Stuart Turner Monsoon 4 Bar Twin Pump - HUNTING

6 Oct 2011
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United Kingdom
Hello all!

From the top...

In August 2009 British Gas to replumbed my entire house. No existing plumbing was left.

They installed a ST Monsoon 4 bar nagative twin head pump to supply the multiple bathrooms as there was insufficient pressure and flow coming in from the street for a megaflow system.

Ignoring the CH system.

The mains comes into the cellar. On the ground floor they plumbed in pumped hot and mains cold in to the kitchen taps (separate hot and cold). On the first floor where the only bathroom existed they plumbed in mains cold to the loo, bath cold tap and sink cold tap and pumped hot into the bath hot tap and sink hot tap. They installed the pump, header tank, hot water storage and whole CH system on the second floor in a room which was to become a bathroom. They also prepared the pipework for the planned layout of this bathroom. This system all worked fine and the pump never hunted.

By December 2010 we had finished a complete rennovation of the house and now the plumbing is as follows: On the ground floor the kitchen hot tap is fed by pumped hot and the cold tap by mains cold. There is a dishwasher connected to the mains cold, a fridge water filter connected to mains cold and a quooker (boiling water tap) whose canister is connected to the mains cold (as it should be).

On the first floor the old bathroom now has a shower, loo and two sink mixer taps. The loo is fed by mains cold. The two sink mixer taps and shower mixer (no inbuilt NRVs) are fed by pumped hot and mains cold.

On the second floor, in the room where the pump ect is housed there is the bathroom which was planned for by BG. The Loo is fed by pumped cold, likewise the washing machine. The sink hot tap fed by pumped hot and cold tap by pumped cold. Ditto for the bath taps and the shower mixer is fed by pumped hot and cold. Basically from the second floor up everything is fed by pumped hot and cold.

The loft conversion is a new extension. They ran new plastic pipes, syphoning water from the copper pipes on the outflow side of the pump. In the loft bathroom there is a loo fed by pumped cold and a shower mixer fed by pumped hot and cold and a sink where the hot tap is pumped hot and the cold tap pumped cold.

The hunting began probably between a month to a month and a half after the completion of the works. They started really far apart and gradually (in the space of a month or two) got closer together. Now it hunts as frequently as every 20 mins just for a few seconds.

The pump was replaced and installed bt ST with a brand new one like for like. This pump has in built NRVs so rules out a problem with the pump as the the hunting continued. Stuart Turner say that for the pump to cycle the pressure needs to drop from 4 bar to 1 bar and so if this was occuring every 20 mintues a leak would become visible. There is no visible leak anywhere in the house and please bear in mind this has been bothering me for almost 2 years!

I have carried out a number of tests with the most poingnant being that if I isolate everything in the house all at once with the exception of the shower valves, which I can not isolate, the hunting continues.

When I close the hot pump outlet the hunting stops. When I close the cold pump outlet the hunting continues.

The original theory was that the shower mixer valve in the 1st floor bathroom (with no NRVs) was allowing cross over and thus causing the pump to hunt. This is no longer an option since I have had the cold pumped supply fed down to feed this bathroom and so all shower mixers and mixer taps are fed by equal pressure.

After this drastic measure I am turning to all of you to see if anyone lives locally to me (Camberwell, London) and feels up to the challenge to help me put an end to these two years of torment.

I would be extremely, extremely grateful if anyone could assist.

Many thanks

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The hot supply will be 3 bar as the cold feed to the megaflow will pass through a pressure reducing valve. If the pumped cold supply does not pass through this valve then the hot and cold will be unbalanced.
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The hot supply will be 3 bar as the cold feed to the megaflow will pass through a pressure reducing valve. If the pumped cold supply does not pass through this valve then the hot and cold will be unbalanced.

What megaflo ?

I would start with the precharge in the expansion vessel, as there is now more height to your system pump up the vessel to 2 bar and see if the hunting stops.

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