Pump flow problem?

[heatme]

I offered to repair a combi boiler that had a diverter valve problem. The hot water side didn't work, but the heating side worked. I changed the Diaphragm in the diverter valve and the diverter now works. On re-starting the boiler, the pump starts when called to, but the pump flow switch doesn't start the flame lighting sequence (operates by the output of the pump on one side of a diaphragm and the input on the other). The water circulates and if the switch is switched on manually the boiler works ok and heats the water, also the hot water when called to. The pump motor is difficult to stall when dismantled. The pump flow diaphragm works when blown by mouth and doesn't leak. There doesn't seem to be any blockages and the components just fairly clean cnsidering.

Any ideas? EM

 

[ChrisR]

But which model of Ariston is it?

 

[heatme]

I'm not sure what make it is. If it's important I can find out in a day or two.

 

[ChrisR]

SOme are quite sensitive to pumps going slow. If your puff is enough to switch the sensor, it should work OK with a 5 or 6 metre head pump!

 

[heatme]

Hi,

ChrisR,

It does seem to be the pump pressure, but I am reluctant to ask them to buy a new one, till I understand that pumps can loose their power. The pump pressure was strong enough last time they tried it.

EM

 

[ChrisR]

Is it the same for CH and HW?

 

[heatme]

Yes.

I tried turning the central heating feed ball valve off, to see if a restriction would help and it tried to lift the diaphragm but not enough.

 

[ChrisR]

Check all the waterways are clear, and any Bundy tubing (the 4mm stuff). Also take a really good look at where the pressure-sampling points, connected across the diaphragm, are taken off the circuit. Check for any auto- or fixed, internal bypass.

A cheap tachometer would have resolved the question ages ago. I presume you don't have one, so get a new pump, check the speeds should be similar (printed on the end or in the data sheet) and run them side by side, then load them with a finger and see what happens. If you rest a small screwdriver point on the rotor you should hear the difference. Quite often it's the Capacitor in the pump which is responsible for them running slowly - pinch one from a new pump to try.
You "shouldn't run pumps dry" but a few trials of a few seconds won't damage anything.

No chance any tubes have got connected swapped over is there? (yes i have managed it!)

 

[heatme]

Ok, thanks. It will be later in the week now. I'll let you know what happens.

 

[heatme]

What happens if the pump is run dry for a longer period?

I mention this because when the boiler was tried immediatelly after the diverter valve repair, the pump probably did run dry for quite a while, before I realised that the pump automatic bleed was blocked and not working properly.

 

[ChrisR]

They certainly CAN get stuffed quite quickly. Eg it has happened to me while bleeding a reluctant ch system. After 15 or so minutes it was full of water but the pump wouldn't then go fast enough to pump. Wasn't making much noise either.

 

[heatme]

I bought a new off the shelf pump today, they told me was a standard fit, but the fittings are 1/2"BSP instead of the existing 1/4" BSP (??). The owner doesn't like the idea of me changing the fittings as I would have to cut off the old ones off his boiler and solder on the new ones. The pipes are silver, and I think they are Brass, but I am pretty confident that I could do it ok. I think he will try to get another pump with the correct ends, so this might take him a while.

 

[chrishutt]

I think you have the pump thread sizes muddled up. The standard thread on domestic sized CH pumps is 1.5", but combi boiler pumps often have smaller threads, typically 1". I'm not sure if 1.25" exists as an option.

The simplest way of replacing your pump is to buy a standard one of the same make as the old one (Grundfos, Wilo, etc.) and then change the head over. This is held on with 4 allen key bolts. Make sure you also match the power rating ("head") of the existing pump - 5 metre or 6 metre.

 

[ChrisR]

I would have to cut off the old ones off his boiler and solder on the new ones. The pipes are silver, and I think they are Brass,

 

[heatme]

I did put (??) after the 1 1/4inch because I wasn't sure, but they are 1" on the existing old pump.

I have tried to get hold of the manufacturer but this is proving a bit difficult, I'll keep trying.

I was thinking about cutting the 1" fittings off the boiler connecting pipes and soldering 1 1/2" fittings to accomodate an off the shelf pump. On further investigation I found that of course valves are normally used each side of the pump. As these would not fit into the boiler housing I realise that i have to get a 'proper' fitting pump.

One thing to consider is that on the old pump there is an automatic bleed valve built into it's casting.

So now I am on the trail of a proper pump or the motor side that will fit onto the old pump.