Hi, TLB plumbers. Drained my central heating system down yesterday, flushed it out and refilled it with a dose of cleaner. So far, so good but when I switched the circulating pump on, I swear it was running backwards as it was drawing cold water from the return and feeding it back to the boiler via the flow route. Is this possible for a bog standard Wilo pump? Strangely, this morning after some head scratching, I turned it back on and it was working correctly, pumping the right way. Anybody know what was going on or is my house built on a vast ley line intersection?
I’ve consulted the font of all knowledge. Old Mr Fudderman from next door. His conclusion is it’s those damn Gremlins again. On a serious note what boiler is it and how old? You might have a pumped return. Not all systems have the pump on the flow.
A pump will never run backwards,do the two hand hot test,place one hand on what you think his the flow and the other hand on what you think is the return,with the boiler running you will find out which is which
The boiler is irrelevant as I have a Dunsley-Baker neutralizer fitted. The pump in the (Worcester) boiler just circulates hot water through the neutralizer. The central heating has a separate pump in the airing cupboard which circulates hot water from the neutralizer through the radiators. By feeling the pump outlet pipe, I can tell how hot the water is. If it's coming from the neutralizer, it's red hot but if it's coming from the ch return, it's cold. This was the case a couple of days ago but it's working normally now. Would an airlock in the pump cause it to run backwards?
Er excuse me. The boiler isn’t irrelevant Depending on the boiler, reversed circulation is impossible as the primary flow switch would cause a lock out if it runs in reverse. It stops numpty installers reversing the flow and returns on the install, or fitting the pump backwards. But in your case yes it’s not relevant. I reckon that you had an air locked pump. The heat you were feeling wasn’t circulated water running in reverse, it was convected heat, and due to how a system is piped that heat can show up in the flow or the return. Once the airlock clears then you’ll get normal flow restored. Water pumps can’t run in reverse. The orientation of the pump head is irrelevant, flow is determined by the pump body (it’s got an arrow on it). So unless you removed the pump then that’s impossible. where do I send my invoice?
I think we all know you should make some metal on metal thumpy noises, a lot of sighing and then some air through teeth sucking noises. Over your third brew, heavily criticise British Gas for a while then suddenly leave at midday to get parts. Return 3 days later and fix the problem.
I presume it must be convection then though I don't understand how cold water from the return could flow through the pump backwards while the pump is running. Can there be an airlock in the pump while water is flowing through it? Send your invoice to @Merlin Cat . She's supposed to be a heating engineer and should have come up with a better answer than ley lines!
Yes this can happen. It’s a process called cavitation. Basically air ‘sticks’ to the pump impeller. The impeller spins but it can’t ‘grip’ the water, so no circulation takes place. It sort of spins in its own bubble of air. The tale tale sign of this is often that the pump head gets very very hot, as it’s only the circulating water that cools it. im not a physicist so I can’t explain the proper process, but this is my understanding of it and I’ve seen it happen many times. It’s why pumps must not be installed with the head pointing upwards
If you have an induction motor based pump with a capacitor on a second winding to encourage it to start in a particular direction, if the capacitor fails or falls off it can either run backwards, forwards or sit buzzing.. Big single phase induction motors on compressors or expensive pressure washers and upwards have a contactor for the capacitor to provide phase shift on a second winding to get them to start in a particular direction then once running rely on the inertia of the rotor to follow the AC in a particular direction, disconnect the capacitor connected winding above a certain RPM. A cheaper solution on smaller induction motors is a cam and a peg that catches the rotor if it spins the wrong way and bounces it off in the correct direction. A bit of dirt on the cam and it becomes a 50:50 which way it spins.
