Operating one end of a twin-ended pump

5 Feb 2007
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United Kingdom
Recently installed a Stuart Turner Monsoon Universal Twin 3.0 Bar pump as a whole house pump. Bath fillers take gravity feed for cold and hot from the HW cylinder. All other taps throughout the house are mains-fed for the cold and are a combination of mixers and separate hot/cold taps. The problem is the mixer shower in the en-suite is mains-fed for the cold (no clue why it was installed like that), so whenever this is in use, the pump will be pumping single-ended for several minutes at a time. Stuart Turner advised it's fine to run the pump single-ended (eg. hot side only) for short periods, such as filling a basin, for example, but definitely not for longer periods such as taking a shower. They advised re-plumbing the shower for gravity-fed cold, or installing a single-ended pump specifically for the shower. My preferred option would be to re-plumb to gravity-fed cold, but although the plumbing job is simple, it would require a lot of making good in terms of retiling, decorating, etc. In the interim, I'm going to just run a pumped cold tap while taking a shower so both ends of the pump are operating.

I'm interested to understand what the issue is with running a twin pump single-ended. I get that the cold side would be operating against a closed valve (no taps open), so the physical resistance would be high against the impeller. I can also see the load on the motor would consequently be higher. So, would pump life be reduced simply because the pump motor would be working harder, or is there a more serious issue that could result in the pump failing more quickly (days/weeks as opposed to it lasting 5 years instead of 10, for example). Just would like to understand the mechanics of what's going-on in the pump.
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As long as both sides are wet there is not a big problem.

But there is a simple solution for you.

That is to put both sides on the hot in parallel!

Only slight drawback is that the turn on flow would be a little higher. Even that could be solved by adding a resistance on one side.

The impeller in the pump housing imparts kinetic energy in the water resulting in the higher pressure/flow to the outlets. If you run with the outlet closed the impeller still imparts kinetic energy in the water but it will be converted to heat risking damage due to high temperature and expansion pressure...the seals can fail. Some pumps (some of the Salamander range) have a bypass around the impeller to reduce the effect and allow true closed end running.

If the mains pressure/flowrate is sufficient have you considered running the bathroom cold outlets off the mains, and just using the pump to boost the gravity hot. You could plumb both sides of the pump in parrallel ie. doubling the flowrate.
As well as both ends needing to be 'wet' the water needs to flow through in order not to burn out the mechanical seals.
Its not sufficient to simply have them wet as if the water is 'static' it will heat up and cause the seal to fail.
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Pump in parallel as said. You'll get even better hot water performance too (not truly double, but improved).
As well as both ends needing to be 'wet' the water needs to flow through in order not to burn out the mechanical seals.
Its not sufficient to simply have them wet as if the water is 'static' it will heat up and cause the seal to fail.

Presumably, that's where the 65C temperature limit on the HW inlet comes from?
No, the cylinder temperature is set at a sufficiently high temperature of 60 degrees in order to prevent Legionaires bacteria buildup but a temperature in excess would lead to increased scale buildup (as well as being even more unsafe from a scalding point of view). But, the reason pump manufacturers quote 60 or 65 degrees maximum is primarily to reduce the risk of cavitation at the impeller eye ie. the pressure reduction at the inlet lowers the water boiler point and there needs to be a sufficient margin (especially given so many installations are woefully sub-standard), secondly the materials used in some pumps are barely adequate...I've seen impellers melt when cylinder elements overheat the water.
Interesting, makes sense. So the temperature limit on the pump is to reduce the risk of cavitation due to the water boiling as a result of a lowered boiling point due to pressure drop.

So, if the pump is operated single ended for periods of time that keep the increase in temp on the static side to a safe level, the pump should be OK? I'm assuming that pumping the hot side leaving the cold side static is better than vice versa where the water temperature on the hot side is already at 60C to begin with.

Any idea what a safe temp is for the pump as measured by the temp of the impeller housing? I could use an infrared thermometer to see how long single ended operation causes the static end to rise to that level.
It'll be the temperature when either the impellor itself, if plastic, or shaft seals melt! I'd 'bite the bullet' and replumb so that both sides are pumping when it's in use.

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