong.

[Dave Liquorice]

Or a single device with a single motor but with an impellor at each end, one for hot, one for cold. Very common, I'd almost say it's easier to find a twin impellor pump than single.

[Chris Holford]

I have a Grundfoss pump like that BUT; the input to the impeller at one end is connected to the hot supply, the output from that end is connected to the input of the impeller at the other end and the output from that end goes to the mixer. So the two impellers are in series.

Presumably this gives an extra boost to the hot supply. However the cold comes out of the mixer at a very low pressure (-it seems to be supplied by the cold water tank in the roof space.).

I have been wondering if it would be a good idea to connect the pump so that one end boosts the hot water and the other boosts the cold water; this seems the logical way to do it, any comments?

[Max Quad]

I'd have thought that the two impellers are on the same shaft, so having the output of one feed the input of the other seems redundant. I'm pretty sure they are only meant to be used with hot at one end, cold the other.

[Max Quad]

I'd suggest that you need a single pump with twin impellers. My pump has only a single impeller. Our cold pressure is very good, but that creates problems at the mixer tap, because the hot water can't compete with the pressure of the cold, so it never flows, and thus can't activate the pump until you turn the mixer to the hot position. Once the hot water's flowing, it works very well. Having a twin impeller pump means that as soon as the cold water's flowing, the hot water gets pumped, too, because they're both activated and pumped together.

[Fredxx]

I would expect the increase in pressure to be doubled?

[Fredxx]

You obviously don't have a thermostaic valve or this would open the hot side preferentially when staring with a cold outlet temperature.

Can I recommend one, it would save any nonsense of turning on the "hot" first!

[Fredxx]

I meant "Thermostatic"!

[Max Quad]

Sure. Always glad to hear a recommendation.

[David.WE.Roberts]

Hmmmm......both impellers are linked.

So if we consider that with the tap open the impellers are increasing the speed of the water flow then the first will bring it up to speed and the second will maintain the speed?

Still, I'm having difficulty visualising it.

If you had two independent pumps I don't think you would get double the pressure increase unless you had an intermediate expanding pressure vessel which could hold enough water at the higher pressure - and I'm not sure that that would work either.

I think the second pump would be limited by the first pump - if the first pump could shift 10l per second then the second pump isn't going to be able to shift 20l per second because it is throttled by the first pump.

I think.

Cheers

Dave R

[Fredxx]

I look at this differently, water is incompressible.

Therefore it's volume and characteristics should be largely independent of pressure, and so the pressure increase for each "side" should be the same. The flow rate is also the same for both sided. I'm assuming that the impellers are physically linked, ie waterways aren't linked, and that the rpm is largely independent for flow rates of interest.

I am conscious though that a small reduction in rpm will have a disproportionate effect on pressure as its a square law of shaft speed. Any flow will start to absorb power from the shaft.