Help with Salamander RP75PT Twin Pump

14 Dec 2019
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United Kingdom
Bought a Salamander Right Pump RP75PT Twin to replace a 20 year old Stuart Turner shower pump. It supplies shower, basin and bath.

Installed in same location, some mods to pipework around pump to allow for different inlet and outlet positions.

Pump ran fine, all working normally. But next morning turned on basin cold tap and got loads of hot water!? Pump had been off all night.

There's something unusual about these Salamander pumps. They have “Crossflow Technology” which is a small ~3mm ID tube directly connecting the hot and cold impeller chambers. It’s to stop one side burning out if just the other side is flowing (i.e. you’ve had just the cold tap on for a long time).

I realized that with the pump off (and even unplugged) that over 2-3 hours I was getting hot water flowing very slowly from the right chamber (hot side) to the cold chamber through this tube.

My pump is around 1.5 m from and just below the cylinder outlet (Surrey flange), and all pipework is 22mm copper so good heat conductance from the cylinder to the pump keeping the hot side hot. The cold supply pipe goes from the side of the pump vertically straight up to the storage tank in the loft, the same tank that supplies the hot cylinder, so there’s no pressure difference between the hot and cold supply pipes. It’s a normal gravity-fed vented system.

I think I’m seeing a thermosiphon effect, where heat from the hot side of the pump is heating up the cold side a little. This causes a small amount of heat to flow up my vertical cold supply pipe, which pulls more hot water through this Crossflow tube, so that over time the cold supply and outlet pipes both heat up to the point where there is equilibrium.

Salamander tell me they have only ever seen this when there is a pressure imbalance between the hot and cold supplies, usually as they are coming from different tanks. But my supplies are from the same tank.

Has anyone ever experienced something like this please??
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Hi, yes there is indeed a small crossover between the hot and cold sides of the pump, this will allow excess pressure to be relived from what we class as the closed head side of the pump (the tap that's turned off between the hot and cold taps if only one is in use) it also replenished that side of the pump with lower temperature water, cooling down the water that gets quite hot by friction on the closed head side. If there were two cold water tanks, for example one that supplies the cylinder and one that supplies the pump, if the cold tank supplying the cylinder was higher then the cold tank supplying the pump, there is the potential for water to cross through. If there's only one tank that supplies both, its unlikely to have an effect.

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