bassanclan Posted March 3 Share Posted March 3 I have a thermal store with an external plate heat exchanger (phe) When a tap is turned on a dlow switch detects the flow of water and turns on a pump to pump water through the phe and heat the mains cold water. There is a problem. The pump isn't coming on, so manually wired the pump to a plug temporarily - it works. Input 240v into the flow switch, 220v coming out. So i got a new flow switch, but that behaves similarly, 240v in and 224v out, this time getting the pump on, but the pump flashing saying low voltage. Pump is a Wilo para RS25. I feel like 220v should be enough to make it work? Specs say 230v +/- 10% Can I have two faulty flow switches? Link to comment Share on other sites More sharing options...
ProDave Posted March 3 Share Posted March 3 You should not be dropping 20V across the switch. I suspect a wiring / measuring error somewhere. Especially as you are getting similar volt drop with the new switch. Post some pictures of where you are taking your measurements and with what. Is it a real flow switch (I have seen some strange setups with a thermostat acting as a flow switch) 1 Link to comment Share on other sites More sharing options...
bassanclan Posted March 3 Author Share Posted March 3 https://uk.farnell.com/te-connectivity/fs-05/flow-switch-brass-ac-10bar/dp/1006768 This is the flow switch. I'm away until Thursday so can't take a photo, but thought I could order a part for my return. I'm taking readings with a basic cheap multimeter. Two wires, brown being live in ans blue (sleeved brown) as live out. It is fed from a fused switched flex outlet. With the switch off there is 0volts Switch on shows 240v going to the flow switch and 0v coming out. When a hot tap is turned on 220v or 224v coming out Link to comment Share on other sites More sharing options...
bassanclan Posted March 3 Author Share Posted March 3 I found this photo which shows the flow switch Link to comment Share on other sites More sharing options...
ProDave Posted March 3 Share Posted March 3 That switch has a reed switch. It would not be my first choice. Though it does say it is rated for 3A which ought to be enough, and the usual trouble with reed switches if you overload them is the contacts weld shut. Link to comment Share on other sites More sharing options...
sharpener Posted March 3 Share Posted March 3 1 hour ago, ProDave said: You should not be dropping 20V across the switch. This. Is this a new installation or has it been working OK for many weeks/months? Maybe there is a bad connection upstream of the switch causing the apparent voltage drop. Try measuring the voltage across the switch not between the live and neutral, does that measure 20V? A remote possibility is your neutral connection is getting pulled up by resistance in the circuit somewhere else. The data sheet shows this is a reed switch rated at 3A. Without qualification I would suspect that is for a resistive load, and designed for a control circuit not an inductive load like a pump, though the list of applications does include them. Link to comment Share on other sites More sharing options...
dpmiller Posted March 4 Share Posted March 4 I'd be adding a wee relay in there... Link to comment Share on other sites More sharing options...
sharpener Posted March 4 Share Posted March 4 1 hour ago, dpmiller said: I'd be adding a wee relay in there... +1 Still need to diagnose the 20V drop. If confirmed that represents a lot of power being dissipated in a reed switch. 1 Link to comment Share on other sites More sharing options...
Originaltwist Posted March 4 Share Posted March 4 I encountered the same issue with a heat bank but the issue was not electrical. The problem was due to a pressure drop when taps etc were turned on and the flow switch didn't get enough of a kick to activate it. The owner was not willing to remove the flow restricting pressure reducer on the mains or to change to a more sensitive flow switch so the situation was left unsolved. The local plumber, of course, knew better so I left them to it. A lot of people don't see they have a pressure problem when the pressure guage looks good and they don't notice the drop when there is flow. I suspect from the dialogue so far that the problem really is electrical but I thought I'd add this just in case. Link to comment Share on other sites More sharing options...
sharpener Posted March 4 Share Posted March 4 2 hours ago, Originaltwist said: The problem was due to a pressure drop when taps etc were turned on Yes probably not the issue here, assuming the OP can get 1 l/min out of the tap which is not a lot, that flow switch is quite sensitive. Though I once had a problem with the pump controller not turning off reliably on my pressurised rainwater circuit. Couldn't find any fault with the flow switch so changed the enire controller, but it turned out the underlying issue was a cracked plastic impeller shroud in the jet pump hence not generating sufficient pressure in the first place. Stuart Turner is a long established brand but the pump in question is just imported rubbish with their name on it. Have had no end of trouble and am on my third, though managed to swap the shroud from one that had a failed shaft gland (ST do not stock spares as they regard the pumps as non-repairable!) Link to comment Share on other sites More sharing options...
Temp Posted March 4 Share Posted March 4 We have two similar flow switches. Have had one fail after 15-16 years. Did you mount it the right way up? Link to comment Share on other sites More sharing options...
Temp Posted March 4 Share Posted March 4 (edited) 16 hours ago, ProDave said: You should not be dropping 20V across the switch. I wonder if the pump is stalled and somehow current limits so fuse doesn't blow? Repeat the voltage measurements. Measure voltage at the pump. At the output of the switch. Input to the switch. Check the neutral to the pump. With power off measure resistance/ continuity from neutral at the pump to neutral as far away from the pump as your meter leads will reach. Edited March 4 by Temp 1 Link to comment Share on other sites More sharing options...
bassanclan Posted March 4 Author Share Posted March 4 The system worked fine for about a year, then the electrical bit of the flow switch went wrong, but of course you have to buy the whole part. Not exactly sure how the switch works, but it looks like a little magnet which moves inside when the water is flowing to complete the circuit. I swapped it over and it has worked for another 10 months without a problem, but there must be an underlying cause of two failures in a short time Link to comment Share on other sites More sharing options...
sharpener Posted March 4 Share Posted March 4 The magnet causes the two switch blades in a sealed glass capsule to be attracted to one another so completing the circuit. As others have pointed out it is not a type of switch really suited to controlling a load like a pump. 9 hours ago, Temp said: Repeat the voltage measurements. Measure voltage at the pump. At the output of the switch. Input to the switch. Check the neutral to the pump. With power off measure resistance/ continuity from neutral at the pump to neutral as far away from the pump as your meter leads will reach. I would add with the pump running measure the voltage across the switch, and between the neutral at the pump and a neutral on a different circuit. Report back with the results of the tests upthread suggested by me and @Temp, then we can help further. 1 Link to comment Share on other sites More sharing options...
bassanclan Posted March 17 Author Share Posted March 17 Just getting round to looking at this again Voltage at the pump 240v Input voltage 240v Output voltage 240v Output voltage when going through the flow switch 220v Neutral from pump to neutral on another circuit is 0v It must just be the flow switch has gone and the new one is faulty! Given both are under a year old I will return to RS and see how a third one works! As they will probably send me a replacement is there a better way it could be used to prevent this problem recurring? Alternatively a completely different setup which would make sense? Link to comment Share on other sites More sharing options...
ProDave Posted March 17 Share Posted March 17 Before you do that, do a sanity check and replace the flow switch with any manual switch, even a standard light switch will do, and confirm the pump turns on and off and runs properly with no excess voltage drop anywhere. 1 Link to comment Share on other sites More sharing options...
John Carroll Posted March 17 Share Posted March 17 As above or with flow switch made check for 220/240V between the (switched) L&N terminals where the pump is connected in, if 2220/240V, get any sort of load like a 60/100W bulb and see that it lights when you wire in to the same terminals. Link to comment Share on other sites More sharing options...
bassanclan Posted March 18 Author Share Posted March 18 Yes I have rewired it temporarily so that the switch on the wall is basically a pump on/off switch and it works well. I have asked for a replacement part to be sent. When it arrives I will initially just put it back as it was to make sure it works, but should the DESIGN change, e.g. relay extra fuse, etc and if so how? Link to comment Share on other sites More sharing options...
dpmiller Posted March 19 Share Posted March 19 Honestly yes, I'd have a relay in there. Link to comment Share on other sites More sharing options...
sharpener Posted March 19 Share Posted March 19 19 hours ago, bassanclan said: When it arrives I will initially just put it back as it was to make sure it works, but should the DESIGN change, e.g. relay extra fuse, etc and if so how? Yes, connect output of flow switch to the coil of a substantial mains relay or small contactor. Connect normally open contacts of relay between mains and the pump. I still think the original fault is very odd as there appears to be some 20V across the reed switch contacts, this implies current is flowing but the contacts are high resistance so power is being dissipated. I would expect either 240V if they are burnt away or virtually nothing if they are making. Did you measure the voltage directly across the contacts? Link to comment Share on other sites More sharing options...
Trojan Posted March 23 Share Posted March 23 Could you give a clearer picture of how it's plumbed into your system? Link to comment Share on other sites More sharing options...
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