In August 2009 I contracted British Gas to replumb the entire out. All the very basic but poorly done plumbing was ripped out and we started from scratch.
At the time there was only a kitchen tap, one bathroom and a couple of radiators. I told BG that we would be adding a further two bathroom. They recommend a powerful pump (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.
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, storage 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 in 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 are fed by pumped hot and mains cold.
On the second floor, in the room where the pump ect is housed there is the planned bathroom. Loos 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 two bath taps and the shower mixer 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.
Thats the entire system. Quite quickly we had teething problems such as a leak from the loft shower tray which was visible from the celing beneath and stopped once rectified.
We also had a leak from the pump flexihose outlet where the flexihose seal meets the copper pipe and so the flexihose was replaced (the leak may have occured around the same time the hunting started but this is not documented). The flexihose seal here is dry now though.
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.
I suppose the first action that was taken was to replace the pump with a brand new one but like for like (replaced by ST engineer). Unfortunately the hunting continued however so it was not the pump. 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!
Hence I turned to this forum. I will try to summarise all the tests I have done and all the results.
Simply, when shutting off the hot outlet alone the hunting stops. Shutting the cold outlet alone and the hunting continues. I once isolated the hot INLET and after a few minutes the pump cycled so immediately opened as I did not want the pump running dry.
I have isolated every tap, wc and appliance which consumes water in the house all at once. All except for the shower mixers which can not be isolated. With everything isolated (except shower mixers) the hunting continues.
The hunting is consistently every 25 minutes (approx) and last for 5 or 6 seconds.
I have turned off the mains water coming into the house and the hunting continues.
I have turned off the mains and opened the cold kitchen tap and left mixers in the cold position and the hunting continued. There was a constant dribble from the mains (flow rate 0.1375L/minute).I also felt for warm pipes leading to the mixers but could not detect any warmth.
I have since found that moving the spout of the kitchen tap interupts and ceases the flow of the dribble. I also reported that if during the dribble I close the hot and cold outlets of the pump (and unplug it) the dribble also stops.
If whilst there is no dribble (with kitchen cold open and pump on and both outlets open) I move all the mixers from hot to cold (but not open) the dribble does not return.
I have repeated the above test (mains off, kitchen cold open) with the mixer taps in the normal position and hunting continues.
One point is that on first opening (ie after the taps has not been run for perhaps the whole day) the mixer tap all the way over to cold in the first floor bathroom the water initially warms up and then goes cold again.
I think that is most of your tests and if there are any screaming tests you think I have missed let me know. Otherwise I really need to help of a pro who has all this info above in mind when looking for the cause.