I have had a pump installed on my shower. It is on a gravity fed system. Whenever you have the shower as hit as possible it's fine, when you try to mix in the cold the pump seems to cut in and out and the water surges. Any idea what it could be? The pump is a salamander for both hot and cold and sits in the loft below the cold water tank
positive, negative or universal head? what is the distance between your shower head and bottom of the storage tank?
had an issue with that years ago at my clients flat, replacing the pump solved the problem, in first place he had positive head and shower could activate the pump only when it has been held on the knee level I guess your problem might be related if you refer 800 as mm is just above minimum for salamander ... btw have installed quiet few of salamander pumps in the past and this isn't the brand that I currently recommend to my clients
Before I buy a replacement pump do you think I should try move it or drop it lower. It's on the loft floor so not that much higher than shower head.
The pump only "pulses" when you mix the feeds. So I know both sides on full bore have enough pressure to maintain the active pump
Small tank will be just be the feed and expansion tank for your boiler/rads. The hot and cold shower feeds should both be coming from the larger CWS tank with the hot connecting slightly higher than the cold.
I have checked and yes that is what I have. Hot it higher on the tank than the cold and both come off the cws tank
Pump should have been installed ideally on floor beside hot cylinder and preferably not in loft. Hot draw off from cylinder should be air free supply - Surrey or Warix flange and a drop loop if pump higher than cylinder. All filters need checked on pump and shower. And should normally have been a negative head pump where little or no head of water is to shower head or where pipes go up and over in lofts and on long pipe runs. Pipes should be given a gradual fall and be all in copper to be best job. Usually with troublesome pumps it is the installer that is at fault and likely not done by a real plumber.
The pipes are all copper. I couldn't have the pump sat on the floor by the cylinder so had to go in the loft. Pipes come off the top of the hot tank with an air free supply and yes there is a drop loop before going up to the pump. I dont have a negative head system so why would that be suitable?
That is a common misconception. You might well in theory have positive head to your shower head, but how would a positive head pump start if the flow is in reality poor? Pipes that are run overhead are normally under very little head of water and might also be very restrictive due to length of pipe runs and poor piping method. Negative or Universal pumps start when an outlet is opened. They are not just for installs where there is little or no head distance from cold tank to shower head. But check that the shower valve isn’t allowing water to flow from the hot to cold side of shower if failed built in check valve on cold side.
Cold pressure pushing back through the valve into the hot supply? You could try adding non-return valves to both outlets from the pump or inlets to the valve.
Shower valve might already have built in check valves, but worth checking. One of them could be faulty. They are basically only a plastic valve with spring and washer and easy to fail
Could it be the unequal pressure from hot and cold causing the issue? Should I limit the cold pressure to match the hot feed
Yes, the feeds to the pump would have a different flow rate, although often not a huge difference. But the pump outlets should be pumping same pressure to both the hot and cold.
That makes sense. I have an update. If it mix the feeds prior to turning the shower on (presenting the temperature on the dial) then turn the shower on I do not get the surging issue. Could this help find the issue?