Pump or just capacitor?

[GeeTee]

Good to see some of the same faces knocking around on here - hopefully I can tease some knowledge out of their experienced heads

I have a Vaillant turboMAX Plus 837E combi (VUW GB362/1-5 R3) (GC# 47-044-028) that is playing up. On drawing hot water, the temperature on the boiler display rises extremely rapidly and results in an F22 code lockout*. On central heating, the radiators only just become warm and the boiler cycles more frequently than normal.

*From the manual: F.22 Dry fire protection. No water in appliance, pump cable defective, pump blocked or defective, pump output too low

A bit of numpty diagnosis later reveals that the pump is not spinning up although the shaft feels free and turns easily. So I drained down and pulled the pump head off (removed the 4 bolts).The impellor has a light black coating, but no major nasty build up. If I apply power to the pump head and manually set the impellor spinning it comes up to speed and stays spinning. It is however remarkably easy to stop spinning with a fairly light finger touch.

So the questions are - how much "oomph" should the pump have, should it be that easy to stop spinning? Or - is it just the capacitor that has given up the ghost so failing to kick the pump into life initially? Oh - and as I am no electronics engineer - does the capacitor only contribute to the inital start of the pump or does it also add some "oomph" when the pump is up to speed??

Other info that might be useful - the pump is type VP5/2 P/N 59866537 (Grundfos 161015).

Grateful for any advice (and grateful I remembered I had some thermal longjohns stashed away from years back!).

 

[petertheplumber12]

New pump head needed, sorry no easier answer, The resistance to stopping it turning is quite high, higher than you describe. capacitor won't make any difference, that is used to absorb the current while the motor builds up speed.

 

[GeeTee]

Thanks for the reply. I thought the pump had probably had it.

Are the pump heads or vaillant pump assemblies on these interchangeable with anything else?

I'm getting lost looking for replacement pumps online as the part numbers don't seem to match up. Some sites list the 160977 pump as the recommended replacement (my pump is marked 161015) whilst others only list the 160977 for the smaller boilers in this range. I'd rather not drop ~£120 on the wrong thing.

e.g. The Grundfos 15/65 pump heads look identical, right down to the location of the wiring box on top and the impellor pattern - impossible to know if it would be right without measuring I guess ~sigh~

 

[DP]

Why not remove the capacitor and measure the capacitance. Does the pump run if you spin the rotor when a demand is applied to the boiler? The capacitor kick starts the pump.

 

[Terrywookfit]

New pump head needed, sorry no easier answer, The resistance to stopping it turning is quite high, higher than you describe. capacitor won't make any difference, that is used to absorb the current while the motor builds up speed.

WRONG!!!!

A single phase a/c supply creates a pulsating magnetic field.
A/C motors require a rotating field to start.

The capacitor is wired in series with the start winding. This gives a 90deg phase difference between the currents in the start and run winding .
When the motor is turned on, the different magnetic fields in the two windings create the turning force to run the motor.

 

[Agile]

Exactly!

The torque depends on the capacitance value.

Those caps have a tendency to lose some of their value after a few years.

Its not such a problem with Grundfoss pumps but particularly common on Wilo and CP units.

Ideally measure the value of the cap.

You can read more in the FAQ on this site about the "finger test" !

Tony

 

[GeeTee]

Why not remove the capacitor and measure the capacitance. Does the pump run if you spin the rotor when a demand is applied to the boiler? The capacitor kick starts the pump.

My multimeter doesn't have the facility for testing capacitance. Yes the pump will run if I spin the rotor manually, although this only works with the pump head off, trying to do this with the pump mounted using a screwdriver in the end of the rotor doesn't achieve anything.

Thanks to Terry and Tony for the explanations. When spinning the rotor can be stopped easily with a very light finger touch.

 

[GeeTee]

It seems my local Maplins has suitable replacement capacitors in stock (N39KG) for a touch over £3, so I'll pick one up in the morning, drop it in the pump and test it out.

 

[matt1e]

New pump head needed, sorry no easier answer, The resistance to stopping it turning is quite high, higher than you describe. capacitor won't make any difference, that is used to absorb the current while the motor builds up speed.

WRONG!!!!

A single phase a/c supply creates a pulsating magnetic field.
A/C motors require a rotating field to start.

The capacitor is wired in series with the start winding. This gives a 90deg phase difference between the currents in the start and run winding .
When the motor is turned on, the different magnetic fields in the two windings create the turning force to run the motor.

As above, and for an extra bit of trivia
A lot of people refer to the capacitor as a start capacitor when actually it is a run capacitor (the motor wont start without it though )

Matt

 

[Agile]

If you are really careful you can measure the value of a capacitor of similar values to that one ( 400 vac and 2.6 uF ) by measuring the current taken from the mains.

But that means working at mains voltages and a silly mistake will take out your meter! Or worse electrocute you!

So its not advisable but it can be done!

Tony

 

[GeeTee]

For the sake of a short walk and £3 for a new capacitor I think I'll avoid messing around with live mains voltage. As you say, one clumsy mistake could see me 6ft under - I'd rather sacrifice the £3 than take the risk, no matter how small.

 

[Agile]

You dont need to use mains voltage though if you have a 12v or 24v ac source like a transformer.

Then the voltages will be non hazardous!

Tony

 

[GeeTee]

I have plenty of DC transformers knocking around, but nothing that outputs AC.

Even using lower voltages, wouldn't the capacitor be capable of delivering a fairly nasty jolt once charged up?

 

[matt1e]

I have plenty of DC transformers knocking around, but nothing that outputs AC.

Even using lower voltages, wouldn't the capacitor be capable of delivering a fairly nasty jolt once charged up?

nope, they only store voltage at the same level at what you put in

 

[GeeTee]

Thanks. This is the problem when you learn something at school and then never use it again for nigh on 30 years, it tends to fall out of memory