Monsoon twin pump installation - wrong pump?

[Tim Farnden]

My house was a bungalow that was extended up into the roof, and a bathroom installed up there. The hot water cylinder remains on the ground floor. The mains water pressure is low. A Stuart Monsoon U3 dual pump was installed to pump both hot and cold.

I am thinking that this is the wrong type of pump for this as it pumps both supplies at once, regardless of which one is called?

Some years ago I had a similar pump fail and when I stripped it down found that one of the woodruff keys that holds the impeller to the motor shaft had sheared off. My assumption was that as it was trying to pump against a closed pipe for the supply that hadn't been called for and this caused the failure.
Am I right in my logic of how this works? If, for instance, I run only the hot tap upstairs then the pump is trying to also pump cold up there but with the cold taps closed (and toilet cistern full) it has nowhere to go?

Reason this has come up is because the pump has started making a squealing noise that sounds like impeller slipping or rubbing (from one end of the pump only).

Surely two separate pumps should have been installed?

 

[fowlerboi]

The U3 is designed to do what you're asking of it

 

[John.g]

I know of a few installations like yours with no apparent problems but I would think that these pumps are not pumping "single ended" for very long, ST advise that short periods of single ended running is fine so filling a sink or toilet cistern should not be a problem but watering your garden might be. Salamander provide a by pass I think back to the suction. The problem arises with prolonged running is that the water heats up as the pump energy is now converted to thermal energy and the water can exceed 65C and damage the pump seals which will occur far more rapidly at the hot end.

 

[Tim Farnden]

I know of a few installations like yours with no apparent problems but I would think that these pumps are not pumping "single ended" for very long, ST advise that short periods of single ended running is fine so filling a sink or toilet cistern should not be a problem but watering your garden might be. Salamander provide a by pass I think back to the suction. The problem arises with prolonged running is that the water heats up as the pump energy is now converted to thermal energy and the water can exceed 65C and damage the pump seals which will occur far more rapidly at the hot end.

Thanks @John.g It is the hot water end that is squealing. I will pull it apart and have a look. ST sell new seals, at £48! Still better than £600 odd for a new pump.

 

[Ric2013]

I know of a few installations like yours with no apparent problems but I would think that these pumps are not pumping "single ended" for very long, ST advise that short periods of single ended running is fine so filling a sink or toilet cistern should not be a problem but watering your garden might be. Salamander provide a by pass I think back to the suction. The problem arises with prolonged running is that the water heats up as the pump energy is now converted to thermal energy and the water can exceed 65C and damage the pump seals which will occur far more rapidly at the hot end.

Hi. Actually had this conversation with ST recently. I can't see quite why the Monsoon can be used as a whole-house pump for reasons you have stated, but apparently it can and I suppose if you were to water your garden off the pump because mains pressure were poor, the warranty would still be valid. Given the Screwfix reviews for anything by Salamander, I wouldn't bother, whatever bypass arrangements they may have in place.

 

[John.g]

A very rough calculation maybe using a UPS 3, the power required when dead heading at 6.4M is 28W (0.028kw), assume the pump body holds say 75ml, (0.075L), if all the absorbed power goes into heating the water then the temperature rise is, 0.028*860/60/0.075, 5.35C/min, A 3 bar ST might absorb 130W (0.13kw) per dead headed end, don't know what the pump casing holds, say 200CC?, 0.2L, then the temp rise is, 0.13*860/60/0.2, 9.3C/min, in practice, the motor probably absorbs and radiates around 50% of the power so maybe a temp rise of say 4.5C/min, a cold end would then run for ~ 10 min before reaching 65C and the hot end only 1 minute, or 9 minutes (from 60C) to reach boiling point. Be interesting if someone took a few temperatures off the dead headed cold end of a 3bar ST.