Cavitation on Rayburn Central Heating

[nathan_b_a]

Hi There,

I have an issue where a pump is constantly cavitating on a central heating system. Please see screenshot of installed schematic.

The system is new and installed as per the schematic supplied by the designer. All lines where bled as well as the pump.

The height between the top of the water in the header tank and the pump is approx 0.5m, so there is a small amount of static at the inlet to the pump.

Any suggestions?

Thank you

Nathan

 

[Johntheo5]

Is this a solid fuel stove/boiler?
The flow pipe should have a open vent that goes up and over the F&E cistern. Why (if) is there a swing check installed?. What height is the C&F cistern above the boiler?, what is the device installed before the manifold? what are the boiler flow & return temperatures?, what is the pump make/model and whar mode and setting is it on?

 

[fixitflav]

The flow pipe should have a open vent that goes up and over the F&E cistern.

Looks like a combined feed and vent. I'm not keen on that arrangement but from previous posts I thought you were a fan.

 

[fixitflav]

Wha

Hi There,

I have an issue where a pump is constantly cavitating on a central heating system. Please see screenshot of installed schematic.

What makes you think it's cavitating, is it the noise? If the pump isn't at minimum speed you could try reducing it.

 

[nathan_b_a]

Hi,

Yes, solid fuel stove (Rayburn 355sfw).

The hot flow pipe goes from the stove up into the cistern via the check valve. The system can thermosiphon with the cistern initially but then when it gets to temperature, the pump turns on and pushes water through the radiator circuit. In theory, the swing check valve closes when the pump turns on preventing circulation in the cistern. The swing check valve can always open and let water into the cistern if the water expands too much.

The pump is the triangle shaped object which is controlled via the thermostat. It is a smallish Wilo pump with 3 speeds.

The pump sounds like gravel going through it when it is cavitating. I have tried it on all different speeds.

I have bled the entire radiator circuit with mains pressure water, one loop at a time and filled the stove from the bottom up with mains pressure as well.

Nathan

 

[Johntheo5]

Looks like a combined feed and vent. I'm not keen on that arrangement but from previous posts I thought you were a fan.

I am but not with solid fuel, mine, even though combined, does carry the vent up and over the F&E cistern

 

[fixitflav]

even though combined, does carry the vent up and over the F&E cistern

Ah yes, I was forgetting that.

 

[Johntheo5]

Hi,

Yes, solid fuel stove (Rayburn 355sfw).

The hot flow pipe goes from the stove up into the cistern via the check valve. The system can thermosiphon with the cistern initially but then when it gets to temperature, the pump turns on and pushes water through the radiator circuit. In theory, the swing check valve closes when the pump turns on preventing circulation in the cistern. The swing check valve can always open and let water into the cistern if the water expands too much.

I have never seen a system like this.
What direction is the flow through the swing check, does it open inwards to the F&E cistern or outwards from the F&E cistern.
I installed a 6M Wilo Yonos Pico in a relations solid fuel system recently, he only runs it in constant pressure (CP) mode at between 0.5M and 1.5M head, depending on how much fuel he is burning, the head (pressure) can be incrementally changed in 0.1M steps, rough idea of his system, below, the cold feed is teed into the coil return and the system works very well.

 

[nathan_b_a]

The check valve opens into the F&E cistern.

 

[Johntheo5]

The check valve opens into the F&E cistern.

So, the theory is that when the pump is running then the check valve closes, which means it would have been pulling air in through a conventional up and over vent pipe?,
I've asked before, what is the item just before the manifold?.

 

[nathan_b_a]

The item on the cold return after the manifold is a dosing pot.

 

[Johntheo5]

Thanks,
If you reduced the stat setting low to say 30C which means the pump should start circulating at a boiler water flow temp of ~ 20/25C, do you still get cavitation?.
At 25C the pump needs a suction head (NPSH) of ~ 0.5M, at 60C it requires ~ 2M and at 90C and ~ 3M at 80C.
NPSH = Atmos pressure (absolute) + suction head - the suction pipe friction loss - the water vapour pressure. At 25C the vapour pressure is 0.3M, at 60C is 1.9m and at 4.7m at 80C, atmospheric pressure (absolute) is 10M, your suction head is 0.5M?.
If the pump is cavitating at 25C then the pipe friction loss/restriction would need to be (10+0.5-0.3)-0.5, 9.7M. At 60C, (10+0.5-1.9)-2, 6.6M, At 80C (10+0.5-3.0)-4.7, 1.8M.
If you use 60C as a typical flow temperature then the pipe friction loss would need to be as high as 6.6M, no way except some restriction or problem elsewhere IMO. Even if you are pulling water at 25LPM through 20M of 1ins ID piping, the pipe friction loss is only ~ 0.8M.

What is the piping ID and length from the boiler to the pump suction/

 

[nathan_b_a]

I get cavitation with cold water (fire turned off) as well as hot.

The pipe ID is 1 inch most of the way with the last ~400mm branch to the pump 20mm. See picture.

The suction pathway when the check valve is closed is all the way from header tank, through the wet back stove then back up to the pump.

 

[Johntheo5]

what height above the boiler top is the pump?

 

[nathan_b_a]

The pump is around 3m above the boiler top. The stove and radiators are on the ground floor and the pump and cistern plus manifold in the loft on the second floor.